Date of Award

Spring 1-1-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

First Advisor

Christopher N. Bowman

Second Advisor

Wayne D. Cook

Third Advisor

Jeffrey W. Stansbury

Fourth Advisor

Kristi S. Anseth

Abstract

Conventional thermosetting photopolymers are ubiquitous in a wide array of industrial applications such as UV coatings and adhesives, 3D printed objects, and dental composites, and these reactions are largely based on acrylate or methacrylate chain-growth free radical photopolymerizations. Despite the significant demand for (meth-) acrylate-based thermosets, problematic characteristics that are inherent in the nature of this polymerization still persist especially in bioapplications. For instance, incomplete conversion of monomers upon early gelation and vitrification in heterogenous networks containing hydrolytically labile ester functionalities, considerable shrinkage stress developed during polymerization, and brittleness of these highly crosslinked (meth-) acrylate-based polymers hinder both the biological and mechanical performance. In an attempt to develop ester-free, photo-curable thermosets with improved material characteristics, photo-initiated copper-catalyzed azide-alkyne cycloaddition (CuAAC) polymerization, in particular, has been utilized due to benefits associated with the step-growth nature of these reactions including formation of homogeneous network structures, readily tailorable backbone functionalities, and delayed gel point conversion, in addition to the efficient and orthogonal ‘click’ nature of CuAAC reactions. Further, CuAAC polymerization results in rigid triazole moieties densely formulated in the networks, which enables the formation of strong, high modulus thermosets with high glass transition temperatures.

This thesis examined structure-property relationships of triazole-based photopolymer networks formed via photo-initiated copper-catalyzed azide-alkyne cycloaddition (CuAAC) polymerization. In particular, molecular design of the monomers and resin formulations, materials characterization on in situ bulk photopolymerization kinetics and mechanical properties, and implementation of polymer-based composites for dental restorative material applications were demonstrated. Firstly, structurally variable multi-functional azide and alkyne monomers were synthesized to analyze structure-property relationships of CuAAC-based polymer networks. Photopolymerization kinetics were directly governed by vitrification, which is dictated by variations in monomer backbone rigidity, monomer functionality, and functional group density. The resulting polymer networks with glass transition temperatures significantly above ambient resulted in exceptional toughness and ductile behavior with the capacity for multiple shape memory recovery cycles upon glassy state deformation, which was enabled by the triazole moieties. Additionally, for dental restorative materials applications, copolymerization of CuAAC-based resins and functionalized glass micro-fillers were readily implemented to form in situ photopolymerizable CuAAC-based composites. Considerably lower polymerization-induced shrinkage stress at quantitative conversion was achieved in CuAAC-based composites despite relatively high volumetric shrinkage, and superior mechanical properties were obtained primarily due to delayed gelation in step-growth mechanism. Lastly, structurally various mono-functional monomers were incorporated in off-stoichiometric, multi-functional CuAAC resin systems to alter the mechanical properties, crosslink density, glass transition temperature, and polymerization kinetics, with their characteristic independently controlled by either or both of the concentration or chemical structure of the mono-functional monomers.

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