Quantitative Analysis of Polymer Surface Restructuring

Polymer surfaces can adapt to surrounding media by reorientation of surface segments ; such motions can produce changes in contact angles (CAs). Building on two existing formalisms for time-independent CAs of heterogeneous surfaces, treatments are derived for the analysis of time-dependent CAs, enabling quantitative evaluation of the restructuring of polymer surfaces. In particular, we treat the case of air/water CAs on polymers which contain some polar surface groups that are immobile and others that are mobile and can migrate away from the surface by rotational and/or translational motions of polymer chains. A first-order process is assumed for the rate-limiting reorientation motion. The resulting equations have three parameters, viz., the fraction of the surface area covered by mobile polar groups, the fraction of the surface area covered by immobile polar groups, and the characteristic time constant (lifetime) of the reorientation process. The theoretical descriptions were applied to sets of contact angles measured on various plasma-modified polymer surfaces over extended periods oftime, enabling quantitative comparison of the effects of identical plasma treatments on the surface restructuring of different polymers.