Anisotropic swelling behavior of copolymeric gel film with crystalline oriented side chains

A copolymeric gel film with swelling anisotropy was prepared on a flat Petri dish by free-radical copolymerization of stearyl acrylate (SA) and acrylic acid (AA) with N,N′-methylenebisacrylamide (MBAA) as the crosslinking agent. An X-ray diffraction study of the gel film revealed that the SA side chains in the gel film SA/AA/MBAA:24.7/74.3/1.0 (in mol) were spontaneously oriented perpendicular to the film surface in order to minimize the interfacial free energy between the gel film surface and air. The one-dimensional swelling ratio perpendicular to the gel film surface in an ethanol/water mixture was greater than that in the direction parallel to the film surface. This swelling anisotropy can be explained on the basis of changes in the state of anisotropic aggregation.