Structural effects on swelling of thin phosphorylcholine polymer films

Spectroscopic ellipsometry (SE) has been used to monitor the time-dependent process of swelling of phosphorylcholine (PC) biocompatible polymer films of different initial dry film thicknesses. Factors including the extent of silyl cross-linker, ratio of hydrophilic and hydrophobic moieties, annealing temperature, and the composition of a set of polymer blends were assessed. The swelling of PC polymer films was found to follow a two-stage mechanism: a fast film expansion at the beginning of film immersion, followed by a much slow process controlled by the relaxation of polymer fragments. The two-stage swelling was well described by the coupled model developed by Berens and Hopfenberg, which assumed a simple superposition between the fast Fickian diffusion and the exponential decay accounting for the slow polymer fragment relaxation. Increase in annealing temperature to films incorporating 5 mol % silyl cross-linkers reduced the rate of swelling, which was attributed to the combined action of structure ordering and the strengthening of silyl cross-linking network. Increase in hydrophilic moieties substantially promoted the rate of swelling and their equilibrated water uptake, as expected. In the case of the blends from the two polymers with and without 5% silyl cross-linker, the swelling behavior was dominated by the crosslinked polymer.