Surface restructuring of polymeric solids and its effect on the stability of the polymer—water interface

Abstract

Polymeric solids possess the distinct capacity to alter their surface structures between different environments in order to minimize their interfacial free energy with a surrounding medium and thereby minimize the free energy of the system as well. This can sometimes lead to very low values of polymeric solid—environmental fluid interfacial free energies. Under such conditions, the solid—fluid interface is likely to become unstable to mechanical and/or thermal perturbations and this interfacial instability can promote the occurrence of undesirable effects such as the absorption of the fluid into the solid and/or the dissolution of the solid in the fluid. The above considerations were investigated on three types of polymeric surfaces, namely, control Teflon FEP, sputtered Teflon, and chemically etched-sputtered Teflon, which gave rise to high, medium, and low interfacial free energies with water, respectively, as a result of their surface restructuring in the aqueous environment. The dynamics of surface restructuring of the three surfaces under water were followed by measuring the changes in the contact angles of water drops on the solid specimen under octane, as a function of time. The solid—water interfacial free energies of the specimen were estimated by a suitable contact angle procedure which was developed for characterizing the wetting properties of such mobile surfaces in an aqueous environment. Based on the dynamics of solid surface restructuring under water and the contact angle procedure, it was seen that, only in the case of the chemically etched-sputtered solid surface (which reduces its interfacial free energy with water to a very low value as a result of its surface restructuring in the aqueous environment), the polymeric—solid interface becomes unstable and gives rise to the penetration of water into the solid and the possible dissolution of the solid in water. These contact angle results were well supported by the physicochemical characteristics of the different polymer surfaces used in this study.