Photo-Cross-Linkable PNIPAAm Copolymers. 2. Effects of Constraint on Temperature and pH-Responsive Hydrogel Layers

Photo-cross-linkable co- and terpolymers of N-isopropylacrylamide, 2-(dimethylmaleimido)-N-ethyl-acrylamide as the photosensitive component, and 3-acryloylaminopropionic acid or N-(2-(dimethylamino)ethyl)acrylamide as ionizable comonomers were prepared by free radical polymerization. Aqueous solutions of the linear un-cross-linked co- and terpolymers showed lower critical solution temperature behavior. The phase transition temperature, which was detected by differential scanning calorimetry, ranged from 23.1 to 39.2 °C depending on the pH of the solution and the composition of the polymer. Surface plasmon resonance and optical waveguide spectroscopy were used to obtain information about the swelling behavior of hydrogel films of the photo-cross-linked polymers, giving a measurement of film thickness and refractive index. The transition temperatures of the cross-linked polymer gels showed similar trends to those of the corresponding linear polymers in solution, and the gels were shown to be both temperature- and pH-responsive, with the transition temperature ranging from 25.3 to 44.9 °C for films having a 200 nm dry film thickness. However, the swelling behavior of the cross-linked gels was found to vary as a function of dry film thickness, and three samples were selected for a more detailed study of how film thickness affects the transition temperature and swelling ratio of hydrogel films. Dry film thicknesses ranged from 9 nm to 2.3 μm, and the swelling behavior of the films fell into two distinct regimes separated by a critical thickness, which ranged from 280 to 500 nm. In the thin-film regime, the transition temperature of the films was independent of film thickness, but the refractive index of the films in the collapsed state decreased as film thickness decreased, indicating that these films are not able to fully collapse. In the thick-film regime, the swelling ratio of the films was independent of film thickness, but the transition temperature decreased as much as 2.6 °C as the film thickness increased. This was explained by the constraint imposed on the film by the presence of a fixed substrate, with the length scale of this constraint related to the critical thickness. In these films, the ionizable comonomers were found to have little effect on the swelling ratio, which is determined primarily by cross-linking density in the swollen state and by film thickness in the collapsed state.