Humidity Driven Swelling of the Surface-Attached Poly(N-alkylacrylamide) Hydrogels

We describe a systematic investigation on the humidity driven swelling behavior of a homologous series of surface-attached N-alkylacrylamide hydrogels. To this, first a series of copolymers is synthesized, each comprising an N-alkylacrylamide with an appropriate chain length of the alkyl substituent and a UV-active cross-linker, (methacryloyloxy)benzophenone (MABP). Thin films of the copolymers are deposited onto silicon substrates precoated with a self-assembled monolayer of benzophenone (BP) silane or directly onto polymer films deposited on solid substrates. Upon irradiation with UV light (lambda = 254 nm), the BP moieties present in the copolymer (and when applicable at the surface of the substrate) are activated, which leads to the simultaneous cross-linking of the polymer chains and surface attachment of the forming gel. The cross-link density of the gels is tuned by varying the mole fraction of MABP in the copolymer ranging from 1% to 10.0% MABP. The swelling of the gels is studied using optical waveguide spectroscopy (OWS) while the (relative) humidity of air in contact with the samples is carefully controlled. To obtain information on the energy of hydration, microcalorimetric experiments are carried out. Swelling in humidity or water and the energy of hydration are compared, and the implications of these parameters on nonspecific protein adsorption and cell adhesion are discussed.