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Design, Synthesis and Characterization of Stimuli-Responsive Polymers of Various Architectures

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Title:
Design, Synthesis and Characterization of Stimuli-Responsive Polymers of Various Architectures
Creator:
Dai, Yuqiong
Place of Publication:
[Gainesville, Fla.]
Florida
Publisher:
University of Florida
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english
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1 online resource (141 p.)

Thesis/Dissertation Information

Degree:
Doctorate ( Ph.D.)
Degree Grantor:
University of Florida
Degree Disciplines:
Chemistry
Committee Chair:
SUMERLIN,BRENT S
Committee Co-Chair:
MCELWEE-WHITE,LISA ANN
Committee Members:
MILLER,STEPHEN ALBERT
SAVIN,DANIEL ANDREW
JIANG,PENG

Subjects

Subjects / Keywords:
architecture -- material -- polymer -- raft -- responsive
Chemistry -- Dissertations, Academic -- UF
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bibliography ( marcgt )
theses ( marcgt )
government publication (state, provincial, terriorial, dependent) ( marcgt )
born-digital ( sobekcm )
Electronic Thesis or Dissertation
Chemistry thesis, Ph.D.

Notes

Abstract:
Stimuli-responsive polymers, because of their high-performance and ability to adapt to their surroundings, have attracted extensive attention of researchers. In this dissertation, the design, synthesis and characterization of stimuli responsive polymer star, organogel, bulk network and photonic crystals are investigated. Star polymers containing thermally-labile azo linkages that dissociate during conventional heating or during localized heating via the photothermal effect upon NIR irradiation was synthesized. Controlled release during conventional heating was investigated for the star polymers loaded with a model dye, with negligible release being observed at 25 degree Celsius and over 80% release at 90 degree Celsius. Star polymers co-loaded with NIR responsive indocyanine green showed rapid dye release upon NIR irradiation (wavelength equal to and greater than 715 nm) due to the photothermally-induced degradation of azo linkages within the cores of the star polymers. This approach provides access to a new class of delivery and release systems that can be triggered by noninvasive external stimulation. Adaptable organogels and transformable bulk polymer networks are designed based on two competitive dynamic-covalent Diels-Alder reactions. The original reversible crosslinked systems could be thermally triggered to rearrange their structure to achieve permanent fixation. These structural transformations and the associated changes of mechanical properties due to thermal triggers are discussed. Due to their ability to regulate the flow of the light, photonic crystals have enabled an extensive variety of applications. A series of photonic crystals with varying crosslinking density are synthesized. The relationship of the crosslinking density and the reflection color of the inverse opal films with varying pore sizes are investigated. By decreasing the crosslinking density of the film, the reflection color could be tuned from green, blue, purple to no color in 300 nm template and from orange, yellow, green to no color in 350 nm template. It is also found that lower crosslinking density is required for films with larger pore sizes. The relationships between crosslinking density and reflection color, which provides threshold of the spacing between the pores, is a footstone for further applications of photonic crystals. Responsiveness of the photonic crystal network to thermal stimuli is also investigated. ( en )
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In the series University of Florida Digital Collections.
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Includes vita.
Bibliography:
Includes bibliographical references.
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Description based on online resource; title from PDF title page.
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This bibliographic record is available under the Creative Commons CC0 public domain dedication. The University of Florida Libraries, as creator of this bibliographic record, has waived all rights to it worldwide under copyright law, including all related and neighboring rights, to the extent allowed by law.
Thesis:
Thesis (Ph.D.)--University of Florida, 2018.
Local:
Adviser: SUMERLIN,BRENT S.
Local:
Co-adviser: MCELWEE-WHITE,LISA ANN.
Statement of Responsibility:
by Yuqiong Dai.

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UFRGP
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Applicable rights reserved.
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D ESIGN, SYNTHESIS AND CHARACTERIZATION OF STIMULI RESPONSIVE POLYMER S OF V AR IOUS ARCHI TECTURES By YUQIONG DAI 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 2018

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2018 Yuqiong Dai

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To my family advisor, committee members and all my friends

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4 ACKNOWLEDGMENTS I would like to express my heartfelt gratitude to my advisor, Dr Brent Sumerlin, who has provided tremendous support and guidance during the course of my PhD me connect the dots on what I found from individual experiments, which were especially critical during my early days of the program. Professionally, Brent gave me the freedom to choose the path that was my passion. Personally, Brent is a caring, charming, and easy going friend with great humor I would also like to extend my gratitude to my committee members Dr s Wagener, McElwee White, Miller, Savin and Jiang Their advice, insi ghts, and encouragement have played a significant role in my academic success and they have taught me much more than I could ever give them credit for here. I a m indebted to all of those whom I have had the pleasure to work with here at UF. My labmates, Dr Hao Sun, Chris Kabb col labo rators Dr. Yin Fang, Dr. Xinyan Zhang and all others in the S umerlin research group would always be ready to lend a helping hand whenever it was needed. The friendly and fun relationship we have developed together have mad e the lab a great place to work. The insightful chats, meaningful tips, and sometimes forthcoming critic ism in various aspects of research and in life generally have undoubt edly helped me learn and grow. I also want to take this opportunity to recognize th e other friends in the c hemistry department whom I have known closely, and who have been important to m y successful completion of this journey, including Jiaming, Weijia, and Yunlu In addition Lori in the c hemistry graduate office has always addressed my questions regarding logistics. The kind and knowledgeable staff members with flexible schedules at the nanoscale research facility

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5 ( NRF ) including Al, Brent, and Andrea have enabled me to learn metrology tools quickly and use them after hours. Nobody is more important in my completion of the dissertation than the members of my family. I am forever indebted to my parents, whose unconditional and boundless love ha s been a constant source of energy and inspiration for me in this vital chapter of my life. They are the ultimate role models who m I will always look up to for the rest of my lif e My husband, Luping, has given me endless support without compromise in finishing up what I set out to achieve. He always believed in me and I could not imagine

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6 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ ....... 4 LIST OF TABLES ................................ ................................ ................................ .................. 9 LIST OF FIGURES ................................ ................................ ................................ ............. 10 LIST OF ABBREVIATIONS ................................ ................................ ................................ 15 ABSTRA CT ................................ ................................ ................................ ......................... 18 C HAPTER 1 INTRODUCTION ................................ ................................ ................................ .......... 20 1.1 Overview ................................ ................................ ................................ ................ 20 1.2 Diels Alder Chemistry in Stimuli Responsive Polymers ................................ ...... 21 1.3 Photonic Crystals ................................ ................................ ................................ ... 23 2 RESEARCH OBJECTIVE ................................ ................................ ............................ 28 3 NEAR IR INDUCED DISSOCIATION OF THERMALLY SENSITIVE STAR POLYMERS ................................ ................................ ................................ .................. 30 3.1 Introduction ................................ ................................ ................................ ............. 30 3.2 Experimental Methods ................................ ................................ ........................... 33 3.2.1 Materials ................................ ................................ ................................ ....... 33 3.2.2 Instrumentation and Characterization ................................ ......................... 33 3.2.3 Synthesis of PEG Based Macro Chain Transfer Agent (macroCTA) ....... 35 3.2.4 Synthesis of Block Copolymers ................................ ................................ ... 35 3.2.5 Star Formation ................................ ................................ ............................. 36 3.2.6 Degradation of Star Polymers by Traditional Heating Method .................. 36 3.2.7 Dye Encapsulation ................................ ................................ ....................... 3 6 3.2.8 Synthesis of Furan Modified Fluorescein (FF) ................................ ........... 36 3.2.9 ICG Encapsulation ................................ ................................ ....................... 37 3.2.10 NIR Triggered FF Release via ICG Photothermal Effect ........................ 37 3.3 Results and Discussion ................................ ................................ ......................... 38 3.3.1 Synthesis and Characterization of Star Polymers ................................ ...... 38 3.3.2 Dissociation of Star Polymers by Traditional Heating ................................ 47 3.3.3 Preparation of NIR Sensitive and Degradable Star Polymers .................. 55 3.3.4 Photothermally Triggered Release ................................ ............................. 60 3.4 Conclusions ................................ ................................ ................................ ............ 65 4 MACROMOLECULAR METAMORPHOSIS IN ORGANOGELS AND BULK NETWORKS ................................ ................................ ................................ ................. 66

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7 4.1 Introduction ................................ ................................ ................................ ............. 66 4.2 Experimental Methods ................................ ................................ ........................... 67 4.2. 1 Materials ................................ ................................ ................................ ....... 67 4.2.2 Instrumentation ................................ ................................ ............................ 68 bis (9 anthrylmethyl) trithiocarbonate (BATTC) ........... 70 4.2.4 Preparation of Furan Protected Maleic Anhydride. ................................ .... 71 4.2.5 Preparation of Furan Protected tris (2 maleimidoethyl) amine (Fur Mal 3 ) ................................ ................................ ................................ ................... 72 4.2.6 Synthesis of tris (2 maleimidoethyl) amine (Mal 3 Figure 4 6) .................. 75 4.2.7 Synthesis of P(S b (S alt MAn) b S) Triblock Copolym er ......................... 78 4.2.8 Functionalization of P(S b (S alt MAn) b S) with Furfurylamine ............... 79 4.2.9 Fur Mal 3 Gel Formation (Figure 4 16) ................................ ........................ 79 4.2.10 Thermally Triggered Gel Expansion ................................ ......................... 80 4.2.11 Fur Mal 2 Gel Formation (Figure 4 19) ................................ ...................... 81 4.2.12 Fur Mal 2 Gel Reversal ................................ ................................ ............... 81 4.2.13 Synthesis of Anthracene Acid ................................ ................................ ... 81 4.2.14 Synthesis of Anthracene Containing Monomer (M3, Figure 4 21) ......... 82 4.2.15 Synthesis of Fur Mal Crosslinker (Figure 4 23) ................................ ....... 83 4.2.16 Synthesis of Anth MalOH f rom Fur MalOH (Figure 4 25) ....................... 84 4.2.17 Synthesis of Anth Mal Crosslinker (Figure 4 27) ................................ ..... 85 4.2.18 Thermally Responsive Bulk Polymer Network Formation ....................... 86 4.3 Results and Discussion ................................ ................................ ......................... 87 4.3.1 Metamorphosis in Organogels ................................ ................................ .... 87 4.3.2 Metamorphosis in bulk polymer networks ................................ .................. 95 4.4 Conclusions ................................ ................................ ................................ .......... 105 5 POLYMERIC PHOTONIC CRYSTALS WITH TUNABLE REFLECTION COLORS ................................ ................................ ................................ ..................... 106 5.1 Introduction ................................ ................................ ................................ ........... 106 5.2 Experimental Methods ................................ ................................ ......................... 107 5.2.1 Materials ................................ ................................ ................................ ..... 107 5.2.2 Instru mentation ................................ ................................ .......................... 107 5.2.3 Synthesis of Polycaprolactone Diacrylate (PCLDA) ................................ 108 5.2.4 Preparation of PCLDA Containing Films ................................ .................. 109 5.3 Results and Discussion ................................ ................................ ....................... 110 5.3.1 Synthesis of PCLDA ................................ ................................ .................. 110 5.3.2 Synthesis of Mesoporous Films ................................ ................................ 112 5.3.3 Change in Reflection Color by Tuning the Crosslink density of Films Prepared from 300 nm Templates ................................ ................................ .. 113 5.3.4 Change of the Tuning Range for Films Prepared with 350 nm Templates ................................ ................................ ................................ ......... 120 5.3.5 Thermal Mechanical Responsiveness of PCL Based Polymer Films ..... 124 5.4 Conclusion s ................................ ................................ ................................ .......... 130 6 CONCLUSIONS AND FUTURE WORK ................................ ................................ ... 131

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8 LIST OF REFERENCES ................................ ................................ ................................ .. 132 BIOGRAPH ICAL SKETCH ................................ ................................ ............................... 141

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9 LIST OF TABLES Table page 3 1 Summary of the information of PEG b PHPMA block copolymers. ...................... 40 3 2 Summary of the data for star polymers synthesized by the carbodiimide coupling of PHPMA block cop olymers with compound 2 ................................ ...... 41 3 3 Summary of photothermally triggered release conditions. ................................ .... 62 4 1 Gel expansion metamorphosis: polymer information. ................................ ........... 89 4 2 Information on Fur Mal 3 and Anth Mal 3 gels from rheology testing. ..................... 92 4 3 Monomer 1 (M1) species with their respective polymer T g ................................ .. 96 4 4 The composition information of films prepared within each M1 species. ............. 98 5 1 The composition of films prepared from 300 nm templates. ............................... 113 5 2 DMA results of films with varying crosslink density. ................................ ............ 115 5 3 DMA results of films created fr om 350 nm templates with varying crosslink density. ................................ ................................ ................................ ................... 123

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10 LIST OF FIGURES Figure page 1 1 The first example of re mendable polymer networks based on Diels Alder chemistry. ................................ ................................ ................................ ................. 22 1 2 An example of photonic crystals in nature.. ................................ ........................... 23 1 3 Illustrations of the structures of three species of photonic crystals.. .................... 25 1 4 Simplified scheme of light reflection from ordered spheres by analogy to the principle of X ray di ffraction. ................................ ................................ .................. 26 1 5 Tunable temperature sensitive inverse opal hydrogel.. ................................ ........ 27 3 1 Design and synthesis of thermal responsive star polymers.. ............................... 39 3 2 1 H NMR spectrum of PEG macroCTA in CDCl 3 at 25 C. ................................ ..... 40 3 3 1 H NMR spectra recorded for linear block copolymer (top) and star polymer (bottom) in MeOD. ................................ ................................ ................................ ... 41 3 4 FT IR spectra of linear polymer (magenta) and star polymer (blue). Arrows indicate hydroxyl and ester functional groups. ................................ ....................... 43 3 5 Gel permeation chromatogram of linear and star polymers. ................................ 43 3 6 Number average hydrodynamic diameter of linear and star polymers measured by DLS. ................................ ................................ ................................ ... 44 3 7 T EM image of the star polymers (stained with uranyl acetate). ............................ 44 3 8 Characterization of star polymers with DP HPMA = 24 (Table 3 1, entry 2). (A) GPC traces and (B) particle size analysis by DLS. ................................ ............... 45 3 9 Characterization of star polymers DP HPM A = 70 (Table 3 1, entry 3). (A) GPC traces and (B) particle size analysis by DLS. ................................ ......................... 46 3 10 Schematic illustration of heat trigger ed star polymer dissociation. ....................... 47 3 11 GPC traces of star polymers (Table 3 1, entry 1) after 1 day (orange) and 60 days (blue) stored at room temperature. ................................ ................................ 47 3 12 Thermal degradation of star polymers DP HPMA = 24, N arm = 11 (Table 3 2, entry 2) in the pres ence of radical scavenger at 90 C for different heating time.. ................................ ................................ ................................ ......................... 50

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11 3 13 Thermal degradation of star polymers DP HPMA = 70, N arm = 47 (Table 3 2, entry 3) in the presence of radical scavenger at 90 C for different heating time.. ................................ ................................ ................................ ......................... 51 3 14 DLS size distribut ions of star polymers DP HPMA = 10, N arm = 6 (Table 3 2, entry 1) in the presence of radical scavenger during thermal dissociation at 90 C. ................................ ................................ ................................ ....................... 52 3 15 GPC traces showing star polymer DP HPMA = 10, N arm = 6 (Table 3 2, entry 1) in the presence of radical scavenger dissociation during heating at 90 C. ......... 52 3 16 Fluorescence emission spectra ( exc = 530 nm) of NR loaded star polymers (blue) and linear polymer (orange). ................................ ................................ ........ 54 3 17 Cumulative NR release profiles from star polymers measur ed by fluorescence spectroscopy ................................ ................................ ..................... 54 3 18 Schematic illustration of the preparation of NIR sensitive star polymers. ............ 55 3 19 Fluorescence intensity kinetics of NR and FF dyes in the presence or absence of ICG in the dark. ................................ ................................ .................... 56 3 20 Synthesis of furan modified fluorescein (FF) by EDC coupling reaction. ............. 56 3 21 1 H NMR spectrum of furan modified fluorescein (FF) in CD 2 Cl 2 : MeOD (1:1 volume ratio) at 25 C. ................................ ................................ ............................. 57 3 22 UV Vis absorption spectra of star polymers loaded with FF and ICG (orange), FF loaded star polymers (green) and free ICG in H 2 O (blue). .............................. 58 3 23 Calibration curve using ICG aqueous solutions with known concentrations at 778 nm. ................................ ................................ ................................ .................... 59 3 24 Schematic illustration of FF and ICG encapsulation and NIR triggered FF release via photothermal effect. ................................ ................................ .............. 60 3 25 Images of the FF and ICG loaded star polymer solution before and after NIR irradiation. ................................ ................................ ................................ ................ 61 3 26 Fluorescence emission spectra (excited at 430 nm) of star polymers loaded with ICG and FF under NIR irradiation for varying times. ................................ ..... 61 3 27 Cumulative release profiles of FF by fluorescence intensity changes under different conditions. ................................ ................................ ................................ 62 3 28 Temperature profiles of the FF loaded star polymer solution in the presence or absence of ICG under NIR irradiation. ................................ ............................... 64 4 1 The reaction scheme of Fur Mal DA and Anth Mal DA. ................................ ........ 67

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12 4 2 The synthetic scheme of BATTC. ................................ ................................ ........... 70 4 3 1 H NMR spectrum of BATTC (CDCl 3 ). ................................ ................................ ... 71 4 4 The synthesis scheme of furan protected maleic anhydride. ............................... 71 4 5 1 H NMR spectrum of furan protected maleic anhydride (CDCl 3 ). ......................... 72 4 6 Scheme of Fur Mal 3 synthesis. ................................ ................................ ............... 72 4 7 1 H NMR spectrum of Fur Mal 3 (CDCl 3 ). ................................ ................................ 73 4 8 13 C NMR spectrum of Fur Mal 3 (CDCl 3 ). ................................ ................................ 74 4 9 The high resolution mass spectrum of Fur Mal 3 ................................ ................... 75 4 10 Scheme of Mal 3 synthesis. ................................ ................................ ...................... 75 4 11 1 H NMR spectrum of Mal 3 (CDCl 3 ). ................................ ................................ ........ 76 4 12 13 C NMR spectrum of Mal 3 (CDCl 3 ). ................................ ................................ ....... 77 4 13 The high resolution mass spectrum of Mal 3 ................................ .......................... 78 4 14 Synthetic pathway for P(S b (S alt MAn) b S) Triblock Copolymer. .................... 78 4 15 Scheme for using furfurylamine functionalization of P(S b (S alt MAn) b S). ..... 79 4 16 Schematic illustration of Ful Mal 3 gel formation. ................................ .................... 79 4 17 Schematic illustration of thermally triggered gel expansion. ................................ 80 4 18 Images of Fur Mal 3 gel before and after the thermal trigger. ................................ 80 4 19 Schematic illustration of the formation of a Fur Mal 2 gel. ................................ ...... 81 4 20 1 H NMR spectrum of anthracene acid. ................................ ................................ ... 82 4 21 The synthetic scheme of anthracene containing monomer (M3). ......................... 82 4 22 1 H NMR spectra of anthracene containing monomer M3. ................................ .... 83 4 23 The synthetic scheme of Fur Mal crosslinker. ................................ ....................... 83 4 24 1 H NMR spectrum of Fur Mal crosslinker (500 MHz, CDCl 3 ). ............................... 84 4 25 The synthetic scheme of Fur MalOH. ................................ ................................ ..... 84 4 26 1 H NMR spectroscopy of Anth MalOH (500 MHz, DMSO d 6 ). .............................. 85

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13 4 27 Scheme of Anth Mal crosslinker synthesis. ................................ ........................... 85 4 28 1 H NMR spectroscopy of Anth Mal crosslinker (500 MHz, DMSO d 6 ). ................ 86 4 29 Gel metamorphosis driven by diene displacement. ................................ ............... 87 4 30 GPC traces of polystyrene and P(S b (S alt MAn) b S). ................................ ...... 88 4 31 GPC traces of P(S b (S alt MAn) b S) triblock copolymer before and after functionalization with furfurylamine. ................................ ................................ ........ 89 4 32 Images of the formation of a Ful Mal 3 gel. ................................ ............................. 90 4 33 Rheology frequency sweep of Ful Mal 3 gel and Anth Mal 3 gel.. ........................... 91 4 34 Fluorescence microscopy image of Ful Mal 3 gel. ................................ ................. 92 4 35 Fluorescence microscopy of Anth Mal 3 gel. ................................ .......................... 93 4 36 Fluorescence spectra during the reversal of Fur Mal 2 gel.. ................................ .. 94 4 37 Gel permeation chromatogram of Anth Mal 2 polymers after the disintegration of the Fur Mal 2 gel.. ................................ ................................ ................................ 95 4 38 Schematic illustration of the formation of bulk networks. ................................ ...... 96 4 39 Structures of three M2 moieties used. ................................ ................................ .... 97 4 40 DSC plots from the second heating cycle of PBMA (green line), PEMA (red line) and PMMA (blue line) films. ................................ ................................ ............ 99 4 41 The storage modulus of BMA based films measured from DMA. ...................... 100 4 42 Solid state fluorescence spectra of BMA based films before and after thermal treatment. ................................ ................................ ................................ ............... 101 4 43 The storage modulus of EMA based films measured from DMA. ...................... 102 4 44 Solid state fluorescence spectra of EMA based films before and after thermal treatment. ................................ ................................ ................................ ............... 103 4 45 Solid state fluorescence spectra of MMA based films before and after thermal treatme nt. ................................ ................................ ................................ 104 4 46 The storage modulus of MMA based films measured from DMA ...................... 104 5 1 Synthetic scheme for PCLDA. ................................ ................................ .............. 109 5 2 Scheme for mesoporous film preparation. ................................ ........................... 110

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14 5 3 1 H NMR spectra of PCL diol and PCLDA (CDCl 3 ). ................................ .............. 111 5 4 FT IR spectra of PCL diol (blue) and PCLDA (orange). ................................ ...... 112 5 5 Cartoon illustration of the main steps in the preparation of microporous polymeric photonic crystals.. ................................ ................................ ................. 112 5 6 Images of films (from left to right corresponding to the composition from entry 1 to 4 in Table 5 1). ................................ ................................ ............................... 115 5 7 Reflectance spectra of films created from 300 nm templates with varying PCLDA content (inversely related to crosslink density). ................................ ..... 116 5 8 Cross sectional SEM images of the films with different PCLDA percentages. .. 117 5 9 Surface modulus of films with varying PCLDA content characterized by nanoindentation.. ................................ ................................ ................................ ... 119 5 10 Reflectance spectra of films with varying PCLDA content prepared from 350 nm templates.. ................................ ................................ ................................ ....... 120 5 11 Images of films with 350 nm template (the films from left to right contain 30%, 40%, 50%, 60% and 65% PCLDA, respectively). ................................ ................ 121 5 12 Cross sectional SEM images of films with varying PCLDA content prepared from 350 nm template. ................................ ................................ .......................... 122 5 13 Modulus of films with different PCLDA% prepared from 350 nm template characterized by nanoindentation.. ................................ ................................ ....... 124 5 14 Optical spectra and photograph of the film containing 25 mol% PCLDA (300 nm template). ................................ ................................ ................................ ......... 125 5 15 DSC plot of the film with 25 mol% PCLDA (endothermal up). ............................ 126 5 16 Schematic of the programming process for the shape memory film. Note the film had different temperatures (T) in different stages.. ................................ ...... 126 5 17 Photographs and cross sectional SEM images of the films before and after thermal programming.. ................................ ................................ .......................... 128 5 18 Reflectance spectra of the film before and after thermal programming.. ........... 129

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15 LIST OF ABBREVIATIONS A CVA 4,4 azobis (4 cyanovaleric acid) AIBN 2,2' azobisisobutyronitrile A nth A nthracene A nth Mal A nthracene maleimide A TR A ttenuated total reflectance ATRP Atom transfer radical polymerization B ATTC S S bi s (9 anthrylmethyl) trithiocarbonate CTA Chain transfer agent DA Diels Alder DCM Dichloromethane DLS Dynamic light scattering DMA D ynamic mechanical analysis DMAc Dimethylacetamide DMAP Dimethyl aminopyridine DMSO Dimethyl sulfoxide D P D egrees of polymerization D SC D ifferential scanning calorimetry EDC 1 (3 dimethylaminopropyl) 3 ethylcarbodiimide, HCl E PR E nhanced permeability and retention F F Furan m odified f luorescein F ur Mal F uran maleimide F ur Mal 2 Furan p rotected N,N 4 Phenylenedimaleimide

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16 F ur Mal 3 Furan p rotected tris (2 maleimidoethyl) amine HRMS High resolution mass spectrometry GPC Gel permeation chromatography I CG I ndocyanine green LCST Lower critical solution temperature MacroCTA Macro chain transfer agent M al M aleimide M al 2 N,N 4 Phenylenedimaleimide M al 3 T ris (2 maleimidoethyl) amine M A n Maleic anhydride M ALLS M ulti angle laser light scattering MWCO Molecular weight cut off N IR N ear infrared NMP Nitroxide mediated polymerization NMR Nuclear magnetic resonance N R N ile red P BMA P oly(n butyl methacrylate) P CL P olycaprolactone P CLDA P olycaprolactone diacrylate PEG Poly(ethylene glycol) P EGMA Poly (ethylene glycol methacrylate) P EMA Poly(ethyl methacrylate) P HEMA P oly(2 hydroxy ethy l methacryla te)

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17 PHPMA P oly( N 2 hydroxypropyl) methacrylamide PMMA Poly(methyl methacrylate) P NIPAM P oly( N isopropylacrylamide) PS Polystyrene RAFT Reversible addition fragmentation chain transfer R DRP R eversible deactivation radical polymerization SC V P S elf condensing vinyl polymerization S EC S ize exclusion chromatography SE M Scanning electron microscopy T EM T ransmission electron microscopy TEA Triethylamine T FF Tangential flow filtration THF Tetrahydrofuran T MPTA T rimethylolpropane triacrylate TMS Tetramethylsilane T REN Tris (2 aminoethyl) amine

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18 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 DESIGN, SYNTHESIS AND CHARACTERIZATION OF STIMULI RESPONSIVE POLYMERS OF VARIOUS ARCHITECTURES By Yuqiong Dai December 2018 Chair: Brent S. Sumerlin Major: Chemistry Stimuli re s ponsive polymers because of their ability to adapt to their surround ing s have attracted extensive attention of researchers In this dissertation, t he design, synthesis and characterization of stimuli responsive polymer star s organogel s bulk network s and photonic crystal s are investigated S tar polymers containing thermally labile azo linkages were synthesized. These materials dissociate during conventional heating or during localized heating via the photothermal effect upon NIR irradiation. Controlled release during con ventional heating was investigated for the star polymers loaded with a model dye, with negligible release being observed at 25 C and > 80% release at 90 C. Star polymers co loaded with NIR responsive indocyanine green showed rapid dye release upon NIR ir radiation ( 715 nm) due to the photothermally induced degradation of azo linkages within the cores of the star polymers. This approach provides access to a new class of delivery and release systems that can be triggered by noninvasive extern al stimulati on. A daptable organogels and transformable bulk polymer networks are based on two competitive dynamic covalent Diels Alder reactions The original reversible

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19 crosslinked systems could be thermally triggered to rearrange their structure to achieve permanent fixation. The se structural transformations and the associated changes of mechanical propert ies due to thermal triggers are discussed Due to the ir ability to regulate the flow of the light, photonic crystals have enabled an extensive variety of applications. A series of photonic crystals based on thermalset polymers with varying crossli nk densit ies w as synthesized and t he relationship of the crosslink density and the reflection color of the inverse opal films with varying pore sizes we re investigated By decreasing the crosslink density of the film, the reflection color could be tuned from green, cyan blue to color less using a 300 nm template and from orange, yellow, green to color less with a 350 nm template. It wa s also found that lower crosslink density is required for films with larger pore size s The relationship between crosslink density and reflection color which provides a threshold value f o r the spacing between the pores, is a f oundation for further applications of photonic crysta ls. The r espons e of the photonic crystal network to thermal stimuli wa s also investigated

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20 CHAPTER 1 INTRODUCTION 1 .1 Overview Stimuli responsive polymers are high performance polymers that change according to the ir immediate environment. 1 The ability to change properties under exter nal stimul ation is an attractive feature 2 which leads to a broad range of applications from controlled drug release 3 5 and self healing materials 6 to sensors 7 actuators 8 and microfluidic devices 9 It is clear that these smart polymers are gaining attention across many fields a nd the stimul ating triggers can originate from a variety of environmental c ues such as changes in pH 10 or temperature 11 or exposure to specific chemical s 12 light 13 mechanical for ces 14 or electric 15 and magnetic fields 16 The architecture of polymers is critical in determining many of their properties such as the possibility of entanglement with neighboring chains 17 the ability to modify solution viscosity 18 19 or the possibility for self assembly into complex nanosized objects 20 2 1 thereby further influenc ing their applications. When designing smart polymers, it is important that both the functionality and the topology be considered. 22 23 Developments in reversible deactivation radical polymerization (RDRP) techniques, including atom transfer radical polymerization (ATRP), 24 reversible addition fragmentation chain transfer (RAFT) polymerization, 25 and nitroxide mediated polymerization (NMP), 26 have opened access to remarkably diverse polymer architectures such as block, gradient, graft, hyperbranched, star, cyclic, and network polymers. 23

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21 1 .2 Diels Alder Chemistr y i n Stimuli Responsive Polymers When considering materials that derive their response from exchange of reversibly covalent bonds, c hoosing the ri ght type of dynamic covalent bonds is critical. 27 The most commonly considered dynamic covalent bond s are esters, alkoxyamines, acylhydrazones imines, boronic esters, disulfides and other linkages 28 Each of these bonds is sufficiently stable to be used for construction or functionalization of polymers, but they can also be readily cleaved via response to an external stimul us such as heat, add ition of water ( i.e. hydrolysis), or the presence of a catalyst or reducing agent that shifts the equilibrium from products to reactants. Diels Alder reaction s involving a [4+2] cycloaddition between a diene and a dienophile, stand out due to their high yields, atomic efficiency, reversibility, freedom from catalyst s and mild conditions needed to carry them out 29 Lehn and coworkers reported the first study on the reversibility of Diels Alder chemistry between a library of dienes and dienophiles within the temperature range from 10 to 50 C. 30 The first example of a re mendable crosslinked polymer film was demonstrated by Wudl and coworkers. 31 The thermal remendability was based on a DA reaction between furan and maleimide groups an d t he healing of the cut was characterized by scanning electron microscopy ( SEM ) The average healing efficiency of the polymer film was reported to be 57%, with a 50% mending efficiency at 150 C (as shown in Figure 1 1). Notably, this re mending process did not require catalyst, solvent, additional monomers or surface treatment. In the last decade, DA reactions have been extensively explored in constructing recyclable networks 32 33 self healing materials 34 and complex polymer architectures 35 36

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22 Figure 1 1. The first example of r e mendable polymer network s based on Diels Alder chemistry. (A) F racture toughness test results of s ample s before and after healing to obtain the m ending efficiency (B) I mage of a cracked s ample (C) I mage of the cracked sample in (B) after thermal treatment. (D) SEM image of the surface of a healed sample: the left side is the as healed surface and the right side is the scraped surface. (E) Enlarged image of the boxed area in (D). Adapted from reference [ 31 ], with permission from The American Association for the Advancement of Science. In the Sumerlin research group, several remarkable studies based on Diels Alder chemistry ha ve been published recently C ore crosslinked star polymer s containing furan maleimide adduct s could assemble or disassemble depending on different t hermal trigger s Good reversibility of the star polymer s was demonstrate d in several thermal cycles. 36 Introduction of a furan maleimide DA adduct at the branch point of a hyperbranched polymer enabled detailed analysis of the linear segments and provided insight s into the mechanism of self condensing vinyl polymerization (SCVP). 37 The topological transformations of macromolecules due to external triggers were also

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23 exploited based on Diels Alder chemistry. 38 Two examples of thermal ly responsive induced transform ations of polymer materials will be discussed in chapter 3. 1.3 Photonic Crystal s Photonic crystals are periodic nanostructures with unique optical properties. 39 Th e ir unique opti cal structure are responsible for many of the colors observed in nature, such as in insects, fishes, birds and so on. 40 When compar ed with dyes and pigments, photonic crystals are often brighter and not susceptible to fad ing M ore interestingly, they do not need power to show color and the color may even be tunable. Indeed, many natural photonic crystals not only show beautiful colors but also can change colors when the surrounding environment changes. As shown in Figure 1 2 the color of a butterfly wings comes from periodic ally arranged structures and could be tune d by changi ng the packing parameter. 41 42 Figure 1 2 An example of photonic crystals in nature a) Image of the wings of Morpho rhetenor after ethanol was poured over the right wing; b) Cross sectional transmission electron microscopy (TEM) image of wing scales on the left (s cale bar is 1.8 m) ; c) TEM image of the cross section of the wing scales on the right (s cale bar is 1. 3 m) Figure (a) adapted from Ref. [ 41 ] with permission; copyr ight 2009 American Chemical Society. Pictures (b) and (c) reprinted from Ref. [ 42 ]; copyright 2003 Macmillan Publishers Ltd.

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24 Photonic crystals have been studied since the initial report in 1887 by Lord Rayleigh, 43 but the term photonic crystal was not i ntroduced until a hundred years later by Yablonovitch and John in 1987. 44 45 According to the structural ar rangement, photonic crystals c an be classified as one dimensional, two dimensional and three dimensional (as illustrated in Figure 1 3 ). A o ne dimensional photonic crystal is a layer by layer structure that can be prepared by spin coating, photolithography and deposition. 46 One dimensional photonic crystals have already been widely used in color changing paints, high or low reflection coatings and optical switch es 47 Two dimensional photonic crystals were first demonstrated by Krauss and coworkers in 1996, with periodicity in two dimensions They can be produced by photolithography and etching methods 48 49 and have been commercialized as optical fibers Three dimensional photonic crystals which maintain periodicity in all three dim ensions, are typically prepared by self assembl y of monodispersed spheres onto a substrate. 50 S till far from commercialization, three dimensional photonic crystals have attracted the most re search interest, with applications ranging from sensors and transistors to optical device s 51 53

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25 Figure 1 3 I llustrat ions of the structures of three species of photonic crystals. The blue and grey color represents two kinds of materials that have different dielectric constant s The difference of the dielectric constant is not required in 2D and 3D photonic crystals. Figure 1 3 adapted from Ref. [ 39 ] with permission; copyright @2014 Wiley VCH Verlag GmbH & Co. KGaA, Weinheim. H ow photonic crystals reflect light of a certain wavelength can be explained by a 1 4 we can imagine th at regularly arranged spheres interfere with the incident light based on Bragg diffraction. As shown in Equation 1 1, where is the center to center distance between the particles, is the angle of the incident light, is the order of diffraction, and is the reflected wavelength. (1 1) Due to the difference s in refractive index of the dispers ing media and the corporated to generate Equation 1 2 where is the mean effective refractive index 54 (1 2)

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26 Figure 1 4 Simplified scheme of light reflection from ordered spheres by analogy to the principle of X ray diffraction. A dapted from Ref. [ 39 ] with permission; copyright @2014 Wiley VCH Verlag GmbH & Co. KGaA, Weinheim. From Eq uation 1 2 it is clear that the reflected wavelength c an be tuned by the packing parameter 55 the effective refractive index 56 and the angle of the incident light 57 By playing with these parameters, the reflection color of photonic crystals c an be easily tuned via external stimuli, which make s them candidate s for sensor applications. 51 58 59 60 Figure 1 5 is an example of thermal ly re sponsive porous hydrogel. 61 Th e poly( N isopropylacrylamide)(PNIPAM) containing porous hydrogel was prepared by polymerizing the pre gel solution on a silica colloidal crystal template followed by HF etching to remove all silica component s Due to the low er critical solution temperature (LCST) of the PNIPAM (~27 C), the hydroge l shrink s with increasing temperature, thereby decreasing the center to center distance which leads to a blue shift of the reflect ed wavelength.

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27 Figure 1 5 Tunable temperature sensitive inverse opal hydrogel (A) Schematic representation of the volume change mechanism for the porous P NIPA M containing gel with a thermal trigger. (B) Images and reflection spectra of the porous P NIPA M containing gel in the dark at diffe rent temperatures. A dapted from Ref. [ 61 ] with permission; Copyright 2007, John Wiley and Sons.

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28 C HAPTER 2 R ESEARCH OBJECTIVE T he purpose of this research was to develop new stimuli responsive polymeric materials with varying architectures, from star polymers and organogels to bulk polymer networks and photonic crystals The ability of these materials to adapt to environmental changes was also characterized. The s pecific goal of each project is summarized below. In C hapter 3, star polymers containing thermally labile azo linkages were synthesized. The dissociat ion during conventional heating or during localized heating via the photothermal effect upon NIR irr adiation w as studied Controlled release during conventional heating was investigated for the star polymers loaded with a model dye, with negligible release being observed at 25 C, but more than 80% release at 90 C. Star polymers co loaded with NIR respo nsive indocyanine green showed rapid dye release upon NIR irradiation ( 715 nm) due to the photothermally induced degradation of azo linkages within the cores of the star polymers. The reliance on low energy NIR light to induce the response could have potential for light mediated release in vivo, since NIR irradiation can penetrate tissue with minimal loss of intensity. Chapter 4 presents two examples of a new type of thermal responsive material enabled by two competitive Diels Alder (DA) reactions, i. e. the DA reaction between furan and maleimide ( forward reaction around 60 C retro reaction above 80 C ) and the DA reaction between anthracene and maleimide ( forward at 120 C and thermally irreversible ) By introducing all three groups (furan, maleimi de and anthracene) into one system, the reversibility and permanent fixation were expected to be achieved under different temperature triggers. The transformability of the ogranogels and bulk polymer networks we re investigated. With careful design, the gel to sol transition and a transition

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29 from gel to a more loosely crosslinked gel w ere characterized in the organogel systems. The initial reversibility and triggered permanent fixation were dem o nstrated in the bulk polymer networks with varying glass transition temperatures ( T g s). The goal of C hapter 5 was to study the fundamental relationship between crosslink density and the reflection color for photonic crystal s based on thermoset polymer films and to develop a multi stage shape memory material wit h visible color changes for possible sensor application s First, the photonic crystal structure was introduced into the polymer networks with different crosslink densities by photo cur ing a pre polymerization mixture on silica colloidal crystal templates. After removing all silica contents, the mesoporous polymer films were achieved with varying reflection wavelength s A blue shift of the reflection color was found with decreasing crosslink density of the film, which was demonstrated in two different templa te particle sizes (300 and 350 nm). Furthermore, a thermal mechanical responsive polycaprolactone based mesoporous film was developed. In the thermal mechanical programming process, the reflection color of the film could change from green (original) to cyan (stage 1) and to being colorless (stage 2).

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30 CHAPTER 3 NEAR IR INDUCED DISSOCIATION OF THERMALLY SENSITIVE STAR POLYMERS 3 .1 Introduction Numerous examples of amphiphilic block copolymers have been r eported over the last decade, with many of the copolymers possessing the ability to self assemble into well ordered nanostructures ( e.g. micelles and vesicles). 62 Such block copolymer based micelles can mimic the structure s of plasma lipoproteins that serve as biological carriers, which has led to extensive consideration of these micelles as drug delivery systems. 63 Generally, block copolymer micelles have a stabilizing hydrophilic shell and a hydrophobic core that entraps and protects drugs ( e.g. chemotherapeutics) from degradation. As micelle size increases, drug circulation times are extended and, in the case of cancer therapy, tumor accumulation can be achieved via the enhanced permeabili ty and retention (EPR) effect. 64 65 However, at low concentrations or in high ionic strength environments, the supramolecular n ature of micelles often leads to thermodynamic instability and potential premature dissociation to linear unimers. 66 Therefore, further stabilization of micelles is often needed to prevent premature dis soci atio n and undesired drug release in vivo 67 68 As an alternative to micelles, stable and non dissociable nanoparticles can be constructed through chemical crosslinking of supramolecularly associated block copolymers to yield core crosslinked star polymers. 69 71 In this approach, polymer assembly and stabilit y is governed not only by hydrophobic interactions but also by covalent bonding in the core. While studies have focused on star polymers that are This chapter is reprinted with permission from Dai, Y.; Sun, H.; Pal, S.; Zhang, Y.; Park, S.; Kabb, C. P.; Wei, W. D.; Sumerlin, B. S., Near IR induced dissociation of thermally sensitive star polymers. Chemical science 2017, 8 (3), 1815 1821. Copyright 2017 Royal Society of Chemistry.

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31 permanently crosslinked, 72 in many cas es it is highly desirable to incorporate stimuli responsive moieties within the nanocarriers to enable (external) triggerable dissociation. In this manner, the star polymers can serve as thermodynamically stable, covalently linked carriers until triggered by an appropriate stimulus. The degradation of crosslinked polymer assemblies also assists in the excretion of polymer residues after delivery, thereby preventing potential complications from the prolonged circulation of non degradable polymers. 63 A variety of stimuli responsive polymeri c nanocarriers have been developed by incorporating (post )cleavable bonding motifs such as disulfide, 73 acetal 74 oxime 75 Diels Alder, 37 and light labile linkages. 76 Nanocarriers that are sensit ive to electromagnetic irradiation are particularly advantageous as they could potentially provide remote ( i.e. external), on demand, and spatiotemporal drug release by tuning the wavelength, power, and the irradiation site. 77 Compared to UV and visible light, near infrared (NIR) irradiation has unique advantages in drug del ivery systems because it penetrates deeply through skin and underlying tissue without sever e ly damaging healthy tissues. 78 Combining NIR responsive photosensitizers and thermally degradable nanocarriers, targeted delivery can be achieved by employing local heat generated via the photothermal effect. Generally, two approaches to photothermally indu ced drug release are considered. The first involves controlling the phase transitions of thermally sensitive polymeric carriers, 79 such as those derived from poly( N isopropylacrylamide). At temperatures above the cloud point, the structural collapse of the carrier leads to the release of loaded drugs. 80 The second method focuses on cleaving thermally labile linkers between nanocarriers and active compounds, allowing the covalently

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32 lations have been investigated and utilized for on demand drug release, including iron oxide nanoparticles, 81 copper monosulfide nano agents, 82 carbon nanotubes, 83 graphene, 84 go ld nanoparticles, 85 86 and indocyanine green (ICG) containing systems. 39 51 87 Among these, ICG is a particularly promising component for photothermal therapy due to its low cytotoxicity and excellent photothermal properties. 88 In this project, we investigated the controlled release behavior of fluorescent dyes from star polymers crosslinked with thermally labile azo functionalities. Cleavage of the crosslinks and consequential dye release can be induced by convention al heating and/or NIR irradiation. Biocompatible poly(ethylene glycol) block poly[( N 2 hydroxypropyl) methacrylamide] (PEG b PHPMA) was synthesized by RAFT polymerization using a PEG based macro chain transfer agent (macroCTA). Subsequently, a carbodiimide coupling reaction between pendent hydroxyl groups from the PHPMA segment of PEG b PHPMA and 4,4 azobis(4 cyanovaleric acid) ( 2 ) was carried out to form star polymers crosslinked in their cores by thermally labile linkages. This approach of installing thermally se nsitive crosslinks was motivated by our previous work that demonstrated polymers with azo linkages in their backbones are susceptible to heat induced degradation. 80 In the current work, after encapsulation of model hydrophobic dyes, 89 degradation of the star polymers and concomitant release were examined via both traditional heating and photothermal heating by NIR irradiation. F urther, the effect of ICG incorporation under these conditions was studied. While the goal of this research is not to prepare specific systems for drug delivery, the results

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33 suggest that NIR responsive star polymers may hold good potential as smart and eff ective delivery platforms. 3 .2 Experimental Methods 3 .2.1 Materials 4 Cyano 4 [( dodecylsulfanylthiocarbonyl )sulfanyl]pentanoic acid ( 1 ) was prepared as previously reported. 90 Poly(ethylene glycol) monomethyl ether ( mPEG 5k, Mn = 5000 gmol 1 azobis(4 cyanovaleric acid) ( 2 98%), 4 (dimethylamino)pyridine trioxane, and 2,2 azobisisobutyronitrile (AIBN) were purchased from Sigma Aldrich. 1 (3 Dimethylaminopropyl) 3 ethylcarbodiimide hydrochloride (EDCHCl, 97%) was purchased from Combi Blocks. Nile red (NR) was purchased from TCI, and indocyanine green (ICG) was purchased from Acros Organics. Deuterated chloroform (CDCl 3 99.8%) was purchased from Cambridge Isotopes. N (2 Hydroxypropyl) methacrylamide (HPMA) was synthesized as previously reported. 70 Diethyl ether, N,N dimethylacetamide (DMAc), and dichloromethane were purchased from Sigma Aldrich. AIBN was recrystallized from methanol, and all other chemicals were purchased with the highest available purities and used as received. 3 .2.2 Instrumenta tion and Characterization 1 H NMR spectra were recorded on a Varian Inova2 500 MHz NMR spectrometer, using the residual solvent signal as reference Fluorescence and UV Vis spectra were obtained on a Molecular Devices SpectraMax M2 multimode microplate reader at 25 C. Measurements were conducted on clear (absorbance) or black (fluoresce nce) 96 well polypropylene microplates (Greiner Bio One). Molecular weights and molecular weight distributions were determined via size exclusion chromatography with multi angle laser

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34 light scattering (SEC MALLS) in DMAc with 50 mM LiCl at 50 C and a flow rate of 1.0 mL/min (Agilent isocratic pump, degasser, and autosampler; ViscoGel I guard column and two ViscoGel I series G3078 mixed bed columns, with molecular 3 6 g/mol, respectively). Detection consisted of a Wyatt Optilab TrEX refractive index detector operating at 658 nm and a Wyatt miniDAWN Treos light scattering detector operating at 659 nm. Absolute molecular weights and polydispersi ties were calculated using Wyatt ASTRA software. Dynamic light scatt ering (DLS) measurements were recorded on a Zetasizer Nano ZS, (Malvern Samples were prepared in pure water and filtered through a 0.45 m nylon syringe filter prior to a nalysis. Each measurement was repeated six times to obtain an average value. Transmission electron microscopy (TEM) was conducted on a Hitachi H7000 microscope operating at 100 kV. A Formvar coated 200 mesh Cu grid that was freshly glow discharged (Pelco e solution for 30 s and wicked off with filter paper. Uranyl acetate (0.5% aqueous solution) was used as a negative stain. Infrared spectra were collected on a Thermo Nicolet 5700 FT IR spectrometer equipped with a single bounce, diamond stage attenuated total reflectance (ATR) accessory. In all the photothermal experiments, a 300 W Xenon lamp (Newport) was used with a long pass filter ( 715 nm, 1.5 W/cm 2 ). Substrates were placed in a glass vial and the solution temperature was measured every 10 min during irradiation.

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35 3 .2.3 Synthesis of PEG Based Macro Chain Transfer Agent (macroCTA) As modified from a previously reported procedure, 91 a solution of mPEG 5k (5.0 g, 1.0 mmol) and 1 (5.3 10 2 mg, 1.3 mmol) in dichloromethane (15 mL) was cooled in an ice bath. A solution of EDCHCl (2.6 10 2 mg, 1.3 mmol) and DMAP (14 mg, 1.2 10 1 mmol) was added dropwise. The reaction was stirred overnight at room temperature. The macroCTA was precipitated into cold diethyl ether (2 100 mL) and collected. The product was then dissolved in dichloromethane (100 mL) and washed with distilled water (3 70 mL ). The organic layer was collected, and the residual water was removed by treating with anhydrous Na 2 SO 4 The mixture was then filtered and concentrated under reduced pressure. Finally, the solution was precipitated in cold diethyl ether. The obtained prod uct was dried in a vacuum oven at room temperature for 48 h. Macro CTA ( 3.2 g 60%) was obtained. 3 .2.4 Synthesis of Block Copolymers An example RAFT polymerization of HPMA is as follows: HPMA (7.0 10 2 mg, 5.0 mmol), PEG macroCTA (1.3 g, 2.5 10 1 mmol) AIBN (16 mg, 1.0 10 1 mmol), 1,3,5 trioxane (1.5 10 2 mg, 1.7 mmol), and DMAc (6.0 mL) were placed in a 25 mL Schlenk flask ([HPMA]/[MacroCTA]/[AIBN] = 20/1/0.2). Oxygen was removed over the cou r se of three freeze pump thaw cycles. At the end of the f inal cycle, the Schlenk flask was filled with N 2 and placed in a preheated oil bath at 70 C. Samples were taken periodically by syringe to determine molecular weight and dispersity ( ) by GPC. Monomer conversion was confirmed by 1 H NMR spectroscopy. The f inal polymer was isolated by precipitating in diethyl ether. The resulting polymers were dried in a vacuum oven overnight. All polymerizations were quenched at 50 % conversion of HPMA.

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36 3 .2.5 Star Formation A solution of the block copolymer and 2 ([ COOH] (from 2 ) /[ OH] (from block copolymer) = 2/5) in dichloromethane (15 mL) was cooled in an ice bath. A solution of EDCHCl (3 equiv. relative to [ 2 ]) and DMAP (0.1 equiv. relative to [EDCHCl]) was added dropwise. The reaction was stirred overnight at room temperature. The solution was then precipitated into cold diethyl ether (4 100 mL), and the resulting product was dried in a vacuum oven for 48 h. Tangential flow filtration (TFF) with a 70 kg/mol molecular weight cut off (MWCO) membrane was used to furt her purify the star polymers. 3 .2.6 Degradation of Star Polymers by Traditional Heating Method An aqueous solution of star polymer (2.5 mg/mL) and the radical scavenger (thioglycerol, 10 equiv. relative to azo moieties) was heated to 90 C in an oil bath. Samples were taken periodically for GPC and DLS characterization. 3 .2.7 Dye Encapsulation Nile red (NR) was encapsulated in the stars by a nanoprecipitation method. Briefly, NR (1.0 mg) and polymer star formed from PEG 113 b PHPMA 10 (5.0 mg) were dissolved in 1 mL THF, and then deionized H 2 O (10 mL) was quickly added under vigorous stirring. The solution was then dialyzed against water (MWCO = 3,500 g/mol) at room temperature for 6 h. The dye encapsulated stars were further purified to remove non encapsulate fluorescein dye (FF, section 2.28) was encapsulated under the same procedure. 3 .2.8 Synthesis of Furan Modified Fluorescein (FF) Fluorescein (1.0 g, 3.0 mmol) was dissolved in a mixture of DCM (9 mL) and DMF (6 mL). Then, EDCHCl (1.3 g, 6.6 mmol) and DMAP (0.9 10 2 g, 0.8 mmol) were

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37 added to the solution. The mixture was stirred for 3 h, followed by the dropwise addition of furfuryl amine (0.7 mL, 7.5 mmol). Thereafter, the reaction was allowed to stir for 24 h. Then the solvent was removed by rotary evaporation, and the residue was dissolved in ethyl acetate (50 mL). The organic solution was then washed first with HCl solution (1.0 M) and then brine. The organic layer was collected and dried over Na 2 SO 4 After removing the solvent under vacuum, the product was collected as a yellow solid (0.7 g, 56 %). 3 .2.9 ICG Encapsulation ICG (1 mg) was dissolved in the dye loaded star polymer solution in the dark, and the resulting solution was stir red for 2 h. To remove the excess ICG, the aqueous mixture was spin filtered three times with 1.2 mL of deionized water using Amicon Ultra 1.5 mL centrifugal filter units (10 kDa MWCO) at 12,000 rpm for 20 min. The ICG calibration curve was obtained using ICG aqueous solutions of known concentration. The ICG weight fraction and encapsulation efficiency were calculated by comparing the absorbance intensity of ICG in the supernatant (10 times diluted) to the calibration curve The absorption spectra in the re gion from 400 nm to 900 nm were also used to characterize the loading property of ICG for the star polymers. 3 .2.10 NIR Triggered FF Release via ICG Photothermal Effect Samples of dye loaded star polymer aqueous solutions (3 mL) with different ICG 1.5 W/cm 2 ) for 1 h. During the irradiation period, samples were removed periodically and the solution temperature was measured at the same time.

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38 3 .3 Results and Discuss ion 3 .3.1 Synthesis a nd Characterization o f Star Polymers Our goal was to synthesize well defined PHPMA based star polymers and study their potential for NIR induced degradation and release. Figure 3 1 illustrates the synthetic pathways to the thermally l abile star polymers. RAFT polymerization was utilized to prepare block copolymers with PEG and PHPMA segments. The PEG macroCTA was prepared by a carbodiimide coupling reaction with CDTPA, ( 1 H NMR spectrum is shown in Figure 3 2 ), and subsequently chain ex tension with HPMA was implemented via RAFT polymerization. Various number average degrees of polymerization (DP) of PHPMA segments were prepared, ranging from 10 to 213 ( Table 3 1). The compositions of the copolymers were determined by 1 H NMR spectroscopy, while the absolute number average molecular weights ( M n ) and the molar mass dispersit y ( ) of the block copolymers were further characterized by gel permeation chromatography (GPC) with multi angle laser light scattering (MALLS) detection. The molecular weights determined by GPC were in good agreement with those determined by NMR analysis ( M n NMR) and those predicted based on reaction stoichiometry and monomer conversion ( M n theory). In all cases, values were below 1.2, suggesting efficient chain exte nsion and good polymerization control. The resulting hydroxyl containing block copolymers were employed to prepare star polymers by carbodiimide coupling with the difunctional azo compound 2 ( Table 3 2). Somewhat surprisingly, the number of arms ( N arm ) per star typically decreased with decreasing arm length, which could be the result of the shorter unimers having fewer hydroxyl groups capable of esterification with the azo containing diacid compound. On

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39 the other hand, the addition of 2 to block copolymers with longer PHPMA segments (DPHPMA > 200) resulted in gelation, presumably due to inter star crosslinking. Figure 3 1 Design and synthesis of therm al responsive star polymers. Synthesis of PEG b PHPMA by RAFT polymerization, followed by crosslinking with 2 led to the formation of core crosslinked star polymers.

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40 Figure 3 2 1 H NMR spectrum of PEG macroCTA in CDCl 3 at 25 C. Table 3 1 Summary of the infor mation of PEG b PHPMA block copolymers. Entry a DP HPMA a M n,NMR (gmol 1 ) b M n,theory (gmol 1 ) c M n ,GPC ( gmol 1 ) d d n /d c d d 1 10 6.8210 3 6.8210 3 6.1210 3 0.0534 1.15 2 24 8.8210 3 8.9710 3 8.0510 3 0.0727 1.12 3 70 1.5410 4 1.6110 4 1.3810 4 0.0802 1.21 4 213 3.6410 4 4.1210 4 4.8310 4 0.0893 1.27 Reaction conditions: [HPMA]/[macroCTA]/[AIBN] = x/1/0.2 (x = 20, 50, 150, and 500 for entries 1 4, respectively); T = 70 C; [macroCTA] = 0.04 M; all the reactions were stopped at 50% monomer conversion. a DP of PEG was 113; b Calculated by comparing peak intensities between PEG backbone ( OC H 2 C H 2 ) and HPMA side chains (CH 3 C H (OH)CH 2 ); c M n, theory = [HPMA] 0 /[macroCTA] 0 conversion MW HPMA + MW macroCTA ; d Determined by SEC MALLS in DMAc by 100% mass re covery technique (ASTRA software, Wyatt Technology).

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41 Table 3 2 Summary of the data for star polymers synthesized by the carbodiimide coupling of PHPMA block copolymers with compound 2 Entry Linear Polymer a N Arm b M w GPC (gmol 1 ) c c Size d (nm) 1 PEG 113 PHPMA 10 6 4.1110 4 1.23 27 2 PEG 113 PHPMA 24 11 9.6410 4 1.45 50 3 PEG 113 PHPMA 70 47 7.8810 5 1.36 60 4 PEG 113 PHPMA 213 Gelled n/a n/a n/a a From Table 1, entries 1 4; b Number of arms ( N arm ) per star polymer was calculated by the following equation: ; c Determined by GPC MALLS, assuming d n /d c values identical to those of the linear polymer p recursors (d n /d c values are listed in Table 3 1); d Determined by DLS in aqueous solution. Figure 3 3 1 H NMR spectra recorded for linear block copolymer (top) and star polymer (bottom) in MeOD.

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42 1 H NMR spectroscopy showed the attenuation of the methine resonance of HPMA (CH 3 C H (OH)CH 2 the PEG segments ( OC H 2 C H 2 Figure 3 3). The secondary amide proton signals ( C(=O)N H CH 2 signals were also no longe r visible, which is consistent with the attenuation of these signals due to the high density and low mobility within the partially desolvated core following esterification. 92 93 FT IR spectroscopy further supported successful esterification of the HPMA units via a reduction in intensity of th e peaks assigned to the 1) and the appearance of the signal corresponding to the 1 Figure 3 4). As shown in the GPC traces in Figure 3 5, a clean peak shift to higher molecular weights (shorter eluti on time) was observed for the star polymers. Dynamic light scattering (DLS) indicated an increase in hydrodynamic diameter after crosslinking the linear copolymers (~2 nm) to form the star polymers (~27 nm) in aqueous media ( Figure 3 6, Table 3 2, entry 1) Transmission electron microscopy (TEM) indicated th at stars were formed with uniform spherical morphology, and their sizes (~20 nm) were similar to those obtained by DLS analysis ( Figure 3 7). Star polymers with larger numbers of arms were characterized in the same manner ( Figure 3 8 and 3 9).

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43 Figure 3 4 FT IR spectra of linear polymer (magenta) and star polymer (blue). Arrows indicate hydroxyl and ester functional groups. Figure 3 5 Gel permeation chromatogram of linear and star polymers.

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44 Figure 3 6 Number average hydrodynamic diameter of linear and star polymers measured by DLS. Figure 3 7 TEM image of the star polymers (st ained with uranyl acetate).

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45 Figure 3 8 Characterization of star polymers with DP HPMA = 24 ( Table 3 1, entry 2). (A) GPC traces and (B) particle size analysis by DLS.

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46 Figure 3 9 Characterization of star polymers DP HPMA = 70 ( Table 3 1, entry 3). (A) GPC traces and (B) particle size analysis by DLS.

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47 3 .3.2 Dissociation o f Star Polymers b y Traditional Heating Figure 3 10 Schematic illustration of heat triggered star polymer dissociation. Figure 3 11 GPC trace s of star polymers ( Table 3 1, entry 1) after 1 day (orange) and 60 days (blue) stored at room temperature The star polymers were thermally stable and showed no degradation by GPC analysis when stored at room temperature for extended periods ( Figure 3 11 ). The

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48 d ecomposition rate constant ( k d ) and half life time ( t 1/2 ) of an azo initiator are defined as: 94 (3 1 ) (3 2 ) W here, A is the pre exponential factor, E d is the activation energy (for dissociation, kJ/mol), R is the gas constant (8.314 J/Kmol). For compound 2 E d = 132 kJ/mol, ln (A) = 36/s. At 25 C ( T = 298.15 K), t 1/2 N,N dimethylformamide. This estimation indicates that the dissociation of the star polymers would be negligible at room temperature over extended time periods. However, because the star polymers contained labile azo moieties in their cores (10 h half life at 69 C in toluene), we examined the t hermally induced dissociation of the stars at 90 C in aqueous media ( Figure 3 10 ). Thioglycerol was added to the solution as a radical scavenger to limit undesired intermolecular (re)crosslinking reactions by radical recombination, thereby allowing the di ssociation kinetics of the star polymers to be evaluated by DLS and GPC. The thermal degradation behavior of three kinds of star polymers ( Table 3 2 entries 1 3) was investigated. Star polymers with larger numbers of arms ( Table 3 2, entries 2 ( N arm = 11) and 3 ( N arm = 47)) underwent slow and/or insufficient degradation on heating ( Figure 3 12 and 3 13 ). Such slow dissociation may be attributed to recombination reactions during degradation, as a result of the high density of azo linkages and insufficient p enetration of the radical scavenger within these densely crosslinked star cores. In contrast, smaller star polymers with an average of only 6 arms ( Table 3 2, entry 1) showed considerable

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49 degradation under identical conditions. A gradual reduction in size was observed by DLS while heating at 90 C ( Figure 3 14 ). After 12 h, the average hydrodynamic diameter was reduced to 3 nm, which is slightly larger than that of the linear block copolymers prior to star formation (2 nm). Similarly, the GPC traces indicat ed gradually decreasing star sizes as a function of heating time ( Figure 3 15 ). The molecular weight of the stars after being treated at 90 C for 12 h was slightly higher than that of the initial linear polymers prior to crosslinking, presumably due to mu ltiple azo linkages per chain, incomplete scission, and/or a small degree of radical recombination within the star cores.

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50 Figure 3 12 Thermal degradation of star polymers DP HPMA = 24 N arm = 11 ( Table 3 2 entry 2) in the presence of radical scavenger at 90 C for different heating time (A) GPC traces and (B) particle size analysis by DLS.

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51 Figure 3 13 Thermal degradation of star polymers DP HPMA = 70 N arm = 47 ( Table 3 2 entry 3) in the presence of radical scavenger at 90 C for different heating time (A) GPC traces and (B) particle size analysis by DLS.

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52 Figure 3 14 DLS size distributions of star polymers DP HPMA = 1 0 N arm = 6 ( Table 3 2 entry 1 ) in the presence of radical scavenger during thermal dissociation at 90 C. Figure 3 15 GPC trace s showing star polymer DP HPMA = 1 0 N arm = 6 ( Table 3 2 entry 1 ) in the presence of radical scavenger dissociation during heating at 90 C.

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53 The successful degradation of the star polymers with 6 arms prompted a further investigation of the release behavior of model compounds using this sample (all the star polymer used in the rest of experiments are the ones with 6 arms). To demonstrate the potential application of the novel thermally degradable star polymers as a controlled release platform, a model compound (Nile red, NR) was encapsulated in the star polymers via nanoprecipitation. Briefly, the star polymers and NR were d issolved in tetrahydrofuran (THF), and deionized water was slowly added under vigorous stirring. The mixture was purified by dialysis against deionized water. Fluorescence spectroscopy was employed to determine the content of NR in the stars ( Figure 3 16 ). According to the fluorescence emission spectra, star polymers were successfully loaded with the dye, whereas linear, uncrosslinked block copolymers showed negligible NR incorporation when subjected to the same loading procedure. This result is not surpris ing, given that only weak interactions are expected between the hydrophobic NR and the hydrophilic PEG b PHPMA. The release of NR from the loaded star polymers was investigated under similar conditions as the star polymer degradation study. Accumulated dye release caused by dissociation of the star polymer was monitored by the decrease in fluorescence intensity as a function of time. 95 At 25 C, negligible dye release (~5%) from the star polymers was observed. However, more than 60% release was observed after 1 h at 90 C, and 80% of the NR was released after 12 h at this temperature ( Figure 3 17 ). These results indicate that the thermally labile star polymers can be stored at room temperature ( Figure 3 11 ) with triggered degradation attainable by conventional heating. Therefore, we reasoned this nanocarrier could potentially be used for stimulus induced release.

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54 Figure 3 16 Fluorescence emission spectra ( exc = 530 nm) of NR loaded star polymers (blue) and linear polymer (orange). Figure 3 17 Cumulative NR release profiles from star polymers measured by fluorescence spectroscopy: control (orange, T = 25 C) and after heating (blue, T = 90 C).

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55 3 .3.3 Preparation o f N IR Sensitive a nd Degradable Star Polymers Figure 3 1 8 Schematic illus tration of the preparation of NIR sensitive star polymers. Indocyanine green is a well known photosensitizer, 96 and as such it would be expected to promote decomposition of the azo groups in the star polymers due to heating by the photothermal effect during NIR irradiation. With this in mind, studies of the photoinduced thermal degradation and triggered release were carried out. The preparation method of these NIR sensitive star polymers is represente d in Figure 3 18. When star polymers contained only encapsulated NR, fluorescence intensities remained consistent for 8 h under ambient temperature in the dark ( Figure 3 19 blue). However, when both ICG and NR were embedded within the stars, quenching was observed ( Figure 3 19 green) which precluded the determination of drug release kinetics via fluorescence spectroscopy. Therefore, an additional hydrophobic dye, furan modified fluorescein (FF), was prepared ( Figure 3 20 and 3 21 ).

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56 Figure 3 19 Fluorescence intensity kinetics of NR and FF dyes in the presence or absence of ICG in the dark. Figure 3 20 Synthesis of furan modified fluorescein (FF) by EDC coupling reaction.

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57 Figure 3 21 1 H NMR spectrum of furan modified fluorescein (FF) in CD 2 Cl 2 : MeOD ( 1:1 volume ratio ) at 25 C. When FF was encapsulated in the star polymers in conjunction with ICG ( as shown in Figure 3 18 ), fluorescence was observed ( Figure 3 20 orange), and any remaining non encapsulated dye could be readily removed by centrifugation and 900 nm) of the supernatant were utilized to characterize the ICG loading in the polymer star. As s hown in Figure 3 22 the FF loaded polymer stars (without ICG) showed no absorption from 550 97 The absorption curve of the ICG loaded star polymer (also loaded with FF) was slightly red shifted as compared to that of the free ICG in water, most likely due to the aggregation of ICG in the cor e. 97 To further quantify the ICG content of the FF loaded star polymers, a calibration curve of absorbance vs con centration was obtained using known max = 778 nm), as shown in Figure 3 23 According to

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58 Equation 3 3 and 3 4, the encapsulation efficiency was determined to be 28% and the loading capacity was 11% for ICG. Figure 3 22 UV Vis absorption spectra of star polymers loaded with FF and ICG (orange), FF loaded star polymers (green) and free ICG in H 2 O (blue).

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59 Figure 3 23 Calibration curve using ICG aqueous solutions with known concentrations at 778 nm. (3 3) (3 4) Mass of total ICG is calculated according to the ICG concentration in the original solution. Mass of ICG in supernatant is calculated with the ICG concentration in the filtrate after the removal of star polymers. Mass of ICG loaded in star polymers can be calculated from the difference between the mass of total ICG and the mass of ICG in supernatant

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60 3 .3.4 Photothermally Triggered Release Figure 3 24 Schematic illustration of FF and ICG encapsulation and NIR triggered FF release via photothermal effect. Ground state ICG can be promoted to the excited state through absorption of a photon of appropriate energy ( i.e. NIR ). The molecule will quickly relax to its ground state, releasing this energy via either fluorescence or internal conversion ( i.e. photothermal effect), the latter of which contribute s to localized heating. Therefore, stated ICG c ould be excited under exposed light ( i.e. NIR), then can release either fluorescence or internal energy conversion (photothermal effect). We utilized the second pathway to accomplish the decomposition of the nanocarriers. Dissociation of the ICG containing star polymers under NIR irradiation for controlled release of model compounds was examined ( Figure 3 24 ). Briefly, ICG and FF dye were successfully encapsulated by a nanoprecipitation technique. We postulate d that the heat generated by NIR irradiation of ICG w ould lead to the cleavage of the azo bonds in the cores of the star polymers, resulting in the release of the FF dye into solution. Therefore, the NIR triggered release was studied by irradiating solutions of star polymers conta ining both FF and ICG ( 715 nm, 1.5 W/cm 2 ). The extent of dye release was determined through a decrease in fluorescence intensity at max = 515 nm ( exc = 430 nm), resulting from precipitation of the hydrophobic dye that was no longer stabilized within the star core ( Figure 3 25 ). In the first 10 min of NIR irradiation, 23% of the dye was

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61 released ( Figure 3 26 Table 3 3, Entry 4). After further irradiation ( 715 nm, 1 h), approximately 43% of the original fluorescence intensity was diminished ( Figure 3 27 ). These results indicate that FF release during NIR irradiation is incomplete, presumably due to trapping of FF through recombination of cyanoalkyl radicals or a reaction between the formed radicals and FF in the absence of radical scavengers. Figure 3 25 Ima ges of the FF and ICG loaded star polymer solution before and after NIR irradiation. Figure 3 26 Fluorescence emission spectra (excited at 430 nm) of star polymers loaded with ICG and FF under NIR irradiation for varying times.

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62 Figure 3 27 Cumulative release profiles of FF by fluorescence intensity changes under different conditions. Table 3 2 Summary of photothermally triggered release conditions. Entry a ICG Incorporation b Dissociation Conditions FF Release in 1 h (%) 1 With ICG 25 C, in the dark < 5 2 With ICG 45 C, in the dark < 5 3 Without ICG NIR c < 5 4 With ICG NIR 43 a From Fig. 3 27 ; b FF loaded star polymer solution with or without ICG; c Temperature during 1 h irradiation is between 25 to 45 C (see Figure 3 28 ). To unequivocally credit the liberation of FF to the localized heat generated by ICG under NIR irradiation, variables were systematically removed, and the results were examined. First, release from star polymers w ith encapsulated ICG and FF was studied in the dark ( i.e. in the absence of NIR trigger). Without NIR irradiation at 25 C,

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63 negligible FF release (< 5%, for 1 h) was observed, indicating th at star polymers are stable at ambient temperature ( Table 3 3, Entry 1). When the temperature was elevated to 45 C, there was still limited FF release (< 5%, Table 3 3, Entry 2), suggesting that the nanocarriers are stable in the absence of NIR irradiation due to a lack of azo bond dissociation. The stability of t he star polymers was further investigated under NIR irradiation in the absence of ICG ( i.e. no photosensitizer). While the bulk solution temperature rose by a few degrees, insignificant FF release was detected over 1 h ( Table 3 3, Entry 3). Irradiated sol utions of ICG encapsulated star polymers showed slightly higher solution temperature increases (T = 45 C) than those of star polymers without ICG ( Figure 3 28 ) H owever, as mentioned previously, no significant release is observed in this bulk temperature range. The results of these control experiments indicate a locally elevated temperature in the core of the star polymers due to photosensitization of ICG. As a result, effective dye release was found only in ICG loaded star polymers under NIR irradiation, where temperatures surrounding the azo moieties are sufficiently high to cause appreciable bond dissociation. Although the actual temperature at the star polymer cores upon NIR irradiation could not be verified, the results indicated that the local tempera ture induced by the photothermal effect of ICG must be much higher than 45 C in order to break the azo linkages and release the cargo.

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64 Figure 3 28 Temperature profiles of the FF loaded star polymer solution in the presence or absence of ICG under NIR i rradiation.

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65 3 .4 Conclusions In this report, we developed responsive azo containing star polymers by combining RAFT polymerization and carbodiimide coupling reactions. This approach resulted in water soluble particles that were uniform in size and displa yed stability in highly dilute solutions. The star polymers were structurally stable up to temperatures of 45 C in the absence of irradiation but would rapidly degrade under elevated temperatures ( T = 90 C). These results provided evidence that these pol ymers may be used as nanocarriers for temperature triggered release. This approach was further extended to include heat generated from the interaction of NIR irradiation and a photosensitizer via the photothermal effect. ICG was encapsulated as a NIR respo nsive photosensitizer in order to generate site specific heating and induce bond cleavage. Effective release during NIR irradiation was observed only for the star polymers that contained ICG. In contrast, removal of either of the essential components [ i.e. the photosensitizer (ICG) or the energy source (NIR)] resulted in minimal disturbance to the polymeric carriers (< 5% release), indicating the nanoparticles can be stored under ambient conditions. Combining the results obtained with the major benefits of NIR irradiation ( e.g. mild energy source and deep tissue penetration), we anticipate that this research can provide an efficient strategy for remote and on demand photothermal therapy with concomitant release of encapsulated cargo.

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66 CHAPTER 4 MACROMOLECULAR METAMORPHOSIS IN ORGANOGELS AND BULK NETWORKS 4 .1 Introduction In this chapter, a new type of thermally responsive material is introduced that relies on two competitive dynamic covalent reactions. The well known Diels Alder (DA) reaction be tween furan (Fur) and maleimide (Mal) is employed. Due to its reversibility, the Fur Mal DA reaction has been widely used in preparing recyclables and self healing polymeric materials 31 32 The other DA reaction studied in this work involved the cycloaddition of anthracene (Anth) and Mal groups. As shown in Figure 4 1, the Fur Mal forward DA reaction proceeds with reasonable rates at around 60 C. When the temperature is elevated to above 80 C, the retro DA reaction becomes favored However, the Anth Mal DA reaction happens at a higher temperature of ~120 C, while the retro DA reaction requires an even higher temperature of ~220 C, which is above the degradation temperature of many polymers 98 As such, the Anth Mal DA reaction can be considered thermally irreversible. Her ein, we designed a system that contains both the Fur Mal adduct and the Anth group and investigated their structural transformations using thermal triggers. At elevated temperatures (>120 C), the retro DA reaction of the Fur Mal adduct resulted in free Fu r and Mal groups, which could further react with Anth moieties to form the Anth Mal adduct. Parts adapted with permission from Sun, H.; Kabb, C. P.; Dai, Y.; Hill, M. R.; Ghiviriga, I.; Bapat, A. P.; Sumerlin, B. S., Macromolecular metamorphosis via stimulus induced transformations of p olymer architecture. Nature C hemistry 2017, 9 (8), 817 823 Copyright 2017, Springer Nature.

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67 Figure 4 1. The reaction scheme of Fur Mal DA and Anth Mal DA. A daptable organogels and transformable bulk polymer networks we re designed and synthesized The se systems were initi ally reversibly crosslinked systems capable of reversible gel to sol transitions and self healing but could be triggered to rearrange their structure to achieve permanent fixation (irreversib ly crosslinked system) on heating to elevated temperatures In t he gel systems, by tuning the functionality of the Mal crosslinker, two transformations were studied, namely the gel to sol transition and the gel expansion. In bulk materials, the interchange of Fur Mal to Anth Mal in a series of polyme rs with varying T g s w as investigated. 4 .2 Experimental Methods 4 .2.1 M aterials 1 (2 hydroxyethyl) 1H pyrrole 2,5 dione 99 (Fur MalOH), S S bi s (9 anthrylmethyl ) trithiocarbonate 100 (BATTC), and tris [2 dimethylamino)eth yl]amine 101 (Me 6 TREN) were synthesized according to literature methods. Poly (ethylene glycol) methacrylate ( M n = 500 g/mol), succinic anhydride (>99%), tris (2 aminoethyl) am ine (TREN, 96%), formaldehyde (37 wt% in water), 9 anthracene methanol (97%), sodium hydride (60% dispersion in mineral oil), and methacrylic anhydride (>99%) were purchased from Sigma Aldrich. Styrene (99%), 9 (chloromethyl) anthracene (>98%), methyl meth acrylate (99%), and 4 dimethylaminopyridine (98%) were purchased from

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68 Acros Organics. Triethylamine (>99%), methyl methacrylate (99%), furan (99%), maleic anhydride (MAn, >98%), and furfurylamine (99%) were purchased from Alfa Aesar. Sodium hydroxide was p urchased from Fisher Scientific. 1 Ethyl 3 (3 dimethylaminopropyl ) carbodiimide hydrochloride (EDC HCl, >98%) was purchased from TCI. All other chemicals were used as received unless otherwise noted. Monomers were passed through a column of basic alumina t o remove inhibitors and a cidic impurities prior to polymerization. 4 .2. 2 Instrumentation Nuclear Magnetic Resonance (NMR). 1 H NMR and 13 C NMR spectra were recorded in CDCl 3 using an Inova 500 MHz spectrometer. Gel Permeation Chromatography (GPC). Molecular weight and polydispersity were determined by gel permeation chromatography in dimethylacetamide (DMAc) with 50 mM LiCl at 50 C and a flow rate of 1.0 mL min 1 (Agilent isocratic pump, degasser, and scoGel I series G3078 mixed bed 3 4 g mol 1 ). Detection consisted of a Wyatt Optilab T rEX refractive index detector operating at 658 nm and a Wyatt miniDAWN Treos light scattering detector operating a t 659 nm. The system was calibrated using 10 poly(methyl methacrylate) (PMMA) standards with M w from 9.88 x 10 5 to 602 g/mol. Absolute molecular weights and polydispersities were calculated using the Wyatt ASTRA software and the dn /dc for polystyrene (0.14 44 mL/g). Fluorescence Spectroscopy. All measurements were taken using a Molecular Devices SpectraMax M2 Multimode Microplate Reader at 25 C. Fluorescence

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69 the sample on black 96 well polypropylene microplates (G reiner Bio One) with an excitation wavelength of 360 nm. Solid State Fluorescence Spectroscopy. Fluorescence spectroscopy of compressed networks was conducted on a Horiba Fluorolog 3 spectrophotometer with a Ushio Xenon arc lamp and Horiba photomultiplier tube (detector range: 290 850 nm). The excitation wavelength was 360 nm and the emission spectrum was scanned from 380 nm to 500 nm. The instrument was equipped with a petite integrating sphere. High Resolution Mass Spectrometry (HRMS). HRMS was carried out using an Agilent 6220 TOF MS mass spectrometer in the electrospray ionization (ESI) mode. Fluorescence Microscopy. Fluorescence microscopy images were taken using a TE 2000 imaging acquisition system (Nikon) with 10x objective lens, X Cite 120 fluoresc ent light source (EXFO) and a Cascade:1K camera (Roper Scientific) at room temperature. Rheometry. Rheological measurements were conducted at 25 C on an ARES LS1 rheometer (TA Instruments) using 50 mm flat plate geometry with a 0.5 mm gap. Dynamic Mechani cal Analysis (DMA). Mechanical measurements were carried out using a Q800 (TA Instrument s ) to determine the modulus with film tension clamp. Samples were heated from 0 C to 250 C at a heating rate of 3 C/min and a frequency of 1 Hz. Differential scanni ng calorimetry (DSC). Thermal analysis was performed from s ).

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70 4 .2.3 bis (9 anthrylmethyl) trithiocarbonate (BATTC) Figure 4 2. T he s ynthetic scheme of BATTC. bis (9 anthrylmethyl) trithiocarbonate was prepared using a previously reported method ( Figure 4 2) 102 A mixture of CS 2 ( 10 mL ) a nd 33% aqueous NaOH solution ( 15 mL ) was stirred vigorously at room temperature in a 50 mL round bottom flask with a magnetic stirrer. The phase transfer catalyst (n Bu4NCl, 3 mol% to 9 chloromethyl anthracene used) was then introduced. After stirring for 10 min, 9 chloromethyl anthracene (2 .0 g 8.8 mmol) was added. The mixture was stirred vigorously at room temperature overnight. To work up the reaction the products were washed with THF (3 x 10 mL). Then recrystallization from THF gave a bright yellow cr ystal. Yield: 92%. 1 HNMR (500 MHz, CDCl 3 Figure 4 3 5.6 9 (s 4H) 7.5 1 (m 4H) 7.6 1 (m 4H) 8.0 5 (m 4H) 8.2 6 (m 4H); 8.5 0 (s, 2H).

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71 Figure 4 3 1 H NMR s pectrum of BATTC (CDCl 3 ) 4 .2.4 Preparation of Furan Protected Maleic Anhydride. Figure 4 4. The s ynthesis scheme of furan protected maleic anhydride. Furan protected maleic anhydride was prepared using a previously reported method ( Figure 4 4) 103 Briefly, maleic anhydride ( 30 g 0. 3 1 mol) and toluene ( 150 mL ) were added to a three neck flask (500 mL) and stirred with a magnetic stir bar under a nitrogen atmosphere, and the reaction mixture was heated to 65 C. Then furan ( 33 mL, 0.46 mol) was slowly injected into the flask via syringe. After the mixtur e had been stirred for 19 h, the reaction was stop ped, and the mixture was cooled to ambient temperature. The white crystals were precipitated from the mixture after 1 h, collected

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72 via vacuum filtration, and washed with petroleum ether (3 30 mL). The res ultant white powder was dried in a vacuum oven for 24 h. Yield: 77%. 1 H NMR (500 MHz, CDCl 3 Figure 4 5 ) : 3.1 7 (s 2H) 5.4 6 (s, 2H) 6. 58 (s, 2H). Figure 4 5. 1 H NMR s pectrum of furan protected maleic anhydride (CDCl 3 ) 4 .2. 5 Preparation of Furan Protected tris (2 maleimidoethyl ) amine (Fur Mal 3 ) Figure 4 6 Scheme of Fur Mal 3 synthesis. A round bottom flask equipped with magnetic stirrer was charged with tris (2 aminoethyl) amine (2.0 mL, 1 4 mmol) and methanol (300 mL). Furan protected maleic anhydride (8.4 g, 50 mmol) was added and the reaction mixture was heated to 65 C. Triethylamine (3.0 mL, 41 mmol) was slowly injected into the flask via syringe. After 72

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73 h, the reaction mixture was co oled to 10 C. The resulting white crystals were collected via vacuum filtration and washed with cold methanol (3 30 mL). The resultant white powder was dried in vacuo Yield: 90%. 1 H NMR (500 MHz, CDCl 3 Figure 4 7 6.50 (s, 2H), 5.19 (s, 2H), 3.41 (t, 2H) 2.89 (s, 2H), 2.60 (t, 2H); 13 C NMR (125 MHz, CDCl 3 Figure 4 8 HRMS ( Figure 4 9) : Calcd. for [M + H] + : 591.2086. Found: 591.2089. Figure 4 7. 1 H NMR s pectrum of Fur Mal 3 (CDCl 3 )

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74 Figure 4 8. 1 3 C NMR s pectrum of Fur Mal 3 (CDCl 3 )

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75 Figure 4 9 The h igh resolution mass spectrum of Fur Mal 3 4 .2.6 Synthesis of tris (2 maleimidoethyl) amine (Mal 3 Figure 4 6 ) Figure 4 10 Scheme of Mal 3 synthesis Fur Mal 3 (2.0 g, 3.4 mmol) and toluene (100 mL) were added to a round bottom flask equipped with a magnetic stirrer. The solution was purged with nitrogen and heated to 1 1 0 C. After refluxing for 48 h, the mixture was cooled to room temperature

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76 and toluene was r emoved under reduced pressure. The resultant white powder was dried in vacuo Yield: 92%. 1 H NMR (500 MHz, CDCl 3 Figure 4 11 2H), 3.52 (t, 2H), 2.72 (t, 2H); 13 C NMR (125 MHz, CDCl 3 Figure 4 12 134.14, 51.73, 35.74. ESI HRMS ( Figure 4 13) : Calcd. for [M + H] + : 387.1299. Found: 387.1311. Figure 4 11. 1 H NMR s pectrum of Mal 3 (CDCl 3 )

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77 Figure 4 12. 1 3 C NMR s pectrum of Mal 3 (CDCl 3 )

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78 Figure 4 13 The h igh resolution mass spectrum of Mal 3 4 .2.7 Synthesis of P( S b (S alt MAn) b S ) Triblock Copolymer Figure 4 14. Synthetic pathway for P( S b (S alt MAn) b S ) Triblock Copolymer Styrene (1 2 mL, 0.1 mol), BATTC (98 mg, 0.20 mmol), and AIBN (8.2 mg, 50 were sealed in a 20 mL reaction vial equipped with a magnetic stirrer. The reaction mixture was purged with nitrogen and placed in a preheated oil bath at 70 C. After 24 h, a purged solution of MAn (1.0 g, 10 mmol) in 1,4 dioxane (3 mL) was

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79 injected into the vial. The polymerization was quenched after 27 h by removing the vial from the heating block and exposing the contents to atmospheric oxygen. The polymer was precipitated in excess of cold petroleum ether (3) and dried in vacuo to afford the purified poly mer. Yield: 98%. 4 .2.8 Functionalization of P( S b (S alt MAn) b S ) with Furfurylamine Figure 4 1 5 S cheme for using furfurylamine functionalization of P( S b (S alt MAn) b S ) A modified version of our previously reported procedure was used ( Figure 4 15) 36 71 Furfurylamine (0.22 g, 2.3 mmol) was dissolved in THF (3 mL) in a vial equipped with a magnetic stirrer. The triblock copolymer P( S b (S alt MAn) b S ) ( P1 0.85 g, 2.3 mmol MAn) was added, and the vial was placed in a heating block at 50 C for 20 h. The furan functionalized polymer ( P2 ) was isolated by precipitation into cold diethyl ether (3) and dried in vacuo. Yield: 90%. 4 .2.9 Fur Mal 3 Gel Form ation ( Figure 4 16) Figure 4 1 6 Schematic illustration of Ful Mal 3 gel formation. A solution of P2 in DMSO (100 mg/mL) was prepared and Mal 3 (1 .1 equiv relative to anthracene ) was added, followed by rapid mixing at room temperature. The

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80 mixture was heated to 55 C, and gelation occurred within 90 min. A gel sol transition was observed upon heating the gel to 85 C. 4 .2.10 Thermally Triggered Gel Ex pa nsion Figure 4 17 Schematic illustration of thermally triggered gel expansion. The pr e formed Fur Mal 3 gel was heated at 120 C for 90 min. A gel sol transition first took place, followed by a new gelation event yielding the darker colored extended organogel Figure 4 18. Images of Fur Mal 3 gel before and after the thermal trigger.

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81 4 .2.11 Fur Mal 2 Gel Formation ( Figure 4 19) Figure 4 19 Schematic illustration of the formation of a Fur Mal 2 gel. A solution of P2 in DMSO (100 mg/mL) was prepared and bifunctional crosslinker Mal 2 (1 equiv relative to furan) was added, followed by rapid mixing at room temperature. The mixture was heated to 55 C, and gelation occurred within 1 h. 4 .2.12 Fur Mal 2 Gel Reversal The pre formed Fur Mal 2 gel was heated at 120 C for 1 h, at which time an irreversible gel sol transition was observed. 16 4 .2.13 S ynthesis o f Anthracene Acid A mixture of 9 anthracene methanol (10 g, 5 0 mmol), succinic anhydride (20 g, 0. 20 mol), and 4 DMAP (6.1 g, 50 mmol) was dissolved in 500 mL DCM. To this solution anhydride pyridine (20 mL, 0. 25 mmol) was added and the reaction was stirred for 16 h at room temperature. The reaction mixture was poured into an ice/water mixture and warmed to room temperature. The organic layer was separated and the aqueous layer was extracted with DCM (2 100 mL). The combined organ ic layers were washed with 5% HCl, sat. NaHCO 3 and brine, dried by anhydrous MgSO 4 filtered and concentrated in a vacuum oven to afford 14 g (yield 97%) of desired anthracene acid as a yellow powder. 1 H NMR (500 MHz, CDCl 3 Figure 4 20 1H)

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82 8.30 (d, 2H) 8.01 (d, 2H) 7.56 (d, 2H) 7.52 (m, 2H) 6.18 (s, 2 H) 2.68 (t, 2H) 2.64 (t, 2H). Figure 4 20. 1 H NMR s pectrum of anthracene acid. 4 .2.14 S ynthesis o f Anthracene Containing Monomer (M3 Figure 4 21 ) Figure 4 21. The s ynthetic scheme of anthracene containing monomer (M3). A solution of anthracene acid and poly (ethylene glycol) methacrylate ([ COOH]/[ OH] = 1/1.2) in dichloromethane (500 mL) was cooled in an ice bath. A solution of EDCHCl (3 equiv to anthracene acid) and DMAP (0.1 equiv to [EDCHCl]) was added dropwise. The reaction was stirred overnight at room temperature. The mixture was then precipitated into cold diethyl ether 4 times. The product obtained was dried in a vacuum oven for 48 h. 1 H NMR (500 MHz, CDCl 3 Figur e 4 22 ) : 51 (s, 1H)

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83 8.3 3 (d, 2H) 8.0 2 (d, 2H) 7.5 7 ( m 2H) 7. 49 (m, 2H) 6.1 7 (s, 2 H) 6.12 (d, 1H), 5.57 (d, 1H), 4.29 (t, 2H), 4.16 (t, 2H), 3.64 ( m 28 H) 2.6 5 ( s 4 H) 1.94 ( s 3 H). Figure 4 22. 1 H NMR s pectr a of anthracene containing monomer M 3. 4 .2.15 Synthesis o f Fur Mal Crosslinker ( Figure 4 23) Figure 4 23 The s ynthetic scheme of Fur Mal crosslinker. Fur Mal diol (1.0 g, 4. 2 mmol), 4 dimethylaminopyridine ( 0. 20 g, 1. 7 mmol) and triethylamine ( 2 5 g, 25 mmol) were mixed in dichloromethane (50 mL) and cooled in an ice bath. Methacrylic anhydride ( 2. 6 g, 1 7 mmol) was then added dropwise to the reaction solution. The mixture was kept under magnetic stirring overnight at room temperature. Purification by chr omatography on silica gel column (hexane/ethyl acetate gradient) afforded 1. 4 g ( 3.8 mmol, yield 9 2 %) as a pale yellow oil. 1 H NMR (500 MHz, CDCl 3 Figure 4 24 6.38 (d, 1H) 6.03 (d, 2H) 5.51 (d, 2H) 5.23

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84 (s, 1H) 4.91 (d, 1H) 4. 46 (d, 1H) 4.23 (t, 2H) 3.75 (t, 2H) 2.91 (d, 1H) 2.87 (d, 1H) 1.91(s, 3H) 1.86 (s, 3H). Figure 4 24 1 H NMR spectr um of Fur Mal crosslinker (500 MHz, CDCl 3 ). 4 .2.16 Synthesis of Anth MalOH from Fur MalOH ( Figure 4 25) Figure 4 25. The s ynthetic scheme of Fur MalOH. Fur MalOH (1.0 g, 4.8 mmol) and 9 anthracenemethanol ( 0. 99 g, 4.8 mmol) were mixed in toluene (50 mL) and heated at 120 C for 24 h. Upon cooling, the product was collected as a white precipitate and dried. 1 H NMR (500 MHz, DMSO d 6 Figure 4 29 (ppm) 7.68 (d, 1H) 7.44 (d, 1 H) 7.17 (m, 6H) 5.33 (t, 1H) 4.85 (m, 2H) 4.70 (s, 1H) 4.60 (t, 1H) 3.26 (s, 2H) 3.01 (t, 2H) 2.57 (m, 2H).

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85 Figure 4 26 1 H NMR spectroscopy of Anth MalOH (500 MHz, DMSO d 6 ). 4 .2.17 Synthesis of Anth Mal C rosslinker ( Figure 4 27) Figure 4 27 Scheme of Anth Mal crosslinker synthesis. Anth MalOH (1.0 g, 2. 9 mmol), 4 dimethylaminopyridine ( 0. 14 g, 1.1 mmol) and triethylamine (1.7 g, 17 mmol) were mixed in dic h loromethane (50 mL) and cooled in an ice bath. Methacrylic anhydride (1. 8 g, 11 mmol) was then added dropwise to the reaction solution. The mixture was kept under magnetic stirring overnight at room temperature. Purification by chromatography on silica gel column (hexane/ethyl a cetate gradient) afforded 1. 3 g (2. 6 mmol, yield 90 %) as a pale white powder. 1 H NMR (500 MHz, DMSO d 6 Figure 4 28 7.37 (d, 1H) 7.20 (m, 6H) 6.01 (d, 2H) 5.71 (d, 2H) 5.46 (m, 2H) 4.82 (s, 1H) 3.37 (m, 6H) 1.91(s, 3H) 1.86 (s, 3H).

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86 Figure 4 28. 1 H NMR spectroscopy of Anth Mal crosslinker (500 MHz, DMSO d 6 ). 4 .2.18 Thermally Responsive Bulk Polymer N etwork Formation Films with different monomer (varying T g s ) and crosslinker species (Fur Mal, Anth Mal or EGDMA) and va rying ratios of co monomers were photo cured by UV irradiation in a rubber mold between two glass slides.

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87 4 .3 Results and D iscussion 4 .3.1 Metamorphosis in Organogels The first aim of this project is to investigate topological transformations in gels (gel to sol transition and gel expansion) that can arise from con s tructing the networks via dynamic covalent bonds We utilized a diene displacement reaction which involves the diene in one Diels Alder adduct being replaced by another, to thermally trig ger a transition in network architecture within a gel ( Figure 4 29 ). Figure 4 29. Gel metamorphosis driven by diene displacement. ( A copolymer with a pendent Fur and terminal Anth functionality was reacted with a Mal 3 ( i ) or Mal 2 ( ii i ) crosslinker to form reversible Fur Mal gels. Increasing the temperature further to 120 C resulted in a reaction of the liberated Mal functionalities with terminal Anth to effect either a gel expansion ( ii ) or disintegration of the gel network ( iv ) via diene displacement reaction. )

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88 In this example, a triblock copolymer of styrene (S) and maleic anhydride (MAn) termini and pendent Fur groups along the central portion of the backbone Synthesis of P( S b ((S alt MAn) c o (S alt FurMAn )) b S ) included two main steps ( Figure 4 14 and Figure 4 15). First the triblock copolymer P( S b (S alt MAn) b S ) was synthesized in one pot, with polystyrene (blue trace in Figure 4 30) was polymerized in the presence of BATTC for 24 h an d an aliquot being taken for GPC analysis. At this time, maleic anhydride was added and the polymerization was continued for 3 h, at which time the triblock copolymer (orange trace in Figure 4 30 ) was obtained. After post polymerization functionalization with furfurylamine, P( S b ((S alt MAn) co (S alt FurMAn)) b S ) was obtained. A s shown in Figure 4 31, a shift in the GPC trace to lower elution time is indicative of the formation of larger molecular weight polymer, suggesting successful functionalization of P( S b (S alt MAn) b S ) with furfurylamine. The detailed polymer information can be found in Table 4 1 Figure 4 30. GPC traces of polystyrene and P( S b (S alt MAn) b S )

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89 Figure 4 31. GPC traces of P( S b (S alt MAn) b S ) t riblock c opolymer before and after functionalization with furfurylamine. Table 4 1 Gel expansion metamorphosis: polymer information. GPC MALLS Entry Topology M n (g/mol) P(S b (S alt MAn) b S) Triblock Copolymer 16 900 1.08 P(S b ((S alt MAn) co (S alt FurMAn) b S)) Triblock Copolymer 19 800 1.13 Treating this P( S b ((S alt MAn) co (S alt FurMAn)) b S ) with a tris maleimide crosslinker (Mal 3 ) ( Figure 4 16 ) at 55 C yield ed a gel within 90 minutes predominantly because of Fur Mal cycloaddition along the backbone of the chains ( Figure 4 29 ( i )). To demonstrate the reversibility of the crosslinked network the gel was heated to 85 C and cyclorev er sion of the Fur Mal linkages caused a slow gel sol transition within one hour ( Figure 4 32 ).

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90 Figure 4 32 I mages of the formation of a Ful Mal 3 gel. ( At 55 C, rapid gel formation due to Fur Mal cycloaddition is observed. The retro Diels Alder reaction at 85 C causes a gel sol transition to occur. ) I maleimide to react with anthracene groups on the termini of the polymer chains, forming an Anth Mal crosslinked gel ( Figure 4 29 ( ii )). For the purpose of characterizing the property c hange of the organogel and to quantify the gel expansion, rheological tests were carried out ( Figure 4 33). In the frequency sweep shown in Figure 4 33, the storage modulus is higher than the loss modulus for both Fur Mal 3 and Anth Mal 3 gels, which indicates both gels at ambient conditions showed solid like behavior (crosslinked). By substituting the plateau value of the storage modulus into equations Equation 4 1 to Equation 4 4, 104 105 the detailed gel information could be achieved, as shown in Table 4 2 An increase in the avera ge distance between two neighboring crosslinking points (Fur Mal 3 gel, 2.73 nm; Anth Mal 3 gel, 8.08 nm) was revealed by rheological testing which indicated that Fur Mal 3 gel transformed to an expanded network (Anth Mal 3 gel).

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91 (4 1) (4 2) (4 3) (4 4) ( : storage modulus; : molecular weight between net points ; : crosslink density ; : crosslink distance ; number of elastically active junctions in the network per unit of volume ; : u niversal gas constant ; : p olymer concentration ; : absolute t emperature ; : Avogadro constant .) Figure 4 3 3. Rheology frequency sweep of Ful Mal 3 gel and Anth Mal 3 gel. Rheology of the Fur Mal 3 gel shows a plateau value of the storage modulus at 200 kPa (filled blue circle s ), consistent with a molecular weight of 1.2 kg / mol between net points ( M e ( Table 4 2 )); rheology after the diene displacement to the Anth Mal 3 gel reveals a plateau modulus of 7.8 kPa (filled orange circle s ), consistent with a molecular weight of 31.8 kg / mol between the net points, which indicates that the Fur Mal 3 gel has a higher degree of crosslinking than the terminally linked Anth Mal 3 gel.

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92 Table 4 2 Information on Fur Mal 3 and Anth Mal 3 gels from rheology testing Moreover, although the dangling Anth end groups of the original gels led to fluorescence, the gels formed at 120 C were not fluorescent, which suggests that the Anth units had been consumed by cycloaddition to the Mal 3 crosslinker ( Figure 4 34 and 4 35 ). Figure 4 34. Fluorescence m icroscopy image of Ful Mal 3 gel Fluorescence imaging of the Fur Mal 3 gel (room temperature) reveals the retention of terminal anthracene groups, as indicated by the blue fluorescence emission.

PAGE 93

93 Figure 4 35 Fluorescence microscopy of Anth Mal 3 gel. Following gel expansion, fluorescence microscopy reveals a substantial decrease in fluorescence intensity of the Anth Mal 3 gel, indicating that the majority of the terminal anthracenyl groups had reacted with the tris maleimide crosslinker. To gain insight into the operable mechanism of gel expansion a control experiment was conducted by treating P( S b ((S alt MAn) co (S al t FurMAn)) b S ) with a bis maleimide crosslinker, Mal 2 ( N,N 4 phenylenedimaleimide ), under the same conditions. First, on heating the solution to 55 C, gelation due to the DA reaction between Fur and Mal functionalities was observed ( Figure 4 29 ( iii )). The gel was then heated to 120 C, which caused dis sociation of the Fur Mal linkages and bonding of Mal moieties with the Anth chain ends of the polymers. This led to the disintegration of the polymer network into soluble polymers (Anth Mal 2 polymers) aft er 48 hours ( Figure 4 29 ( iv )). The reaction between anthracene groups and the released maleimides should lead to a decrease in the fluorescence intensity of the system. Fluorescence spectra w ere taken to monitor the gel decomposition ( Figure 4 36). The fi nal product showed a decreased fluorescence intensity caused by consumption of terminal anthracene units.

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94 Figure 4 36 Fluorescence s pectra during the reversal of Fur Mal 2 gel. During gel decomposition, anthracene was consumed via Diels Alder reaction with maleimide. The resulting cycloadduct was no longer fluorescent, resulting in a decrease in fluorescence intensity as a function of reaction time. After 72 h, the fluorescence signal was negligible, indicative of consumption of the majority of terminal anthracene moieties. Additionally, a multimodal trace of Anth Mal 2 polymers was observed in the GPC chromatogram. The shift to higher molecular weight i s attributed to the DA step growth polymerization of the terminal anthracene groups of P( S b ((S alt M An) co (S alt FurMAn)) b S ) and Mal 2 which led to multiple oligomeric distributions ( Figure 4 37 ). These results clearly confirmed that gel expansion was driven by in situ rDA /DA reactions ( i.e. diene displacement reaction).

PAGE 95

95 Figure 4 3 7 Gel permeation chromatogram of Anth Mal 2 polymers after the disintegration of the Fur Mal 2 gel. The trace (black line) shifted to a higher molecular weight region because of step growth condensation of the original polymer (red line) and bis maleimide crosslinker. Decon volution of the trace that results from the disintegrated gel gave rise to four distributions that can be attributed to a unimer (purple dashed line), dimer (green dashed line), trimer (orange dashed line) and tetramer (blue dashed line) of the original po lymer. The complex distributions of the Anth Mal 2 polymer are in good agreement with the inherent nature of step growth polymerization. 4 .3.2 Metamorphosis in bulk polymer networks The second goal of this project is the preparation of bulk polymer networks that can be thermally triggered to reverse and then lock the ir network structure by exchange of a reversible Diels Alder adduct for an irreversible one These dynamic structural changes are induced by competitive Diels Alder and retro Diels Alder reactions between furan maleimide and anthracene maleimide functionalities. The Diels Alder reaction between furan and maleimide is thermally reversible (form ation at 60 C and while the Diels Alder reaction between anthracene and maleimide is not considered to be thermally reversible (form ation at 120 C). By having furan maleimide

PAGE 96

96 Diels Alder adducts and anthracene groups within a polymer network, material disassembly and structural locking can be achieved at specific temperatures. Figu re 4 3 8 Schematic illustration of the formation of bulk networks. ( R = CH 3 CH 2 CH 3 (CH2) 3 CH 3 ; R = CH 2 CH 2 Fur Mal adduct, Anth Mal adduct; R = (CH 2 CH 2 O) 8 ) Films with different monomer and crosslinker species and varying ratios of comonomers were photo cured by UV irradiation (as shown in Figure 4 38) M1 is the major monomer (> 90 mol%) used to prepare the network, which dominates the glass transition temperature ( T g ) range of the films. By changing the M1 species, we can tune the T g of the synthe sized film to be in an appropriate range for a variety of applications. The M1 choices used in this project are summarized in Table 4 3 Table 4 3 Monomer 1 (M1) species with their respective polymer T g M1 R T g (C) Methyl methacrylate CH 3 105 Ethyl methacrylate CH 2 CH 3 65 n Butyl methacrylate (CH 2 ) 3 CH 3 20 M2 is the crosslinker used to form the covalently bonded networks, which is the key component that dictates the ability of the film to transform. By tuning the crosslinker

PAGE 97

97 species and ratios, films with different degrees of crosslinking can be obtained ; namely uncrosslinked, permanently crosslinked, and dynamic covalently crosslinked polymer networks When us ing a reversible crosslinker ( e.g. Fur Mal adduct), the network formed is re mendable. However, if a permanent crosslinker is used, the film will behave like traditional thermosets, whose network structures are locked when formed. Photo polymerizable Fur Mal crosslinker was synthesized as shown in Figure 4 23. The films containing Fur Mal c rosslinker are expected to be reversible when elevating the temperature due to the rDA reaction between furan and maleimide groups. There are two permanent crosslinkers used in preparing control films. One is ethylene glycol dimethacrylate (EGDMA), used as control groups for DMA tests. The other is Anth Mal crosslinker, which is used as a control in solid state fluorescence measurements. M3 is the anthracene containing monomer, which enables locking of the system by reacting with the released maleimide moie ties from rDA at a higher temperature. The chemical structures of three crosslinker s used in the film preparation are shown in Figure 4 39. Figure 4 39. Structures of three M2 moieties used. The detailed information of f ilm compositions is shown in Table 4 4 The n aming of the films is according to the entry number from the composition table, e.g ., Film 3 contains 9 8 .75 mol% M1 1 mol% Fur Mal crosslinker and 0.25 mol% M3.

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98 Table 4 4 The composition information of films prepared within each M1 species Entry M1 (mol%) M2 (mol%) M3* (mol%) 1 99.75 0 0.25 2 99 .0 1 .0 (Fur Mal) 0 3 98.75 1 .0 (Fur Mal) 0.25 4 99.5 0.25 (Fur Mal) 0.25 5 99.75 0 .25 (Anth Mal) 0 6 9 9.0 1 .0 (EGDMA) 0 7 9 9.75 0 .25 (EGDMA) 0 8 100 0 0 The percentage of M3 was limited due to poor solubi lity of the anthracene group in M1. Thermal properties of the films we re characterized by DSC (as shown in Figure 4 40 ). It is observed that the glass transition agreed well with theoretical values (PBMA ~ 25 C, PEMA ~65 C, PMMA ~101 C).

PAGE 99

99 Figure 4 40. DSC plots from the second heating cycle of PBMA (green line), PEMA (red line) and PMMA (blue line) films. Thermal mechanical propertie s of the films were characterized by DMA within each M1 species. Figure 4 41 shows typical DMA results for BMA based films. The uncrosslinked PBMA film (Film 1) yielded at 110 C. For furan maleimide adduct crosslinked films, the transition at 150 C corre sponds to the retro Diels Alder reaction between furan and maleimide groups (Film 2 and Film 3). The higher storage modulus of Film 3 as compared to Film 2 can potentially be attributed to the reformation of crosslinks via Diels Alder reactions between rel eased maleimide and anthracene groups.

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100 Figure 4 41. The s torage modulus of BMA based films measured from DMA. Based on the DMA results, films with different maleimide and anthracene ratios (1:1 and 4:1) were kept at 150 C for varying time periods und er Ar flow To better understand the thermal stability of the anthracene maleimide adduct, the crosslinker containing an Anth Mal adduct was synthesized (as shown in Figure 4 27 ) and characterized by 1 H NMR spectroscopy (as shown in Figure 4 28) Films con taining Anth Mal adducts at the crosslink ing points were prepared and used as control groups for fluorescence testing. Solid state fluorescence was used to further illustrate the consumption of the anthracene groups from reaction with released maleimide moieties during thermal treatment.

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101 Figure 4 42. Solid state fluorescence spectra of BMA based films before and after thermal treatment. The fluorescence spectra of BMA based films before and after thermal treatment are shown in Figure 4 42 Films with the same anthracene content (Film s 1,3,4) showed similar f luorescence intensity before thermal treatment. With increasing heating time, the fluorescence intensity of Film s 3 and 4 decrease d Furthermore, it was found that the fluorescence intensity of Film 3, which contained a higher maleimide to anthracene ratio (4:1) than F ilm 4 (1:1), decrease d to a greater extent than Film 4 under similar thermal treatment conditions. After incubating at 150 C overnight, the fluorescence intensity of Film 3 was virtually undetectable, suggesting complete consumption of the anthracene groups. The fluorescence intensity of the control films showed a negligib le increase. These fluorescence results further support the reaction between anthracene and the released maleimide from rDA reaction in the solid state.

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102 Figure 4 43. The s torage modulus of EMA based films measured from DMA. Films prepared by other M1 sp ecies and all films after thermal treatment were further characterized by DMA to investigate their mechanical properties. All EMA based films followed the same trend as BMA based film s ( Figure 4 43 and Figure 4 44) Uncrosslinked film s yielded during DMA test, and the rDA transition of Fur Mal adduct occurred around 160 C. The film containing both Fur Mal crosslinker and anthracene monomer exhibited a higher modul u s than the film containing only Fur Mal crosslinker and without anthrace ne groups to react with the released maleimide groups from rDA. The fluorescence intensity of the films decreased with increasing thermal treatment time. It is not surprising that the fluorescence intensity decreased more when the film had a high maleimide to anthracene ratio (4:1) than a low maleimide to anthracene ratio (1:1) under similar thermal treatment conditions.

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103 Figure 4 44. Solid state fluorescence spectra of EMA based films before and after thermal treatment. In MMA based films, the fluoresce nce intensity decreased at a similar rate ( Figure 4 45) However, the storage modulus change d because the rDA reaction was not obvious in DMA measurements ( Figure 4 46) It is hypothesized that the glass transition temperature range overlapped with the rDA temperature range. Additionally, the higher the T g of the material, the lower the possibili ty of the released maleimide groups from the rDA reaction to further react with the anthracene groups.

PAGE 104

104 Figure 4 45. Solid state fluorescence spectra of MMA based films before and after thermal treatment. Figure 4 4 6 The s torage modulus of MMA based films measured from DMA

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105 4 .4 C onclusion s I n summary, thermally responsive gels and bulk polymer films were successfully synthesized. The specific combina tion of rDA and DA reactions was used to alter the transformations in gels and bulk polymer networks. In t he gel systems, the gel formed at 55 C, which the forward DA reaction between furan and maleimide was favored. With elevating temperature to 85 C, t he gel to sol transition was observed, which the rDA between furan and maleimide groups was favored. While continuing increase the temperature to 120 C, the gel expansion (transform to a more loosely crosslinked gel) was achieved by the DA reaction betwee n released maleimide groups and anthracene groups at chain ends By tuning the degree of functionality of the maleimide crosslinker ( i.e ., changing the crosslinker from Mal 3 to Mal 2 ), the gel can be dissociated into linear analog ue s permanently instead of expand ing to another loosely crosslinked network. In bulk polymer network systems, the reversibility of the crosslinked system (enabled by the rDA reaction of Fur Mal adduct in the crosslinker) and the permanent fixation (through the DA reaction between An thracene groups and released maleimide moieties) were investigated in a variety of networks with varying T g s. These materials are examples of covalently bonded networks capable of topological changes, which bring the architecture of a crosslinked network i nto a variate that can response to an external stimulus. Beyond these examples, polymer networks with a lower response temperature by similar or different chemistry, or response to other stimuli within a relatively moderate condition are needed to be explo red in the future.

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106 CHAPTER 5 POLYMERIC PHOTONIC CRYSTAL S WITH TUNABLE REFLECTION COLORS 5 .1 Introduction This chapter first introduces the relationships between crosslink density and reflection color of photonic crystals. The design and optimization of the m ulti stage shape memory sensors are discussed, in which the properties of visible reflection color change s from photonic crystals and stimuli responsiveness from traditional shape memory materials were combined. The response s of these shape memory sensors to changes in temperature, solvent and contact force a re also examined Due to their strong interactions with light, photonic crystals have promise in a variety of applications ranging from optical devices to sensors. 39 52 53 Recently, it was reported that polymeric photonic crystals could be used as light weight and power free mechanochromic sensors. 51 By applying monomer to pre assembled SiO 2 nanoparticle templates, followed by photo cur ing and particle removal, the inverse opal mesoporous polymer films showed reflection colors that were dictated by the starting nanoparticle sizes (from orange, green to purple), indicating that the reflection color of these photonic crystal films is determined in part, by the spacing between the pores left behind a fter nanoparticle removal. When forces of different magnitude were applied, the film would be correspondingly strained to different degrees. The larger the force, the higher the stain, le a d ing to shrinking pore sizes and ultimately a blue shift in the refl ected light. However, one drawback was these sensors were for one time use only. U sing a similar method, Jiang and coworkers fabricated tunable photonic crystals with adjustable reflection color respon sive to external stimuli, such as the vapor of solvents 106 Nevertheless, their materials could achieve only two stages ( colored and colorless ). In

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107 the present research we combine d traditional thermal responsive shape memory polymer s with periodic nanostructures to visualize shape changes in the nano scale, with the ultimate goal of fabricating multi stage shape memory sensors. Inspired by Lendlein and coworkers who reported a mult i stage temperature memory polymer actuator based on a crosslinked copolymer network with a broad melting temperature range, 107 we designed a polycaprolactone (semi crystalline) based crosslinked polymeric photonic crystal system. In order to better understand the working mechanism of the photonic crystal system, the relationships between the crosslink density and reflection color w er e studied The responsiveness of the photonic crystal system to external stimuli, including temperature, solvent, and vapor pressure was also investigated 5 .2 Experimental Methods 5 .2.1 Materials Polycaprolactone (PCL) diol with a molecular weight of 200 0 g/mol acryloyl chloride (96%), trimethylolpropane triacrylate (TMPTA) and triethylamine (TEA) (+99%) were purchased from Sigma Aldrich. Darocur 1173 (2 hydroxy 2 methyl 1 phenyl 1 propanone) was purchased from BASF. All chemicals or solvents were used w ithout further purification. 5 .2.2 Instrumentation SEM imaging was carried out on a n FEI XL 40 FEG SEM. A 15 nm thick gold layer was sputtered onto the samples before imaging. Normal incidence optical reflection spectra were obtained using an Ocean Optics HR4000 high resolution vis NIR spectrometer with a n R600 7 reflection probe and a tungsten halogen light source (LS 1). Absolute reflectivity was calculated as the ra tio of the sample spectrum and a

PAGE 108

108 reference spectrum, which was the optical density from a silicon wafer covered by a 1000 nm thick aluminum film by sputtering. 1 H NMR spectra were recorded on a Varian Inova2 500 MHz NMR spectrometer, using the residual sol vent signal as the reference. Infrared spectra were collected on a Thermo Nicolet 5700 FT IR spectrometer equipped with a single bounce, diamond stage attenuated total reflectance (ATR) accessory. Nanoindentation tests were performed on an MFP 3D Nanoinden ter (Asylum Research, Inc.) using a spherical sapphire indenter (tip radius ~125 m). Dynamic mechanical analysis (DMA) was carried out using a Q800 (TA Instrument s ) to determine the modulus with film tension clamp. Samples were heated from 25 to 250 C at a heating rate of 3 C/min with a frequency of 1 Hz. Differential scanning calorimetry (DSC) was (TA Instrument s ). 5 .2.3 Synthesis of P oly caprolactone Diacrylate (PCLDA ) PCLDA were synthesized following a previously reported method 108 and the main steps are shown in Figure 5 1. Briefly, PCL diol (10 g 5.0 mmol M W =2000 g/mol ) was dissolved in 100 m L of toluene in a 250 mL round bottomed flask. Then, TEA (1.6 mL 11 mmol) was added to the flask, and acryloyl chloride (0.9 0 mL 11 mmol) was subsequently added dropwise followed by stirr ing for 3 h at 110 C. Afterwards, t he reaction mixture was condensed by rota ry e vap oration PCLDA was obtained by dropping the condensed mixture into an excess of n hexane. Finally, the precipitated PCLDA was dried at 50 C in a vacuum oven for 24 h. The product was characterized by FT IR and 1 H NMR with > 95% functionalization and 80% yield.

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109 Figure 5 1. Synthe ti c scheme for PCLDA. 5 .2.4 Preparation of PCLDA C ontaining Films PCLDA and TMP TA were mixed in different ratios and the temperature of the mixture was elevate d to 55 C to melt the PCLDA. Photo initiator (2 hydroxy 2 methyl 1 phenyl 1 propanone 1 wt%) was then added to the mixture. The films were prepared by injecting the macromer mixtures into a Teflon mold covered with a photonic crystal template. UV cur ing at 360 nm was conducted for 2 h. The films were then etched with 2% HF aqueous solution for a period between overnight to 3 days to remove the silica m i crospheres as shown in Figure 5 2

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110 Figure 5 2. Scheme for mesoporous film preparation. 5 .3 Results and Discussion 5 .3.1 Synthesis of PCLDA Polycaprolactone diacrylate (PCLDA) was synthesized following a method previously reported in literature 108 as shown in Figure 5 1. The synthesized PCLDA was characterized by 1 H NMR and FT IR spectroscopy As shown in Figure 5 3, 1 H NMR spectroscopy confirmed the production of the vinyl resonance of PCLDA (CH 2 CH 6.32 range (g)) after esterification, whereas the other resonances from the PCL diol segments ( OCH 2 (CH 2 ) 4 OCH 2 CH 2 O ) were preserved. By comparing the integration of g and f >95% of the terminal hydroxyl groups in PCL diol w ere converted to acrylate groups. FT IR spectroscopy ( Figure 5 4 ) further supported the successful esterification of the PCL diol as evidenced by a reduction in int ensity of the peaks assigned to the hydroxyl groups

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111 3400 cm 1 ) and the appearance of two peaks assigned to vinyl groups ( 162 0 cm 1 and 81 0 cm 1 ) Figure 5 3. 1 H NMR spectra of PCL diol and PCLDA (CDCl 3 ).

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112 Figure 5 4. FT IR spectra of PCL diol (blue) and PCLDA (orange). 5 .3.2 Synthesis o f M es oporous Films Figure 5 5 Cartoon illustration of the m ain step s in the preparation of microporous polymeric photonic crystals. The photocurable mixture (monomers and initiator) was applied t o the interstitial space of a silica colloidal crystal template. After a 2 h UV irradiation, the silica content was removed by HF etching to achieve the mesoporous polymeric photonic crystal film. The general procedure to prepare the polymeric photonic crystal films is illustrated in Figure 5 5. The colloidal crystal templates were assembled by a convective self assembly technology using silica m i crospheres with diameters ranging from 300 to

PAGE 113

113 400 nm. 109 The n the interstitial air between the silica microspheres was replaced by viscous oligomer mixtures of PCLDA and trimethylolpropane triacrylate (TMPTA) with varying molar ra tios from 65:35 to 15:85 and 1 wt% photo initiator for all films The photo curable mixture was then irradiated under UV light (360 nm) at ambient conditions for 2 h. After curing, the silica microspheres inside the template were selectively dissolved in a 2 vol% hydrofluoric acid aqueous solution, leaving behind a freestanding mes oporous membrane with periodic arrays of m es opores, as depicted in Figure 5 5 The diameter of the starting silica microspheres in the colloidal crystal template determine d the si ze of the templated m es opores, which in turn determine d the reflection color of the final mesoporous photonic crystal film 5 .3.3 Change in Reflection Color b y Tuning the Crosslink density o f Films Prepared from 300 nm Templates Table 5 1. The composition of films prepared from 300 nm templates Entry PCLDA (mol%) TMPTA (mol%) 1 30 70 2 35 65 3 40 60 4 50 50 To the best of our knowledge, the relationships between crosslink density and film properties for thermoset based photonic crystals, especially optical activity, has not been investigated In this work, the crosslink density of the film was found to be critical for the optical properties of the mesoporous photonic crystal film prepared with a 300 nm colloidal crystal template. To contro l the crosslink density, two monomers with

PAGE 114

114 varying ratios were used: PCLDA with a molecular weight of 2000 g/mol as a difunctional monomer and TMPTA with a molecular weight of 296 g/mol as a trifunctional monomer. The crosslink density of the networks decr eases with decreasing concentration of TMPTA (i.e., increasing PCLDA content ). From the tensile modulus of the films at the rubbery plateau ( characterized by DMA), the crosslink density ( ) of the films could be calculated according to the equations ( Equation 5 1 to 5 2). 110 111 is the molecular weight between crosslinks, is the density of the film (1.16 0 .02 g/mL), is the universal gas constant (8.314 ), is the absolute temperature at which the modulus is determined (423 K in this work), is the storage modulus of the film at rubbery plateau, and i s the crosslink density. The results are summarized in Table 5 2. As expect ed the crosslink density decreases as the PCLDA content increased. But the crosslink density calculated for a ll films was higher than expected, which need more investigation to confirm the cause of the increase. (5 1) (5 2)

PAGE 115

115 Table 5 2. DMA results of films with varying crosslink density Entry PCLDA (mol%) TMPTA (mol%) Modulus (MPa) M c (g/mol) (mol/mL) 1 15 85 88.58 138 0.0084 2 30 70 49.03 250 0.0046 3 35 65 45.46 269 0.0043 4 37.5 62.5 36.58 335 0.0035 5 40 60 29.24 419 0.0028 6 45 55 25.11 487 0.0024 7 50 50 20.32 602 0.0019 Figure 5 6. Images of films (from left to right corresponding to the composition from entry 1 to 4 in Table 5 1).

PAGE 116

116 Figure 5 7 Ref le ctance spectra of films created from 300 nm templates with varying PCLDA content (inversely related to crosslink density). The reflection colors of films were found to be highly dependent on the crosslink density of the films, changing from green (30% PCLDA) to cyan (35% PCLDA) to blue (40% PCLDA) to colorless (50% PCLDA) (Figure 5 6) with decreasing crosslink density (Table 5 1). The reflectance behavior of the films was characterized by optical spectroscopy (Figure 5 7). The reflectance spectra underwent a continuous blue shift as the PCLDA content increased ( i.e. decreasing crosslink density). With a higher crosslink densi ty (30% PCLDA or less) the reflection color of the films was all green, which indicated the pores are fully open. In the range of 30% to 50% PCLDA content, the reflection color of the films was very sensitive to the crosslink density, which showed a blue shift of the reflection color with a decreasing crosslink density. While keep increasing PCLDA content to equal and above 50%, the reflection color of films disappeared, which indicated the collapsing of the porous structure on the film surface.

PAGE 117

117 We hypothesized that with a higher crosslink density, there are more fixed net points around the mesopores and that the increa sed stiffness allows the open pore structure to be preserved over the course of removing the template and subsequently processing the films. As the crosslink density decreases, there are fewer and fewer net points to hold the pore structure leading to collapse of the pores and ultimate loss of the periodicity of the porous structure of the film. the Cross sectional SEM images of the fil ms shown in Figure 5 8 further confirmed our hypothesis that the observed color change was due to the pore size change (or corresponding to d spacing change). Figure 5 8 Cross sectional SEM images of the films with different PCLDA percentages

PAGE 118

118 To fur ther investigate the unusual behavior of these photonic crystal films, t he modulus at the film surface (pore region) and in the bulk ( beneath pore region) film were characterized by nanoindentation and the results are shown in Figure 5 9. In this test, th e indenter compresses the surface of the film to 0.5 depth. The red dots in Figure 5 9 represent the storage modulus measured from the region without pores, whereas, the black dots are measured from the region with porous structures. It is seen that the storage modulus of the bulk films de creased following the same trend as the DMA results, with increasing PCLDA content (decreasing crosslink density) However, the storage modulus in the porous region remained one magnitude lower and relatively stable probably because most of the region is filled with air. Because the modulus in the porous region did not change much throughout all film compositions, we concluded that the crosslink density is the key to maintaining the pores structure and tuning the de gree of pore opening, which in turn control s the reflection color

PAGE 119

119 Figure 5 9 Surface m odulus of films with varying PCLDA content characterized by nanoindentation. Red dots stand for the storage modulus in the bulk region (no pores), which decrease wit h decreasing crosslink density. And the black squares represent the storage modulus in the porous region, which is relatively stable.

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120 5 .3.4 Change of the T uning R ange for F ilms P repared with 350 nm T emplates Figure 5 10 Reflectance spectra of films with varying PCLDA content prepared from 350 nm templates. The reflectance wavelength blue shifted with increasing PCLDA content (decreasing crosslink density). To further confirm that the color range can be tuned by crosslink density, f ilms with a larger pore size were prepared and tested by starting with m icr osphere diameters of 350 nm. T he optical spectra in Figure 5 10 show that the reflection wavelength undergoes a blue shift with decreasing crosslink density, as expected. The refle ction color of the film was orange when the pores we re fully open ( Figure 5 11) and shift ed to greenish and no color with decreasing crosslink density

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121 Figure 5 11 Images of films with 350 nm template (the films from left to right contain 30%, 40%, 50 %, 60% and 65% PCLDA, respectively) SEM imaging was also used to confirm that the blue shift of the reflection color is due to the d spacing change. From the SEM images of the cross sections of films, we can tell that the porous structure is gradually collapsing in films with decreasing crosslink density (as shown in Figure 5 12). The modul i of the films at the rubbery plateau from DMA are presented i n Table 5 3 The modulus decrease d with in creasing PCLD A percentage in agreement with the feed ratio. From the nanoindentation results shown in Figure 5 13 the modulus in the bulk region decrease s with decreasing crosslink density and the modulus in the porous region remains low and relatively stable which followed the same trend as the films prepared by 300 nm templates But a higher value of modulus in pore region was found, when compared with films made by 300 nm templates. We also note that, with 35 0 nm templates, a lower crosslink density limit is required to maintain the porous structure (65% PCLDA), when compare d with 300 nm templates (50% PCLDA). We attributed this to the larger sized templates that lead to larger interstitial spaces resulting i n more polymer in the pore walls and, therefore, greater rigidity and preservation of the expanded pore structure

PAGE 122

122 Figure 5 12 Cross sectional SEM image s of films with varying PCLDA content prepared from 350 nm template.

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123 Table 5 3 DMA results of films created from 350 nm template s with varying crosslink density Entry PCLDA (mol%) TMPTA (mol%) Modulus (MPa) M c (g/mol) (mol/mL) 1 30 70 49.03 250 0.0046 2 35 65 45.46 269 0 .0043 3 40 60 29.24 419 0.0028 4 45 55 25.11 487 0.0024 5 50 50 20.32 602 0.0019 6 55 45 12.35 991 0.0012 7 60 40 9.06 1351 0.0009 8 65 35 6.96 1758 0.0007

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124 Figure 5 13 Modulus of films with different PCLDA% prepared from 350 nm template characterized by nanoindentation. Black squares stand for the storage modulus in the bulk region (no pores), which decrease with decreasing crosslink density. And the red dots represent the storage modulus in the porous region, which is relatively stable. 5 .3.5 Thermal Mechanical Responsi veness o f PCL Based Polymer Films Based on the results from these fundamental stud ies on the effect of crosslink density on the pore size behavior of the mesoporous films, a film with 25 mol% of PCLDA formed using 300 nm template ( below the crosslink densi ty limit 50 mol% PCLDA ), was prepared for the responsiveness study. The optical spectra and a photograph of the original film are shown in Figure 5 14 The image indicates that the film has a green reflection color The differential scanning calorimetry (D SC) plot in Figure 5 15 show s that the T g of the copolymer network is around 41 C and the melting temperature is around 42 C. Based on the DSC plot, 70 C was chosen as the progra m ming temperature, which is above all transitions (chain mobility limited only by crosslink s ). Room temperature, which is between the T g ( 41 C) and the melting

PAGE 125

125 temperature (42 C), was chosen to lock the first stage of shape memory process (partial chain mobility). Liquid nitrogen cooling, which is below T g ( 41 C), was used to lock the second stage of temperature memory (all chain s locked). Figure 5 14 Optical spectra and photograph of the film containing 25 mol% PCLDA (300 nm template)

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126 Figure 5 15 DSC plot of the film with 25 mol% PCLDA (endothermal up) Figure 5 16 Schematic of the progra m ming process for the shape memory film Note the film had different temperature s ( T ) in different stages. Typical progra m ming temperature ( T p ) and pressure (P) were 70 C and 7 kPa respectively. The progra m ming proce ss of the film is shown in Figure 5 16 The film was first kept at 70 C for 3 h on a thermal pres s with a pressure of 7 kPa and then naturally

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127 cooled to room temperature. Figure 5 17 shows the photographs and SEM images before and after the thermal press treatment. From the photographs it is seen that the compression treatment caused the reflection color to shift from green to cyan which is attributed to the pore size change as witnessed by the SEM images. Correspondingly, a blue shift was observed in the optical spectr um of the film after the press treatment ( Figure 5 18 ). After being kept at 70 C overnight (~16 h) on the press m achine with a pressure of 7 kPa, the film was briefly dipped in liquid nitrogen T he reflection color was colorless imme diately after film removal from liquid nitrogen, which is in agreement with the minimum reflection intensity seen from the spectrometer (Figure 5 18). As shown in Figure 5 17, the cyan reflection color first started to recover from the edges of the film (w ithin 30 s of removal from liquid nitrogen). Full recovery to cyan color was observed in 30 min. Correspondingly, the reflection spectra of the fully recovered film with cyan color is similar to the film treated at 70 C for 3 h, which also had cyan color. I t is interesting that the recovered film change back to the original green reflection color even after further treatment at 70 C for 3 h. When the recovered film was further treated at 70 C for 48 h, the film changed from cyan to colorless permanently The cross sectional SEM image suggested that the photonic crystal str ucture was permanently damaged, which was due to collapse of the polymer mainframe under extended heat exposure.

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128 Figure 5 17 Photograph s and cross sectional SEM images of the films before and after thermal progra m ming From left to right, the original film having green reflection color, where the pores were fully open as confirmed by SEM image; after pressing with 7 kPa at 70 C and cooled to room temperature, the film showed a cyan reflection color ; the film with cyan reflection color was then pressed with 7 kPa at 70 C overnight and cooled in liquid nitrogen, when the film turned to colorless and quickly recovered to cyan. T he pores collapsed as the film temperature rose per the SEM image

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129 Figure 5 18 Ref le ctance spectra of the film before and after thermal progra m ming The reflection color was characterized by optical spectroscopy. The o riginal film showed green reflection color (green trace); after pressed with 7 kPa at 70 C for 3 h, the reflecti on wavelength blue shifted (cyan trace); after removal from liquid nitrogen the film showed no reflection color (orange trace) and quickly rec overed to cyan color (blue trace)

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130 5 .4 C onclusion In this chapter, photonic crystals of varying crosslink density were prepared and t he relationship s between the crosslink density and the reflection color of the photonic crystal films with different pore sizes w as investigated By decreasing the crosslink density of the film, the reflection color could be changed from green to cyan, to blue, or to color less using 300 nm templates. The reflection color coul d be tuned from orange to yellow, to green, to color less using 350 nm templates. It was found that with a larger pore size, which also provided a larger interstitial volume that more polymers could be fit into the space to increase the local rigidity, a lo wer crosslink density was required to preserve the pore structure The thermal mechanical responsiveness of PCL based polymer films was also investigated The reflection color changed from green to cyan and lastly to colorless upon continued pressure tre atment at 70 C, which was induced by the change of pore size as evidenced by cross sectional SEM images. This fundamental study demonstrated a practical approach to tuning the reflection color of photonic crystals with multiple color stages. This behavior differs significan tly from the limited wavelength shift exhibited by traditional tunable photonic crystals.

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131 CHAPTER 6 C ONCLUSIONS AND FUTURE WORK Stimuli responsive polymers have garnered tremendous interest from researchers worldwide In this dissertation, several examples of responsive materials with different architectures were developed and characterized. In t he NIR dissociable star polymer system, rapid release of the payload was triggered by irradiation under conditions that would, in prac tice, be noninvasive and provide spatial control. Future studies should consider the encapsulation of clinically relevant drug formulations and consideration of the potential biocompatibility and release behavior of the star based carrier system in vivo T he reversible transitions and permanent fixation enabled by two competitive DA reactions were investigated in the organogel systems and bulk polymer network systems. In the organogel systems, by tuning the maleimide crosslinker functionality, gel to sol tr ansition and gel expansion were achieved. In the bulk networks, the re mendable and permanent ly crosslinked materials with varying T g s were demonstrated. However, the triggering temperature required is too high for many applications. In the future, material s which transform at a lower temperature or respond to other stimuli need to be explored. In the end, the fundamental study of photonic crystals indicates a blue shift of reflection wavelength with decreasing crosslink density of the material, whic h provides another approach to control the reflection color of photonic crystals. A multi stage shape memory sensor was prepared and the reflection color could be tuned from green (original) to cyan (stage 1) and to color less (stage 2) by thermal programm ing. However, the reason why the film cannot recover from stage 1 to the original stage remains unclear and needs more investigation in the future.

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141 BIOGRAPHICAL SK ETCH Yuqiong Dai was born in Hebei, China. Growing up with much love and curiosity Engineering from Xiamen University. Her childhood dream of becoming a scientist was s trengthened at the beautiful campus of Xiamen University, and she decided to chase that dream further in the United States. She was admitted to the graduate program in the Department of Materials Science and Engineering at University of Florida (UF) in 201 program in chemistry and joined a wonderful group of researchers in the UF chemistry department, where she worked on the design, synthesis and characterization of stimuli respons ive polymers. The internship in Intel in summer 2017 led her to seek a job in the semiconductor industry. Yuqiong will start working for Applied Materials in California shortly after completing her PhD in December 2018.