Static and dynamic swelling of grafted poly (2-alkyl-2-oxazoline)s

Poly(2-methyl-2-oxazoline) (PMOX) and poly(2-ethyl-2-oxazoline) (PEOX) bearing trimethoxysilane end groups for surface coupling were synthesized and grafted onto silicon/silicon dioxide substrates. Static and dynamic hydration of these films by water vapor was quantitatively studied by ellipsometry. The static swelling behavior was analyzed by measuring the equilibrium film thickness as a function of the relative humidity to obtain quantitative force-distance curves. The disjoining pressure measures the sum of all operating forces in equilibrium with the chemical potential of the surrounding atmosphere, where the dominant forces could be determined. The hydration of polymers with different initial thickness was compared by the normalized swelling ratio p and the calculated decay constant lambda*. All the measured parameters exhibited a clear dependence upon the chain length. The dynamic swelling behavior was studied by monitoring the film thickness as a function of time under an osmotic shock, switching from dry to humid atmospheric conditions (relative humidity from similar to4% to similar to90%). The characteristic time constants were analyzed for each derivative semiquantitatively, indicating that the kinetics of the water uptake is dependent on the polymer length rather than on the side chains.