Surface Dynamics of Polymer Glasses: Sub-Tg Surface Reorganization in End-Functional Polymers

Abstract

Dynamic water contact angle analysis is applied to characterize the surface reorganization of glassy polystyrenes (PS), terminated with either a fluorosilane or carboxylic acid end group, upon exposure to an atmosphere of saturated water vapor. Fluorosilane end groups are initially adsorbed preferentially at the surface and diffuse away from the surface when exposed to water vapor. Carboxylic acid end groups are initially depleted from the surface and are drawn to the surface when exposed to water vapor. The time dependence of the surface composition during reorganization, determined by application of Cassie’s equation, scales with the square root of time, consistent with a diffusive process. Angle dependent X-ray photoelectron spectroscopy (ADXPS), applied to characterize the surface concentration depth profiles of the fluorosilane-terminated PS before and after exposure to water vapor, indicates that reorganization in the glassy state involves motions on a length scale of about 2 nm. Modified lattice model calculations, assuming that reorganization can occur only over length scales of this magnitude, are found to provide a reasonable representation of ADXPS surface composition depth profiles, supporting the conclusion that the length scale for surface reorganization of end-functional polymers in the glassy state is of the order of a few nanometers. When this length scale is coupled with the dynamic contact angle data, apparent diffusion coefficients in the range of 10–13 to 10–10 cm2/s are obtained. Analysis of the temperature dependence of the apparent diffusion coefficients, found to be an activated process following Arrhenius behavior, yields activation energies of 137 kJ/mol for fluorosilane-terminated PS and 39 kJ/mol for PS terminated with a carboxylic acid end group, considerably lower than experimental values determined from analysis of either polymer–polymer interdiffusion or free surface dynamics. These activation energies are a closer match to those of the β-relaxation of bulk PS than they are to the α-relaxation of either the PS surface or the PS bulk, suggesting that surface reorganization in glassy PS can occur by virtue of short-range motions characteristic of a surface activated β-relaxation occurring over length scales of a few nanometers.