Photo-cross-linkable co-and terpolymers of N-isopropylacrylamide (NIPAAm), 2-(dimethylmaleimido)-N-ethylacrylamide (DMIAAm) as the chromophore, and N,N-dimethylacrylamide (DMAAm) were prepared by free radical polymerization. Aqueous solutions of the co-and terpolymers showed lower critical solution temperature (LCST) behavior, and the corresponding phase transition temperature (Tc) was detected by differential scanning calorimetry (DSC). Tc decreased with increasing amount of DMIAAm, as low as 24.7 °C for 9.2 mol % DMIAAm, and increased with increasing DMAAm content, as high as 59.5 °C for 52.6 mol % DMAAm. The resulting polymers were shown to be photo-cross-linkable, and the sensitivity of the polymers toward UV light was studied by monitoring the photo-cross-linking reaction with ATR-FTIR. With 2 wt % thioxanthone as the photosensitizer, nearly full conversion could be achieved with 10 min of irradiation even though the photo-cross-linking was performed in the glassy state. Surface plasmon resonance (SPR) spectroscopy and optical microscopy were used to obtain information about the swelling behavior of thin hydrogel films. In the SPR scans the plasmon resonance minimum and the first waveguide mode were fit to Fresnel calculations to determine the refractive index (n) and the layer thickness (d) of the hydrogel. The volume degree of swelling was calculated from the refractive index, and the swelling ratio was calculated from the layer thickness. Changes in the degree of swelling, T c, and the width of the transition (∆Tc) were observed by changing the chromophore content and, as a result, the gel cross-linking density. For a hydrogel film with a dry thickness of 200 nm, the collapsed film thickness above Tc was around 220 nm and only weakly dependent on chromophore content. However, at temperatures below Tc, the swollen film thickness was strongly dependent on the chromophore content and ranged from 1200 nm for 2.4 mol % DMIAAm to 800 nm for 9.2 mol % DMIAAm. The reverse is true for the refractive index, which increases as the film thickness decreases. A comparison of volume degree of swelling and swelling ratio was utilized to demonstrate the high anisotropy of swelling in these hydrogel layers that were physisorbed to the substrate and therefore constrained from expanding or contracting laterally. The swollen film expanded 9.5% laterally as compared to the dry film, and this value appeared to be independent of temperature. The swelling perpendicular to the substrate ranged from 6.4% for 9.2 mol % DMIAAm at temperatures above T c to 630% for 2.4 mol % DMIAAm at temperatures below Tc.
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.
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