QUANTITATIVE-ANALYSIS OF POLYMER SURFACE RESTRUCTURING

Polymer surfaces can adapt to surrounding media by reorientation of su rface segments; such motions can produce changes in contact angles (CA s). Building on two existing formalisms for time-independent CAs of he terogeneous surfaces, treatments are derived for the analysis of time- dependent CAs, enabling quantitative evaluation of the restructuring o f 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 r otational and/or translational motions of polymer chains. A first-orde r process is assumed for the rate-limiting reorientation motion. The r esulting equations have three parameters, viz., the fraction of the su rface area covered by mobile polar groups, the fraction of the surface area covered by immobile polar groups, and the characteristic time co nstant (lifetime) of the reorientation process. The theoretical descri ptions were applied to sets of contact angles measured on various plas ma-modified polymer surfaces over extended periods of time, enabling q uantitative comparison of the effects of identical plasma treatments o n the surface restructuring of different polymers.