Photo-cross-linkable PNIPAAm copolymers. 4. Effects of copolymerization and cross-linking on the volume-phase transition in constrained hydrogel layers

Photo-cross-linkable temperature and pH-responsive polymers based on N-isopropylacrylamide and 2-(dimethylmaleimido)-N-ethyl-acrylamide were synthesized and spin-coated to produce uniform thin films of cross-linked, responsive hydrogels. Surface plasmon resonance and optical waveguide spectroscopy were used to determine the effect on the volume-phase transition when these materials are confined to a thin film. The film thicknesses ranged from 10 nm to 2.5 mum, and the volume-phase transition temperature and swelling ratio of the films fell into two overlapping regimes separated by at least one critical thickness. In the thick-film regime, greater than 100-760 nm, depending on the cross-linking density, ionizable comonomer concentration, and reference state, the films exhibited two transition temperatures. This can be explained by the stress imposed on the hydrogel as it swells perpendicular to the substrate. In the thin-film regime, less than 270-440 nm, depending on cross-linking density, the presence of a fixed substrate also limited the collapse of the gel at temperatures above the volume-phase transition temperature. Nonequilibrium chain conformations resulting from the spin-coating process may explain this effect. Two different mechanisms for constraint are likely in the thin- and thick-film regimes, and existing models for the anisotropic swelling of hydrogel layers were used to explain these trends. The spin-coated hydrogel layers were also compared to dip-coated and free-radical polymerized hydrogel layers to provide additional insight to the constraint mechanism. The elastic modulus of the hydrogel layers was measured with atomic force microcopy. The modulus in the swollen state was a function of both the cross-linking density and the ionizable comonomer concentration and ranged from 4.48 to 26.6 kPa. The modulus in the collapsed state was primarily a function of the cross-linking density and ranged from 466 to 1540 kPa. The effect of the ionizable comonomer concentration on the modulus also suggests the presence of non-Gaussian chain conformations in the swollen network.