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Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2014-12-31.

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Permanent Link: http://ufdc.ufl.edu/UFE0043720/00001

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Title: Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2014-12-31.
Physical Description: Book
Language: english
Creator: Muthiah, Palanikkumaran
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Statement of Responsibility: by Palanikkumaran Muthiah.
Thesis: Thesis (Ph.D.)--University of Florida, 2011.
Local: Adviser: Sigmund, Wolfgang M.
Electronic Access: INACCESSIBLE UNTIL 2014-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0043720:00001

Permanent Link: http://ufdc.ufl.edu/UFE0043720/00001

Material Information

Title: Record for a UF thesis. Title & abstract won't display until thesis is accessible after 2014-12-31.
Physical Description: Book
Language: english
Creator: Muthiah, Palanikkumaran
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2011

Subjects

Subjects / Keywords: Materials Science and Engineering -- Dissertations, Academic -- UF
Genre: Materials Science and Engineering thesis, Ph.D.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Statement of Responsibility: by Palanikkumaran Muthiah.
Thesis: Thesis (Ph.D.)--University of Florida, 2011.
Local: Adviser: Sigmund, Wolfgang M.
Electronic Access: INACCESSIBLE UNTIL 2014-12-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2011
System ID: UFE0043720:00001


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1 RAPID WETTABILITY SWITCHING INDUCED BY THERMAL CHANGES IN NANOFIBERS By PALANIKKUMARAN MUTHIAH A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2011

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2 2011 Palanikkumaran Muthiah

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3 To m y l ovely f amily

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4 ACKNOWLEDGMENTS I would like to thank Dr Wolfgang Sigmund, who is not only just an adviser but an incredibly energetic mentor and scientist. His vision, compassion, support, understanding and guidance helped me through out the entire work. I would also like to thank my committee members : Dr Brij Moudgil, Dr Christopher Batich Dr Hassan El Shall, and Dr Anuj Chauhan for their constructive comments and support I would like to thank Dr Laurie Gower for her support and help in using the Optical Microscope. I also extend thank to Dr. Timothy J Boyle group at Sandia National Labs, New Mexico, for their support in carrying o ut several experiments there. I would like to recognize the help of the staff members of MAIC (Materials Analytical Instrument Center) and PERC (Particle Engineering Research Center) regarding the characterization of the surface morphology and contact angl e measurements. I would like to acknowledge all the past and current students from group for assisting me in many ways during my work. I particularly thank Shu Hau Hsu and Rui Qing for th eir help in capturing videos and Krishna for his help with MATLAB and guidance in calculating temperature decrease in silicon (Si) wafer. Special thanks would also need to go to the people who had lived on campus for sharing the ir life in Gainesville. Last but not least, I am extremely indebted to my parents and my wife (Pravina) for their love and unselfish support throughout my study in United States. Without them, this dissertation would have never been accomplished.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 8 LIST OF FIGURES ................................ ................................ ................................ .......... 9 LIST OF ABBREVIATI ONS ................................ ................................ ........................... 12 ABSTRACT ................................ ................................ ................................ ................... 16 CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 18 Motivation ................................ ................................ ................................ ............... 18 Hypothesis and Rapid Response Time Expected ................................ ................... 18 Specific Objectives ................................ ................................ ................................ .. 22 Overview of the Dissertation ................................ ................................ ................... 22 2 BA CKGROUND ................................ ................................ ................................ ...... 27 Stimuli Responsive Polymers ................................ ................................ ................. 27 Reversible Hydrogen Bonding in Poly(N isopropylacrylamide) ........................ 27 Lower Critical Solution Temperature ................................ ................................ 28 Gap Analysis for Quick Response Temperature Responsive Polymers ........... 29 Electrospinning ................................ ................................ ................................ ....... 30 Developments in Electrospinning: His tory ................................ ........................ 31 Forces Involved at the Cone Formation ................................ ............................ 32 Factors Affecting Electrospinning Performance and Nanofiber Quality ............ 32 Polymer concentration in electrospinning solution ................................ ..... 33 Solvent and solution conductivity ................................ ............................... 33 Surface tension ................................ ................................ .......................... 34 Dielectric constant ................................ ................................ ...................... 34 Applied voltage ................................ ................................ .......................... 34 Feed rate ................................ ................................ ................................ .... 35 Capillary tip diameter ................................ ................................ ................. 35 Gap distance ................................ ................................ .............................. 35 Wettability ................................ ................................ ................................ ............... 36 3 FABRICATION OF NAN OFIBERS VIA ELECTROSPINNING ................................ 43 Experimental ................................ ................................ ................................ ........... 44 Materials ................................ ................................ ................................ ........... 44

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6 Synthesis of Crosslinked Poly[(n isopropylacrylamide) co (methacrylic acid)] Fibermats ................................ ................................ ............................ 44 Synthesis of PS/PNIPA Blended Fibermats ................................ ..................... 45 Characterization ................................ ................................ ................................ ...... 46 Morphology and Structure Analysis ................................ ................................ .. 46 Mean Fiber Size Analysis. ................................ ................................ ................ 46 Results and Discussions ................................ ................................ ......................... 46 Synthesi s of Crosslinked PNIPAMAA Fibermats ................................ .............. 47 Electrospinnability of PNIPAMAA copolymer solution ................................ 47 Electrospinnability of PNIPAMAA formulation ................................ ............ 49 Crosslinked PNIPAMAA fibermats with different diameter fibers ............... 51 Synthesis of Polystyrene/Poly(N isopropylacrylamide) Blended Fibermats ...... 52 PS/PNIPA blended fibermats w ith various diameter fibers ......................... 52 PS/PNIPA blended fibermats with different thickness fibermats ................ 53 Chapter Summary ................................ ................................ ................................ ... 54 4 THERMALLY TUNA BLE SURFACE WETTABILITY OF ELECTROSPUN FIBERMATS ................................ ................................ ................................ ........... 70 Experimental Section ................................ ................................ .............................. 70 Determination of Transition Temperature ................................ ......................... 70 Contact Angle Measurements ................................ ................................ .......... 71 Setup for temperature dependent contact angle measurements ............... 71 Determination of fraction of the wet solid contact area .............................. 72 Determination of immobilization in fibermats ................................ .............. 73 Results and Discussion ................................ ................................ ........................... 73 Fibermat Morphology ................................ ................................ ........................ 73 Fibermats Thermoresponsive Reversible Wettability ................................ ....... 73 Fiberm ats Integrity ................................ ................................ ............................ 75 Mechanism behind Reversible Wettability ................................ ........................ 78 Chapter Summary ................................ ................................ ................................ ... 79 5 THERMORESPONSI VE SWELLING DESWELLING PROPERTIES OF ELECTROSPUN CROSSLINKED FIBERMATS ................................ ..................... 91 Experimental ................................ ................................ ................................ ........... 91 Characterization of Thermoresponsive Transitional Properties ........................ 91 Characterization of Thermoresponsive Cyclability ................................ ............ 92 Results and Discussion ................................ ................................ ........................... 92 Thermoresponsive Transitional Properties ................................ ....................... 92 Thermoresponsive Cyclability ................................ ................................ ........... 94 Mechanism for Thermoresponsive Swelling Deswelling ................................ .. 94 Chapter Summary ................................ ................................ ................................ ... 95 6 RESPONSE TIME STUDIES ON ELECTROSPUN FIBERMATS .......................... 99 Experimental ................................ ................................ ................................ ........... 99

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7 Characterization of Response Time on Crosslinked PNIPAMAA Fibermats .... 99 Contact Angle Measurments on PS/PNIPA Blended Fibermats ..................... 100 Characterization of Response Time on PS/PNIPA Blended Fibermats .......... 100 Results and Discussions ................................ ................................ ....................... 101 Response Time on C rosslinked PNIPAMAA Fibermats ................................ 101 Response Time on PS/PNIPA Blended Fibermats ................................ ......... 102 Mechanism for Droplet Spreading Over the Fibermat ................................ .... 105 Chapter Summary ................................ ................................ ................................ 106 7 CONCLUSIONS AND SUGGESTIONS FOR FUTURE WORK ............................ 136 Conclusions ................................ ................................ ................................ .......... 136 Suggestions for Future Work ................................ ................................ ................ 137 APPENDIX: TEMPERATURE DECREASE IN SILICON WAFER ............................... 139 LIST OF REFERENCES ................................ ................................ ............................. 146 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 153

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8 LIST OF TABLES Table page 1 1 The estimated characteristic swelling time to achieve equilibrium for different diameter fibers ................................ ................................ ................................ .... 25 3 1 Solvents used in electrospinning PNIPAMAA copolymer and observations made during and after electrospinning ................................ ............................... 55 3 2 Attempts made to produce a formulation of copolymer solution, crosslinker and crosslinking a id and trials made on electrospinning the formulation ............ 57 3 3 Formulations used and electrospinning operating conditions to synthesize PNIPAMAA fibermats with different diameter fibers and different thickness fibermats ................................ ................................ ................................ ............. 59 3 4 Blend solution concentrations and operating conditions to produce different diameter fibers in electrospinning ................................ ................................ ....... 60 3 5 Electrospinning operating conditions to fabr icate PS/PNIPA blended fibermats with various thickness fibermats ................................ ......................... 61 4 2 Amount of immobilization or crosslinking of PS/PNIPA blended and crosslinked PNIPAMAA fibermats. ................................ ................................ ..... 82 6 1 Comparison of calculated contact angle values with Cassie Baxter con tact angle values (for contact angle images see Figure 6 2) ................................ ... 108 6 2 Response time studies on PS/PNIPA blended fibermats with differe nt diameter fibers ................................ ................................ ................................ .. 109 6 3 Response time studies on PS/PNIPA blended fibermats with different thickness fibermats ................................ ................................ ........................... 110 A 1 The temperature decrease in silicon (Si) wafer was considered as an initial Si,t =0 = 65 C and iteratively calculated using MATLAB as shown here. ................................ ................................ ................................ 142 A 2 The time for temperature to reach 30 C is reported for various dimension silicon (Si) wafers ................................ ................................ ............................. 143

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9 LIST OF FIGURES Figure page 1 1 Schematic diagram showing the concept behind temperature responsive polymer switching ................................ ................................ .............................. 26 2 1 Poly(N isopropylacrylamide) (PNIPA) chemical formula depicted with hydrophilic gro up and hydrophobic groups. ................................ ........................ 38 2 2 Schematic diagram showing the temperature dependent conformational change occurs in a temperature responsive polymer (TRP) ............................... 38 2 3 Schematic diagram showing the lower critical solution temperature (LCST) in a binary system ................................ ................................ ................................ .. 39 2 4 Schematic diagram showing the electrospinning process with a setup. ............. 40 2 5 Schematic diagram showing the forces acting on a droplet as it emerges from the electrospinning needle. ................................ ................................ ......... 41 2 6 Cassie Baxter (CB) model: wetting behavior of vapor pockets that are trapped between the grooves and the liquid droplet ................................ ........... 42 3 1 P oly[(n isopropylacrylamide) co (methacrylic acid)] (PNIPAMAA) chemical formula ................................ ................................ ................................ ............... 62 3 2 Digital photographic image showing electrospinning setup used in this dissertation ................................ ................................ ................................ ......... 63 3 3 Scanning electron microscopy (SEM) images of PNIPAMAA fibermat. .............. 64 3 4 Optical and SEM micrographs of PNIPAMAA fibermats ................................ ..... 65 3 5 Optical and SEM micrographs of PNIPAMAA fibermats ................................ ..... 66 3 6 Crosslinked PNIPAMAA fibermats morphology ................................ .................. 67 3 7 SEM micrographs of polystyrene ( PS ) /PNIPA blended fibermats obtained with various diameter fibers ................................ ................................ ................ 68 3 8 SEM micrographs of PS/PNIPA blended fibermats synthesized with various thickness fibermats ................................ ................................ ............................. 69 4 1 Digital photographic images of PS/PNIPA blended fibermats ............................ 83 4 2 SEM micrographs of PS/PNIPA blended fibermat s ................................ ............ 84

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10 4 3 A photographic image of contact angle Goniometer instrument used in this dissertation ................................ ................................ ................................ ......... 85 4 4 Differential scanning calorimetry ( DSC ) graph of PNIPAMAA dissolved in water and hydrated crosslinked PNIPAMAA fibermat ................................ ......... 86 4 5 Temperature responsive switchability of PS/PNIPA blended fibermats and crosslinked PNIPAMAA fibermats ................................ ................................ ...... 87 4 6 Temperature responsive reversibili ty of PS/PNIPA blended and crosslinked PNIPAMAA fibermats ................................ ................................ ......................... 88 4 7 SEM images of the section of PS/PNIPA blended and crosslinked PNIP AMAA fibermats after 5 heating and cooling cycles ................................ ... 89 4 8 The fraction of contact area is estimated by adjusting the threshold of top view SEM pictures ................................ ................................ .............................. 90 5 1 Temperature dependent transition of crosslinked PNIPAMAA fibermat s ............ 96 5 2 Temperature responsive cycles on crosslinked PNIPAMAA fibermats ............... 97 5 3 Video graphic image demonstrating the t hermoresponsive swelling deswelling behavior of crosslinked PNIPAMAA fibermat ................................ .... 98 6 1 Transition kinetics on crosslinked PNIPAMAA fibermats ................................ 111 6 2 Contact angle (CA) images showing 4 l water droplet placed on PS/PNIPA fibermats ................................ ................................ ................................ ........... 112 6 3 Photographic image showing the setup for conducting response time studies on PS/ PNIPA blended fibermats ................................ ................................ ....... 113 6 4 Video images showing frames captured for every 0.5 s of fibermat BLD380nm ................................ ................................ ................................ ........ 114 6 5 Video images showing frames captured for every 0.5 s of fibermat BLD990nm ................................ ................................ ................................ ........ 11 5 6 6 Video images showing frames captured for every 0.5 s of fibermat BLD1.5m 116 6 7 Video images showing frames captured for every 0.5 s of fibermat BLD16m. 117 6 8 Video images showing frames captured for every 0.5 s of fibermat BLD16m on cold water stage ................................ ................................ .......................... 123 6 9 Video images showing frames captured for every 0.5 s of fibermat BLD380nm on cold water stage ................................ ................................ ........ 129

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11 6 10 Video images showing frames captured for every 0.5 s of fibermat BLt83 m. 132 6 11 Video images showing frames captured for ev ery 0.5 s of fibermat BLt163 m. ................................ ................................ ................................ ........ 133 6 12 Video images showing frames captured for every 0.5 s of fibermat BLt258 m. ................................ ................................ ................................ ........ 134 6 13 Schematically drawn fibermat cross section with dye solution droplet sitting above the fibermat ................................ ................................ ............................ 135 6 14 Changes in water contact angle with changes in diameter of capillary pores at 1 atm pressure. ................................ ................................ ............................. 135 A 1 Setup showing how silicon (Si) wafer was placed on metal bar and description on how this was considered as one body ................................ ....... 144 A 2 With changing time, the temperature decrease in Si wafer is plotted for various dimension Si wafers ................................ ................................ ............ 145

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12 LIST OF ABBREVIATION S Al foil Aluminium foil BLD380nm Polystyrene / poly(N isopropylacrylam ide ) blended fibermat with fiber diameter of 380 nm BLD990nm Polystyrene/poly(N isopropylacrylamide ) blended fibermat with fiber diameter of 990 nm BLD1.5m Polystyrene/poly(N isopropylacrylamide ) blended fibermat with fiber diameter of 1.5 m BLD16m Polystyrene/poly(N isopropylacrylamide ) blended fibermat with fiber diameter of 1 6 m BLD30m Polystyrene/poly( N isopropylacrylamide) blended fibermat with fiber diameter of 30 m BLt83 m Polystyrene/poly(N isopropylacrylamide) blended fibermat with fibermat thickness of 83 m BLt163 m Polystyrene/poly(N isopropylacrylamide ) blended fibermat with fibermat thickness of 1 6 3 m BLt258 m Polystyrene/poly(N isopropylacrylamide ) blended fibermat with fibermat thickness of 258 m PS/PNIPA Polystyrene/poly(N isopropylacrylamide) BL PS/PNIPA Polystyrene/poly(N isopropylacrylamide) blended BTEAC B enzyl triethyl ammonium chloride c P olymer concentration c* C ritical polymer concentration required for chain overlap CA C ontact angle CB Cassie and Baxter D coop Cooperative diffusion coefficient of the network in the swelling solvent D cp Gap distance /d istance between the terminus of the capillary tip to the surface of the collector DC Direct current

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13 DI H 2 O Deionized water DMF D imethylformamide DMSO D imethyl sulfoxide DSC Differential scanning calorimetry DSHP D isodium hydrogen phosphate EF E lectric field Espg Electrospinning ETEOS E thylene triethoxy silane FR F low rate f s Fraction of the wet solid contact area LCST L ower critical solution temperature MAA M ethacrylic acid Mn Number average molecular weight Mw Weight average molecular weight MW Molecular weight MWD M olecular weight distribution NaCl Sodium chloride NIPA N isopropylacrylamide OHAc A cetic acid PAAc P oly(acrylic acid) PAN Polyacrylonitrile PEG P oly ( ethylene glycol ) PLGA P oly(lactide co glycolide) PMMA P oly(methyl methacrylate) PNIPA P oly(n isopropylacrylamide)

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14 PNIPAMAA P oly[(n isopropylacrylamide) co (methacrylic acid)] with with 9 0 mol% of n isopropylacrylamide and 10 mol% of methacrylic acid PS P olystyrene PVA Poly(vinyl alcohol) r Smallest gel dimension in the swollen state RT R oom temperature SE S econdary electron SEM Scanning e lectron m icroscope Si S ilicon SRPs Stimuli responsive polymers NG N eedle gauge TEM Transmission e lectron m icroscope THF Tetrahydrofuran TRPs Temperature responsive polymers UCST Upper c ritical s olution t emperature V A pplied voltage XL PNIPAMAA C rosslinked p oly[(n isopropylacrylamide) co (methacrylic acid)] fibermats XLD2m Crosslinked poly[(n isopropylacrylamide) co (methacrylic acid)] fibermat with fiber diameter of 2 m XLD241nm Crosslinked poly[(n isop ropylacrylamide) co (methacrylic acid)] fibermat with fiber diameter of 2 42 n m XLD870nm/t50m Crosslinked poly[(n isopropylacrylamide) co (methacrylic acid)] fibermat with fiber diameter of 870 nm and fibermat thickness of 50 m XLD870nm/t150m Crosslinked poly[(n isopropylacrylamide) co (methacrylic acid)] fibermat with fiber diameter of 870 nm and fibermat thickness of 1 50 m

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15 CB A pparent contact angle on a rough surface /Cassie Baxter contact angle Y E surface Characteristic t ime for diffusion to achieve equilibrium w i Initial weight W f Final weight

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16 Abstract of Dissertation Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy RAPID WETTABILITY SWITCHING INDUCED BY THERMAL CHANGES IN NANOFIBERS By Palanikkumaran Muthiah December 2011 Chair: Wolfgang M. Sigmund Major: Ma terials Science and Engineering Temperature responsive polymers ( T RPs) show a transformation from one conformation al state to another as a response to temperature stimuli from the environment This transformation is reversible. These polymers offer a big chance for creating intelligent materials Mostly, TRPs are polyme rized with crosslinker and used in hydrogel form. This results in number of drawbacks including a slow response R esponse rate of hydrogel is a diffusion controlled process as it depends on hydro gel thickness A simple way to improve the response rate is to reduce the size of the hydrogel. Electrospinning is a promising way to reduce the size to nanometer dimensions as this process produce s nanofibers using electrostatic forces In this dissertation p oly[( N isopropylacrylamide) co (methacrylic acid)] (PNIPAMAA) and poly( N isopropylacrylamide) (PNIPA) were selected as temperature responsive polymers and two different approach es one by crosslinking and another by blending these polymers were processed into nano fibers PNIPAMAA was mixed with a crossli nker and a crosslinking aid in acet ic acid (OHAc) to prepare a formulation. T his was electrosp u n and heat treated at 160 C to synthesize crosslinked PNIPAMAA fibermat s In another approach, PNIPA was simply blended with polystyrene (PS) in

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17 dimethylformamide ( DMF ) to make a blend solution. This was electrosp un to synthesize PS/PNIPA blended fibermat with different diameter fibers ranging between 100s nm and 10s of m Both the PS/PNIPA blended and crosslinked PNIPAMAA fibermat s were characterized for thermoresponsive surface wettability in Goniometer by sessile drop method. Particularly, PS/PNIPA blended fibermat s swithcing behavior was between (approaching) F ibermat s c haracterized for their structural integrity in 10 C water demonstrated that the crosslinked PNIPAMAA fibermat s as expected had better integrity T hus the crosslinked PNIPAMAA fibermat s were further characterized for thermoresponsive swelling deswelling b ehavior R esponse time on crosslinked PNIPAMAA and PS/PNIPA blended fibermat s were studied by gravimetric method and maximum to minmum contact angle change method respectively Both the fibermat s have shown that at the nanometer dimensions the response was quick Importantly, PS/PNIPA blended fibermat s demonstrated that the re is an influence of fiber diameter on response time w ith 380 nm diameter fiber exhibited 4.20.9 s response time

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18 CHAPTER 1 INTRODUCTION Motivation The main motivat ion for this research work was when responsive polymers processed into nanostructures or nanocomposites may enhance their responsive properties F or example, nanofibers with nanoparticle system may be designed in such way that nanoparticles move in and out of the structure when nanoparticles are tethered to responsive polymer chain and provide an improved responsive wettability property This change can be rapi d with nanostructures. Such functional property may not be achieved with current materials Further motivation on the stand point of potential application was making responsive tires. For example, car tires may skid during rain R esponsive tires will redu ce skidding by read ing the road as the tires are rotating They are designed to adjust them se lves respond to wet /dry or water/ice conditions and provide an enhanced safety to the driver by improving their adhesion with road It is important to discuss some economic aspect s associated with tire industry to understand the significance of this research. T here are about 45 tire manufacturing factories in the world. I n 2004, $80 billion worth of tires were sold worldwide This figure was raised to $140 billi on in 2010 [ http://en.wikipedia.org/wiki/Tire ]. The bottom line is t ire manufacturing has a multi billion dollar market and p roducts that are designed to improve the tire performance will hav e a significant e conomic potential Hypothesis and Rapid Response Time Expected The main hypothesi s of this work is that nanofibers of responsive polymers will improve response rate by six orders of magnitude over bulk materials and show rapid

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19 response (sub second) in surf ace energy, water absorption capacity and mechanical properties with changes in environmental conditions (e.g. temperature). This six order improvement in response rate was theoretically calculated for 500 nm diameter fiber. Temperature as the stimulus was of interest from the potential application where these materials can be applied as stated above in the motivation. The idea was with changes in temperatures the system should changes from a hydrophilic state to hydrophobic a nd vice versa In the hydrophilic state, the system should absorb water and swell and in the hydrophobic state, it should release the absorbed water and collapse. T emperature responsive polymer ( TRP ) gives such property The concept behind the temperature responsive switching is schematically shown in Figure 1 1. Poly(N isopropylacrylamide) ( PNIPA ) is a typical TRP. PNIPA has lower critical solution temperature ( LCST ) at 32 C in water. Its polymer chains change from hydrophilic state to hydrophobic and vic e versa at 32 C with water This change occurs in TRP due to its polymer chain conformational change between expanded and collapsed state at the transition. TRPs show reversible hydrogen bonding capabilit y with water molecules as they transform through L CST [1 3] T his transformation is reversible. Since on e of the transition state s is in soluble form, TRP chains have to be either chemically or physically integrated to make use of their responsive polymers when processed into useful structures. In general TRPs are polymerized into crosslinked network structure s and used in hydrogel form Polymer gel is a cross linked polymer network immersed in a liquid that can change their volume in response to external stimuli [4, 5] When they are used in the hydrogel form, they show a slow response This is due to skin layer

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20 formation at the surface of thick hydrogels T his leads to internal stress build up during swelling This slows down the response rate of hydrogels to several hours [4 6] The response time of a hydrogel is strongly in fluenced by the diffusion rate at which water go in and come out of the hydrogel structure This in turn depends upon the gel thickness. Faster response time is essential for almost in every application R esponse time of a hydrogel is directly proportional to the square the gel dimension and inversely proportional to the network diffusion coefficient Th is relation is expressed as : = r 2 / D coop (Equation 1 1 ) where, is the characteristic swelling time for diffusion to achieve equilibrium, r represents the smallest gel dimension, and D coop is the cooperative diffusion coefficient of the network D coop for PNIPA macrogel s in water is 5.0 0.3 10 11 m 2 s 1 [7] and for PNIPA mini gel s in water is 6. 0 0.2 10 11 m 2 s 1 [8] The numerical value of D coop varies between the10 1 2 10 10 m 2 s 1 depending upon the crosslinking density, polymer concentratio n and temperature etc Tanaka et al. found an inverse correlation between D coop and temperature, an order of difference in diffusion coefficients equal to 5 10 12 and 2 10 11 m 2 s 1 for de swollen ( temperature > 32 C) and swollen ( temperature < 32 C) PNIPA gels in water respectively [9] It is not easy to increase the value of D coop by a factor of 10 2 or more [10 13] [7, 8, 14 20] Thus the actual response time is largely depends upon gel thickness. The response time to reach a steady state is lower than the time to achieve equilibrium. The simplest way to improve the response time is to reduce hydrogel dimensions It is reported that w hen hydrogels are processed into thinner structures, they show poor

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21 mechanical properties. Several other approaches which result in a faster response time have been reported [6, 13, 21 30] but the response rate achieved by these methods showed only a modest improvement It is expected that w hen TRPs are processed with high aspect ratio s fibers the diffusion path for the transformation is shorter This will show a fast response rate Characteristic swelling time calculated for various diameter fibers using (Equation 1 1) is reported in Table 1 1. D coop value denoting a cooperative diffusion coefficient of PNIPA minigels in water 6.00.210 11 m 2 s 1 was used in the calculation. A study concludes that D coop values found for PNIPA macrogels and minigels were on the same order of magnitude [17] In that study, disk type PNIPA macrogels dimension was 15 mm diameter and spherical PNIPA minigels dimensions were ranging between 8 and 60 m It is important to know that even though there were 3 orders of difference between gel dimension s no significant difference in D coop values obtained. Although t here are orders of magnitude difference in dimensions between PNIPA minigels and nanofibers as this dissertati on propose s to produce the chosen D coop value in the calculation is a closer approximation to nanofibers However, the difference in structure of material itself (gels vs electrospun fiber mat ) difference in polymer (PNIPA vs. blend of PNIPA vs. copolyme r of PNIPA as this proposes to use), curvature in nanofibers, voids present in electrospun fibermat difference in crosslinking density may influence the D coop value in this study From the table it can be observed that fibers ranging between 100 nm and 1000 nm are capable of reaching the c haracteristic time for swelling to achieve equilibrium in microseconds to milliseconds

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22 There are several ways to process responsive polymers into thinner fibers Electrospinning is o ne very promising way It uses electrostatic forces to produce fibers typically in the ranges of few microns to 10s of nanometer s Electrospun fibers with this smaller dimension are expected to have a large surface area to mass ratio F ibers obtained through electrospinning process have potential to be used in various applications including creating intelligent materials [31 33] Specific Objectives With above thoughts the following are the specific objectives of this research: 1. Selecting suitable temperature responsive polymer and fabricating into various diameter fiber s ranging between 10s m and 100s nm by electrospinn ing process 2. Characterizing the obtained fibermat s for their structural integrity by leaching experiments in 10 C water and further characterizing these fibermats for temperature responsive wettability by Sessile drop method in Goniometer. 3. Characterizing selected fibermat s for thermoresponsive swelling deswelling properties by gravimetrically in terms of their transitional properties and cyclability 4. Characterizing electrospun fibermats response time for these electrospun fibermat s to change from one physical state to another Overview of the D issertation This research work address es the is sues and limitations of stimuli responsive hydrogels. This dissertation is divided into seven chapters. The first chapter, the present one, introduces the motivation behind this research work, hypothesis assumed followed by specific objectives The second chapter titled discuss es in detail temperature responsive polymers (TRPs) in particular mechanism behind reversible hydrogen bonding in TRP drawbacks and limitations of hydrogels, electrospinning process and factors that influence its performance, and finally on stimuli responsive wettability

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23 The third chapter titled, Fabricati on of Nanofibers via Electrospinning discusses the synthesis of temperature responsive polymers into nanofibers P olystyrene (PS) /poly(N isopropylacrylamide) (PNIPA) and poly [ (N isopropylacrylamide ) co (m ethacrylic acid) ] (PNIPAMAA) fibermats were synthesized by blending and crosslinking respectively This chapter reports, m ore on electrospinnability of these polymers with a right formulation with what conditions that produce fibers Further details on characteriz ing the obtained fib ermat s for their surface morphology and fiber size are discussed The fourth third chapter titled, Tunable Surface Wettability of Electrospun Fibermat discusses the temperature responsive wettability of PS/PNIPA blended and crosslinked PNIPA MAA fibermat s. Fibermat s synthesized were characterized for contact angle values in Goniometer by Sessile drop method The chapter further discusses the fibermat s integrity by leaching experiments and f inally, it closes with discussing the mechanism behind this reversible wettability The fifth chapter titled, Thermoresponsive Swelling Deswelling Properties of Crosslinked PNIPAMAA Fibermat s discusses the temperature responsive transitional and cyclabi lity properties of crosslinked PNIPAMAA fibermat s characterized by gravimetric method. The sixth chapter titled, Fibermat discusses the response time for the crosslinked PNIPAMAA fibermat s and PS/PNIPA blended fibe rmat s to change from one state to another P articularly for the PS/PNIPA blended fibermat s, th is chapter adopts a method called contact angle change to study the response time The response time obtained for different diameter fibers were

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24 analyzed subsequently The seventh chapter titled, concludes the findings from this research work with their interpretations Finally, the dissertation ends with suggestions for future work.

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25 Table 1 1. The e stimated characteristic swelling time to achieve equilibrium for different diameter fiber s F iber dia meter ( m) = r 2 /D coop (s) a 0.1 42 0.5 1 042 1 4 167 10 416 667 50 10 416 667 a r is the fiber radius; D coop value used in the calculation was 6.00.210 11 m 2 s 1 ( cooperative diffusion coefficient for the poly(N isopropylacrylamide) (PNIPA) minigel s in water ) ; is the characteristic swelling t ime

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26 Figure 1 1 .Schematic diagram showing the concept behind temperature responsive polymer switching. D iameter increase by 1,000 % achievable [34] This much increase is expected to show an approximately 10 times increase in diameter as depicted in the diagram.

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27 CHAPTER 2 BACKGROUND Stimuli Responsive Polymers Stimuli responsive polymers (SRPs) are a class of polymers that show a transformation from one state to another as a response to various stimuli from the environment As mentioned earlier this conversion is reversible They are also known as stimu li sensitive polymers or smart polymers or intelligent polymers. These polymers are playing an important role in various ranges of applications, not limited to such as drug delivery, diagnostics, tissue engineering, biosensors, coatings and textiles [2, 3, 35 38] T emperature responsive pol ymers (TRPs) are the most widely studied among the stimuli responsive polymers [e.g. poly( N isopropylacrylamide ) (PNIPA)] [39, 40] These polymers offer a big chance for creating intelligent materials through various processing routes. Reversible Hydrogen Bonding in P oly(N isopropylacrylamide) PNIPA has been explored to a large extent for its temperature responsive properties. The chemical structure of PNIPA is shown in Figure 2 1 It has a lower critical solution tem perature (LCST) at 32 33 C in water. Its polymer chains show reversible hydrogen bonding capability with water molecules, which is caused by the enthalpic and entropic contribution to the free energy of PNIPA chains [41 43] Below LCST, the enthalpic contributions overtake the entropic contributions, the polar groups (C = O and N H) of PNIPA make intermolecular hydrogen bonding with water molecules. This causes the expanded and to dissolve in water. Above LCST, entropic contributions are greater than enthalpic contributions, hydrogen bonding formed bet ween the polymer chains and water

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28 molecules break. Intramolecular hydrogen bonding form within the polar groups of PNIPA chain instead, and the polymer conformation. Also, above the LCST Van der Waals interactions tak e between hydrophobic moieties of PNIPA. This results in the polymer chains precipitating out from the solution. This transformation in polymer chain conformation is schematically shown in Figure 2 2. Lower Critical Solution Temperature The phase behavior of polymer solutions is an important property involved in the development and design of most polymer related processes. Polymer solutions often show two solubility boundaries the upper critical solution temperature (UCST) and lower critical solution temp erature (LCST). In general, polymer solution (single phase) exhibit the combinatory free energy of mixing less than the native phases at all concentrations above a critical temperature called UCST. Temperatures lower than the UCST, phase separation occur d ue to above stated thermodynamic condition is not achieved. Even though phase behavior studies on polymer solutions have mostly been concerned with upper critical phase behavior, the occurrence of widening solubility gap leading to lower critical phase behavior has also been known for some time. In this case, polymer solution is homogeneous at lower temperature, a macroscopic phase separation occurs when the temperature exceeds a critical value called cloud point of the mixture. The LCST can be consider ed as the lowest cloud point of the system as it corresponds to the minimum of phase diagram as shown in (Figure 2 3) The occurrence of lower critical phase behavior was first noticed on polar systems in which the phase separation was attributed to the fo rmation of hydrogen bonded complexes when the temperature was raised PNIPA has LCST at 32 C in water, which is closer to

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29 room temperature and thus can be easily accessible [44 46] There are several techniques for determining the LCST including differential scanning calorimetry ( DSC ) [46] Gap Analysis for Quick Response Temperature Responsive Polymers As mentioned earlier that one of the transition states of TRP is in water soluble form, their polymer chain has to be physically or chemically integrated in o rder to use them. Usually, TRPs are synthesized using crosslinker, such as methylenebisacrylamid e during polymerization and used in (crosslinked) gel form. This limits their performance where a fast response is required because of the crosslinked bul k physical structure. For example, disk shaped crosslinked PNIPA gel having 1.5 cm diameter requires 1 month for the deswelling to achieve equilibrium [6] In another example, 0.7 cm diameter disk shaped gel based on poly(N isopropylacrylamide) co poly(ethylene oxide) requires 75 hours to attain equilibrium swelling [4] This slow response of hydrogels at the transition is due to the f ormation of skin layer restricting either water permeation from hydrogel during deswelling or water entrance during swelling. Improving the response rate is essential in all applications. S everal studies have been reported that improve the performance of TRP hydrogels that include [6, 13, 21 30] : (i) comb type grafted PNIPA hydrogels [6, 12, 24, 25] where free ends of grafts form water release channels that accelerate the deswelling rate; (ii) macroporous PNIPA hydrogels synthesized either by pore forming agent or by phase separation method [21 23] that give faster macromolecule permeation rates; (iii) cold polymerization of N isopropylacrylamide (NIPA) [28] synthesis of cryogels [30] enables again to have a porous and regularly arranged network structure to improve hydrogel

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30 performance; (iv) use of crosslinking agents, su ch as ethylene triethoxy silane (ETEOS) [26, 27] during polymerization/crosslinking reaction which provide interconn ected water release channels in the hydrogel structure; (v) micro fabrication of photo cross linkable PNIPA copolymers [10] With these modifications the response time of hydrogels showed only a modest improvement Likewise, the swelling ratios of the hydrogels have varied up to 3000 wt. % based on the structure, thickness and crosslinking density of the hydrogels. T ill date improvements in response time have been limited because these polymers have not been subjected to proce ssing into fine fibrous structure As described in the diffusion equation, the response time of a hydrogel is directly proportional to the square the dimension of the gel (Equation 1 1). The simplest way to improve the hydrogel response rate is reducing t he structure to thinner shapes. This usually makes the hydrogel fragile and mechanically weak to use them This is the limitation that answers the question why hydrogels are being made in thicker dimensions. As stated in the hypothesis reducing the hydrog el size to 500 nm diameter fiber will improve the response rate by 6 orders of magnitude. This can be achieved when TRPs processed into nanofibers. Electrospinning is the promising way produce nanofibers. Nanofibers of responsive polymers are expected to o vercome the limitations and drawbacks of hydrogels. Electrospinning Electrospinning is a simple and versatile process that can produce diameter fibers in ranges from a few micrometers to 100 s nanometers. Electrospinning process works by applying a high el ectrical field ( 1 5 kV/cm) to a drop of polymer pushed through a needle. The applied high electrical force overcomes the surface energy of the droplet

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31 and forms a Taylor cone which, through a whipping motion, forms mats of fibers that possess a high surfac e area to mass ratio. Nanofibers produced by electrospinning porcess has vast applicatoin aeras ranging from biomedical, filtration, nanosensors, protective clothing to other industrial applications [33, 47 49] For more details on electrospinning in general, the reader is advised to look int o these literature work [50 52] An electrospinning setup with fiber formation process is schematically drawan in Figure 2 4 Electrospinning process is related to electrospraying, the later process has been studied in some detail over several decades. Both the processes comprise a larger field known as electrohydrodynamic forming [53] Electrospraying is the formaiton of nan oparticles, and electrospinning as described above the fabrication of high aspect ratio s (over several meters) fiber s Both processes convert liquid droplets into nanomaterials thorugh a strong electical field. The major difference between the two processe s is on chain entanglement and the resulting elongational visous forces that operate in polymer solutions undergoing electrospinning. This reults in the synthesis of fibers in electrospinning, as opposed to the production of droplets in electrospraying. De velopment s in E lectrospinning: H istory The first documented wrok on electrospinning polymer solutions into nanofibers was described in 1902 by J.F Cooley and by W. J. Morton (US Patent # 692,631 & 705,691) In the 1960s, Sir Geoffrey Taylor occurs when an electric field is applied to a liquid that led to the fundamental understanding of the behavior of droplets in an electric field This shape is specifically a cone with a semi vertical angle of 49 .3 or apex angle of 98.6. This conical shape has been coined the Taylor Cone [54, 55] The present understanding on electrospinning is

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32 contributed from the rese arch work carried out in this field for the past 10 15 years on understanding the fundamentals, fluid dynamics, electrostatics, bending instability, modeling [56] fiber morphology (e.g. bead formation) [52] Fo rces Involved at the Cone F ormation Electrospinning has three distinct sections in fiber formation, which are: (i) the cone formation (ii) the thinning of the fiber (iii) the drying of the fiber. In the cone formation stage, overall there is a force balanc e associated with the droplet as it emerges from the needle, which is complex in nature as shwon in Figure 2 5 During the the cone formation, electric polarization stress and gravity tend to elongate the droplet from within with the aid of tangiatial elec tric stress on the surface of the droplet. Viscous drag of the polymer solution slows down the formation of the cone. In addition to the above forces discussed, surface tension of the polymer solution tries to minimize the surface area by pulling the surfa ce area vertically in and an oppositing force that is normal electrical stress which tries to maximize the distance between the same electical charges on the surface by enlarging the surface area. When the electrical field acting on the droplet is large en ough, the droplet is deformed into the Taylor cone. As soon as the electrostatic repulsive forces over come the surface tension, a fine jet is ejected from the Taylor cone that is typically 1/100 in diameter of the needle opening. After leaving the cone th is jet move towards the counter electrode, during the following travelling time the jet will become unstable [50, 57, 58] Factors Affecting Electrospinning Performance and Nanofiber Quality Variables affect electrospinning can be conviniently catagorized into two: materials variable and process variables. The mater ial parametes are: polymer solution (concentration, solvent mix and additives), enviroment and collector (geometry, dielecric

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33 properties). The process parameters are: applied potential, feed rate, tip and gap distance [52] How some of the important variables influence the fiber quality will be discussed here. P olymer concentration in electro spinning solution The minimum solut ion viscosity (adjusted by varying polymer concentration in a solvent) that allows for adequate chain entanglement is requierd for continous production of nanofibers in electrospinning T his is different for different polymer solvent solutions. Gupta et. a l. [59] report an uniform fiber formation for poly(methyl methacrylate) (PMMA) dimethylformamide (DMF) solutions with PMMA molecular weight distribution (MWD) 1.03 1.35 at c/c* approximately 6 (where is c th e polymer concentration and c* is the critical chain overlap concentration). The work further reports the dependence of fiber diameter on concentration, i.e. fiber diameter ~(c/ c* ) 3.1 It also generally identified that solution viscosity is the dominant variable that determines the fiber diameter [60] In general with increasing polymer concentration, the mean fiber diameter increas es Solvent and solution conductivity The choice of solvent primarily determines conformation of the dissolved polymer chains, ease of charging the spinning jet, cohesion of the solution due to surface tension forces and rate of solidificatino of the jet on evaporation of the solvent [52] In a s tudy, 18 different solvents were investeigated on the electrospinnability of polystyrene (PS) and it was identified that dipole moment and conductivty of the solutions were key properties [61] Electrospinning process requires the transfer of electrical charge from the electrode to spinning droplet, a minimum conducti vity is requiered for the solution to be electrospun and in addition to this improving the conductivity produces a smooth

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34 continuous fibers [52] Typically, an inorganic salt sodium chloride ( NaCl ) at very low concentrations might be used to increase the solution conductivity and improve the electrospinning [62] About 31 0 nm decrease in mean fiber diameter of was obsereved with the additon of benzyl triethyl ammonium chloride (BTEAC) upto 4 wt.% of poly(lactide co glycolide) (PLGA) copolymer in CHCl 3 solution [63] Surface tension In electrospinning surface tension is the primary force opposing C oulomb repulsion (the force responsible for creating more surface area). Therefore, the importance of surface tension cannot be overstated in the discussion of electrospinnability of a polymer solution. All other forces being equal, it is desirable to have a polymer solvent system which is lower in surface tension. Surface tension is temperature dependent, also changes with concentration of a po lymer solution and changes as jet travels towards collector electrode [52] Dielectric constant In a practical sense dielec tric constant is a measure of how much electrical charge the solvent is capable of holding. A solvent with a high dielectric constant is preferred in electrospinning The h igher the dielectric constant of a polymer solution, the higher would be surface ele ctrical charge evenly distributed on the jet. This results in high nanofiber quality and pr oductivity in electrospinning. Applied voltage It is the applied voltage (V) that suppl ies the surface charge required for the production of nanofibers in electrosp inning to polymer solution. Instability and eventually the stretching of the polymer jet after the jet emerges from the Taylor cone increases with increases in applied voltage. The decrease in mean fiber diameter with an

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35 increased applied voltage is report ed for various polymer solutions (e g. polyacrylonitrile ( PAN ) /DMF, polystyrene ( PS ) / tetrahydrofuran ( THF) [64, 65] However, one should bear in min d that the electric field attenuated decrease in fiber diameter is smaller than changing the polymer concentration of the spinning solution [66] Feed rate Feed rate is the rate at which polymer solution replenishes the Taylor cone. F eed rate should be equal to removal rate of polymer jet at the Taylor cone for continuous synthesis of nanofibers Lower the feed rate results in intermittent electrospinning process and higher the feed rate results in large fibers and may be beads along w ith fibers Thus, feed rate is a crucial factor in determining the fiber morphology in electrospinning. Other conditions being the same, increasing the feed rate resulted in increasing the mean fiber diameter [52] Capillary tip diameter The practical considerations for selecting the right needle gauge or capillary tip diameter are the throughput desired and possible clogging due to solvent evaporation. In general, studies have reported that smaller the capillary tip diameter resulted in smaller diameter fiber s in electrospinning [67, 68] Gap distance The gap distance or working distance (D cp ) is the distance between the terminus of the capillary tip to the surface of the collector. D cp decides the strength of the electric field and the time available for the solvent to evaporate as the polymer jet travels towards the collector. W hile keepin g other parameters same, increasing the D cp would decreas e the mean fiber diameter [69] Too small the D cp (less time available for the

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36 solvent to evaporate) would lead to deposition of wet fibers that fuse on the collector [64, 70] It needs to be understood that there are many variables that affect the electrospinning process and resulting nanofiber morphology. T hese variables are interrelated and any set of variables is specific to certain polymer solvent system. Thus it requires a great caution when trying to improve the nanofiber quality. Wettability Materials that demonstrate switchable surfaces, oscillating between superhydrophilic and superhydrophobic have garnered great attention fo r both fundamental studies and practical applications [71 75] such a s self healing surfaces [76] selective molecular separation [77] controlled drug delivery [78] and smart textiles [79] In general, these surfaces are engineered to respond to an external stimulus such as light irradiation, pH changes, solvent exposure, electrical potential, magnetic field, mechanical force, or temperature [76, 80, 81] Among the various stimuli that have been explored, temperature stimulus gives a narrow temperatu re range response at the transition [33, 82] To fabr icate surfaces with reversible wettability characteristics, several processing routes have been explored, including layer by layer, hydrothermal, surface entrapment, phase separation, self assembled monolayers, electrochemical deposition, and others [74, 76, 83 89] These techniques are complicated and typically require more than one process to produce the desired responsive surface that demonstrates a reversible wettability. Of the key issues associated with surfaces showing reversible wettability properties [73] a simple fabrication technique to produce them, and better structura l integrity of these surfaces are paricularly important in view of practical applications.

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37 Of the routes investigated to generate the desired responsive nanomaterials, electrospinning has been underexplored. Electrospinning is a simple, versatile, and cont inuous process that can produce diameter fibers in ranges from a few micrometers to 100 nanometers as explained above [31, 50, 51, 90, 91] Electrosp inning process generates nonwoven type fiber network with three dimensionally interconnected pores How a liquid droplet interacts with a structure which is porous in nature has been studied by Cassie and Baxter (CB) CB model which establishes a relatio ns hip between the contact angle ( CA ) and the porosity of the structure as schematically shown in (Figure 2 6) [92, 93] : ( Equation 2 1 ) CB = apparent contact angle on a rough surface; Y f s = fraction of the wet solid contact area.

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38 Figure 2 1. P oly(N isopropylacrylamide) (PNIPA) chemical formula depicted with hydrophilic group and hydrophobic group s present in its polymer chain Figure 2 2 Schematic diagram showing the temperature dependent conformational change occurs in a temperature responsive polymer (TRP) due to reversible hydrogen bonding of polymer and water molecules Hydrophobic group Hydrophobic group Hydroph ilic group

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39 Figure 2 3 Schematic diagram showing the lower critical solution temperature ( LCST ) in a binary system [44] LCST is considered t he lowest point in the phase diagram. ___________________________________________________________________ Reprinted with permission from Elsevier Limited. Durand A and Hourdet D Polymer 1999;41(2):545 557. Temperature Concentration of polymer in water

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40 Figure 2 4 Schematic diagram show ing the electrospinning process with a setup. A polymer jet is ejected from the needle when a high electric field is applied between the syringe containing a polymer solution and the collector

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41 Figure 2 5 Schematic diagram showing the forces acting on a droplet as it emerges from the electrospinning needle [57] ___________________________________________________________________ Reprinted with permission from John Wiley a nd Sons Sigmund W, Yuh J, Park H, Maneeratana V, Pyrgiotakis G, Daga A, Taylor J, and Nino JC Journal of the American Ceramic Society 2006; 89(2):395 407.

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42 Figure 2 6 Cassie Baxter (CB) model : w etting behavior of v apor pockets that are trapped between the grooves a nd the liquid droplet where C B is the apparent contact angle on a rough surface or also called as CB contact angle [94] ___________________________________________________________________ Reprinted with permission from Elsevier Limited Hsu S H, Woan K, and Sigmund W Materials Science and Engineering: R: Reports 2011;72(10):189 201. Gas

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43 CHAPTER 3 FABRICATION OF NANOFIBERS VIA ELECTROSPINNING In order to achieve rapid wettability switching of responsive polymer s the right selection of polymer and the right synthesis proc ess to make the polymers into naonfibers is required. A good temperature responsive polymer ( TRP ) should have the following desirable properties: should be processable into fiber by electrospinning, have the transition temperature at a desired point, can be chemically crosslinkable or physically blended together to prevent or reduce the solubliilty as one of the transition state is in so luble form and importantly should have good temperature responsive properties Based on the above cr i teria, poly[(n isopropylacrylamide) co (methacrylic acid)] (PNIPAMAA) and poly (n isopropylacrylamide) ( PNIPA ) were selected Both polymers hydrogels show t emperature responsive swelling deswelling behavior [6, 14, 95 101] Both have lower critical solution temperature (LCST) around 32 C in water. PNIPAMAA chemical formula is shown in Figure 3 1 It has carboxylic acid pendent group that can b e chemically crosslink ed after electrospinning where as PNIPA does not have a reactive side group and thus provide s an oppourtunity to blend with a non responsive polymer and electrospin. Polystyrene (PS) is non responsive hydrophobic polymer and was selected to blend with PNIPA to give a physical immobolization to PNIPA chain after blend electrospinning With above thoughts in mind, the major challenges addressed in this chapter are: 1. Finding the right formulation to synthesize crosslinked PNIPAMAA fib ermat s by electrospinning process that yields insoluble fibermat s post electrospinning by heat treatement. 2. Challenges in synthesizing crosslinked PNIPAMAA fibermat s and PS/PNIPA blended fibermat s with different diameter fibers that will be later used in s tudying response time

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44 Experimental Materials The following chemicals were used as received from (i) Sigma Aldrich: polystyrene (PS) of M n 170,000 g/mol and M w 350,000 g/mol, poly[(n isopropylacrylamide) co (methacrylic acid)] ( PNIPAMAA ) of M n 60,000 g/mol with 90 mol% PNIPA and 10 mol% MAA, disodium hydrogen phosphate (DSHP) and dimethylformamide (DMF) (ii) Polyscience Incorporation: poly(N isopropylacrylamide) (PNIPA) of M v 40,00 0 g/mol, (iii) Acros Organics: p oly(vinyl alcohol) (PVA, 75% hydrolyzed) of MW 2,000, tetrahydrofuran (THF) (stabilized, 99.5 % extra dry) (iv) MP Biomedicals : dimethyl sulfoxide ( DMSO ) and (v) Fisher: glacial acetic acid ( 99.9% HOAc ). Synthesis of Crosslinked P oly[(n isopropylacrylami de) co (methacrylic acid)] Fibermat s A typical process to sysnthe size crosslinked poly[(n isopropylacrylamide) co (methacrylic acid)] ( PNIPAMAA ) fibermat with a mean fiber diameter of 8700 150 nm is given here (Table 3 3 & Figure 3 6 B) PNIPAMAA / acetic acid ( HOAc ) p oly(vinyl alcohol) ( PVA ) / deinonized water ( DI H 2 O) and disodium hydrogen phosphate ( DSHP ) / DI H 2 O stock solutions were prepared in 15 %, 15% and 10 wt. % respectively. To the PNIPAMAA /HOAc solution (0.68 g) PVA (5 % wt. to PNIPAMAA ) and DSHP ( 30 % wt. to PVA) were added and mixed well to prepare a formulation. This was placed in a 3 mL syringe, fitted with an 18 gauge stainless steel needle (inner diameter of 0.965 mm). The syringe was fixed horizontally on a syringe pump (Model: BSP 99M, Braint ee Scientific Inc.), and an electrode connected to a high voltage power supply (Model: ES30N 5W, Gamma High Voltage Research) was attached to the tip of the metallic needle. A grounded stationary square collector (size 10 cm x 10 cm) covered

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45 by a piece of clean aluminum foil was used for the fiber collection. The e lectrospinning setup used in this dissertation is shown in Figure 3 2. Electrospinning of formulation was carried under the following operating conditions: flow rate (FR) was 0.43 mL/h; electric field (EF) was1 kV/cm and distance between needle tip and collector surface (D CP ) was 20 cm. Electrospun fibermat s were collected on aluminium ( Al ) foil. The collected electrospun fibermat s were kept in a vacuum oven at room temperature (RT) overnight, fol lowed by a heat treatment at 160 C for 30 minutes again in a vacuum oven. The experimental designing in adjusting electrospinning processing and operating condtions to produce different diameter fibers and differentent thickness fibermats are discussed in Results and Discussions section. Synthesis of PS/PNIPA B lended Fibermat s A typical process to sysnthesi ze p olystyrene ( PS ) / Poly(N isopropylacrylamide) ( PNIPA ) blended fibermat with a mean fiber diameter of 380 100 nm is given here (Table 3 4 & Fgiure 3 7 A) PS and PNIPA (PNIPA/PS 30/70 w/w) were dissolved in d imethylformamide ( DMF ) to make 15 % wt. solution. Electrospinning to produce PS/PNIPA blended was carried out using the blend solution under the following operating conditions: flow rate ( FR) was 0.3 L / min ; electric field (EF) was 0.43 kV/cm and the distance between needle and the collecting plate (D CP ) was 20 cm. The collected electrospun fibermat s were kept in a vacuum oven at RT. This vacuum dried fibermat s were used for further characterization The experimental designing to produce different diameter fibers and different thickness fibermats are discussed in Results and Discussions section.

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46 Characterization Morphology and Structure Analysis The surface morphology of PS/PNIPA ble nded and crosslinked PNIPAMAA fibermat s were examined under Optical Microscope (O lympus ) and field emission gun Scanning Electron Microscope (SEM) (Jeol 6335F). For optical microscope analysis, fibers were collected on glass slide for about 3 5 minutes and examined. For SEM analysis, a small section of electrospun fibermat was cut and placed on SEM stub using a double sided adhesive carbon tape. Sample was sputter coated with a thin film of gold palladium to aid in SEM analysis, and analyzed with an acceler ating voltage of 10 kV. Mean F iber S ize A nalysis. Mean fiber diameter of electrospun fibermat s were analyzed from SEM images using Image J, a general purpose image processing software. SEM images upto 7 10 of same sample obtained but at different places Each fiber present in an image was considered for determining the mean fiber diameter. This many number of spots ensured that at least 1 50 different individual fiber s were measured for the analysis. Results and D iscussions In the search of temperature re sponsive polymer s PNIPAMAA and PS/PNIPA were chosen to synthesize crosslinked PNIPAMAA fibermat s and PS/PNIPA blended fibermat s by electrospinning process. The electrospun b lended and crosslinked fibermat s obtained were then characterized for their morphology and fiber size.

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47 Synthesis of Crosslinked PNIPAMAA Fibermat s In order to fabricat e crosslinked PNIPAMAA fibermat s first step was to find a solvent that dissolves the copolymer and to examin e the electrospinnability of the copolymer solution thus obtained. Electrospinnability of PNIPAMAA copolymer solution There were several solvents had been individually tried to see whether or not they dissovle the copolymer. From these initial attempts, solvents t etrahydrofuran ( THF ) d imethyl sulfoxide ( DMSO ) deionized water (DI H 2 O), DMF and OHAc were found to dissolve the copolymer PNIPAMAA. The next step was to examine the electrospinnabilty of the these copolymer solutions separately The observations made during and after the electrospinning process are summarized in Table 3 1. THF dissolves the copolymer upon heating to 30 40 C. It was noticed that the electrospinnability of THF copolymer solution was found to be poor as THF very quickly evaoparated leaving a precipitation at the needle. THF has a boiling point at 66 C. THF is a volatile solvent with vapor pressure of 17.9 kPa [102] Common isssue with using volatile solvents in electrospinning is polymer precipitation at the needle exit as the solvent evaporates from the solution and blockage to flow [65] Another solvent tried was DMSO, this solvent also dissolved the copolymer upon heating to 30 40 C. It wa s surprising to notice that DMSO produce fibers in electrospinning as the polymer solution deposited at the collector. DMSO has a boiling at 189 C. This may be due to the high boiling point of DMSO and it was not evaporat ed durin g electrospinning. DI H 2 O was another solvent tried to dissovle the copolymer. It dissolved the copolymer at 10 C Electrospinning the 10 wt.% PNIPAMAA /DI H 2 O solution resulted in

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48 primarily producing elongated beads with thin fibers connecting the beads as shown in Figure 3 3 The e lectrospinning was not continuous and was dripping intermittently The poor spinnabil it y of PNIPAMAA / DI H 2 O solution attributed to the high surfa ce tension of water. Water has a surface tension of 71.99 mN/m at 25 C The poor spinnability of water as a solvent is reported in literatures attributing to its surface tension [52] The high surface tension force works against fiber formation in electrospinning DMF was another solvent that dissolved the copolymer. It has a dielectric constant value of 36.7 [52] As the concentration of copolymer in PNIPAMAA /DMF solution influenc ed the fiber forming properties in electrospinning, PNIPAMAA /DMF solution was electrosp u n with 10 wt % 15 wt % and 25 wt % polym er concentration s The observations made during electrospinning and the morphology obtained is reported in Table 3 1 & Figure 3 4 It was observed that only at the 25 wt % polymer concentration mostly fibers with fewer number s of beads obtained. OHAc was another solvent that dissolved the copolymer It has a dielectric constant of 6.19 [52] Again, a s the concentration of co p olymer in PNIPAMAA / OHAc solution was influencing the fiber forming properties in electrospinning, PNIPAMAA / OHAc solution was electrosp u n with 10 wt % and 15 wt % polymer concentration s The observations made in electrospinning and the morphology obtained is reported in ( Table 3 1 & Figure 3 5 ) It was noticed that only at 15 wt % polymer concentration a large amount of fibers with fewer numbers of beads achieved To summarize the electrospinnability of PNIPAMAA copolymer solution, t he minimum solution viscosity that allows for adequate chain entanglement is requierd for continous production of uniform fibers in electrospinning In general, the plot between

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49 polymer solution viscosity and c/c* (where is c the polymer concentration and c* is the critical c hain overlap concentration) gives three different regimes: (i) dilute (ii) semidilute unentangled (iii) semidilute entangled Only polymer droplets obtained in the dilute regime due to insufficient polymer chain overlap. As with increasing polymer concetra tion chain entanglement also increases. This results to d roplets and beaded fibers formation in the sem idilute unentangled regime Further, b eaded and uniform fibers production in the semidilute unentangle d regime U niform fibers alone are obtained well a bove the semidilute entangled regime. For the poly(methyl methacrylate) (PMMA) dimethylformamide (DMF) solutions this was at c/c* approximately 6 [59] Th e critical c/c* vlaue to attain uniform fiber formation is different for different polymer solvent systems. This is due to the different nature of chemical interaction between different polymer solvent systems Electrospinnability of PNIPAMAA formu lation From the above studies it was decided that DMF OHAc and DI H 2 O found to be potential medium to prepare the formulation (i.e. copolymer solution with crosslinker and crosslinking aid ) that later can be electrospun to sysnthesize PNIPAMAA fibermat s. Next step was to find a crosslinker and a crosslinking aid A low molecular weight ( MW ) PVA (MW 2000) and DSHP were selected as crosslinker and crosslinking aid to mix with copolymer solution, re s pectively. PVA has been reported to crosslink poly(acryli c acid) (PAAc) in a esterfication type crosslinking mechanism where hydroxyl side groups of PVA and carboxylic side groups of PAAc react to form ester groups [96, 103 106] A low MW PVA was se lected in this study particularly as it is helpful: (i) for the same mol% of hydroxyl side groups of PVA, there would be more number of polymer chains that effectively participat e in the crosslinked network (ii) there will be a lower probability for

PAGE 50

50 the ge l formation while mixing PVA with copolymer solution as the critical mass concentration per unit area is less with low MW PVA When high MW PVAs were added to the copolymer solution, there was an immediate precipitation occured indicatating a gel formation between copolymer and high MW PVA. This will be explained in detail later DSHP has been reported as crosslinking aid used in esterification crosslinking reactions and in crosslinking cellulose fabrics to synthesize crease recovery fabrics [103] Attempts made to produce the formulati on, observations made during and after electrospinning are report e d in Table 3 2 DI H 2 O dissolved the copolymer, PVA and DSHP. However, this formulation was not tried in electrospinning as t here were issues with temperature dependent solubility of copolym er in DI H 2 O and also copolymer /DI H 2 O solution was not electrospinning well as reported earlier DMF dissolved the copolymer and DSHP It was decided to use PVA/H 2 O and DSHP/ H 2 O solutions to mix with copolymer solution to prepare a formulation. It was noticed that the addition of PVA/H 2 O and DSHP/H 2 O solutions into PNIPAMAA /DMF solution yields a precipitation again indicating a gel formation between copolymer and PVA It was not iced several times that the precipitation was a common problem associated with DMF based formulation s in either case with or without the crosslinking aid This issue was very prominent with high MW PVA s Addition of PVA (MW 11000 31000, 98 99 % hydrolyzed obtained from Alfa Aesar, USA) ( 3 wt % of PNIPAMAA ) to 25 wt % PNIPAMAA /DMF solution resulted to a precipitation immediately Prior to mixing, PVA was dissolved in DI H 2 O to produce 10 wt % solution and this solution added to copolymer solution It was decided not to electrospin the

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51 precipitate as precipitation indicated a reaction between copolymer and PVA and a gel type of product was the result. In electrospinning usually a polymer solution is spun to produce fibers. Gel is already a crosslin ked network and cannot be electrospun to synthesize fibers. Again, the interest was to electrospin the formulation to produce fibers and later through a post treatment process to synthesize a crosslinked fiber. OHAc was similar to DMF, it dissolved the dissolve PVA. Again, as similar to DMF, i t was decided to use PVA/H 2 O and DSHP/H 2 O solutions to mix with copolymer solution to pr epare a formulation. PNIPAMAA/HOAc ( PVA, 5 wt.% of PNIPAMAA; and DSHP, 30 wt.% of PVA) were mixed well to prepare a 15 wt.% formulation. T he formulation prepared in OHAc found to be a stable This was electrospin to produce fiber s Electrospinning this formulation did not produce beaded fibers as shown in Figure 3 6 A. DSHP helped in improving the formulation conductivity and electrosp inning performance. As studies have shown that salt at very low concentrations used to increase the solution conductivity and improve fiber quality [62] Fibermat s thus obtained from the formulation were heat treated in vacuum at 160 C for 30 minutes in vacuum to produce crosslinked PNIPAMAA fibermat s as shown in Figure 3 6 B&C. The mean fiber diameter of the crosslinked fiber mat was 870 150 nm ( Figure 3 6 B&C ). Crosslinked PNIPAMAA fibermat s with different diameter fibers PNIPAMAA formulation prepared in OHAc found to be stable and electrospinning the formulation produced crosslinked fibermat s. In experimental designing to synthesize crosslinked fibermat s with different diameter fibers concentration of formulation flow rate gap distance and other related parameters that would affect the fiber diameter were adjusted in electrospinning as given in Table 3 3.

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52 As pointed out before formulation having 15 wt.% ( PNIPAMAA /HOAc; PVA, 5 wt.% of PNIPAMAA ; and DSHP, 30 wt.% of PVA) with the flow rate of 0.43 mL/h produced fiber diameter 870 150 nm This same formulation with 22 mL/h flow rate only imp ro ved the fiber diameter to approximately 1.5 2 m as most of the formulation was dripping off from the needle at this high flow rate However, electrospinning still produced fibers as shown in Figure 3 6 D. The viscosity of the formulation was already on th e higher side and therefore, no further efforts were carried in this direction to produce higher diameter fibers However, a n attempt was made to produce smaller diameter fiber with 10 wt.% formulation at the given electrospinning conditions Table 3 3 Wit h 0.23 mL/h flow rate and with almost similar electrospinning conditions, the mean fiber diameter achieved was of 241 30 nm ( Figure 3 6 E). Synthesis of Polystyrene/Poly(N isopropylacrylamide) Blended Fibermat s Another approach adopted to produce temperature responsive polymer fibers was by blending. As previously mentioned, one of the tranistion state of PNIPA is in soluble form. In order to electrospin and use the temperature responsive properties of PNIPA in f iber form, it needs to be either chemically or physically integrated PNIPA does not have a reactive group in its polymer chain it gives an oppourtunity to blend with a non responsive polymer. PS/PNIPA blended fibermat s with various diameter fibers A gain the interest was to produce different diameter fiber s to study the influence of diameter fiber on response time especially at the nanometer dimensions as the main goal of this research work With this idea PS and PNIPA blend solutions were prepared at various concentrations F low rate, gap distance and other operating conditions that

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53 would influence the fiber diameter were adjusted in electrospinning to obtain various diameter fibers ( Table 3 4& Figure 3 7 ) The change in the needle gauge with the similar electrospinning conditions produced two different diameter fibers with 18G, the mean diameter fiber achieved was 0.990.3 m (sample: BLD990nm) and with 24G, the mean diameter fiber achieved was 0.380.1 m (sample: BLD380nm) Sample code BLD380nm means PS/PNIPA blended fibermat with mean diameter fiber of 380 nm and likewise for others For the BLD1.5m sample, the FR was increased to 15 L/min and the D cp was decreased to 11 cm. This resulted in producing a mean diameter fiber of 1.50.9 m as in shorter distance more material pushed by the pump had to be stretched thus resulted producing relatively a bigger diameter fiber as compared to sample BLD990nm In the similar line in achiev ing high er diameter fibers the PS/PNIPA blend solution concentra tion was increased to 30 wt.% and also t he FR was increased to 150 L/min to produce sample BLD16m with a mean fiber diameter of 161.2 m In order to further increase the fiber diameter the FR was increased to 250 L/min with the similar electrospinning conditions as sample BLD16m but these conditions yielded wet fibers that were fused together as shown in Figure 3 7 E PS/PNIPA blended fibermat s with different thickness fibermats It was also of interest to study the in fluence of fibermat thickness on response time as the method devised to study the response time demanded that In order to achieve this, the experimental planning was to produce different thickness fibermats but with same diameter fiber To increase the fi bermat thickness in electrospinning process, it simply require s prolong ing the electrospinning process. The electrospinning time varied

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54 was 1 h 3 h and 10 h to produce three different thickness fibermats as given in (Table 3 5 & Figure 3 8 ) The electrospinning process was not stable for longer time, especially for 3 h and 10 h samples. Electrospinning was discontinuous with dripping. This may be due to changes in the electrospinning forces acting on the polymer jet as previously accumulated f ibers covered the collecting plate especially with longer electrospinning in this blend solvent system As the result beads appear ed along with fibers It can be seen that appearence of beads decreased the fiber diameter. Chapter S ummary C rosslinked PNIPAMAA and PS/PNIPA blended fibermat s have been fabricated by electrospinning processes PNIPAMAA / OHAc and PNIPAMAA/DMF produced fibers with fewer number of beads in electrospinning The OHAc based formulation found to be stable Fibermat s thus obtained from OHAc based formulation w ere later heat treated at 160 C to synthesize a chemically crosslinked PNIPAMAA fibermat s DMF was found to be a good solvent to electrospin PS/PNIPA blend solutions into fibers. By changing the PS/PNIPA blend solution concen tration, flow rate and working distance in electrospinning PS/PNIPA blended fibermat s wiht different diameter fibers in the ranges from 100s of nm to 10s of m were synthesized.

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55 Table 3 1 Solvents used in electrospinning PNIPAMAA copolymer and observations made during and after electrospinning Solvent Solubility of copolymer PNIPAMAA Remarks THF 10 wt% PNIPAMAA /THF was heated to 30 40 C to quicken the dissolution. Solution was stable at RT. While trying to electrospin the 10 wt% PNIPAMAA /THF solution, THF evaporate d very quickly and was difficult to electrospin the copolymer solution. DMSO 10 wt% PNIPAMAA /DMSO was heated to 30 40 C to quicken the dissolution. Solution was stable at RT. Whil e trying to electrospin the 10 wt% PNIPAMAA /DMSO solution, electrospinning did not yield fibers. Polymer solution deposited at the collector. DMSO may be was not evaporating due to its high boiling point. DI H 2 O 10 wt% PNIPAMAA / DI H 2 O was brought down to 10 C to quicken the dissolution. Solution was stable at RT. Electrospinning the 10 wt% PNIPAMAA / DI H 2 O solution resulted primarily in producing elongated beads with thin fibers connecting the beads (Figure 3 3). Electrospinning was not continuous and drip ping Electrospinning conditions: 10 wt% PNIPAMAA /DI H 2 O solution; FR: 2 mL/h ; EF: 1 kV/cm; D CP: 20 cm; NG 20.

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56 Table 3 1. Continued DMF 15 30 wt% PNIPAMAA /DMF d issolved at RT Electrospinning 15 wt% PNIPAMAA /DMF solutions resulted in spraying and produced beads (Figure 3 4 A). Electrospinning conditions: 15 wt% PNIPAMAA /DMF solution; FR: 0.4 mL/h ; EF: 0.8 kV/cm; D CP: 10 cm; NG 18. Electrospinning 25 wt% PNIPAMAA /DMF solution resulted in producing mixture o f fibers and beads (Figure 3 4 B). Electrospinning conditions: 25 wt% PNIPAMAA /DMF solution; FR: 0.2 mL/h ; EF: 0.6 kV/cm; D CP: 20 cm; NG 20. Electrospinning 30 wt% PNIPAMAA /DMF solution resulted in producing mostly fibers (Figure 3 4 C& D). Electrospinning conditions: 30 wt% PNIPAMAA /DMF solution; FR: 0.4 mL/h ; EF: 0.6 kV/cm; D CP: 20 cm; NG 20. OHAc 10 15 wt% PNIPAMAA/ OHAc d issolve d RT E lectrospinning 10 wt% PNIPAMAA/ OHAc solution resulted in producing mixture of beads and fibers (Figure 3 5 A&B). Electrospinning conditions: 10 wt% PNIPAMAA/ OHAc solution; FR: 0.2 mL/h ; EF: 1.4 kV/cm; D CP: 10 cm; NG 18. Electrospinning 15 wt% PNIPAMAA/OHAc solution resulted in producing primarily fibers (Figure 3 5 C&D). Electrospinning conditions: 15 wt% PNIPAMAA/OHAc solution; FR: 0.4 mL/h ; EF: 1.2 kV/cm; D CP: 10 cm; NG 20. PNIPAMAA :p oly [(n isopropylacrylamide) co (methacrylic acid)] ; THF : t etrahydrofuran ; DMSO : d imethyl sulfoxide ; DI H 2 O : d eionized water ; DMF : d imethylformamide ; OHAc : a cetic acid ; FR: flow rate, EF: electric field, D CP : distance between the needle tip and collector surface, NG: needle gauge; RT: room temperature

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57 Table 3 2. Attempts made to produce a formulation of copolymer solution, crosslinker and crosslinking aid and trials made on electrospinning the formulation Solvent Solubility of copolymer Solubility of PVA Solubility of DSHP Remarks DI H 2 O 10 wt % PNIPAMAA /DI H 2 O was brought down to 10 C to quicken the dissolution. Solution was stable at RT. 10 15 wt % PVA / DI H 2 O was heated to 85 90 C for 15 mins to quicken the dissolution. Solution was stable at RT. 10 20 wt.% DSHP / DI H 2 O r eadily d issolve d at RT Discontinuity in electrospinning the copolymer/DI H 2 O solution with dripping. DMF 15 30 wt.% PNIPAMAA /DI H 2 O dissolved at RT 10 wt.% PVA/ DMF not dissolved on heating to 30 40 C 10 wt.% DSHP/ DMF dissolve d at RT Addition of PVA/H 2 O and DSHP/H 2 O solutions into copolymer/DMF solution yield ed gel formation. Did not electrospin the gel.

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58 Table 3 2. Continued OHAc 10 15 wt.% PNIPA MAA/OHAc dissolved at RT 10 wt.% PVA/OHAc not dissolved on heating to 30 40 C 10 wt.% DSHP/OHAc dissolved at RT 15 wt % formulation ( PNIPAMAA/HOAc; PVA, 5 wt % of PNIPAMAA; and DSHP, 30 wt % of PVA; were mixed well to produce a formulation). The formulation was electrosp un to pro duce fibers Electrospinning conditions: FR: 0.43 mL/h; EF: 1 kV/cm; D CP: 20 cm. Fiber diameter 870 150 nm (Figure 3 6 A, B & C). PNIPAMAA:p oly[(n isopropylacrylamide) co (methacrylic acid)]; PVA : p oly(vinyl alcohol) ; DSHP : d isodium hydrogen phosphate ; DI H 2 O: deionized water; DMF: dimethylformamide; OHAc: acetic acid; FR: flow rate, EF: electric field, D CP : distance between the needle tip and collector surface, NG: needle gauge; RT: room temperature

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59 Table 3 3 Formulations used and electrospinning operating conditions to synthesize PNIPAMAA fibermat s with different di a m eter fiber s and different thickness fibermats Formulation FR (mL/h) Dcp (cm) EF (kV/cm) N G Espg. time (mins) Fiber dia (nm) Sample code Remarks 15 wt.% (PNIPAMAA/HOAc; PVA, 5 wt.% to PNIPAMAA; and DSHP, 30 wt.% to PVA) 0.43 20 1 18 45 870 150 XLD870nm/t50m S table electrospinning produced fibers with 4716 m thick fibermat (Figure 3 6 A, B&C) 15 wt.% (PNIPAMAA/HOAc; PVA, 5 wt.% to PNIPAMAA; and DSHP, 30 wt.% to PVA) 22 20 1 18 45 870 150 XLD2m Electrospinning produce d fibers but excess solution dripp ed off from the needle (Figure 3 6 D) 10 wt.% (PNIPAMAA/HOAc; PVA, 5 wt.% to PNIPAMAA; and DSHP, 30 wt.% to PVA) 0.23 20 0.6 20 24130 XLD241nm S table electrospinning produced fibers. (Figure 3 6 E) 15 wt.% (PNIPAMAA/HOAc; PVA, 5 wt.% to PNIPAMAA; and DSHP, 30 wt.% to PVA) 0.43 20 1 18 210 870 150 XLD870nm/t150m E lectrospinning p roduced 14843 m thick fibermat PNIPAMAA:p oly [(n isopropylacrylamide) co (methacrylic acid)]; PVA: poly(vinyl alcohol); DSHP: disodium hydrogen phosphate; OHAc: acetic acid; FR: flow rate, EF: electric field, D CP : distance between the needle tip and collector surface, NG: needle gauge; Espg: electrospinning

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60 Table 3 4 Blend solution concentrations and operating conditions to produce different diameter fibers in electrospinning Concentration wt.% (PS/PNIPA 70/3 0 w/w) FR (L/min) D cp (cm) EF (kV/cm) NG Espg. time (mins) Fiber dia (m) Fibermat thickness (m) Sample code Remarks 15 3 20 0.43 24 180 0.380.1 446 BLD380nm Figure 3. 7 A 15 3 20 0.43 18 150 0.990.3 5510 BLD990nm Figure 3. 7 B 15 15 11 0.77 18 45 1.50.9 5310 BLD1.5m Figure 3. 7 C 30 150 11 0.77 18 5 161.2 70 10 BLD16m Figure 3. 7 D 30 250 11 0.77 18 3 Yielded w et fibers ( Fiber dia: 308.7 m ) BLD30m Figure 3. 7 E PS / PNIPA : p olystyrene /p oly(N isopropylacrylamide) ; FR: flow rate, EF: electric field, D CP : distance between the needle tip and collector surface, NG: needle gauge; Espg: electrospinning

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61 Table 3 5 Electrospinning operating conditions to fabricate PS/PNIPA blended fibermat s with various thickness fibermats Blend solution wt.% (PS/PNIPA 70/30 w/w) FR (L/min) D cp (cm) EF (kV/cm) NG Espg. t ime ( hr ) Fiber d ia (m) Fibermat thickness ( m) Sample code Remarks 15 3 20 0.43 18 1 1.5 0. 4 8314 BLt83 m S table electrospinning produced primarily fibers ( Figure 3. 8 A ) 15 3 20 0.43 18 3 0.6 0. 1 16316 BL t163 m Not a stable electrospinning and spitting observed ( Figure 3. 8 B) 15 3 20 0.43 18 10 0. 8 0. 2 25832 BL t258 m Not stable electrospinning and spitting observed ( Figure 3. 8 C) PS/PNIPA: polystyrene/poly(N isopropylacrylamide) ; FR: flow rate, EF: electric field, D CP : distance between the needle tip and collector surface, NG: needle gauge; Espg: electrospinning

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62 Figure 3 1 P oly[(n isopropylacrylamide) co (methacrylic acid)] ( PNIPAMAA ) chemical formula x y

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63 F igure 3 2 Digital photographic image showing electrospinning setup used in this dissertation

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64 Figure 3 3 Scanning electron microscopy ( SEM ) images of PNIPAMAA fibermat E lectrospun from 10 wt % PNIPAMAA / deionized water ( DI H 2 O ) solution. For the electrospinning conditions and other details see Table 3 1

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65 Figure 3 4 Optical and SEM micrographs of PNIPAMAA fibermat s obtained using d imethylformamide ( DMF ) as solvent in electrospinning. Electrospun from 15 wt.% PNIPAMAA /DMF solution (A); 25 wt.% PNIPAMAA /DMF solution ( B ); 30 wt.% PNIPAMAA /DMF solution ( C) ; & 30 wt.% PNIPAMAA /DMF solution spun on Al foil for 30 minutes (D). A, B, and C are optical micrographs and D is SEM micrograph. For the electrospinning conditions and other details see Table 3 1 A C D B

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66 Figure 3 5 Optical and SEM micrographs of PNIPAMAA fibermat s obtained using a cetic acid ( OHAc ) as solvent in electrosp inning. Electrospun from 1 0 wt % PNIPAMAA / OHAc solution (A & B ); 1 5 wt % PNIPAMAA / OHAc solution (C ); & solution spun on aluminum ( Al ) foil for 30 minutes ( D) A, and C are optical micrographs B and D are SEM micrograph s For the electrospinning conditions and other details see Table 3 1 C B A D

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6 7 Figure 3 6 Crosslinked PNIPAMAA fibermat s m orphology before heat treatment (A); after heat treatment (B) & ( C ) After heat treatment fibers that meet at the junction fused together as shown in a magnified image (C). fiber dia meter approximately 1.5 2 m (D); Fiber dia meter 241 30 nm (E). D is an optical micrograph and A, B, C and E are SEM micrograph For the electrospinning conditions and other details see ( Table 3 2 & Table 3 3) A B C D E

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68 Figure 3 7 SEM micrographs of PS/PNIPA blended fibermat s obtained with various diameter fibers 0.380.1 m (A); 0.990.3 m (B); 10.9 m (B); 161.2 m (D); 298.7 m (E) For the electrospinning conditions and other details see Table 3 4 A B C D E

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69 Figure 3 8 SEM micrographs of PS/PNIPA blended fibermat s synthesized with various thickness fibermats 8314 m (A); 16316 m (B); 25832 m ( C). For the electrospinning conditions and other details see Table 3 5. A B C

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70 CHAPTER 4 THERMALLY TUNABLE SURFACE WETTABILITY OF ELECTROSPUN FIBERMAT S In the previous chapter p olystyrene (PS) /poly(N isopropylacrylamide) (PNIPA) blended and crosslinked poly(N isopropyl acrylamide co methacylicacid) ( PNIPAMAA) fibermat s were fabricated by electrospinning. In this chapter, these fibermat s are characterized for thermoresponsive wettability by contact angle Goniometer and for structural integrity by leaching experiments. Experimental Section The electrospinning conditions used to synthesize PS/PNIPA blended and crosslinked PNIPAMAA fibermat s is given in Table 4 1. These fibermat s were used for characterizing thermoresponsive wettability in Goniometer and structural integrity by leaching experiments in 10 C water in this Chapter In order to remove the unreacted polymer chains before characteri zing for wettability especially with PNIPAMAA fibermat s they were collected on a 1 mm thick glass slide (size 7.6 cm x 2.5 cm) for about 3 mins electrospinning The bottom of the glass slide was glued to aluminum foil using a double sided copper tape and this was made as collecting plate The collected electrospun fibermat s were kept in a vacuum oven at room temperature (RT) overnight, followed by a heat treatment at 16 0 C for 30 minutes in a vacuum After this time, the samples were washed in cold water (10 C) followed by a hot water (100 C) wash and this similar washing cycle was repeated twice. These fibermat s were then used for contact angle (CA) measurements. Determination of T ransition T emperature Hydrated crosslinked PNIPAMAA fibermat weighing approximately 20 mg was taken for differential scanning calorimetry ( DSC ) analysis. A temperature scan was

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71 performed between 5 C and 100 C at the heating rate of 2 C/min to analyze the sample transition temperature in DSC. PNIPAMAA copolymer dissolved i n deinoized ( DI ) water to make 7.5 wt. % was also analyzed in DSC to determine its transition temperature. Contact A ngle M easurements Contact angle ( CA ) measurements were carried out on electrospun samples by Goniometer (Model: VCA Optima, AST Products, Inc.) instrument equipped with an automated dispensing system, and a 30 gauge flat tipped stainless steel needle. The probe fluid water having resistivity cm was collected from nanopure Milli Q purification system (Millipore Inc.). Sessile drop images were captured by placing a 2 l volume water droplet over the fibermat at 5 different places for measuring CAs. Goniometer used in this dissertation to measure CA values is shown in Figure 4 3 A. CA were measured in a room enviroment where relative humidity and temperature were 42 % and 25 C, respectively. The CA data were then obtained by Drop Snake analysis [107, 108] a plug in for Image J software as shown in Figure 4 3 B. More information about this plugin can be obtained from web site [ http://bigwww.epfl.ch/demo/dropanalysis/ ] Setup for t emperature d ependent contact angle m easurements For temperature dependent CA measurements a unique setup was used to bring the fibermat s to the temperature of interest either by heating or cooling. Heating was performed using a thin flexible Kapton heater (Model: KH 203/10, O mega Engineering, Inc.). Cooling was performed by a cryostage (Product Number: 39467506, Subzero Leica) attached to a cooler. Fibermat s either collected on aluminum foil or a glass slide was glued to a silicon wafer with the help of

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72 double sided carbon adhesive tape. This arrangement was glued to a flexible heater and again to a cryostage with the help of scotch tape and the whole setup was placed on Goniometer stage. A thin thermocouple (Model: SA1 K SRTC, Omega Engineering, Inc. ) connected with a temperature meter (Model: BS5001k2, Omega Electronics, Inc.) was glued on top of the fibermat to read its surface temperature. The flexible Kapton heater was powered by a DC power supply (Model: 6218A, Agilent HP), the temperature on th e fibermat was adjusted by controlling the voltage and current. Thus, during the heating cycle the DC power supply was switched on and during the cooling cycle the cryostage was on. The temperature on top of the fibermat surface was measured in degree Cels ius with 1C accuracy by the thermocouple attached to it This entire setup ensured a good control on fibermat temperature. Determination of fraction of the wet solid contact area The fraction of electrospun fibermats wet solid contact area was obtained from SEM images using Image J software. The SEM images used for this purpose were secondary electron (SE) images. The SEM image was first converted to 32 bit type image in Image J software by: image > type > 32 bit. A threshold level (only the top layer of fibermat in SEM image) was adjusted and measured to obtian the fraction of the wet solid contact area in Image J software by : i ma ge > adjust > threshold. It was assumed that water droplet contacts only the top layer of the fibermat while measuring CA valu es by Sessile drop method in Goniometer. The top layer of SE image of SEM was relatively brighter than bottom layers as the SE collector is situated closer to the top layer in SEM.

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73 D etermination of i mmobilization in fibermat s To a vial of de ionized water (25 mL at 10 o C), approximately 10 mg of the PS/PNIPA blended or crosslinked PNIPAMAA vacuum dried fibermat was added and stirred (50 rpm) for 24 h. After this time, the fibermat was collected and dried overnight in a vacuum oven at RT. The value reported a re an average of three fibermats sample. The amount of crosslinking or immobilization (I) was calculated using the gravimetric method : (Equation 4 1) w i = initial weight; w f = final weight after treatment Results and Discussion In this section the PS/PNIPA blended and crosslinked PNIPAMAA fibermat s characterized for reversible wettability properties and integrity in 10 C water will be analyzed with their results The thermoresponsive wettability of these fibermat s in response to a dye solution is shown in Figure 4 1. Fibermat Morphology The surface morphologies of the PS/PNIPA blended and crosslinked PNIPAMAA fibermat s were initially analyzed by scanning electron micro scopy ( SEM ) in a similar way as reported in Chapter 3 Experimental section and shown in Figure 4 2. Mean diameter fiber of PS/PNIPA blended and crosslinked PNIPAMAA fibermat s was found to be 1.70.4 m and 870 150 nm, respectively. Fibermat s Thermoresponsive Reversible Wettability CA measurements were undertaken for both the PS/PNIPA blended and crosslinked PNIPAMAA electrospun fibermat s. Superhydrophilic and s uper hydrophobic

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74 [76] At 15 o C, the PS/PNIPA blended fibermat showed a response in agree ment with a superhydrophilic surface. At 65 o C, the CA value was switched to 138.04.5 ( Figure 4 5 ). The switching occurred between 30 C and 45 C At 45 C, the sample was already at 90 % of the steady state CA value. This reversible wettability of PS/PNIPA blended fibermat was cycled 5 times between superhydrophilic and superhydrophobic with no loss of value ( Figure 4 6 ) The CA values observed are consistent with those values reported by Wang et. al [109] In this study, PS/PNIPA blend was electropsu n using dimethylformamide (DMF) solvent that yielded fibers alone. In contrast, Wang et. al. work, PS/PNIPA was electrospun using tetrahydrofuran (THF) solvent that yielded mostly beads. Overall, DMF has relatively high dielectric constant, that helped in storing more charges and produced a relatively high quality fibermat with fibers alone. Before characterizing the reversible wettability properties of the crosslinked PNIPAMAA fibermat s, determination of the transition temperature was done by DSC measurements. The transition temperature is the temperature where the fibermat changes from hydrophilic to hydrophobic properties during heating cycle and from hydrophobic to hydrophilic proper ties during cooling cycle. Agrawal et. al. [34] reported a decrease in transition temperature after crosslinking the poly(N tert butylacrylamide co acryl amide) solution spun fiber using butanetetracarboxylic acid from 37 C reported for the polymer in water. Surprisingly, the crosslinked PNIPAMAA fibermat exhibited an upward shift in transition temperature to a strong peak at 82.7 C from approximately 32 C typically reported for the PNIPAMAA in water, which indicated that after crosslinking the transition temperature increased ( Figure 4 4 ). In our case, the presence of poly(vinly

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75 alcohol) ( PVA ) in the crosslinked network caused the fiber network to be mor e hydrophilic and shifted the transition point to approximately 51 C to a higher temperature In contrast to PS/PNIPA blended fibermat the CA measurements of the crosslinked PNIPAMAA fibermat revealed a response between 20.56.5 and 87.03.0 CA value s at 20 C and 9 0 C ( Figure 4 5 ). The transition point observed for the crosslinked PNIPAMAA fibermat as, determined by DSC, was approximately 83 C, necessitating the need to have CA measurements up to 9 0 C. Interestingly, at 70 C the crosslinked PNIPA MAA fibermat attained 94 % of the steady state CA value with a relatively broader switching temperature range from 40 to 70 C. This response was also repeated for the 5 tested cycles with a similar switching behavior between hydrophilic and closer to hydrophobic for the cooling an d heating cycle at 25 C and 80 C temperatures respectively ( Figure 4 6 ). To the best of the author knowledge this is the first time PNIPAMAA is electrospun into fiber form and its reversible wettability properties are studied. PNIPAMAA is a dual tempera ture and pH responsive copolymer whose responsive properties are usually exploited in hydrogel form [99, 100, 110 112] In one study, a blend of two different homo polymers of PNIPA and poly(acrylic acid) (PAAc) w as electrospun and crosslinked to produce a dual temperature and pH responsive fibermat is reported [103] From the CA measurements PS/PNIPA blended fibermat surface was found to be more sensitive to reversible wettability properties and occurred in a relatively narrower temperature range of 15 o C in comparison to 30 o C temperature range of the crosslinked PNIPAMAA fibermat Fibermat s Integrity One of the key developments in this project was to generate fibermat s that have reversible wettability properties while being insoluble below the transition temperature.

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76 As mentioned previously, at temperatures lower than the lower critical solution temperature ( LCST ) P NIPA dissolves in water due to its polymer chains hydrogen bonding with water molecules. This study used crosslinking and blending to reduce or avoid solubility. This means that we can use leaching experiments to confirm the mass of insoluble chains that a re responsible for reversible wettability properties. This was done in cold water where fibermat s were washed (10 C) for 24 h. The amount of immobilization (or crosslinking) was indirectly measured based on the weight loss of the PS/PNIPA blended and cro sslinked PNIPAMAA fibermat s, i.e. weight loss and the amount of immobilization (or crosslinking) would be inversely proportional to each other. For the PS/PNIPA blended fibermat ( Table 4 2 ), 83.34.2 % weight loss was noticed and the amount of immobilization was 16.74.2 % which is surprising considering this fibermat is merely a physical blend of polymers. The theoretical weight ratio of PS/PNIPA in the blended fibermat was 70/30 and for the calculation only PNIPA weight was considered since it was expected to solely contribute to the weight loss as PS is a hydrophobic polymer and expected not to dissolve in water. In contrast, for the crosslinked PNIPAMAA fibermat 53.30.5 % weight loss was observed and the amout of crosslinking was 46.70.5 % which indicates that only half of the fibermat was retained after the wash. Thermal integrity of the PS/PNIPA blended and crosslinked PNIPAMAA fibermat s was further studied using the section of the fibermat s that had undergone 5 heating and cooling cycles. SEM analysis of this section revealed that the PS/PNIPA blended fibermat had started to disintegrate from its blended fibrous structure, whereas the crosslinked PNIPAMAA fibermat maintained its crosslinked network structure ( Figure 4

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77 7 ). Interestingly for the PS/PNIPA blended even while its blended structure started to disintegrate, the fibermat still demonstrated reversible wettability properties for the 5 tested cycles ( Figure 4 6 ). In contrast, crosslinked PNIPAMAA fibermat integrity was strong and maintained its crosslinked fibrous network structure even after 5 tested cycles. From the leaching experiments and thermal integrity analysis, crosslinked PNIPAMAA fibermat showed strong structural integrity in comparsion to PS/PN IPA blended fibermat A possible mechanism of the crosslinking function to insolubility of crosslinked PNIPAMAA fibermat s can be explained by the work performed by Chen et. a l [103] A ll the possible reactions that can occur between carboxylic acid, alcohol, and amide groups in the presence of the d isodium hydrogen phosphate ( DSHP ) crosslinking aid need to be considered when proposing a crosslinking mechanism. Only intermolecular bond formation that result s in a 3 dimensional network would contribute for the crosslinked PNIPAMAA fibermat s to become insoluble in water not intramolecular bond formation. All the possible crosslinking reactions are: intermolecular anhydride formation between carboxylic acid groups of PNIPAMAA; esterification between carboxylic acid groups of PNIPAMAA and alcohol groups of Poly(vinyl alcohol) (PVA); intermolecular imidation between carboxylic acid groups and amide groups of PNIPAMAA. It is plausible that all the three reactions have contributed to crosslinking the electrospun fiber during heat treated at 160 C in a vacuum oven. However, reactive species concentration and steric hindrance would determine the ultimate reaction proporti on. In addition to this, during handling, the crosslinked PNIPAMAA fibermat was observed to

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78 be more mechanically stronger than the PS/PNIPA blended fibermat as it was more tightly held together. Mechanism behind R eversible W ettability The mechanism behind the reversible wettability behavior of these mats may be explained by the PNIPA chemistry and the Cassie Baxter (CB) model. The temperature responsive wettability is obtained from the PNIPA chemistry [43, 73, 82, 113] One of the transition states of PNIPA is in soluble form so its polymer chains have to be either chemically or physically immobilized in order to realize its beneficial temperature responsive properties. Basically, it is the competition between inter and intra molecular hydrogen bonding between PNIPA (or PNIPAMAA) polymer chains and water molecules that is chemically responsible for the fibermat showing thermally tunable surface wettability. At lower temperatures th e PNIPA (or PNIPAMAA) molecules remain unreacted and easily undergo H bonding with water as enthalpic contributions are greater than entropic contributions; whereas at higher temperatures, the PNIPA molecules undergo intramolecular H bonding and cannot be wetted as entropic contributions are now greater than enthalpic contributions Micro/nano structured surface roughness achieved through electrospinning along with the void pockets present in the fibermat s is further believed to be helping these substrates show reversible wettability properties. This is supported by the Cassie and Baxter (CB) model which establishes a relationship between the CA and the porosity of the structure as described in ( Equation 2 1 ) [92, 93] Surface roughness obtained through electrospinning process played a major in improving the CA values to a large extent at both cooling and heating sides. This is evident as, f or a native PS/PNIPA film prepared with (PS/PNIPA 70/30 w/w) solution, thermally responsive properties were

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79 barely noticed, in which the CA was increased from 29 4 to 52 3 when the temperature changed from 25 C to 65 C. The fraction of the wet solid contact area (f s ) was obtained from SEM images using Image J software to theoritically calcualte C assie B axter contact angle ( CB ) is shown in Figure 4 8 For the fibermat s discussed here, the CB model indicate d 113 4 ( intrinsic contact angle ( Y ) at 65 C for PS/PNIPA film was 52 3 ) and 111 1 Y at 8 0 C for PNIPAMAA film was 5 4 3) CA values for the PS/PNIPA blended and crosslinked PNIPAMAA fibermat s. There are studies that report CB style surface architecture achieved with CA values closer to o r greater than 150 by electrospinning hydrophobic polymers In these studies CA measurements were done at room temperature (RT). Superhydrophobic surface properties are attained first by the low surface energy of hydrophobic polymer s and CB style roughnes s created through electrospinning process [114 118] It is interesting to observe f rom this dissertation that CB model still holds true even at higher temperatures as well In this study, electrospun PS/PNIPA fibers mats surface properties were approaching a superhydrophobic state at about 65 C primarily due to CB style roughness. This is clearly evident from the earlier discussions on CA vlaues of PS/PNIPA film. Chapter Summary PS/PNIPA blended and crosslinked PNIPAMAA fibermat s thermally tunable reversible wettability properties and integrity were characterized. The PS/PNIPA blended f ibermat s demonstrated a switching behavior between superhydrophilic and approaching superhydrophobic with 0 and 138.04.5 CA values respectively. The crosslinked PNIPAMAA fibermat s showed a switching behavior between hydrophilic and

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80 approaching hydroph obic with 20.56.5 and 87.03.0 CA values, respectively. The PS/PNIPA blended fibermat surface was more sensitive to reversible wettability properties and this response also occured in a relatively narrower temperature range than crosslinked PNIPAMAA f ibermat surface. The leaching experiments conducted in 10 C water showed that crosslinked PNIPAMAA fibermat s had a stronger structural integrity than PS/PNIPA blended fibermat s. The mechanism behind these fibermat reversible wettability is believed to be influenced by the hydrogen bonding induced change in contact angle (surface wettability) of PNIPA in combination with the Cassie Baxter style micro/nano surface architecture made by electrospinning.

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81 Table 4 1 E lectrospinning conditions used to synthesize PS/PNIPA blended and crosslinked PNIPAMAA fibermat s Polymer solution FR (mL/h) D cp (cm) EF (kV/cm) NG Espg. time (mins) Fiber dia ( m or nm) Remarks 15 wt. % blend solution of PS/ PNIPA in DMF 0.90 11 0.8 18 45 1.70.4 m Stable electrospinning produced fibers (Figure 4 2 A) 15 wt.% formulation (PNIPAMAA/HOAc; PVA, 5 wt.% to PNIPAMAA; and DSHP, 30 wt.% to PVA) 0.43 20 1 18 5 870150 nm Stable electrospinning produced fibers (Figure 4 2 B) PS/PNIPA: polystyrene/poly(N isopropylacrylamide); PNIPAMAA : p oly[(n isopropylacrylamide) co (methacrylic acid)] ; DMF: Dimethylformamide ; PVA: Poly(vinyl alcohol) ; DSHP: Disodium hydrogen phosphate ; FR: flow rate, EF: electric field, D CP : distance between the needle tip and collector surface, NG: needle gauge; Espg: electrospinning

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82 Table 4 2 Amount of immobilization or crosslinking of PS/PNIPA blended and crosslinked PNIPAMAA fibermat s. Fibermat sample Weight loss SD (%) (A) Amount of immobilization or crosslinkingSD (%) B = 100 A PS/PNIPA blended (amount of immobilization, calculated excluding PS) 83.32 .0 16.74.2 crosslinked PNIPAMAA (amount of crosslinking) 53.3 0.5 46.70.5 PS/PNIPA: polystyrene/poly(N isopropylacrylamide); PNIPAMAA: poly[(n isopropylacrylamide) co (methacrylic acid)] ; SD: standard deviation

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83 Figure 4 1 Digital photographic images of p olystyrene ( PS ) / p oly(N isopropylacrylamide) ( PNIPA ) blended fibermat s (A & B ) and crosslinked p oly[(n isopropylacrylamide) co (methacrylic acid)] ( PNIPAMAA ) fibermat s ( C & D ) showing thermoresponsive wettability in response to a dye solution The temperatures reported are fibermat s surface temperature. PS/ PNIPA blended and crosslinked PNIPAMAA fibermat s mean diameter fibers were 1.70.4 m and 870150 nm respectively. 7 L and 2 L volume of 50 ppm P rocion red dye/water was placed on PS/ PNIPA blended and crosslinked PNIPAMAA fibermat s respectively. Ruler shown is in mm scale Temperature 25 C Temperature 80 C Temperature 25 C Temperature 6 5 C A) D) C) B)

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84 Figure 4 2 SEM micrographs of PS/PNIPA blended fibermat (a) and crosslinked PNIPAMAA fibermat (b). Electrospinning conditions: PS/PNIPA blended fibermat was prepared from 15 % wt. blend solution of PS/ PNIPA in DMF (PNIPA/PS 30/70 w/w ); FR was 0.90 mL/h ; EF was 0.8 kV/cm and D CP was 11 cm. The crosslinked PNIPAMAA fibermat was synthesized by first preparing PNIPA MAA/HOAc, PVA/DI w ater and DSHP/DI water stock solutions in 15 % 15 % and 10 % wt. respectively. To the PNIPA MAA/HOAc solution (0.68 g), PVA (5 % wt. to PNIPA MAA) and DSHP (30 % wt. to PVA) were added and mixed well to produce a formulation; FR was 0.43 mL/h ; EF was 1 kV/cm and the D CP was 20 cm.

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85 Figure 4 3. A photographic image of c ontact angle Goniometer instrument used in this dissertation (A); how contact a ngle is obtained is shown using Drop Snake analysis, a plug in for Image J software (B). A B

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86 Figure 4 4 DSC graph of PNIPAMAA dissolved in water ( PNIPA 10MAA as shown in red line) and hydrated crosslinked PNIPAMAA fibermat (xl PNIPAMAA as shown in dashed blue line) 35000 32500 30000 27500 25000 22500 20000 17500 15000 12500 10000 7500 5000 2500 0 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Endo Heat flow ( W/mg) Temperature ( C) PNIPA 10MAA xl PNIPAMAA

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87 Figure 4 5 Temperature responsive switchability of PS/PNIPA blended fibermat s (bl PS/PNIPA) and crosslinked PNIPAMAA fibermat s (XL PNIPAMAA ) The temperature range studied on contact angle ( CA ) measurements for PS/PNIPA blended fibermat was between 15 C and 65 C whereas for crosslinked PNIPAMAA it was between 20 C and 9 0 C. 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 Contact angle ( ) Temperature ( C) bl PS/PNIPA XL PNIPAMAA

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88 Figure 4 6 Temperature responsive reversibility of PS/PNIPA blended fibermats (bl PS/PNIPA) and crosslinked PNIPAMAA fibermat s (xl PNIPAMAA ) The cooling and heating temperature studied on CA measurements for PS/PNIPA blended fibermat was 15 C and 65 C whereas for the crosslinked PNIPAMAA was 2 5 C and 80 C ,respectively 0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Contact angle ( ) Number of cycles bl PS/PNIPA xl PNIPAMAA

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89 Figure 4 7 SEM images of the section of PS/PNIPA blended (a) and crosslinked PNIPAMAA (b) fibermat s after 5 heating and cooling cycles, images show the spot where CA measurements where carried out.

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90 Figure 4 8 The fraction of contact area is estimated by adjusting the threshold of top view SEM pictures Original image (A); threshold adjusted image (B). A B

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91 CHAPTER 5 THERMORESPONSIVE SWELLING DESWELLING PROPERTIES OF ELECTROSPUN CROSSLINKED FIBERMAT S In the previous chapter s polystyrene ( PS ) / poly(N isopropyl acrylamide) ( PNIPA ) blend solution and poly[( N isopropylacrylamide ) co ( methacrylic acid )] ( PNIPAMAA ) formulation were separately electrospun into fibermat s and their thermoresponsive wettability and integrity were studied. It was found that crosslinked PNIPAMAA fibermat s was showing a switching behavior between hydrophilic and close to hydrophobic with better structural integrity. In this chapter the crosslinked PNIPAMAA fibermat temperature responsive transitional properties and temperature responsiv e cyclability will be studied Experimental Preparation of crosslinked PNIPAMAA fibermat s is sam e as described in chapter 2 in E xperimental S ection It was found from SEM analysis that the crosslinked PNIPAMAA had a mean diameter fiber of 870150 nm. Ch aracterization of T hermo responsive T ransitional P roperties The t hermoresponsive t ransitional p roperties of crosslinked PNIPAMAA were characterized by gravimetric method. Fresh fibermat s with 4413 m thickness (weighing 5.21 mg) were taken for this study. Temperature responsive transition properties were studied by placing this fibermat inside 10 C water to 100 C water for 5 minutes. The amount of water absorption or release was calculated after this time : (Equation 5 1) w i = initial weight; w f = final weight after water absorption o r release

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92 The data reported is an average of three fibermat s having more or less similar thickness and weight. Characterization of Thermoresponsive C yclability The thermoresponsive c yclability of crosslinked PNIPAMAA was again characterized by gravimetric method. Fibermat s with 3426 m thickness (weighing 4.31 mg) having 870150 nm diameter fiber was taken for this study. Before th e characterization, the fibermat s were prepared by immersing first immersing in 10 C water for about 1 min ute and then in 90 C water for about 1 minute. This procedure was repeated for another 2 times. Therm oresponsive cyclability properties were studied by placing this fibermat inside at 10 C water for a minute and then removed to calculate water absorption Then the fibermat was placed in 90 C water for 1 minute and removed to calculate change in water content as per Equ ation 5 1 This cycle was repeated for another 9 times. Final data reported is average of three fibermat s having similar thickness fibermat and weight Results and Discussion Crosslinked PNIPAMAA fibermats thermoresponsive transitional and cyclability will be discussed here. Thermoresponsive Transitional Properties Temperature dependent transition properties of crosslinked PNIPAMAA fibermat s for the temperature scan from 10 C (cold water) to 100 C (hot water) is shown in Figure 5 1 The highest water absorption was observed in 10 C water The lowest water expulsion was noticed in 100 C water. At 100 C the fibermat expelled 99.1 % of water from their initial absorption at 10 C water

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93 While observing the c rosslinked PNIPAMAA fibermat s transitional properties three trends with three different slopes can be observed ( Figure 5 1 ) The trend between 10 C and 50 C, and between 75 C and 100 C are away from the from the transition point and therefore the slopes in the regions are shallow. In the region between 10 C and 50 C the fibermat s are hydrophilic and absorb more water In the region between 75C and 100 C, the fibermat s are hydroph obic and release the whatever absorb ed water The trend between 50 C and 75 C is relatively steep and there lies the transition point. In this region, fibermat s transfer from a hydrophilic to hydrophobic state and start release the absorbed water. When observing the overall trend, the transition appears to be between 60 65 C for the crosslinked fibermats This transition temperature is almost in agreement with thermoresponsive switchability observed for these fibermats (Figure 4 5 ) However, differential scanning calorimetry (DSC) analysis showed a transition point a t around 83 C ( Figure 4 4). PNIPAMAA is a copolymer of N isopropylacrylamide and methacrylic acid. Since it has both temperature and pH responsive groups in its polymer chain, it s hydrogels respond to both temperature and pH stimuli [99 101, 110, 111] pH/temperature responsive electrospun fibermats synthesized from the blends of PNIPA and poly(acrylic acid) ( PAAc ) have shown that to achieve a maximum swelling with swelling degree of 15 times fibermats took 15 min in 25 C water for temperature responsive behavior The fibermats were 0. 2 mm thick containing 700 nm diameter fiber. The swelling degree was calculated by dividing hydrated fibermat mass with dry fibermat mass [1 19]

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94 Thermoresponsive Cyclability Temperature responsive cyclability of crosslinked PNIPAMAA fibermat s is shown in Figure 5 2 Fibermat was cycled between 10 C water and 90 C water. The fibermats were kept for a min ute in each temperature. Fibermat s during the cooling cycle in 10 C water have showed water absorption around 4000 5000 wt.% and during the heating cycle in 90 C they expelled 94 % of water from its initial absorption within a minute to reach approximately 250 wt.% change in water conte nt for that temperature This temperature responsive swelling deswelling property was reversible for the 10 cycles studied. The data obtained for the at least 10 cycles vary within the error reported I n cooling cycle the error was ranging about 1500 wt % whereas in the heating cycle the error was ranging about 500 wt % A study on c haracterizing the e lectrospun p oly(N isopropyIacrylamide co stearyl acrylate) fibermat ( containing 270 nm diameter fibers for thermoresponsive cyclabilit y reported, nanofibermat exhibit ed rapid and significant shrinkage by 34 % of its initial length within 2 min after a jump in temperature from 25 to 40 C [120] This suggested that nanofibermats to show fast swelling deswelling with no skin layer formation Mecha nism for Thermoresponsive Swelling Deswelling Thermoresponsive swelling deswelling of crosslinked PNIPAMAA fibermat s captured in a videographic image is shown in Figure 5 3 PNIPAMAA fibermat s when they are chemically crosslinked as the temperature decrea sed below the transition temperature, polymer chain s make hydrogen bonding with water molecules and they are in an expanded chain conformation hydrophilic and absor ped water into to the structure instead of dissovling in water since each polymer chains are now a part of the

PAGE 95

95 crosslinked network. As the temperature increased above the transition temperature, the hydrogen bond broke rather intramolecular hydrogen bonding is formed and polymer chain s conformation swit ch es to a collapsed (globular conformatio n) and become hydrophobic and the crosslniked fiber mat now releases the absorbed water [1, 3, 38, 121] This is conceptually drawn for a single fiber and shown in Figure 1 1 Chapter Summary The cr osslinked PNIPAMAA fibermat s thermoresponsive swelling deswelling properties were characterized by gravimetic method. T hermoresponsive transitional properties study showed that at 100 C the fibermat s contracted by expelling approx. 99 % from its initial absorption in 10 C water Thermoresponsive c ycles on crosslinked PNIPAMAA fibermat s showed that water uptake upto 4000 wt % during cooling cycle and expelled about 94 % of absorbed water during heating cycle in 90 C water to reach about 250 wt % change in water content for that temperature. T his property was reversible for the 10 cycles studied.

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96 Figure 5 1 Temperature dependent transition of crosslinked poly[(N isopropylacrylamide) co (methacrylic acid)] ( PNIPAMAA ) fibermat ( 4414 m thick fibermat containing 870 150 nm). At each temperature fibermat was kept for about 5 minutes. 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 6000 6500 7000 7500 0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 Change in Water Content (wt. %) Temperature ( C)

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97 Figure 5 2 Temperature responsive cycles on crosslinked PNIPAMAA fibermat s characterized 1 min ( fibermat of 3426 m thick containing individual fiber diameter of 870150 nm ; 3 times cycled first in 10 C and in 90 C water for about a min ) The cooling cycle and heating cycle was carried out in 10 C and in 90 C water respectively 500 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 5500 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5 9 9.5 10 Change in water content (wt. %) Cycles 1 min Temp 10 C Temp 9 0 C

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98 Figure 5 3 Video graphic image demonstrating the t hermoresponsive swelling deswelling behavior of crosslinked PNIPAMAA fibermat In 10 C water fibermat was swollen transparent and not sticky where as in 100 C water the fibermat was shrunk opaque and sticky

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99 CHAPTER 6 RESPONSE TIME STUDIES ON ELECTROSPUN FIBERMAT S In the previous chapters electrospinning to fabricate p olystyrene (PS) / p oly(N isopropylacrylamide) ( PNIPA) b lended and c rosslinked p o ly[(N isopropylacrylamide) co (m ethacrylic acid)] ( PNIPAMAA ) fibermat s and their thermoresponsive wettability and crosslinked PNIPAMAA fibermat s thermoresponsive swelling deswelling have been discussed. In this chapter, the response time for crosslinked PNIPAMAA fibermat s to change from a swollen state to deswollen state and vice versa is studied by gravimetric method. Further, response time for the PS/PNIPA blended fibermat s to change from their maximum to minimum contact angle (CA) value is characterized by capturing and analyzing the video for this change Experimental In order to study response time on crosslinked PNIPAMAA fibermat s with different thickness fibermats crosslinked PNIPAMAA fibermat s were produced as explained in C hapter 2 ( Table 3 3 ) with different electrospinning tim es Further, in order to learn response time on PS/PNIPA fibermat s with different diameter fibers PS/PNIPA blended fibermat s were synthesized as described in Chapter 3 (Table 3 4 ) Characterization of Response Time on C rosslinked PNIPAMAA Fibermat s Crosslinked PNIPAMAA fibermat s were characterized for response time by gravimetric method. Before the characterization fibermat s were prepared by immers ing in 10 C water for one min ute and then in 90 C water for o ne minute. This procedure was repeated for another 2 times. Response time was later then investigated by immersing the crosslinked PNIPAMAA fibermat s in 10 C water first and weighing the amount of water uptake intermittently. Once the fibermat s achieve a swollen steady

PAGE 100

100 state, then they were immersed in 90 C water The change in water content was calculated intermittently till they reach a deswollen steady state in 90 C water The change in water content was calculated as described in Equ ation 5 1 The r esponse time reported is the average of three fibermat s values Contact Angle Measurments on PS/PNIPA Blended Fibermat s CA mesurements on PS/PNIPA fibermat s with different diameter fibers were carried out as described in Chapter 4. A 4 L volume of water d roplet was placed over the fibermat instead of 2 L described. This change was done to match with the droplet volume used in response time studies on PS/PNIPA b lended fibermat s Characterization of Response Time on PS/PNIPA B lended Fibermat s PS/PNIPA Blended fibermat s response time was investigated by capturing and analyzing the video for the transition to happen from a maximum to minimum CA value The camera used in this study ha d a capacity to capture 60 frames per second The setup used to study the response time is shown in Figure 6 3. Sample preparation: fibermat collected on aluminum (Al) foil was glued to silicon (Si) wafer using double side adhesive carbon tape as shown in the inset in Figure 6 3. This ensured a flat fibermat surface that facili tated the response time study. A thermocouple (Model: SA1 K SRTC, Omega Engineering, Inc.) was glued on top of the fibermat. The thermocouple was connected to a temperature meter (Model: BS5001k2, Omega Electronics, Inc.) that To observe change from maximum to minimum CA value fibermat was first heated to 65 1 C by resistive heating using a thin and flexible Kapton heater (Model: KH 2 03/10, Omega Engineering, Inc.) with help o f a DC power supply (Model: 6218A, Agilent HP) (Figure 6 3) Once the fibermat reached 65 C, a 4 L volume dye solution was placed above the fibermat 50 ppm

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101 concentration Procion red dye prepared in water was used for this purpose. The fibermat was then placed on top a metal bar The metal bar temperature was maintained at 303 C using liquid N 2 The start time for the response was noted when the Si wafer substrate now with fibermat fully contacted the metal bar and the end time was noted when the dye solution reach ed a minimum CA value. The response time reported is average of 5 values from 5 different spots. This study was conducted in a room enviroment where relative humidity and temperature were 4 5 % and 25 C, respec tively. This way to study the response time has not been reported in any literature and it was devised particularly in this dissertation work Results and Discussions The results obtained on response time studies conducted with crosslinked PNIPAMAA fiberma t s by gravimetric method and PS/PNIPA blended fibermat s by maximum to minimum CA change method will be discussed here. Response Time on C rosslinked PNIPAMAA Fibermat s The transition kinetics studies carried out on crosslinked PNIPAMAA fibermat s is shown in Figure 6 1 Sample XLD870nm/t150m with the mean fibermat thickness of 14843 m (weighing 4.90.5 m g) demonstrated that about 25 s for the fibermat s to reach a swollen steady state in the 10 C water and it expelled about 75 % of their absor bed water in about 5 s For this sample with approx. 150 m thick fibermat cooling cycle ws carried out till 40 s and after this time heating cycle started in 9 0 C water In contrast, sample XLD870nm/t50m with the mean fibermat thickness of 4716 m (weighing 4.21 m g) took about 3 5 s to achieve a swollen steady state with water an up take of approximately 4000 wt % and in less than 3 s it expelled about 94 wt % of the absorbed water from the fibermat to reach the change in water content to about 2 50

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102 wt % For this sample with approx. 50 m thick fibermat cooling cycle carried out was till 25 s and after this time heating cycle started in 90 C water It has to be mentioned here that 3 s was found to be the minimum time limit a response can be inv estigated by this method. As this minimum time was re quired to place the fibermat in 90 C water and take out immediately for weighing From th is study it is clear that th e response time to achieve a steady state is influenced by the fibermat thickness a s thinner fibermat took relatively less time for the transition in both cooling and heating cycles As discussed in the background temperature responsive polymers ( TRP s) polymerized in gel form have a slow response rate at the transition. This is in the o rders of several hours When TRPs are processed into nanofibermat s they show fast swelling deswelling cycles within a min ute ( Figure 5 2 ) Nanofibers have high surface area to volume ratio and t his allows them to provide fast water diffusion in and out of the fiber structure with no skin layer formation W ater expulsion from the fibermat occurred relatively faster than water uptake, this may be due to an order of increase in water diffusion rate at higher temperatures [122, 123] and positive pressure provided by the swollen fiber [124] S olution spun and crosslinked poly(N tert butylacrylamide co acrylamide) fiber having 30 50 m diameter t ook 20 s and < 2 s to reach the swollen steady state and de swollen steady state respectively [3 4] The difference is, the study report ed the response time on a single fiber but this dissertation work report s on approx. 50 m thick fibermat containing 870 150 nm diameter fiber Response Time on PS/PNIPA B lended Fibermat s CA measurements carried out on various fibermat s with different diameter fibers and their respective CA images are reported in (Table 6 1 & Figure 6 2 ) CA values measured compared with theoretically calculated using CB model. A weak correlation

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103 was observed with increasing fiber diameter, CA value decreases T his relation was in agreement CA values calculated using CB model This is attributed to as thinn er the fiber diameter lesser was the fraction of wet solid contact area the droplet contacts. Response time for the PS/PNIPA blended fibermat s to change from maximum CA value to minimum is reported here The method as described in the experimental section was devised for this purpose It appear ed to be a good method from a few methods explored to study the response time. The response time for the PS/PNIPA blended fibermat s with various diameter fibers is give n in Table 6 2 It is report ed with fiber diameter fibermat thickness, CA values measured at 65 C and 25 C and what was the cold source used to study the response time. Samples with fiber diameter of 380 nm ( BLD380nm ), 990 nm ( BLD990nm ) and 1.5 m (BLD 1.5m) can be grouped in one category as they show quicker response time 4 5 s and sample with fiber diameter of 16 m ( BLD16m ) appeared to have relatively a slower response time as shown in ( Figure 6 4 to Figure 6 7). T h e experiment was difficult to conduct on bigger diameter fiber as the dye solution placed above the BLD16m fibermat was frozen after > 25 30 s times. This was concluded after placing dye solution on several spots By using a forceps t he frozen droplet was co nfirmed after 30 s as shown in Figure 6 7 Therefore, it was decided to modify the method with a cold stage instead of metal bar to study response time on this particular sample. It is ideal that if the fibermat experience s a strong cold source such as met al bar (temperature 30 3 C maintained using liquid N 2 ) T hat will act as a strong step function for the change to occur. As mentioned earlier, by this method the droplet was

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104 frozen after > 25 30 s. Therefore, the response time for the sample BLD16m was measured using cold sta g e (temperature of 24 1 C maintained using a cold water bath circulator) This ensured that the dye solution placed over the sample will not freeze. Response time measured by this modified method on 16 m diameter fiber was 47.4 1.9 s ( Table 6 2 & Figure 6 9 ) In order to have a comparison between response time obtained for sample BLD16m and to a sample that has fiber diameter at the nanometer scale, sample BLD380nm response time was also studied using cold stage source By this way the response time measured on 380 nm diameter fiber was 13.30.9 s The obtained response time was slower than the procedure where metal bar was used ( Figure 6 9 ) O verall, there was a significant difference in the response time between fibermat containing 16 m diameter fiber and fibermats containing less than 1.5 m diameter fibers W ith the fine fibers the response time to change from a maximum to minimum CA value was faster. This is attributed to shorter diffusion path for the fine f ibers to spread and wet the fibermat quickly. The response time for nanofibers may be even quicker than what achieved as slow measurement was a limitation The setup where heat had to lose was through fibermat to aluminum (Al) foil to carbon tape to silico n (Si) waf er to metal bar. This may take some time and it is not accounted in the readings reported. It has been calculated particularly for the Si wafer (assumed to have larger share) and discussed in Appendix A. It was also of interest to study the inf luence of fibermat thickness on response due to different PS/PNIPA thickness fibermats taken for response time studies (Table 6 2) In order to study this, PS/PNIPA fibermat s having different thickness fibermats were fabricated as reported in Chapter 3 (Table 3 5 & Figure 3 8 ) It was surprising to find

PAGE 105

105 that the reported thickness fibermats an influence in response time ( Table 6 3 & Figure 6 10 to Figure 6 11) The response time all reported were within the error expected by this method Mec hanism for Droplet Spreading Over the Fibermat In order to understand how the dye solution droplet placed over the fibermat would spread as it become s superhydrophilic, two possible routes can be hypothetically assumed: (i) into fibers (ii) into pores (Fig ure 6 13). One way to address this question is using Washburn equation [125, 126] : (Equation 6 1) D is the capillary pore diameter [m]; P is the pressure [Pa]; water surface tension [N/m]. The c hange in water CA with changes in capillary pore diameter is calculated according to Washburn equation and plotted in Figure 6 14 : With increasing capillary pore diameter the water CA increases For example, deinoized water (surface tension 72 mN/m [52] ) at 1 atm pressure require s at least 135 CA value to penetrate inside the 2 m diameter capillary pore. Sample BLD380nm top layer pore diameter (assuming circular pore) found from the scanning electron microscopy (SEM) images to be roughly 2 m. The CA value for the droplet to penetrate inside the 2 m could not be theoretically calculated when the surface tension was changed to 50 m N/m value It suggest s that at all CA values the droplet would penetrate inside the 2 m diameter capillary pore. 50 ppm concentration Procion red dye in water was used for the

PAGE 106

106 response time studies on PS/ PNIPA blended fibermats. About 30 % decrease in wa ter surface tension was expected when dye present along with water. This was found to be 50 mN/m from the 72 mN/m surface tension value for the deionized water. D ye solution was used as it imparted a better contrast when analyzing the captured videos PS/ PNIPA film demonstrated a CA change from 294 to 523 when the temperature changed from 25 C to 65 C. At these CA values droplet should have penetrated into the film according to the calculation (Figure 6 14 ). However, droplet did not penetrate. It can be concluded from this analysis that the spreading of droplet on the fibermat surface at lower temperatures is primarily due to reversible hydrogen bonding capability of PNIPA. The contribution for the droplet to spread and wet the fibermat predominantly driven from the thermodynamic conditions ( hydrogen bonding ) and less likely driven from p ores present in the fibermat. Thus, when droplet wets the fibermat the droplet largely goes into the fiber rather than into pores. A diameter swelling of about 1000 % can be achieved in TRP [34] However, due to blending ( PS/PNIPA 70/30 w/w) about 300 % diameter swelling is expected in this case. Droplet quickly spread the fiber surface and later had gone inside the fiber Chapter Summary The crosslinked PNIPAMAA and PS/PNIPA blended fiberma t s have been characterized for response time by gravimetric method and maximum to minimum CA change method, respectively. Crosslinked PNIPAMAA fibermat s having approximately 50 m thick fibermat containing 870 nm diameter fiber response time for transition in the cooling cycle was 3 5 s and in the heating cycle was less than 3 s Response time studies on PS/PNIPA blended fibermat s demonstrated that t here was a significant influence of fiber diameter on response time With less than 1.5 m diameter fibers, the

PAGE 107

107 response was quicker with 4 5 s response time W ith 16 m diameter fiber, the response was slow er with greater than 25 s response time. There was no influence of reported PS/PNIPA blended fibermat s thickness on response time by maximum to minimum CA change method The mechanism behind the droplet spreading over the PS/PNIPA blended fibermats largely driven from the thermodynamic conditions (enthalpic dominated hydrogen bonding between PNIPA and water molecules) and less likely due to pores present in the fibermat.

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108 Table 6 1 Comparison of calculated contact angle values with Cassie Baxter contact angle values (for contact angle images see Figure 6 2) Sample code Fiber dia ( m) Mat thickness w/Al foil ( m) c at 65 C () C B at 65 C () ( calculated using Y at 65 C for PS/PNIPA film was 523 ) C B at 50 C () (calculated using Y at 50 C for PS/PNIPA film was 943 [109] ) Fraction of wet solid contact area (f s ) BLD380nm 0.380.1 446 144 4.6 1123 1532 0.390.03 BLD990nm 0.990.3 5510 1464.1 1122 1531 0.390.02 BL D1.5m 1.50.9 5310 1404.3 1111 1521 0.4 0 0.01 BL D16m 16 .0 1.2 70 10 1355.7 1033 1491 0.480.03 c : contact angle ; CB : Cassie Baxter contact angle ; Y : intrinsic contact angle on the original smooth surface ; PS/PNIPA : p olystyrene /p oly(n isopropylacrylamide )

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109 Table 6 2 Response time studies on PS/PNIPA blended fibermat s with different diameter fibers Sample code Fiber dia ( m) Mat thickness w/Al foil ( m) c at 65 C () c at 2 4 C () Cold source a Response time (s) BLD380nm 0.380.1 446 144 5 0 metal bar 4.20.9 BLD990nm 0.990.3 5510 1464 0 m etal bar 4.30.6 BL D1.5m 1.50.9 5310 1404 0 m etal bar 4.80.7 BL D16m 16 .0 1.2 70 10 135 6 0 m etal bar f r o z en ( > 25 30 s) BLD16m 16 .0 1.2 70 10 135 6 0 stage 47.41.9 BLD380nm 0.380.1 446 144 5 0 stage 13.30.9 a metal bar temperature of 303 C was maintained using liquid N 2 ; stage temperature of 241 C was maintained using water cold water bath circulator c : contact angle

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110 Table 6 3 Response time studies on PS/PNIPA blended fibermat s with different thickness fibermats Sample code Fiber dia ( m) Mat thickness w/Al foil ( m) Cold source a Response time (s) BLt83 m 1.5 0. 4 8314 m etal bar 4.60.4 BL t163 m 0.6 0. 1 16316 m etal bar 4.70.5 BL t258 m 0. 8 0. 2 25832 m etal bar 4.40.7 a metal bar temperature of 303 C was maintained using liquid N 2

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111 Figure 6 1 Transition kinetic s of 14843 m and 4716 m thick crosslinked p oly[(n isopropylacrylamide) co (methacrylic acid)] ( PNIPAMAA ) fibermat s containing 870 nm diameter fiber The cooling cycle and heating cycle was carried out in 10 C and 90 C water, respectively. approximately 50 m thick fibermat sample undergone 3 times cooling (10 C water )/heating (100 C water ) cycles for about 1 min at each cycle where as approximately 1 50 m thick fibermat sample was fresh 500 0 500 1000 1500 2000 2500 3000 3500 4000 4500 5000 0 5 10 15 20 25 30 35 40 45 50 Change in water content (wt. %) Time (s) thick(150um) thick(50um)

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112 Figure 6 2 Contact angle (CA) images showing 4 l water droplet placed on fibermat sample of BLD380nm at 65 C (A) and at 24 C (B); sample of BLD990nm at 65 C ( C ) and at 24 C ( D ); sample of BLD1.5m at 65 C ( E ) and at 24 C ( F ); sample of BLD16m at 65 C ( G ) and at 24 C ( H). For details on CA values see Table 6 1 &Table 6 2 A G H E F D C B

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113 Figure 6 3 Polystyrene (PS)/poly(N isopropylacrylamide) (PNIPA) blended fibermat sample prepared for response time studies by gluing fibers deposited on aluminum (Al) foil with silicon (Si) wafer using double sided adhesive carbon tape to make flat fibermat surface ( A); Photo graphic image showing the setup for conducting response time studies on PS/PNIPA blended fibermat s (B) A) B)

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114 ` Figure 6 4 Video images showing frames captured for every 0.5 s of fibermat BLD380nm CA on this sample measured in Goniometer changed from 144 4.6 to 0 for the temperature change from 65 C to 24 C 0 s 0.5 s 1.0 s 1.5 s 2.5 s 3.5 s 3.0 s 4.0 s 4.5 s 5.0 s 2.0 s

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115 Figure 6 5 Video images showing frames captured for every 0.5 s of fibermat BLD990nm CA on this sample measured in Goniometer changed from 14 6 4. 1 to 0 for the temperature change from 65 C to 24 C 0 s 0.5 s 1.0 s 2.0 s 1.5 s 3.0 s 3.5 s 2.5 s

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116 Figure 6 6 Video images showing frames captured for every 0.5 s of fibermat BLD1.5m CA on this sample measured in Goniometer changed from 140 4. 3 to 0 for the temperature change from 65 C to 24 C 0.5 s 0 s 4.0 s 4.5 s 2.0 s 3.0 s 3.5 s 1.0 s 2.5 s 1.5 s

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117 Figure 6 7 Video images showing frames captured for every 0.5 s of fibermat BLD16m CA on this sample measured in Goniometer changed from 1 35 5.7 to 0 for the temperature change from 65 C to 24 C (cont.) 0 s 0.5 s 1.0 s 1.5 s 2.0 s 2.5 s 5.5 s 5.0 s 4.5 s 4.0 s 3.5 s 3.0 s

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118 Figure 6 7 11 .5 s 7.5 s 9.0 s 9.5 s 10.0 s 11.0 s 10 .5 s 8.0 s 7.0 s 6.5 s 6.0 s 8.5 s

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119 Figure 6 7 17 .5 s 15 .5 s 1 5 .0 s 17.0 s 16 .5 s 13.0 s 14.0 s 12 .5 s 13 .5 s 12.0 s 14 .5 s 16.0 s

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120 Figure 6 7 23 .5 s 20.0 s 19 .5 s 21.0 s 21.5 s 22.0 s 23.0 s 22.5 s 19.0 s 20 .5 s 18 .5 s 18.0 s

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121 Figure 6 7 29 .5 s 25 .5 s 28.0 s 27 .5 s 27.0 s 29.0 s 28 .5 s 25.0 s 24 .5 s 24.0 s 26 .5 s 26.0 s

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122 Figure 6 7 End 30.0 s 34.0 s 33 .5 s 33.0 s 32 .5 s 32.0 s 31 .5 s 31.0 s 30 .5 s 34.5 s

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123 ` Figure 6 8 Video images showing frames captured for every 0.5 s of fibermat BLD 16m on cold water stage CA on this sample measured in Goniometer changed from 1444.6 to 0 for the temperature change from 65 C to 24 C (cont.) 0 s 7.5 s 7.0 s 6.5 s 6.0 s 5.5 s 5.0 s 4.5 s 4.0 s 2.0 s 2.5 s 3.0 s 3.5 s 1.5 s 1.0 s 0.5 s

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124 Figure 6 8 Continued 8.0 s 15.5 s 15.0 s 14.5 s 14.0 s 13.5 s 13.0 s 12.5 s 12.0 s 11.5 s 11.0 s 10.5 s 10.0 s 9.5 s 9.0 s 8.5 s

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125 Figure 6 8 16.0 s 23.0 s 23.5 s 22.5 s 22.0 s 21.5 s 21.0 s 20.5 s 20.0 s 18.0 s 18.5 s 19.5 s 19.0 s 17.5 s 17.0 s 16.5 s

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126 ` Figure 6 8 C 24.0 s 24.5 s 31.0 s 30.5 s 30.0 s 29.5 s 29.0 s 28.5 s 28.0 s 26.0 s 26.5 s 27.5 s 27.0 s 25.5 s 25.0 s 31.5 s

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127 Figure 6 8 32.0 s 39.5 s 39.0 s 38.5 s 38.0 s 37.5 s 34.5 s 37.0 s 36.5 s 36.0 s 34.0 s 35.0 s 35.5 s 33.5 s 33.0 s 32.5 s

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128 Figure 6 8 End 40.0 s 40.5 s 41.5 s 42.0 s 43.0 s 42.5 s 41.0 s

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129 Figure 6 9 Video images showing frames captured for every 0.5 s of fibermat BLD380nm on cold water stage CA on this sample measured in Goniometer changed from 1444.6 to 0 for the temperature change from 65 C to 24 C (cont.) 0 s 4.5 s 4.0 s 3.5 s 3.0 s 2.5 s 2.0 s 1.5 s 1.0 s 0.5 s 5.5 s 5.0 s

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130 Figure 6 9 6.0 s 11.5 s 11.0 s 10.5 s 10.0 s 9.5 s 9.0 s 8.5 s 8.0 s 7.5 s 7.0 s 6.5 s

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131 Figure 6 9 End 13.5 s 12.0 s 12.5 s 13.0 s

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132 Figure 6 10 Video images showing frames captured for every 0.5 s of fibermat BLt83 m 0 s 0.5 s 1.0 s 1.5 s 2.0 s 2.5 s 3.0 s 3.5 s 4.0 s 5.0 s 4.5 s

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133 Figure 6 11 Video images showing frames captured for every 0.5 s of fibermat BLt163 m 0 s 5.0 s 4.5 s 4.0 s 3.5 s 3.0 s 2.5 s 2.0 s 1.5 s 1.0 s 0.5 s

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134 Figure 6 12 Video images showing frames captured for every 0.5 s of fibermat BLt258 m 0 s 4.5 s 4.0 s 3.5 s 3.0 s 2.5 s 2.0 s 1.5 s 1.0 s 0.5 s

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135 Figure 6 13. Schematically drawn fibermat cross section with dye solution droplet sitting above the fibermat. Arrow marks indicate the two possible paths dye solution droplet would spread over the fibermat as it become superhydrophilic. Figure 6 14 Changes in water contact angle with changes in diameter of capillary pores at 1 atm pressure. 80 90 100 110 120 130 140 150 0 0.5 1 1.5 2 2.5 Contact angle ( ) Pore diameter (m) mN/m mN/m

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136 CHAPTER 7 CONCLUSIONS AND SUGG ESTIONS FOR FUTURE W ORK Conclusions Fabrication of p olystyrene ( PS ) / Poly(n isopropylacrylamide) ( PNIPA ) blended and crosslinked p oly[(n isopropylacrylamide) co (methacrylic acid)] ( PNIPAMAA ) fibermat s in Chapter 3 have shown that electrospinning is a promising way to process responsive polyemrs into nanofibers. In addition to this finding, there were several other accomplishements achieved in this dissertation, including the thermally tunable surface wettability from PS/PNIPA blended and crosslinked PNIPAMAA fibermat s. The response time studies conducted in Chapter 6 have proved that PS/PNIPA blended and crosslinked PNIPAMAA fibers synthesized at the nanometer dimensions have a quicker respone rate Wi th the above discussions and analysis the following are the conluctions arrived from this reserach work: 1. Synthesis of PS/PNIPA blended and crosslinked PNIPAMAA fibermats by electrospinning process have shown that e lectrospinning is a promising way to produce responsive polymers into nanofibers. By simply adjusting the electrospinning process ing and operating conditions it has been shown that fibers with different diameters can be achieved. This was demonstrated with PS/PNIPA blended fibermats. B y chan ging the polymer concentration flow rate, gap distance and electric field PS/PNIPA blended fibermats with different diameter fibers in the ranges of 100s nm to 10s of m were synthesized 2. Characterizing the fibermats for temperature responsive wettabili ty have shown that the PS/PNIPA blended fibermats demonstrated a switching behavior between superhydrophilic and approaching superhydrophobic with 0 and 138.04.5 contact angle ( CA ) values, respectively, whereas the crosslinked PNIPAMAA fibermats showed a switching behavior between hydrophilic and approaching hydrophobic with 20.56.5 and 87.03.0 CA values, respectively. Further characterizing the fiber mats for integrity by leaching experiments have shown that crosslinked PNIPAMAA fibermats had a s tronger structural integrity than PS/PNIPA blended fibermats as expected Overall, it can be concluded that PS/PNIPA blended fibermat s may be suitable for applications where strong reversible wettability properties are required but for a few cycles and crosslinked PNIPAMAA fibermat s may be suitable for applications with higher cycle numbers where less strong reversible wettability properties are required.

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137 3. The mechanism behind PS/PNIPA blended and crosslinked PNIPAMAA fibermat s temperature tunable revers ible wettability is believed to be the Cassie Baxter (CB) style micro/nano surface architecture achieved by electrospinning process with the hydrogen bonding induced change in CA (surface wettability) of PNIPA in combination. The presence of air pockets (p roviding non polar characteristics) in fibermats act as a repelling force especially at higher temperatures to show hydrophobic/superhydrophobic surface properties. 4. T he crosslinked PNIPAMAA fibermat s swelling deswelling studies have showed that water upta ke upto 4000 wt.% during cooling cycle and water expulsion down to approx. 250 wt.% during heating cycle. When the fibermat s changed from a swollen state in cooling cycle to collapsed state in heating they expelled about 94 wt.% of the water from their initial absorption with in one minute This was reversible at least for the 10 cycles studied without any loss in value It can be concluded that n ot only the crosslinked PNIPAMAA fibermat s surface properties have temperature respon sive properties but also whole fibermat This again confirmed that crosslinked PNIPAMAA fibermat s can be used applications where many cycles are required. 5. Response time studies conducted on crosslinked PNIPAMAA and PS/PNIPA blended fibermats have shown tha t when PNIPAMAA and PS/PNIPA polymers processsed into nanofibermats the response time was fast. In addition to this finding, t here was an influence of fiber diameter on response time. This was demonstrated with PS/PNIPA blended fibermats. When the b lended fibermat s were less than 1.5 m diameter fibers, response time for the change from a maximum to minimum CA was 4 5 s From this information, it can be concluded that finer the diameter fibers shorter diffusion path for the water to spread and wet quickly the fibermats Suggestions for F uture W ork Determining r esponse t ime on a single f iber : Initially, the goal of this dissertation work was to determine response time on a single fiber (100s of nanometers) to transit from one conformational state to anothe r. The e fforts made in since there is no specific characterization method available that will help in characterizing a quick response estimated from nanofibers For example, few fibers collected on a glass slide were examined under optical microscope with a camera fixed through eye piece of microscope and a setup which had provision to push cold and hot water alternatively to effect the transition The transition from

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138 swelling to deswelling and vice versa was video capture d When water was pushed on top of the glass slide, for some reason the optical path to observe fiber was disturbed and was not able to see the fibers. The future focus should involve modifying an existing instrument that facilitate response time studies or devising a new method. Setup for t racking temperature while measuring contact angle: As the maximum to minimum contact angle (CA) change method used to study response time in this dissertation did not have a provision to track down the temperature as the CA being changed Electrospun fibers can be deposited on the tip of a thermocouple and this setup can be placed in a c uvette type container with water. According to the wettability nature of fibermat water would interact with the fibermat. T he c urvature of the meniscus gives the fibermat CA In this way, there is a pr ovision for tracking fibermat temperature as the CA changes using recorder This can be video graphed to study the response time. To effect the transition either cold or hot water can be provid ed to the container without disturbing the thermocouple setup.

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139 APPENDIX TEMPERATURE DECREASE IN SILICON WAFER The aim of this section of the work was to calculate how fast the Si wafer temperature decreases with time. The actual condition as used in the polystyrene (PS)/poly( N isopropylacrylamide) ( PNIPA ) response time studies was assumed In that study, t he Si wafer substrate was heated to 65 C and then placed on a metal bar maintained at 30 C. The Si wafer substrate cools due to heat lost to the metal bar F ollowing are the assumptions made for the purpose of this calculation : 1. The metal bar is assumed to be an infinite cold reservoir 2. The thermal conductivity of Si wafer is assumed to be constant for the whole temperature range 3. Heat loss primaril y due to conduction (or radiative heat transfer is negligible) The s etup showing how silicon (Si) wafer was placed on metal bar is shown in Figure A 1 (A). The whole setup was considered as one body having Si wafer side and t is the temper ature of Si res is the reservoir (metal bar) temperature and is the thickness of Si wafer. The following are the variables with their definition and if required with their values [127] : : thermal conductivity of Si wafer = 149 W m 1 K 1 : Thickness of Si wafer = 0.5 mm t: time s: specific heat capacity of Si wafer = 0.7110 3 J kg 1 K 1 : density = 2.3310 3 kg m 3 A: Si wafer area m: mass = A

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140 The constitutive equations for this system are: (i) e quation for heat transfer between Si and metal bar (Equation A 1) (ii) h eat lost due to decrease in temp of Si (Equation A 2 ) (iii) the heat energy lost by the Si substrate to the metal heat reservoir (Equation A 3 ) The final equation is after combining Equation A 1, Equation A 2 and Equa tion A 3 : (Equation A 4 ) Now the situation was considered as a n initial value problem Si,t =0 = 65 C and the decrease in Si wafer according to (Equation A 4) is iteratively calculated using MATLAB (Table A 1 ). When the t =0 s, the Si wafer temperature was 65 C. Then for the 0.01 s time change, the 1 was calculated using the Equation A 4. Likewise, this pro cedure was repeated. T he decrease in Si Wafer temperature w ith changing time is plotted for various Si wafer dimensions as shown in Figure A 2. T he time for the temperature to reach 30 C is reported for various dimension silicon (Si) wafer s in Table A 2 u sing (Figure A 2) Around 30 C t emperature PS/PNIPA fibermat s were expected to show the transition from hydrophobic to hydrophilic state as the PNIPA has lower critical solution temperature (LCST) around this temperature There will some lag time for first Si wafer to reach 30 C and followed by the fibermat to reach 30 C. The Si wafer area typically used in this study was 1"2.5" and the time for this particular dimension Si wafer to reach 30 C was ranging between 6 and 9.5 s.

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141 The calculated time f or 30 C temperature decrease in Si wafer is longer than the response time measured for PS/PNIPA fibermats having less than 1.5 diameter fibers This suggest ed the following: (i) the response could have start ed happen ing even at higher temperatures as obse rved in Figure 4 5; (ii) c onvective type heat loss could have occurred to the droplet with the surrounding cold environment I t can be inferred from this analysis that the fibermats might have even respond ed quicker than reported time.

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142 Table A 1. The tem perature decrease in silicon (Si) wafer was considered as an initial value problem with Si,t =0 = 65 C and i teratively calculated using MATLAB as shown here. Time (s) Temperature (C) 0 65 0.01 1 0.02 2 0.03 3

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143 Table A 2. The time for temperature to reach 30 C is reported for various dimension silicon (Si) wafer s Si wafer area (inch inch) Time for temperature to 30 C (s) 11 3 1 2 6 1 3 9.5 1 4 12

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144 Figure A 1. Setup showing how silicon (Si) wafer was placed on metal bar (A); the whole setup was considered as one body having Si wafer side and metal bar t is the temperature of Si res is the reservoir (metal bar) temperature and is the thickness of Si wafer (B) A) B)

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145 Figure A 2 With changing time the temperature d ecrease in Si w afer is plotted for various dimension Si wafers (inch inch).

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153 BIOGRAPHICAL SKETCH Palanikkumaran(PK) Muthiah was born in India in 1980. He received B.Tech. in Textile Technology from PSG college of Technology, Coimbatore, India. Before pursuing his postgraduate education, he joined as Research Associate at Indian I nstitute of Technology Delhi (IITD) and worked in a research project for about a year and half, then received his M.Tech. in Fiber Science and Technology from the same institute. He t hen again joined at IITD for Research Associate position and worked on a project for a year. After Aug ust 2007, he became a Ph.D. student in the Department of Materials Science and Engineering at University of Florida till 2011. At the University of Florida he first joined to Dr. Laurie Gower group and worked for a year and With Dr. Sigmund, he was assigned by his advisor. After about three years of hard working in the project, PK and his advisor ha ve made a few remarkable breakthroughs. They have published two scientific articles and one US patent based on the results. In addition, he is preparing manuscripts for publications from the noteworthy results in the assigned projects.