Swelling behavior of thin, surface-attached polymer networks

The swelling behavior of thin, surface-attached cross-linked dimethylacrylamide (DMAAm) films in contact with water has been characterized with multiple-angle nulling ellipsometry and compared to the swelling of nonattached, bulk DMAAm. networks. The polymer networks are fabricated by cross-linking thin films of statistical copolymers composed of DMAAm and a photoreactive benzophenone derivative monomer. Covalent attachment of the film to the surface is achieved by means of a benzophenone-based silane monolayer. UV illumination simultaneously cross-links and chemically links the layer and chemically links the layer to the surface, forming stable networks that do not delaminate upon swelling. It is observed that the surface-attached networks swell less than the nonattached, bulk networks at the same cross-link density; however, the swelling of surface-attached networks is larger than that suggested by simple geometric considerations for swelling in one dimension. The results are in qualitative agreement with Flory-Rehner theory extended to one-dimensional swelling.