A Photocurable Polyurethane System with Reconfigurable Disulfide Bonds Featuring High Transmittance and Low Shrinkage

The shrinkage of UV-curable polymers is directly related to the structural changes in the monomer behavior of the system. The issue of shrinkage in these systems can be mitigated by leveraging the dynamic behavior of disulfide bonds. However, it remains unclear how light induction affects the contraction of the system through processes such as bond cleavage, migration, and recombination of disulfide bonds. In this study, we modified the polyurethane core of a UV-curable system by incorporating three different topologies of disulfide bonds. We examined the impact of these different arrangements and their dynamic adjustment on reducing shrinkage using scanning electron microscopy, shrinkage rate tests, and theoretical analysis based on the Gauss-type radial distribution probability density function. Our findings show that the modified sample exhibited lower curing shrinkage and a smoother surface compared to unmodified polyurethane. Specifically, while the shrinkage rate before modification (PU) was 5.16%, after modification (DSBT-PU, DSBP-PU, DSBE-PU) it decreased to 1.24% for DSBT-PU, 0.67% for DSBP-PU, and 0.95% for DSBE-PU. We also observed that longer chain lengths in disulfide-containing fragments aided in dynamically adjusting the system structure. However, the increase in the molecular fragment length makes it more difficult to migrate dynamic disulfide bonds. Furthermore, our synthetic sample demonstrated high transmittance levels, exceeding 98%. Additionally, we conducted comprehensive examinations including testing 180° peel strength, differential scanning calorimetry, thermogravimetric analysis, tensile characteristics, and transmittance properties, among others. This work not only sheds light on how dynamic bonds influence system structure but also holds practical implications for applications in liquid optical clear adhesive (LOCA) and unique pressure-sensitive adhesives.