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Effects of Crosslinker and Filler on the Mechanical Properties of 1,2,3-Triazole-Polymers as Potential Rocket Propellant...

Permanent Link: http://ufdc.ufl.edu/UFE0024409/00001

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Title: Effects of Crosslinker and Filler on the Mechanical Properties of 1,2,3-Triazole-Polymers as Potential Rocket Propellant Binders
Physical Description: 1 online resource (59 p.)
Language: english
Creator: Wang, Ling
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: 1, crosslink, crosslinking, filler, mechanical
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Crosslinked Triazole polymers are potential novel binders for high-energy explosives and rocket propellants. 1,2,3-triazole linkages can be realized by Huisgen 1,3-dipolar cycloaddition of alkynes and azides. The reaction of E300 dipropiolate with diazide from tetraethylene glycol was selected to study the effects of crosslinker and filler on the mechanical properties of resulting triazole polymers. The modulus of the polymers increases while the strain decreases with increasing crosslinker concentration and functionality. However, the effects of crosslinker functionality are getting much less at functionality higher than 6. Addition of filler also increases the modulus but decreases the strain. The increase in elastic modulus of resulting triazole polymers due to the addition of aluminum powder could be well predicted by the equation of Guth and Smallwood. The resulting triazole polymers showed that the desired mechanical properties could be obtained by selecting an appropriate crosslinker and tuning the concentration of the crosslinker and filler. The mechanical properties of these triazole polymers are comparable to typical polyurethane elastomeric matrices for rocket propellants. Thus, the triazole polymers are of potential application as novel rocket propellant binders.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: 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.
Statement of Responsibility: by Ling Wang.
Thesis: Thesis (M.S.)--University of Florida, 2009.
Local: Adviser: Duran, Randolph.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024409:00001

Permanent Link: http://ufdc.ufl.edu/UFE0024409/00001

Material Information

Title: Effects of Crosslinker and Filler on the Mechanical Properties of 1,2,3-Triazole-Polymers as Potential Rocket Propellant Binders
Physical Description: 1 online resource (59 p.)
Language: english
Creator: Wang, Ling
Publisher: University of Florida
Place of Publication: Gainesville, Fla.
Publication Date: 2009

Subjects

Subjects / Keywords: 1, crosslink, crosslinking, filler, mechanical
Chemistry -- Dissertations, Academic -- UF
Genre: Chemistry thesis, M.S.
bibliography   ( marcgt )
theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
born-digital   ( sobekcm )
Electronic Thesis or Dissertation

Notes

Abstract: Crosslinked Triazole polymers are potential novel binders for high-energy explosives and rocket propellants. 1,2,3-triazole linkages can be realized by Huisgen 1,3-dipolar cycloaddition of alkynes and azides. The reaction of E300 dipropiolate with diazide from tetraethylene glycol was selected to study the effects of crosslinker and filler on the mechanical properties of resulting triazole polymers. The modulus of the polymers increases while the strain decreases with increasing crosslinker concentration and functionality. However, the effects of crosslinker functionality are getting much less at functionality higher than 6. Addition of filler also increases the modulus but decreases the strain. The increase in elastic modulus of resulting triazole polymers due to the addition of aluminum powder could be well predicted by the equation of Guth and Smallwood. The resulting triazole polymers showed that the desired mechanical properties could be obtained by selecting an appropriate crosslinker and tuning the concentration of the crosslinker and filler. The mechanical properties of these triazole polymers are comparable to typical polyurethane elastomeric matrices for rocket propellants. Thus, the triazole polymers are of potential application as novel rocket propellant binders.
General Note: In the series University of Florida Digital Collections.
General Note: Includes vita.
Bibliography: Includes bibliographical references.
Source of Description: Description based on online resource; title from PDF title page.
Source of Description: 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.
Statement of Responsibility: by Ling Wang.
Thesis: Thesis (M.S.)--University of Florida, 2009.
Local: Adviser: Duran, Randolph.
Electronic Access: RESTRICTED TO UF STUDENTS, STAFF, FACULTY, AND ON-CAMPUS USE UNTIL 2010-05-31

Record Information

Source Institution: UFRGP
Rights Management: Applicable rights reserved.
Classification: lcc - LD1780 2009
System ID: UFE0024409:00001


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1 EFFECTS OF CROSSLINKER AND FILLER ON THE MECHANICAL PROPERTIES OF 1,2,3 T RIAZOLE POLYMERS AS POTENTIAL ROCKET PROPELLANT BINDERS By LING WANG A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PART IAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2009

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2 2009 Ling Wang

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3 To my family

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4 ACKNOWLEDGMENTS First, I would like to thank my advisor Randolph. S. Dur an for his support and guidance through the past few years. My thanks also go to Dr. Alan R. Katritzky and my group members for NAVY project: especially Yuming S ong Reena Gyanda, and Rajeev Sakhuja for their kindness and helpful discussion. I also thank D r. David A. Ciaramitaro and Dr. Clifford D. Bedford for their help and guidance I also appreciate the help from Dr. Anthony B. Brennans group in Materials Science and Engineering department, especially Dave Jackson for he lping me run mechanical tests. I n addition, I would like to thank all the group members in Durans group for their support. Lastly, I wish to thank my parents and my husband for their selfless love. They never stopped encouraging and supporting me during my most difficult times.

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS .................................................................................................................... 4 LIST OF TABLES ................................................................................................................................ 7 LIST OF FIGURES .............................................................................................................................. 8 ABSTRACT ........................................................................................................................................ 11 CHAPTER 1 INTRODUCTION ....................................................................................................................... 12 Solid Composite Propellant Binders .......................................................................................... 12 Triazole polymers ........................................................................................................................ 13 Factors that affect mechanical properties of rocket propellants ............................................... 16 Effect of filler ....................................................................................................................... 16 Effect of crosslinking of binder .......................................................................................... 20 Thesis overview ........................................................................................................................... 24 2 EXPERIMENTAL AND OPTIMIZATION OF POLYMERIZATION ................................. 25 Experimental ................................................................................................................................ 25 General ................................................................................................................................. 25 Method of manual testing strain and modulus ................................................................... 26 General procedures for preparation of unfilled linear triazole polymer mini samples .... 26 General procedures for preparation of crosslinked triazole polymer mini samples ........ 27 General procedures for preparation of dogbone samples .................................................. 29 Optimization of polymerization ................................................................................................. 29 Selection of monomors ........................................................................................................ 29 Stoichiome try effect on the mechanical properties of triazole polymers ......................... 30 Summary ...................................................................................................................................... 31 3 EFFECT OF CROSSLINKER ON THE MECHANICAL PROPERTIES OF UNFILLED TRIAZOLE POLYMERS ..................................................................................... 33 Effect of crosslinker concentration ............................................................................................ 33 Effect of crosslinker functionality .............................................................................................. 36 Conclusion ................................................................................................................................... 38 4 EFFECT OF FILLER ON THE MECHANICAL PROPERTIES OF TRIAZOLE POLYMERS ................................................................................................................................ 43 Effect of filler type ...................................................................................................................... 43 Effect of aluminum filler content ............................................................................................... 45

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6 Conclusion ................................................................................................................................... 51 5 SUMMARY AND FUTURE WORK ....................................................................................... 53 Summary ...................................................................................................................................... 53 Future work .................................................................................................................................. 54 Optimization of fillers size and composition .................................................................... 54 Optimization of plasticizers ................................................................................................ 54 Preparation and characterization of hig h filled triazole polymers .................................... 54 LIST OF REFERENCES ................................................................................................................... 56 BIOGRAPHICAL SKETCH ............................................................................................................. 59

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7 LIST OF TABLES Tab le page 2 1 Effect of stoichiometry on the mechanical properties of triazole polymers (data from mini samples) .......................................................................................................................... 32 3 1 S train and modulus of unfilled triazole polymers (data from dogbone samples) .............. 35 3 2 Strain and modulus values of crosslinked triazole polymers having different crosslink functionality (data fro m dogbone samples) .......................................................... 41 4 1 Triazole polymers with different fillers (mini samples) ...................................................... 43 4 2 Effect of Al filler content on the mech anical properties of triazole polymers (Strain and modulus data from dogbone samples) ........................................................................... 46 4 3 Strain and modulus of filled triazole polymers (43wt%Al, data from dogbone samples) .................................................................................................................................. 48 4 4 Experimental and predicted values of the modulus of filled triazole polymers (27 vol% Al) .................................................................................................................................. 51

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8 LIST OF FIGURES Figure page 1 1 Urethane linkage and polyurethane formation ..................................................................... 13 1 2 Triazole linkage and1,2,3 triazole polymer formation ....................................................... 14 1 3 Crosslinked triazole polymer formation in Reeds group .................................................... 15 1 4 Crosslinked triazole polymer formation in Manzaras group.............................................. 15 1 5 Variation trend of e longation at break of HDPE -A1 composite versus volume percent of Al ........................................................................................................................................ 19 1 6 The variation trend of m odulus of elasticity of HDPE -A1 composite versus volume percent of Al ........................................................................................................................... 19 1 7 Formation of crosslinking ...................................................................................................... 20 1 8 Variation trend of modulus of the HTPB based polyurethane elastomer with i ncreasing triol concentration ................................................................................................ 21 1 9 Variation trend of elongation at break of the HTPB -based polyurethane elastomer with increasing triol concentration ........................................................................................ 21 1 10 Scheme of a hexafunctional network formation................................................................... 23 1 11 Tensile stress -strain curves for the three different functionalities ...................................... 24 2 1 Dimension of dogbone mold ................................................................................................. 25 2 2 Method of manual testing strain.. .......................................................................................... 26 2 3 Method of manual tes ting modulus.. ..................................................................................... 27 2 4 Formation of crosslinked triazole polymers with different crosslinkers ............................ 28 2 5 Formation of different triaz ole polymers and their properties ............................................ 28 2 6 Dogbone mold with unfilled and filled samples .................................................................. 29 2 7 Triazole polymer formation at diff erent stoichiometry ....................................................... 30 2 8 Reaction of pure acetylene ..................................................................................................... 32 2 9 Reaction of diacetylene and diazide at 1:1 ratio................................................................... 32 3 1 Formation of triazole polymers with varied crosslinker concentration .............................. 33

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9 3 2 TGA curve of unfilled crosslinked triazole polymer (with 16mol% tetra propoilate) ..... 33 3 3 Effect of crosslinker concentration on the mechanical properties of unfilled triazole polymers (data from mini samples) ....................................................................................... 34 3 4 Effect of crosslinker concentration on the mechanical properties of unfilled triazole polymers (data from dogbone samples) ................................................................................ 35 3 5 Crosslinkers with different function ality f (f =3, 4, 6, 16, 32, 64) ....................................... 36 3 6 Formation of unfilled triazole polymers with different crosslinkers.. ................................ 37 3 7 Effect of crosslinker functionality on the strain of triazole polymers (data from mini samples) .................................................................................................................................. 39 3 8 Effect of crosslinker functionality on the modulus of triazole polymers (data from mini samples) .......................................................................................................................... 39 3 9 Effect of crosslinker functionality on the strain of triazole polymers (data from dogbone samples) ................................................................................................................... 40 3 10 Effect of crosslin ker functionality on the modulus of triazole polymers (data from dogbone samples) ................................................................................................................... 40 3 1 1 Stress -strain curves of the polymers having different crosslink functionality (data from dogbone sampl es) .......................................................................................................... 41 4 1 Formation of triazole polymers with different fillers ........................................................... 43 4 2 Mini samples of triazole polymers filled with NH4NO3 wit h and without degassing ....... 44 4 3 Triazole polymers filled with different content of aluminum powder ................................ 45 4 4 Formation of triazole p olymers with different aluminum content ...................................... 45 4 5 Stress -strain curves of triazole polymers with different aluminum content ....................... 46 4 6 Formation of filled triazole polymers with different crosslinker concentration ................ 47 4 7 Effect of crosslinker concentration on the mechanical properties of filled triazole polymers (43wt% Al, da ta from mini samples) ................................................................... 48 4 8 Effect of crosslinker concentration on the mechanical properties of filled triazole polymers (43wt% Al, data from dogbone samples) ............................................................. 49 4 9 Stress -strain curves of filled triazole polymers with increasing crosslinker concentration (4mol%14mol%) ........................................................................................... 49

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10 4 10 Comparison of mechanical properties of unfilled and filled (with 43% Al) triazole polymers (data from dogbone samples) ................................................................................ 50 4 11 Comparison of predicted modulus and experimental modulus. .......................................... 52

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11 Abstract o f Thesis Presented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Master of Science EFFECTS OF CROSSLINKER AND FILLER ON THE MECHANICAL PROPERTIES OF 1,2,3 T RIAZOLE POLYMERS AS POTENTIAL ROCKET PROPELLANT BINDERS By Ling Wang May 2009 Chair: Randolph S. Duran Major: Chemistry Crosslinked Triazole polymers are potential novel binders for high energy explosive s and rocket propellants. 1,2,3 -triazole linkages can be realiz ed by Huisgen 1,3 -dipolar cycloaddition of alkynes and azides. Th e reaction of E300 dipro piolate with diazide from tetraethylene glycol was selected to study the effects of crosslinker and filler on the mechanical properties of resulting triazole polymers The modulus of t he polymers increases while the strain decreases with increa sing crosslinker concentration and functionality. However, the effects of crosslinker functionality are getting much less at functionality higher than 6. Addition of filler also increases t he modulus but decreases the strain The increase in elastic modulus of resulting triazole polymers due to the addition of aluminum powder could be well predicted by the equation of Guth and Smallwood. The resulting triazole polymers showed that the desired mechanical properties co uld be obtained by selecting an appropriate crosslinker and tuning the concentration of the crosslinker and filler The mechanical properties of these t riazole polymers are comparable to typical polyurethane elastomeric matri ces for rocket propellants Thus, the triazole polymers are of potential application as novel rocket propellant binders.

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12 CHAPTER 1 INTRODUCTION Solid Composite Propellant Binders Solid rocket propellants have two ty pes: double -base and composite.1 Composite solid propellant contains three basic components: ( I ) a fuel which is an o rganic polymer and also serves as a binder ; ( II) an oxidizer such as ammonium per c hlorate or ammonium nitrate; ( III) various additives such as metallic filler, plasticizers, wetting agents, and stabilizers, for improving propellant processability, efficiency or physical properties.1,2 A ll solid ingredie nts are distributed uniformly in a matrix provided by the polymeric binder. Various organic polymers have been used as binders such as polyesters, polysulphides, polyurethanes, polybutadienes, polyvinyl chloride, synthetic rubbers, epoxy resins and acrylat es.1P olyurethane binders are widely used in modern rocket propellants due to their excellent mechanical properties, shrink free low temperature controlled cure and acceptable compatibility with metal powders and oxides.3 Polyurethanes are organic polymers with urethane linkages, which are formed from the reaction of di or multifunctional isocyanates with polymeric diol s (Figure 1 1) However, they have several drawbacks : First, isocyanate is toxic Second, both reactants are moisture sensitive: i socyanate can react with water and hydroxyl group s includ ing even trace amount of moisture in t he binder or other ingredients .4 Thi rd, p olyurethane rocket propellant binders can undergo side reactions during and after polymerization that degrade the mechanical properties of the resulting propellant, e.g., loss of elasticity .3Last, t he process of preparing polyurethane binders also presents environmental concerns. Their formulations, once mixed, must be cured quickly or side reac tions will degrade the ultimate properties of resulting polymer. Excess material cannot be reused and must be disposed of. Consequently, not only the environmental burden is increased but the cost of

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13 disposal thes e hazardous energetic materials.4Therefore, new polymer propellant binders that are effective, economical, environmentally fri endly and compatible with other ingredients are desired .4 1. Urethane formation 2. Polyurethane formation Figure 1 1 U rethane linkage and polyurethane formation Triazole polymers Triazole polymers are novel macromolecules that have received growing interest in the area s of polymer, material, and surface science.5 6 T hey can be used as high energy binders for explosives or rocket propellants, destructible adhesives, or other high energy and destructible products.7 T he t riazole linkage can be synthesized by 1,3 -dipolar cycloaddition of azide s with terminal alkyne s following Cli ck C hemistry ( Figure 1 2 ), which has been explored as an efficient tool for the synthesis of functionalized monomers8, polymers9and post -polymerization modification of polymers ,5 10 such as the synthesi s of dendrimers, dendronized polymers,11 12 biohybrid amphiphiles,13 chromophores,14 conjugated polymers,15 block copolymers16 17, glyco polymers,18 macrocyclic polymers,19 and adhesives.20 1. Triazole formation

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14 2. T riazole polymer formation Figure 1 2 T riazole linkage and1,2,3 triazole polymer formation 1,2,3 triazole p olymers have been studied recently as novel bi nders for explosives and high energy propellant s .3 21, 22 Compare d to polyurethane linkages, p olymerization through thermally and chemically stable triazole linkages proceeds readily and the components of the triazole cure (ethynyl groups and azido groups) react preferentially with each other3, which largely avoids the possibility of side reactions. The reaction between acetylenes and azides is insensitive to mois ture, and no special precautions are needed for preventing moisture from air or other ingredients.7 The r eactions of certain azid e s and acetylenes such as acetylenes having long carbon chain s between end groups proceed slowly at the temperature between 100140oF ((38 60C)), temperatures below which the mixture of acetylenes and azides can sit at least one week without significant reaction. One desi rable consequence of this is that excess mixed starting materials can be stored at ambient temperature (about 77 oF (25C) ) and reused the next day, which reduces the cost and environmental contamination.4 Reeds group reported the synthesis of cross linked triazole polymers as energetic binders with improved mechanical properties and st ability. The triazole polymers are formed from azido

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15 and azido nitrato polyether prepolymers crosslinked by diacetylenes without a catalyst. ( Figure 1 3 ) The resulting triazole binder is soluble in energetic plasticizers and has excellent mechanical proper ties like polyurethane s, but has improved compatibility wit h other propellant ingredients.3 Figure 1 3 Cross linked triazole polymer formation in Reeds group (adapted from reference 3) Crosslinked triazole polymers were also prepared as rocket propellant binders in Manzaras group by crosslinking azido polymers with a multifunctional dipolaroph ile having a reactive group selected from acrylic and acetylenic esters or amides (Figure 1 4), which enhanced the burn r ate of the crosslinked polymer.7 Figure 1 4 Cross linked triazole polymer formation in Manzara s group (adapted from reference 7) Huangs group synthesized and characterized a series of novel low -temperature curing and heat resistant polytriazole resins by the 1,3 -d ipolar cycloa ddition reaction of various acetylenes and azides as advanced composits. The curing reaction s and the properties of developed

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16 polytriazole resins such as decomposition temperature, tensile strength, modulus and elongati on at break were characterized.2325 P revious work in ou r group developed strategies fo r low temperature synthesis of oligo triazoles as binder ingredients.26 T riazole-cured polymers were prepared with v arious alkynes and azides without any solvent or copper catalysts under mild conditions near room temperature. The resulting triazole polymers were characterized by NMR spectroscopy, elemental analysis, and gel permeation chromatography. Five triazole polymers were selected as potential rocket propellant binders for future work.21 However; the mechanical properties of those triazole polymers were not quantified T he above work on triazole poymers was mainly focused on the polymer synthesis rather than characterization. Thus, the properties of novel traizole polymers need to be i nvestigated.5 Factors that affect mechanical properties of rocket propellants Mechanical proper ties of the propellants are important for rocket motors. Poor mechanical properties such as formation of cracks may lead to unstable combustion and m alfunctioning of rocket motors.1All the ingredients in the formulation can affect the mechanical properties of final propellant. A good propellant formulation should give a viscous mix that is castable and cures to a rubbery composite. The mechanical properties of rocket propellants can be modified by varyi ng the type and proportion of filler (solids) and plasticizer.27 The binder polymer s nature and crosslinking degree of binder also affect propellants properties. To predict the performance of propellants, a knowledge of the mechanical properties of unfilled and filled binder polymers is necessary.1 Effect of filler Fillers are usually used to improve the stre ngth of the polymer matrix, reduc e the creep and lower the cost. Generall y, fillers have higher elastic modulus than polymer matrix. The size,

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17 shape of filler particles and the content of filler can affect t he mechanical behavior of the resulting filled s ystem.28 Various mathe matical equations have been developed to predict the increase in the modulus of an elastomer resulting from the addition of fillers, most of these relationships originate from the theory of the viscosity of suspensions.29 The simplest theoretical form is Einsteins well known equation for the viscosity of a suspension.28,29 (Equation 1 1 Viscosity of fluid; 0: Viscosity of suspension The volume concentration of suspended particles ) This equation can also be used to describe a fillers effect on increasing the rigidity of the polymer matrix by substitutin g and 0 with E and E0, respectively (Equation 1 2 E: The elastic modulus of filled elastomer; E0: The elasti The volume conc entration of filler ) The Einstein equation does not consider the interaction of adjacent par ticles; and assumes that the filler is more rigid than matrix This expression is valid only at low filler concentration when there is a perfect adhesion between filler particles and polymer. The Einstein equation was developed by Guth and Smallwood to explain rubber reinforcement, which takes account of the interaction between a pair of filler particles. (Equation 1 3)28, 29 Another equation that has been widely used is the empirical Eilers -Van Dijck equation, (Equation 1 m is the maximum volume fraction of filler. (Equation 1 5) The m m varied with particle shape and state of m m is generally me asured by sedimentation technique and the values range from 0.31 to 0.85.30 1 1 1 2 1 3

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18 1 5 Nicholas and Freudenthal investigated the relationship between mechanical properties of polyurethane elastomer s with filler contents from 0 to 50% by volume. An equation was proposed for the increase in modulus due to the addition of filler. (Equation 16 ) Based on Equation 1 4, a more general form ( E q uation 1 7 ) was suggested to describe the increase in modulus of an elastomer resulting from the addition of more rigid filler, where m is dependent on each individual material and filler.29 1 6 1 7 The above equations (1 1 to 1 6) generally do not agree with experimental data well at very high concentration of filler because the suspensions become non-Newtonian in behavior, a nd the coefficient of viscosity is not constant .30 Compared to theories for the increase in modulus, the theory explaining the decrease in elongation at break due to the addition of rigid fillers is more complex and semiquantitative. Assuming there is a perfect adhesion between the phases, the following simple model developed by Nielsen predicts the elongation at break of filled system in a qualitative or semiqu antitative manner. (Equation 1 8 )28 30 1 8 Tavman inve stigated the mechanical properties such as tensile strength, elongation at break, and elastic modulus of aluminum powder -filled high density polyethylene composites 1 4

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19 experimentally in the range of filler content 0 50% by volume 31 With increasing filler content, the elongation at break of the filled polymer decreased (Figure 1 5); elastic modulus first increased but then decreased as filler content became higher than 30%, which may be attributed to the formation of cavities around filler particles during stretching in tensile tests (Figure 1 6) Figure 1 5 Variation trend of e longation at break of HDPE -A1 composite versus volume percent of Al (adapted from the data in reference 31) Figure 1 6 The variation trend of m odulus of elasticity of HDPE -A1 composite versus volume percent of Al (adapted from the data in reference 31)

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20 Effect of c rosslinking of binder The b inder polymer is critical for rocket propellant operation The mechanical proper ties of binder polymer are significantly influenced by their molecular structures such as chain length between the triazole.21 Cross linking is another structural feature that can drastica lly affect polymer s behaviors. Uncrosslinked polymers are randomly packed and entangled linear c hains.10 Adjacent polymer chains can be connec ted to form a vast molecular network by forming chemical bonds between them. These connections are called crosslinks. (Figure 1 7) The number of crosslinks per unit volume or length is called crosslink density. Crosslinkers are crosslinking reagents that l ink the linear polymer chains together to form network structure, which limits the mobility of the individual polymer chain. Accordingly, the mechanical properties of polymers, such as tensile strength, modulus and e lasticity, are changed by cross linking. The effect of crosslink density on the mechanical properties of rubbery polymers has been studied extensively and well understood.32 Increasing crosslinker density, usually, increases a polymers strength and modulus but decr eases elongation to failure .32 Besides natural rubber, the relationship between crosslinker density and mechanical properties has been studied on various pol ymers such as polyurethanes,32,33 hydrogels ,34. 35 and acrylate networks.3537 Figure 1 7 Formation of crosslinking S. zkars gro up developed a polyurethane elastomer using h ydroxyl terminated polybutadiene (HTPB ) as the prepolymer and Isophorone Diisocyanate (IPDI) as the curing

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21 agent for liner applications in solid rocket propellants. Triethanolamine was used as the crosslinker of the polyurethane composition matrix. The crosslinker density was increased by varying the triol/diol ratio from 0 to 0.5 and the mechanical properties of resulting polymers were invesgated As the crosslink density in the elastomeric polyurethane matrix i ncreases, the modulus of the matrix decreases initially and starts to increase due to an initial decrease in the average molecular weight with inclusion of cros slinking into system (Figure 1 8 ); The elongation at break increases first with the increasing c oncentration of triol, and after reaching a maximum at the triol/diol ratio of 0.031, it starts to decrease. (Figure 1 9 )38 Figure 1 8 Variation trend of modulus of the HT PB -based polyurethane elastomer with increasing triol concentration (adapted from the data in reference 38) Figure 1 9 Variation trend of elongation at break of the HTPB based polyurethane elastomer with increasing triol concentration (adapted from the data in reference 38)

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22 The crosslinker functionality f also plays an important role in polymer network structure. However, compared to the studies on crosslinker density, much less effort has been given to investigating the relationship between crosslinker functionality and physical properties of crosslinked polymers. James E. Marks group did a series of studies on effect of crosslink functi onality on the elastomeric properties of cr osslinked polymers decades ago.39 41 Unlike previous method of preparing polymer network, by which the crosslinks are usually introduced in a highly random, uncontrolled ma nner, they prepared model networks by linking the polydimthylsiloxane (PDMS) chains exclusively through functional groups placed at their two ends with crosslinkers having different functionality. An example of the formation of a hexafunctional network i s given by Figure 1 10. Since the chain length of PDMS chains between crosslinks is the same as before their incorporation into the network structure, the molecular weight Mc between crosslinks is then simply known. Crosslinkers with different functionalit y ranging from 3 to 37 were used to investigate the relationship between crosslinker functionality and elastomeric properties of resulting polymers. The semiempirical equation of Mooney and Rivlin described the relationship between modulus and elongation. (Equation 1 9 ) In Equation 1 9 f is defined as reduced is values of elongation. 2C1 and 2C2 According to new Florey theory of rubber like elasticity, 2C1 should be proportional to the factor (1 2/ f ), where f is the functionality of the crosslinker. The other constant 2C2, which is a measure decreases with increasing crosslinker functionality f Thus in a network having higher junction functionality, its modulus should decrease less with increasing elongation. Marks experimental results are in satisfactory agreement with Florys theory.39

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23 Figure 1 10. Scheme of a hexafunctional network formation (adapted from reference 39) 1 9 Mesfin Tsige and Mark J. Stevens studied the effect of crosslinker functionality and interfacial bond density on the fracture behavior of highly crosslinked polymer networks bonded to a s olid surface. Three different cross linker functionalities ( f = 3, 4, or 6) were studied. They used molecular dynamics (MD) simulations of highly crosslinked polymer networks bonded to a rigid surface to directly determine the local structure of the adhesiv e as a function of deformation and simultaneously calculate the stress -strain curves based on the bead-spring model. The simulations showed that the plateau regime in the stress -strain curve was found to depend on f (Figure 1 11) The strain range of the p lateau regime is found to decrease with an increase in f As f increased, failure stress increased while failure strain decreased. Experiments results showed that the simulation values of failure strain are larger due to the flexibility of the model (i.e., there is no stiffness in the bead -spring strands). However, this bead -spring model can still give us a systematic understanding of the role of crosslinker functionality in a crosslinked polymer network.42 The studies on the effects of filler and crosslinker on the mechanical properties of triazole polymers have not been systematically performed. The reac tions to form triazole polymers are good models to investigate the relationship between crosslinking and polymer mechanical properties since acetylene and azide groups should react with each other at 1:1 molar ratio, no small molecules are produced, the re action should not be influenced by residual moisture, and side reactions should not occur. Understanding the relationship between crosslinking and

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24 mechanical properties of triazole polymers, the synthesis then can be optimized to result in polymers with de sired mechanical properties. Figure 1 11. Tensile stress -strain curves for the three different functionalities (adapted from the data in reference 42) Thesis overview This thesis has five chapters. Chapter 1 is the introduction to the research. Chapter 2 illustrates the experimental method and explains how a reaction of polymerization was selected as a model for future studies. The third and fourth chapters discuss the effects of crosslinker and filler on the mechanical properties of triazole polymer s, respectively. The conclusions of this research and future work are summarized in the last chapter.

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25 CHAPTER 2 E XPERIMENTAL AND OPTI MIZATION OF POLYMERI ZATIONa Experimental General Commercially obtained reagents were used without further purificat ion. All monomers except 1 4 were prepared following reported procedures21, 26 43 Solvents were distilled by standard methods. 13C and 1H NMR spectra were recorded at 75 and 300 MHz, using tetramethylsilane (TMS) as an internal standard. In view of the stringent stoichiometry requirements for step polymerization, the monomers were systematically dried by aziotropic distillation and lyophilization. The uniaxial test specimen was a standard microtensile dogbone, with dimensions 0.88 0.19 0.13 44(Figure 2 1 ) S train (% elongation at break) and e lastic modulus (Youngs modulus) in Figure were measured by Instron Universal Tensile Testing Machine ( Upgrade package, Model no. 1122). Ea ch data point in the figures is an average of at least two measurements. Th e error bars in F igure 3 3, 4 and 4 7, 8 indicate mean standard deviation for each data set. Figure 3 7 to 3 10 dont show error bars for readability. The mean stand ard deviation for the measurements in Figure 3 7 and 3 8 are 0.13 Mpa and 7 0 % respectively. Figure 2 1 Dimension of dogbone mold a The article related to the work in Chapter 2, 3 and 4 has been submitted to the journal as follow ing : Preparation and Mechanical Properties of Crosslinked 1,2,3TriazolePolymers as Potential Propellant Binders, submitt ed to Journal of Applied Polymer Science 2009

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26 The mechanical properties of triazole polymers were compared with two gumstocks obtained from U.S. NAVY. Gumstock 1 used Desmodur N 100, a polyfunctional aliphatic isocyanate as crosslinking agent, and Gumstock 2 used hexamethylene diisocyanate ( HDI), a difunctional curative. The isocyanate -to -hydroxyl (NCO:OH) ratio was in both instances 1.1 The mechanical properties of the gumstocks were tested by Instron Universal Tensile Testing Machine with a strain rate of 50mm/min. Method of manual testing strain and modulus The strain of the polymers was estimated by subjecting much smaller thin film samples to external deformation and measuring the original and increased length of the samples by a fine scale. (Figure 2 2) This allowed for the testing of many more compositions on the tens of milligram scale and the concomitant gains in safety, sample waste, and efficiency. The process of tests of mod ulus on the mini samples is illustrated in Figure 2 3 A ) Original sample B) Sample under extension Figure 2 2 Method of manual testing strain The sample was stretched by two tweezers. The original lengt h (L0) and increased length ( scale. The strain at failure was calculated by the equation 2 1 General procedures for preparation of unfilled linear triazole polymer mini samples The monomer s were mixed in an aluminum pan at a small scale (<3 00mg) un til a homogenous mixture was obtained. The pan was cured in a vacuum oven under the conditions 2 1

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27 described in Figure 2 5 The reactions were monitored by NMR. The strain and modulus of the polymers were estimated manually as described in Figure 2 2 and 2 3 Figure 2 3 Meth od of manual testing modulus. A) A typical mini sample film was obtained from the polymerization at a scale of hundreds of milligrams. B) The film was cut into a 5mm wide rectan gular strip. The strip was marked by two green lines. The length between the green lines was the original length (L0). C) The sample was then loaded with a certain weight. The force (F) loaded on the sample should be the w eight attached to the sample. D) T he original length, the original width, the thickness of cross section and the increased length under the tensile load were measured by a fine scale. The value of modulus is then calculated by equation 2 2. 2 2 (F: force loaded on the sample, A0: original area of sample cross section) General procedures for preparation of crosslinked triazole polymer mini samples In an aluminum pan, diacetylene (1 ) was weighed and different concentrations of crosslinker ( Figure 2 4 ) w ere added and stirred until it dissolved completely. This was followed by the addition of diazide (2 ), which on stirring gave a homogeneous mixture. For an unfilled system this pan w as degassed under vacuum at room temperature for 3 hour s and the curing was done in a vacuum oven at 55 oC for 72 h our s. Fo r the filled system s, alumin um powder was added to the homogeneous mixture of diacetylene ( 1 ), diazide ( 2 ) and crosslinker then mixe d uniformly and degassed followed by curing in a vacuum oven at 55 oC for 1h our After that the mixture was stirred again and cured at 55 oC for an additional 71 hours

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28 Figure 2 4 F ormation of crosslinked triazole polymer s with different crosslinkers Figure 2 5 Formation of different t riazole polymers and their properties

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29 General procedures for preparation of dogbone samples For preparing dogbone samples the above mentioned procedure was used with larger amounts (approx. 2g). The mixture was ca st ed into a dogbone mold (Figure 2 6 ) and cured as described above. The dogbone samples were carefully removed from the mold. After they cooled they were tested using Instron tensile testing machine. Results for the trends from mini and dogbone samples were compared to assure reproducibility. Figure 2 6 Dogbone mold with unfilled and filled samples Optimization of polymerization Selection of monomors Figure 2 5 lists thirteen reactions of the polymerization of different diazides and diacetylenes. These reactions were screened by the following criteria: As per the specification of rocket propellant binders, the monomers should be reactive at low temperatures (r oom t emperature to 60 oC) with no or l ittle side reaction s ; T he polymerization process should pr oceed in the absence of any solvent or heavy metal catalyst s ; In addition the polymerization should be easily scaled up. A s shown in Figure 2 5 diacetylenes with acetylene groups connected to electron withdrawing groups showed better activit i es than pro pargyl esters or ethers ( Figure 2 5 Reaction b g l ) that needed higher temperature and/or catalyst to react with azides. Benzyl azides ( Figure

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30 2 5 Reaction e, m) can react with propiolates easily at room temperature. However, the reactions are more vig orous and sometimes too violent. Reactions c and j ( Figure 2 5 Reaction c, j ) were also ruled out because of difficulties in scaling up the preparation of starting material ( 7 ). Moreover, the reactions of monomers with longer spacers between end groups ( F igure 2 5 Reaction d f h i k ) need higher temperature or longer reaction time. Based on the above discussion, Reaction a was finally selected as a model system to study the relationship between crosslinker concentration and mechanical properties of triazole polymers. Stoichiometry effect on the mechanical properties of triazole polymers The effect of diacetylene to diazide stoichiometry on the mechanical properties of triazole polymers was studied. The ratio of diacetylene to diazide was varied from 0.98:1 to 1.12:1. (Figure 2 7 ) Since many of the resulting polymers were too soft and tacky to cast into dogbone molds, the strain and modulus of the polymers were estimated by the manual method described earlier. The results are listed in Table 2 1 Figure 2 7 Triazole polyme r formation at different stoichiometry As shown in Table 2 1 the polymer s with azide stoichimetry in excess are not elastic (Entry 1 ). With slight excess of diacetylene, the polymer becomes viscoelastic ( Entry 3 ). If the excess acetylene is further increa sed the elasticity decrease s and the modulus increased and reach ing the maximum value at the ratio of 1.10:1 ( Entry 7 ), then finally decreases. These results were unexpected because i n a step polymerization involving two difunctional monomers, the

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31 polymer should give the best mechanical properties at 1:1 stoichiometry of the monomers since any imbalance in stoichiometry should significantly lowe r the degree of polymerization. 45 46 However, these results showed that the ratio of 1.10:1 gave the highest value of modulus and a systematic shift towards excess acet ylene was observed in spite of the purification noted in the experimental section S ide reactions may account for this shift. For instance, it has been reported that the reaction of propiolate coupling to form enyne could be catalyzed by Lewis b as es such a s aromatic and aliphatic amines due to the lone pairs on the nitrogens.47 In this case such excess unr eacted diacetylenes might then undergo self condensation or oxidative coupling in the presence of triazole a cting as a catalyst to result in int ernal crosslinking, increase in molecular weight and ultimately shifting the stoichiometry from the expected 1:1 toward excess acetylene. To study this systematic shift towards excess acetylene, two reactions were perfo rmed. The first is the self reaction of pure diacetylene ( 1 ) (Figure 2 8 ); the second is the reaction of diacetylene (1 ) with diazide ( 2 ) at 1:1 molar ratio. ( Figure 2 9 ) After three days of reaction, the first reaction seemed incomplete while the second r eaction gave tacky and soft gel. The first reaction resulted in a non tacky brittle solid on the eighth day while the triazole polymer from the second reaction stayed the same. NMR results suggested that no side reaction occurred for the second reaction. U nfortunately, the solid from the first reaction was insoluble in any o rganic solvent thus unanalysable. However, the reaction of pure acetylene clearly indicates that side reactions may occur in the case of excess acetylene, which could result in unstable triazole polymers. Therefore, the ratio of acetylene to azide groups was kept at 1:1 to avoid any potential side reactions in the following experiments. Summary A method of manual testing strain and modulus on mini samples was developed for safety, cost, and sample waste. The results from mini samples will be compared with dogbone samples

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32 to assure reproducibility. Th e reaction of E300 dipropiolate ( 1 ) with diazide from tetraethylene glycol (2 ) was selected to study the effects of crosslinker and filler on the mechanical properties of resulting triazole polymers. In the following studies, the ratio of acetylene to azide groups was kept at 1:1 to avoid any potential side reactions. Table 2 1. Effect of stoichiometry on the mechanical properties of triazole p olymers (data from mini samples) Entry Diacetylene : diazide Modulus(psi) Strain (%) Sample description 1 0.98:1 N/A N/A Non elastic 2 1:1 N/A N/A Non elastic 3 1.02:1 N/A N/A Viscoelastic, very tacky 4 1.04:1 <1 1470 Elastic, very tacky and soft 5 1. 06:1 1 3180 Elastic, tacky 6 1.08:1 4 2980 Elastic, tacky 7 1.10:1 7 2800 Elastic, tacky 8 1.12:1 <1 1580 Elastic, very tacky and soft Figure 2 8 Reaction of pure acetylene Figure 2 9 Reaction of diacetylene and diazide at 1:1 ratio

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33 CHAPT ER 3 E FFECT OF CROSSLINKER ON THE MECHANICAL PROPERTIE S OF UNFILLED TRIAZOLE POLYMERS Effect of crosslinker concentration The effect of increasing concentration of crosslinker (3 ) on the strain and modulus of the resulting polymers was studied in unfilled system having end groups in the ratio of 1:1 ((diacetylene + tetrapropiolate ):diazide Figure 3 1 ). T hermal g ravimetric a nalysis (TGA) shows t he decomposition temperature of unfilled triazole polymers is around 300 C. ( Figure 3 2 ) Figure 3 1 Formatio n of triazole polymers with varied crosslinker concentration Figure 3 2 TGA curve of unfilled crosslinked triazole polymer (with 16mol% tetra propoilate)

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34 The variation trend s of strain and modulus value s with increasing crosslinker concentration for u nfilled triazole polymers is shown in Figure 3 3 and 3 4 The data in Figure 3 3 are from manual tests on mini samples while the data in Figure 3 4 are from tests on dogbone samples by Instron tensile tester with 20mm/min strain rate and listed in Table 3 1 T he results from mini samples and dogbone samples show the same variation trend s of the strain and modulus. T he difference in strain and modulus values between Figures 3 3 and 3 4 is likely due to lack of controlled strain rate in the manual measuremen t, though errors associated with the different sample size of mini samples ( <1 00 mg) and dogbone samples ( ), and measuring errors during manual testing may also contribute. Figure 3 3 Effect of crosslinker concentration on the mechanical properties of unfilled triazole polymers (data from mini samples) As shown in Figure 3 4 and Table 3 1 as crosslinker concentration was increased from 4 to 12mol% the modulus increaed from 0.18 to 1.0 Mpa while the strain decreased from 400 to

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35 70%. This trend is due to increasing crosslink density. As the amount of crosslinker was increased, crosslink density was also increased, which means more crosslinks to restrict the individual chain mobility. Th i s five -fold modulus increase and correlated stiffness increase occurs over a convenient range of crosslinker concentration and gives a facile met hod to control mechanical properties in these systems Table 3 1 Strain and modulus of unfilled triazole polymers (data from dogbone samples) E ntry Crosslinker concentration (mol %) Strain (%) Modulus (Mpa) 1 4 400 0.18 2 6 200 0.38 3 8 120 0.6 2 4 10 88 0.93 5 12 70 1.0 Figure 3 4 Effect of crosslinker concentration on the mechanical properties of unfilled triazole polymers (data from dogbone samples)

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36 Effect of crosslinker functionalityb To study the effect of crosslinker functi onality on the mechanical properties of triazole polymers, crosslinkers having different functionality f (f =3, 4, 6, 16, 32, 64) (Figure 3 5 ) were used with varied concentrations. Mini samples and dogbone samples were prepared (Figure 3 6), and their mech anical properties were tested manually (for mini samples) and by Instron tensile tester with a strain rate of 50mm/min (for dogbone samples). Figure 3 5 Crosslinkers with different functionality f ( f =3, 4, 6, 16, 32, 64) b The article related to the work in this section has been submitted to the journal as following: Effect of Crosslink Functionality on the Mechanical Properties of Crosslinked 1,2,3TriazolePolymers as Potential Binders for Rocket Propellants submitted to Polymer 2009

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37 Figure 3 6 Formation of u nfilled triazole polymers with different crosslinkers. X = (The n umber of a cetylene groups provided by crosslinker) / ( Total number of a cetylene groups provided by crosslinker and diacetylene) The variation trends of strain and modulus with increasing co ncentration of crosslinkers having different functionalities are illustrated in Figure 3 7 3 8 (data from mini samples) and 3 9, 3 10 (data from dogbone samples) Figure 3 11 compares the stress -strain curves of the polymer having different crosslinker fu nctionality For figure 3 7 to 311, the amount of diacetylene was kept at 80 mol%, thus x ( x in Figure 3 6) equals to 20 mol %. Table 3 2 lis ts the strain and modulus values corresponding to figure 3 9 and 3 10. As expected, for each crosslinker, with inc reasing crosslinker concentration, the strain decreases while modulus in creases, which is consistent with previous results. (Chapter 3.1 ) Comparing the trends of different crosslinkers, at the same concentrati on of acetylene groups (x = 20 mol% in Figure 6 ) provided by the crosslinker the polymers having higher crosslink or junction functionality give lower strain and highe r modulus than those having lower crosslink functionality. For example, at 68 mol % diacetylene, as crosslinker functionality increased from 3 to 6, the corresponding modulus of the resulting triazole polymers also increased from 0.87 to 2.73 Mpa while the failure strain dcreased from 113 to 72%, respectively. Junctions of higher functionality will be more firmly embedded within the pol ymer network .39 With higher cros slinker functionality,

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38 more polymer chains extend from each crosslink, which should also introduce more entanglements and steric hindrance.39 Therefore such networks should be stiffer and less flexible thus have higher modulus and lower strain. However, f or crosslinker s having higher ave rage functionalities of 16, 32 and 64 end groups, such trends are not obvious. In fact, the modulus vs functionality appears to level off. T his behavior may be explained by the following reasons. First, Peppas established a modified theoretical model for t he determination of the average molecular weight between crosslinks (Mc) of highly crosslinked polymers that also includes functionality. Mc showed a strong dependence of on the junction functionality f As f increased from 2 to 20, Mc undergoes the great est variation near f =4. It is also observed that Mc levels off if f is further increased.48 It should also be noted that the crosslinkers ( f =16, 32, 64) are derived from commercial available Boltorn polyols that are, of course, mixtures of highlybranched polymers and the functionality is an average value. In addition, the highest crosslink functionality can introduce possible s teric difficulties in terms of many chains terminating within a relatively small volume, which may cause incomplete end link reaction during the polymerization. Conclusion Th e reaction of E300 dipropiolate ( 1 ) with diazide from tetraethylene glycol (2 ) was selected from thirteen reactions of various organic diazides and diacetylenes as a model reaction to study the effects of crosslinker on the mechanical properties of triazole polymers. The variation trends obtained by manual testing and mechanical testing agreed with each other. The modulus of unfilled polymer s increased while the strain decreased with increasing percentage of crosslinker. At the same amount of acetylene groups provided by different crosslinkers (x in Figure 3 6 ), higher junction functiona lity polymers show lower strain and higher modulus, although the mechanical properties change much less at functionalities higher than 6 Thus, the

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39 triazole polymers with desired mechanical properties as potential rocket propellant can be obtained by selec ting appropriate crosslinker and adjusting the crosslinker concentration during the polymerization. Some of the resulting triazole polymers can obtain comparable mechanical properties to those of polyurenthanes. Figure 3 7 Effect of crosslinker functionality on the strain of triazole polymers (data from mini samples) Figure 3 8 Ef fect of crosslinker functionality on the modulus of triazole polymers ( data from mini samples)

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40 Figure 3 9 Effect of crosslinker functionality on the strain of triazole polymers (data from dogbone samples) Figure 3 10. Effect of crosslinker functionality on the modulus of triazole polymers (data from dogbone samples)

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41 Figure 3 11. Stress -strain curves of the polymers having different crosslink functionality (data f rom dogbone samples) Table 3 2 Strain and modulus values of crosslinked triazole polymers having different crosslink functionality (data from dogbone samples) Entry Crosslinker functionality ( f ) Diacetylene 7 (mol %) X (mol %) Modulus (Mpa) Strain (%) 1 3 68 32 0.87 113 2 3 64 36 1.14 99 3 3 60 40 1.36 92 4 3 56 44 2.01 71 5 3 52 48 2.04 66 6 3 48 52 2.16 78 7 4 88 12 0.18 488 8 4 84 16 0.43 207 9 4 80 20 0.72 166 10 4 76 24 1.20 116 11 4 72 28 1.57 85 12 4 68 32 1.99 65 13 6 8 8 12 0.29 298 14 6 84 16 0.61 171 15 6 80 20 0.97 128 16 6 76 24 1.69 82 17 6 72 28 2.03 88 18 6 68 32 2.73 72

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42 Table 3 2. Continued Entry Crosslinker functionality ( f ) Diacetylene 7 (mol %) X (mol %) Modulus (Mpa) Strain (%) 19 16 92 8 0. 17 270 20 16 88 12 0.47 155 21 16 84 16 0.48 108 22 16 80 20 0.82 73 23 16 76 24 0.90 74 24 32 92 8 0.064 269 25 32 88 12 0.21 124 26 32 84 16 0.38 95 27 32 80 20 0.51 75 28 32 76 24 0.98 59 29 64 92 8 0.074 242 30 64 88 12 0.18 126 31 64 84 16 0.46 89 32 64 80 20 0.57 77

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43 CHAPTER 4 EFFECT OF FILLER ON THE MECHANICAL PR OPERTIES OF TRIAZOLE POLYMERS Effect of filler type To study the effect of different fillers on the mechanical properties of resulting fi lled triazole polymers, several fillers including inorganic salts and metals were added during the polymerization. ( Figure 4 1 ) The concentration of filler was calculated by Equation 4 1 Samples with different fillers are compared in Table 4 1 4 1 Figure 4 1 Formation of triazole polymers with different fillers Table 4 1 Triazole polymers with different fillers (mini samples) Entry Filler Sample description 1 NH4NO3 Not uniform, bubbles, elastic, easy to break from the interface between binder and crystals 2 NaSO 4 easy to break from the interface between binder and crystals 3 NaCl easy to break from the interface between binder and crystals 4 Al Much more uniform c ompared to above binders, elastic Ammonium nitrate is commonly used as oxidizer in rocket propellant.49 To investigate its effect on the mechanical properties of resulting triazole polymers, different concentrations of ammonium nitrate were added to the polymer The resulting mini samples with ammonium nitrate filler were not uniform and had poor elasticity. The filled samples were easy to break

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44 from the interface between ammonium nitrate crystals and triazole polymer when they were stretched. The poor mechanical properties of triazole polymers filled with ammonium nitrate crystals result ed from their large siz e. Large particles have smaller specific surface area compared to small ones The tensile strength and elongation of filled polymers decrease with decreasing specific surface area of oxidizer, or increasing fil l er part icle size .50 In addition, the air bubbles in the samples also degraded their mechanical properties. Degassing the samples in a pre vacuum for several hours prior curing could greatly reduce the bubbles. (Figure 4 2) A) Samples not degassed prior curing B) Sample degassed prior curing Figure 4 2 Mini samples of t riazole polymer s filled wi th NH4NO3 with and without degassing Since ammonium nitrate itself or in combination with other compounds such as aluminum powder is high explosive, sodium sulfate and sodium chloride were used to replace ammonium nitrate to study the effect of inorganic o xidizer on triazole polymers mechanical properties. To imitate real rocket propellant sy stem, which is highly filled with 75 85wt % solids,51 Attempts were made to maxim ize the amount of sodium sulfate or sodium chloride included in the triazole polymer. The content of sodium sulfate or sodium chloride can be maximized to 80 wt%. However, the resulting filled polymers presented the same problem as the polymers filled with ammonium nitrate: samples are not uniform and lack of good interfacial adhesion between the

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45 binder and filler. Continuing research attempts to solve t hese problems by optimizing fillers size and adding wet ting/coupling agents in the future work.28 30 Compared to the polymers filled with inorganic salts, triazole polym ers filled with aluminum powder were much more uniform due to smaller filler particle size, and better adhesion between filler particles and polymer binder was observed (Figure 3 3) 43wt% Al 75w t% Al 80wt% Al Figure 4 3 Triazole polymers filled with different content of aluminum powder Effect of aluminum filler content To st udy the effect of filler content on the mechanical properties of triazole polymers, the content of aluminum powder was varied from 0 to 74wt%. ( Figure 4 4) The modulus and strain of resulting triazole polymers are listed in table 4 2. Figure 4 5 compares their stress and strain curves. Figure 4 4 Formation of triazole polymers with differ ent aluminum content

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46 Table 4 2 Effect of Al filler content on the mechanical properties of triazole polymers (Strain and modulus data from dogbone samples) Al content by weight % Al content by volume % Strain (%) Modulus (Mpa) 0 0 396 0.18 43 27 308 0.50 74 58 53 5.5 Figure 4 5 Stress -strain curves of triazole polymers with different aluminum content As shown in T able 4 2 and Figure 4 5 with increasing aluminum content, t he modulus of the polymers increased due to the addition of much sti ffer aluminum powder to the polymer matrix while the strain decreased, and this is in accordance with literature which states that the filler restricts the mobility of polymer chains leading to decrease of strain.52 For unfilled triazole polymers, the modulus increases while strain decreases with increa sing crosslinker concentration. ( Ch apter 1.3) The strain and modulus of filled triazole polymers should give the same variation trends To verify this, 43wt% of aluminum powder was added to the crosslinked binder as a filler. (Figure 4 6) With 43wt% aluminum filler, the polymer ha d good pro cessability, which facilitate d the study of the crosslinker effect on the mechanical properties of filled triazole polymers. Manually tested and mechanical tested data (strain rate:

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47 20mm/min) are plotted in Figures 4 7 and 4 8 Table 4 3 gives the specific values of strain and modulus corresponding to the data points in Figure 4 8 Compared to unfilled polymers, more measuring errors occurred for filled triazole polymer mini samples during manual testing of strain and modulus Therefore, more mini samples w ere tested (with crosslinker concentration from 3 to 15%) so that a more reliable variation trend of strain and modulus of filled triazole polymers was achieved ( Figure 4 7 ) F or unfilled and filled triazole polymers, crosslinker has the same effect on the ir strain and modulus: with increasing crosslinker concentration, the modulus increases while the strain decreases. As shown in Table 4 3 and Figure 4 8, f or triazole polymers with 43% aluminum filler, with increasing crosslinker from 4 to 12mol%, the modulus increased from 0.5 to 3.9Mpa, an 8-fold change, while the strain decreased from 310% to 50%. ( Table 4 2 ) Since the strain and modulus for potential propellant binders should be at least 50% and 1.4Mpa respectively, the ideal range of crosslinker co ncen tration to achieve the goal is around 9 12mol%. For more highly filled triazole -based systems, the modulus can be expected to show further increases. Compared to a typical polyurethane elastomer matrix for rocket propellants (with 83wt% filler) having mo dulus around 2.8 Mpa, t he modulus of these filled t riazole polymers is comparable. It should be noted that dogbone samples with 43% aluminum filler were somewhat hard to keep uniform because heavier aluminum powder tended to settle during polymerization. T his may be why larger error bars were observed in Figure 4 8 compared to Figure 3 4 Figure 4 6 Formation of filled triazole polymers with different crosslinker concentration

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48 Table 4 3 Strain and modulus of filled triazole polymers (43wt % Al, data from dogbone samples) Entry Crosslinker concentration (mol%) Strain (%) Modulus (Mpa) 1 4 310 0.5 2 6 180 1.1 3 8 120 1.2 4 10 95 2.2 5 12 50 3.9 Figure 4 9 compares the stress -strain curves of filled triazole polymers with different crosslink er concentration measured on the dogbone samples at controlled strain rate The change of mechanical properties of the polymers with different crosslinker concentrations can be clearly seen from these stress-strain curves. With lower crosslinker concentrat ion, the polymer is softer and more elastic. Likewise the samples become stiffer and more prone to elongational failure as the crosslinker concentration was increased. F igure 4 7 Effect of crosslinker concentration on the mechanical properties of f illed triazole polymers ( 43wt% Al, data from mini samples)

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49 Figure 4 8 Effect of crosslinker concentration on the mechanical properties of filled triazole polymers ( 43wt% Al, data from dogbone samples) Figure 4 9 Stress -strain curves of filled tria zole polymers with increasing crosslinker c oncentration (4mol% 14mol%)

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50 The mechanical properties of unfilled and filled triazole polymers are compared in Figure 4 10. The modulus of filled polymers is at least two times that of the unfilled polymers due to the addition of much stiffer aluminum powder to the polymer matrix. Generally, the decrease in strain of the polymers with 43wt% Al filler is not drastic compared to the unfilled polymers and this may be due to the good tack of these materials noted above and adhesion of binder to the filler. Figure 4 10. Comparison of mechanical properties of unfilled and filled (with 43% Al) triazole polymers ( data from dogbone samples) As mentioned in the introduction (Chapter 1.3), various mathematical equations h ave been developed to predict the increase in the modulus of an elastomer resulting from the addition of filler Three simple equations, GuthSmallwood equation (1 3), Eilers -Van Dijick equation (1 4) and modified Roscoe equation (1 6), were used to predic t the modulus of triazole polymers with 43wt% (27vol%) aluminum filler. The data predicted by the three equations and the data

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51 obtained experimentally were compared in Table 4 4 and Figure 4 11. As shown in Table 4 4 and Figure 4 11, the data from all thre e equations fit the experimental data reasonably and GuthSmallwood equation appears to be the best. However, it i s hard to say that the Guth -Smallwood equation is the best theory because the experimental data usually show great variability on filled sys tems.30 m 0.74 used in the equations is not the true value because the filler particles are not perfectly spherical or of exactly the same size.29 m.30 The difference between theoretical and true va m may cause lager discrepancies between the experimental data and the data predicted by Eilers -Van Dijick equation and modified Roscoe equation. Table 4 4 Experimental and predicted values of the modulus of filled triazole polymers (27 vol% Al) Crosslinker concentration (mol%) Modulus a (MPa) Modulus b (MPa) Modulus c (MPa) Modulus d (MPa) 4 0.5 0.48 0.41 0.55 6 1.08 1.01 0.88 1.16 8 1.19 1.65 1.43 1.88 10 2.16 2.48 2.14 2.83 12 3.86 2.72 2.35 3.10 a. data obtained from dogbone samples; Modulus b. data predicted by Guth-Smallwood equation; c.data predicted by Eilers -Van Dijick equation; ddata predicted by modified Roscoe equation. Conclusion The content of filler can modify the mechanical properties of resulting filled polyme rs. Among the fillers of ammonium nitrate, sodium sulfate, sodium chloride and aluminum, aluminum powder gave triazole polymers the best mechanical properties due to much smaller size (14um). Generally, increasing filler content increases the modulus of tr iazole polymers but decreases their strain. Therefore, the mechanical properties of triazole polymers can be modified by adjusting filler content. The triazole polymers with 74 wt% aluminum filler and 4mol% tetra -

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52 propiolate crosslinker can reach 5.5Mpa modulus and 53% strain, which fulfills the requirements for rocker propellant. The effect of crosslinker concentration on filled triazole polymers is the same as that of unfilled triazole polymers. The modulus values of triazole polymers filled with 43wt% (27vol%) aluminum powder predicted by Guth -Smallwood equation approach to the experimental data. Figure 4 11. Comparison of predicted modulus and experimental modulus.

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53 CHAPTER 5 S UMMARY AND FUTURE WO RK Summary The reactions of azides with terminal alky ne s to for m triazole polymers following C lick chemistry are good models to investigate the relationship between crosslinking and polymer mechanical properties since acetylene and azide groups should react with each other at 1:1 molar ratio, no small mole cules are produced, the reaction should not be influenced by residual moisture, and side reactions should not occur. The reaction of E300 dipropiolate (1 ) with diazide obtained from tetraethylene glycol (2 ) was selected from thirteen reactions of various organic diazides and diacetylenes as a model reaction to study the effects of crosslinker and filler on the mechanical properties of triazole polymers. The modulus of triazole polymers increased while the elongation at break decreased with incr easing cross linker concentration or functionality although this trend is not obvious at crosslinker functionality higher than 6. Thus, the triazole polymers with desired mechanical properties as potential rocket propellant can be obtained by choosing a crosslinker w i th appropriate functionality and adjusting the crosslinker concentration during the polymerization. Some of resulting unfilled triazole polymers have comparable mechanical properties compared to commonly used polyurethane binders. For example, w ith the cro sslinker of hexapropiolate, one of the crosslinked triazole polymers can reach 2.73 MPa modulus and still have 72% strain. The type, size and content of fillers also affect mechanical properties of triazole polymers. Fillers of inorganic salts such as amm onium nitrate, sodium sulfate and sodium chloride don t have good adhesion with triazole polymers due to their large filler particle size, which degrade s the mechanical properties of triazole polymers. Polymers filled with alu minum powders are uniform and thus was selected to study the effects of filler and crosslinker on the filled the

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54 triazole polymers. Generally, t he addition of filler improve d the modulus of tria z ole polymers but decreased strain. The increase in the modulus due to the addition of alumi num powder predicted by the equation of Guth and Smallwood approach the experimental data. The modulus of polymers with 74wt% aluminum filler and 4mol% tetra propiolate ( 3 ) can reach nearly 5.5 Mpa modulus while the strain is still acceptable (53%). Compar ed to a typical polyurethane elastomeric liner matrix with 83wt% solids for rocket propellants having modulus of 2.8 MPa, these unoptumized triazole polymers have comparable modulus Future work Optimization of fillers size and composition As mentioned in chapter 4, the large size of fillers degrades filled polymers mechanical properties. Filler particles such as sodium sulfate or sodium chloride will be gr ound to defined smaller sizes.53 The portion of each filler size will be optimized to give the polymers best mechanical properties. Since metallic additive and oxidizer are essential ingredients in rocket propellant, aluminum powder and inorganic salts such as sodium chl oride will be mixed and the fraction of each ingredient will also be optimized. Optimization of plasticizers Plasticizers are usually used to improve the processability of binder matrices. The mechanical properties of polymers can be affected by the type a nd content of plasticizers. Different plasticizers will be tried and screened. The type of the plasticizer and its concentration will be selected according to the mechanical properties of resulting triazole polymers. Preparation and characterization of hig h filled triazole polymers To imitate real rocket propellant that is highly filled triazole polymers having 75% 85wt % fillers will be prepared and fully characterized. Tensile tests, thermal analysis such as DSC (Differential Scanning C alorimetry) and TG A (Thermo Gravimetric Analysis ), and

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55 d ynamic mechanical analysis will be performed to study the properties of highly filled triazole polymers.

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59 BIOGRAPHICAL SKETCH Ling Wang was born in 1978 in Nantong, China. She gra duated from Soochow Un iversity, China, in 2000 with a Bachelor of Science degree in chemistry. From the sam e university, she received her masters degree in e ducation in 2003, under the supervision of Dr. Liming Shen. Then she taught chemistry in Nantong N o.1 High School of Jiangsu Province, China during 2004. Ling enrolled as a graduate student in c hemistry a t the University of Florida in the s pring of 2006 under the direction of Dr. Randolph S. Duran. She gr aduated in May of 2009 with a Master of Science degree in analytical chemistry.