Power law in swelling of ultra-thin polymer films

Abstract

Steady state and dynamic behaviour of swelling of spin coated polyacrylamide films have been studied at room temperature in a precisely controlled environment of 12–99% relative humidity (RH). Swelling of the films was monitored by measuring the thickness of the films using X-ray reflectivity and Atomic Force Microscopy. Swellibility of the films was observed to be independent of thickness of the films indicating no significant influence of substrate polymer interaction or confinement effect in swelling. The water content of the films swelled at different humidity conditions was calculated comparing the electron density of the dry and the swelled films. The volume fraction of water in the swelled films and their swellibility were found to follow double power law behaviour as a function of relative humidity and the transition from one regime to the other was observed around 75% RH value. The behaviour was explained in terms of transition from short to long diffusing channels in the films. Comparison of time evolution of relative humidity of the experimental chamber with that of the dynamics of swellings of the films shows that diffusion of water into the environment was several orders of magnitude faster than the swelling dynamics. The observed values of the diffusion coefficients for the films at different humidity were same whereas the excluded volume parameter, that determine the saturated thickness of the films, was found to have dependence on relative humidity and film thickness.