Quick reaction kinetics under atmospheric conditions and low energy-cost of fabrication have made thiol-ene networks a popular material choice for research on protective coatings and films. Physical properties of thiol-ene materials produced from bulk monomer have been widely explored, though solubility issues may arise in situations which require a solid monomer or filler. Because thiol-ene materials are often fabricated in the bulk, the effect of solvent on network formation remains unknown. Interestingly, we have found thiol-ene films fabricated through photopolymerization exhibit higher glass transition temperatures (Tg) when processed in good solvents vs in the bulk. To investigate this phenomenon, we fabricated networks by combining tetrafunctional tetramethylolpropanetetra(3-mercaptoproprionate) (4T) and trifunctional 1,3,5-triallyl-1,3,5-triazine-2,4,6-trione (TTT) in a 1:1 thiol:alkene ratio (adding 20 wt% of good solvent when needed) and curing under UV light (l = 340 nm). Our results show solvent-processed films to exhibit higher Tg than bulk processed-films, as determined through differential scanning calorimetry, dynamic mechanical analysis, and thermomechanical analysis. Additionally, our 4T-TTT films fabricated in good solvents have shown greater thermal dilation and lower storage moduli than their bulk-processed counterparts, deviating from trends in literature which typically show thermomechanical values increase for materials with higher glass transition temperatures. We have looked for chemical differences between our bulk- and solvent-processed thiol-ene networks, but through IR spectroscopy have found all systems reach &gt;99% functional group conversion. Continuing structural investigations through positron annihilation lifetime spectroscopy will elucidate on the differences in free hole volume and connectivity between bulk- and solvent-processed networks together these results highlight the importance of photopolymerization processing conditions and demonstrate their ability to lead to vastly different network connectivity and physical properties, further expanding the modularity of thiol-ene-based materials.

Investigating solvent effects on thiol-ene network formation

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