Stress Relaxation by Addition-fragmentation Chain Transfer in Glassy Polymer Networks

Park, Hee Young

Graduate Thesis Or Dissertation

Stress Relaxation by Addition-fragmentation Chain Transfer in Glassy Polymer Networks Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/xw42n802w

Abstract

Since polymer materials have come to dominate our life by replacing many other, more traditional materials, any endeavors to improve their performance have heightened importance. Here, we have strived to develop methodologies to reduce and understanding of the processes relevant to the origins of polymerization-induced shrinkage stress. In this thesis, addition-fragmentation chain transfer (AFCT) was utilized to reduce the shrinkage stress through network relaxation and adaptation by radical-mediated bond rearrangement that occurs throughout the polymerization. In previous studies, the presence of the allyl sulfide functional group was shown to reduce the final stress in thiol-ene resins via AFCT. Unfortunately, thiol-ene polymerizations often yield elastomeric materials with low glass transition temperatures (Tgs) that are illsuited for structural applications. To extend the utility of AFCT into structural applications, AFCT-capable monomers have been incorporated into many systems to achieve both low stress and excellent mechanical properties by designing chemical structure of monomers and by exploration of the polymerization and relaxation mechanisms. In particular, incorporation of AFCT-capable monomers into conventional dental resins was investigated to generate novel low stress dental composites. This thesis has demonstrated the development and implementation of thiol-ene and thiol-yne based monomers that are capable of undergoing AFCT to reduce polymerization stress further in a glassy polymer and its application for the conventional glassy methacrylate-based dental resins. Additionally, novel monomers that undergo AFCT were developed and implemented in methacrylate-based resins that undergo propagation exclusively through chain growth mechanisms. Since direct incorporation of the allyl sulfide in methacrylatebased systems was not effective for stress relaxation with large amounts of methacrylate, trithiocarbonates were used in place of allyl sulfides in dimethacrylate monomers, where the trithiocarbonate enables reversible reactions with the methacrylic radicals. The trithiocarbonate-based dimethacrylate demonstrated 65 % stress reduction compared with the standard BisGMA-TEGDMA composite while demonstrating fracture toughness which was slightly higher than the control BisGMA-TEGDMA composite indicating that TTCDMA and related monomers represent excellent candidates for reactive diluents in BisGMA-based dental materials. The improved understanding of the trithiocarbonate functional group will broadly expand its utility for a wide range of applications such as coatings, microelectronics, photoresists, and dental materials.

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