Transparency of Temperature-responsive Shape-memory Gels Tuned by a Competition between Crystallization and Glass Transition

Transparency is often an important property in the practical applications of temperature-responsive shape-memory gels. We investigated the mechanism of significant transparency improvement upon a change in two copolymer gels with their molar ratios between stearyl acrylate and N, N -dimethylacrylamide from 1:1 to 0.75:1. By means of Flash DSC measurement, we made the thermal analysis characterization of crystallization and glass transition in two copolymer gels and compared the results to the parallel experiments of corresponding homopolymers. The results showed that the slightly lower content of stearyl acrylate sequences suppresses crystallization in their side chains due to the chemical confinement of comonomers on copolymer crystallization; meanwhile it shifts up the glass transition temperature of the backbone N, N -dimethylacrylamide sequences. Eventually on cooling, crystallization gives its priority position to glass transition in copolymer gels, resulting in a higher transparency of the gel without losing the shape-memory performance. To confirm the chemical confinement, we further compared the isothermal crystallization kinetics of stearyl acrylate side chains in the copolymer gel to that of their homopolymer. Our observations facilitate the rational design of the temperature-responsive shape-memory gels for the transparency property.