Photo-cross-linkable PNIPAAm copolymers. 1. Synthesis and characterization of constrained temperature-responsive hydrogel layers

Photo-cross-linkable co- and terpolymers of N-isopropylacrylamide (NIPAm), 2-(dimethylmaleimido)-N-ethylacrylamide (DMIAAm) as the chromophore, and NN-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 (T-c) was detected by differential scanning calorimetry (DSC). T-c decreased with increasing amount of DMIAAm, as low as 24.7 degreesC for 9.2 mol % DMIAAm, and increased with increasing DMAAm content, as high as 59.5 degreesC 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 (DeltaT(c)) 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 T-c was around 220 nm and only weakly dependent on chromophore content. However, at temperatures below T-c, 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 T-c.