On the nature of volume-phase transitions in photo-cross-linked poly(cyclopropylacrylamide) and poly(N-vinylisobutyramide) coatings

Abstract

Ellipsometry in an inverted configuration was used to characterize the temperature-dependent swelling of thin (100 nm) coatings of photo-cross-linked poly(cyclopropylacrylamide), or poly(CPAAm), and poly(N-vinylisobutyramide), or poly(NVIBAm). Both polymers contained 3 mol% of methacroylaminobenzophenone (MnBP) as the photo-cross-linking unit. Poly(CPAAm-co-MnBP) showed a continuous, 2^nd order deswelling transition between 10 and 70 °C with no hysteresis. Poly(NVIBAm-co-MnBP), on the other hand, showed a discontinuous, 1^st order deswelling transition at 45 °C with a hysteresis in the swelling curve. The two types of swelling transitions are consistent with the cloud-point measurements of the uncross-linked polymers, which both showed lower-critical solution temperature (LCST) behavior. The LCST for poly(CPAAm-co-MnBP) is located at almost zero polymer concentration, and hence cross-linked poly(CPAAm) cannot cross its cloud point curve during swelling or deswelling. The LCST for poly(NVIBAm-co-MnBP), in contrast, is located at a significant off-zero polymer concentration (>10 wt% polymer), meaning cross-linked poly(NVIBAm) can cross its cloud point curve and experience a discontinuous jump in swelling. Concurrent measurements of the infrared vibrations of the amide groups in both systems further revealed that the amide I band in poly(CPAAm-co-MnBP) showed a single sub-band throughout the swelling transition. Poly(NVIBAm-co-MnBP), on the other hand, showed two distinct sub-bands, one of which grew at the expense of the other throughout the swelling transition, which may explain the off-zero concentration in the cloud point curve and the discontinuous swelling transition.