Swelling of zwitterionic polymer films characterized by spectroscopic ellipsometry

The swelling of thin biocompatible polymeric films under water has been characterized by spectroscopic ellipsometry. The films were formed by coating a copolymer (PC 100B) onto optically flat silicon oxide surface. PC 100B has a polymethacrylate backbone bearing pendent zwitterionic phosphorylcholine (PC) groups (23 mol %), lauryl chains (47%), 2-hydroxypropyl groups (25%), and trimethoxysilane groups as cross-linker (5%). Film swelling as a result of water sorption was monitored by following the change in two ellipsometric angles (psi and Delta) with time using dry films of a wide range of thickness. The swelling pattern was characterised by two distinct stages, an initial diffusion-controlled fast swelling, followed by a subsequent slower process controlled by the relaxation of polymer fragments. Although the fast initial stage occurred within the first minute of film immersion in water, some 50% water was absorbed into the films during this period. The rate of swelling in the second stage showed a steady decrease with the increasing annealing temperature. The equilibrated water content (f(w,infinity)) was found to be some 60% for films annealed at 50 degreesC, 45% at 100 degreesC, and 30% at 150 degreesC, thus indicating that as the silyl network is strengthened with temperature its capacity for water uptake is suppressed. The values of f(w,infinity) calculated from the refractive indices of the films were broadly consistent with those determined separately from the increase in film thickness. This observation suggests that the swelling is almost linear over the period of the experiment and scales with the initial sample volume. The result is consistent with the assumption of homogeneous swelling used in the modeling and rules out any case II type of water distribution within these PC hydrogel films.