Higher order structure and thermo-responsive properties of polymeric gel with crystalline side chains

Crosslinked polymeric gel films were prepared by a radical copolymerization of stearyl acrylate (SA) and acrylic acid (AA) with N,N'-methylenebisacrylamide (MBAA) as a cross-linker. Aggregation structure of the (SA/AA/MBAA, 24.7/74.3/1.0; molar ratio) gel film was investigated on the basis of X-ray diffraction study. The gel films in both a dried and a swollen states in dimethyl sulfoxide (DMSO) formed crystalline lamellar structure at room temperature owing to intermolecular aggregation of stearyl acrylate side chains. The thermo-responsive behavior of the (SA/AA/MBAA) gel film was examined by the temperature dependence of the swelling ratio. The swelling ratio of the (SA/AA/MBAA, 24.7/74.3/1.0) gel film increased with an increase in temperature up to 320 K (the melting temperature of stearyl acrylate side chain crystals), whereas it decreased above 320 K. In order to improve the thermo-responsive behavior, the stretched gel film was prepared by uniaxial stretching. The magnitude of melting endotherm for the stretched (SA/AA/MBAA) gel film increased with the stretching ratio and also, the crystallinity and/or the regularity of layer structure of the stretched gel him was improved by stretching, i.e. the lamellar long axis in the stretched gel aligned along the stretching direction. The thermo-responsive properties of the stretched gel films were also examined by the temperature dependence of the swelling ratio and the length of the gel film. The stretched gel film abruptly shrinked to the original length upon heating at melting temperature of stearyl acrylate side chain crystals.