Stress Relaxation by Addition-Fragmentation Chain Transfer in Highly Crosslinked Thiol-Ynes

Objectives: The aim of this study is to examine the effect of allyl sulfide functional groups and addition-fragmentation on shrinkage stress during photopolymerization reactions in highly crosslinked networks. The addition-fragmentation mechanism of allyl sulfides has been previously shown to reduce shrinkage stress in sub-ambient glass transition temperature thiol-ene systems. We hypothesized that incorporation of allyl sulfide functional group into thiol-yne networks will exhibit reduced shrinkage stress as well as super-ambient glass transition temperatures by increasing the network's crosslinking density while also enabling addition-fragmentation.

Methods: Stoichiometric mixtures of tetrathiol (PETMP) and allyl sulfide-core diyne monomers (MDTBY) were formulated. Additionally, a propyl sulfide-core diyne monomer(MeDTBY), which is not able to undergo addition-fragmentation, was formulated with PETMP and evaluated as an analogue to MDTBY, allowing for a comparison between allyl sulfide- and propyl sulfide-based networks. Simultaneous shrinkage stress and functional group conversion measurements were performed during polymerization using a cantilever-type tensometer coupled with an FTIR spectrometer. The glass transition temperature and elastic modulus were determined by dynamic mechanical analysis (DMA). ANOVA (CI 95%) was conducted to determine differences between the means.

Results: Both allyl and propyl sulfide-based materials exhibited super-ambient glass transition temperatures. A comparison of the thermomechanical properties of the fully cured allyl and propyl sulfide-based materials demonstrated excellent similarity between the two network structures. PETMP-MDTBY formulations exhibit significantly reduced polymerization stress (43%) compared to PETMP-MeDTBY, attributable to the addition-fragmentation reaction effecting network connectivity rearrangement during the polymerization. The difference between the allyl sulfide and the propyl sulfide containing materials was statistically significant (P<0.0005).

Conclusions: The resulting networks were highly crosslinked, possessed super-ambient glass transition temperatures, and exhibited significantly reduced polymerization-induced shrinkage stress when compared with analogous propyl sulfide-based system that are incapable of addition-fragmentation. Supported by NIH DE Grant #DE 10959