A Photocurable Polyurethane\nSystem with Reconfigurable\nDisulfide Bonds Featuring High Transmittance and Low Shrinkage

The shrinkage of UV-curable polymers is directly related\nto the\nstructural changes in the monomer behavior of the system. The issue\nof shrinkage in these systems can be mitigated by leveraging the dynamic\nbehavior of disulfide bonds. However, it remains unclear how light\ninduction affects the contraction of the system through processes\nsuch as bond cleavage, migration, and recombination of disulfide bonds.\nIn this study, we modified the polyurethane core of a UV-curable system\nby incorporating three different topologies of disulfide bonds. We\nexamined the impact of these different arrangements and their dynamic\nadjustment on reducing shrinkage using scanning electron microscopy,\nshrinkage rate tests, and theoretical analysis based on the Gauss-type\nradial distribution probability density function. Our findings show\nthat the modified sample exhibited lower curing shrinkage and a smoother\nsurface compared to unmodified polyurethane. Specifically, while the\nshrinkage rate before modification (PU) was 5.16%, after modification\n(DSBT-PU, DSBP-PU, DSBE-PU) it decreased to 1.24% for DSBT-PU, 0.67%\nfor DSBP-PU, and 0.95% for DSBE-PU. We also observed that longer chain\nlengths in disulfide-containing fragments aided in dynamically adjusting\nthe system structure. However, the increase in the molecular fragment\nlength makes it more difficult to migrate dynamic disulfide bonds.\nFurthermore, our synthetic sample demonstrated high transmittance\nlevels, exceeding 98%. Additionally, we conducted comprehensive examinations\nincluding testing 180° peel strength, differential scanning calorimetry,\nthermogravimetric analysis, tensile characteristics, and transmittance\nproperties, among others. This work not only sheds light on how dynamic\nbonds influence system structure but also holds practical implications\nfor applications in liquid optical clear adhesive (LOCA) and unique\npressure-sensitive adhesives.