title: Thermo-reversible swelling of thin hydrogel films immobilized by low-pressure plasma.
abstract: Thin films of graft copolymers consisting of poly(N-isopropylacrylamide) (PNiPAAm) or poly(N,N-diethylacrylamide) (PDEAAm) as polymer backbone and poly(ethyleneglycol) as side chains were cross-linked on fluoropolymer substrates by low-pressure plasma treatment. All immobilized polymers exhibit a lower critical solution temperature between 34 and 40 degrees C. The swelling and collapsing of the hydrogels was examined with temperature-dependent spectroscopic ellipsometry. Two time ranges of swelling were observed: a fast 'dynamic' and a slow 'equilibrium' swelling. The dynamic swelling occurs within minutes or less, whereas the equilibrium swelling needs several days to complete. The surface-bound hydrogels show a shift in the transition temperature toward lower temperatures compared with the behavior in solution. Full reversibility of the dynamic swelling/collapsing was found, but the temperature scan exhibits a hysteresis between heating and cooling cycles. The PNiPAAm-containing hydrogels show a sharper transition compared to the PDEAAm-containing hydrogels, which is almost linear over a wide temperature range.
authors.affiliation: Welsh School of Pharmacy, Redwood Building, King Edward VII Avenue, Cardiff University, Cardiff, CF10 3XF, Wales, UK. schmaljohannd@cf.ac.uk
authors.forename: Dirk
authors.initials: D
authors.lastname: Schmaljohann
authors.forename: Detlev
authors.initials: D
authors.lastname: Beyerlein
authors.forename: Mirko
authors.initials: M
authors.lastname: Nitschke
authors.forename: Carsten
authors.initials: C
authors.lastname: Werner