Contact angle hysteresis on oxygen plasma treated polypropylene surfaces

Abstract

Due to the preparation method or to surface treatments, solid surfaces are very often far from equilibrium. In all materials the thrust is to minimize the free energy, therefore surface properties are in principle time dependent, but it is possible to observe modifications only if relaxation mechanisms are fast enough to be observed. Many materials (e.g., metals or ceramics) are very rigid and surface immobility is implicitly assumed. On the contrary, a growing body of experimental data points to the mobility of polymer surfaces. Back in 1938 Langmuir described the rearrangement of hydrophilic groups on a solid surface (1). In the last few years increasing attention has been devoted to polymer surface dynamics, as reflected for instance in two books edited by J. D. Andrade (2, 3 ). In these studies the measurement of contact angle hysteresis is fundamental. Despite the simplicity of experimental apparatus (especially when compared with other surface sensitive techniques), contact angle data provide precious information about the behavior of polymer surfaces. Plasma treatments are a well-known method to increase the wettability of polymer surfaces; it is also well known that plasma treated polymer surfaces are subject to aging (4-7). In this note we present a study of contact angle hysteresis during aging in air of oxygen plasma treated polypropylene at different temperatures. Following contact angles as a function of aging time at different temperature