A physicochemical study of oxygen plasma-modified polypropylene

The physicochemical properties of oxygen treated polypropylene (PP) films have been studied using a series of analytical techniques with different degrees of surface sensivity. The treated polymer surfaces were found to be acidic by X-ray photoelectron spectroscopy (XPS) in conjunction with the molecular probe technique, a result in agreement with previous wettability studies. Dimethyl sulfoxide (DMSO) has been used for the first time as a molecular probe to investigate the surface acidity of O-2 plasma treated PP. The retention of DMSO increased with plasma treatment time and is thus governed by surface modification which permits acidic groups to be grafted onto the host polymer. Such retention is depicted in the binding energy of S2p from DMSO. DMSO contact angle measurements showed that the treated surfaces exhibit acid-base character from the initial stage of plasma treatment as judged by the acid-base contribution of the total work of adhesion (W-DMSO-PP(ab)) which was found to readily reach a plateau at 14 mJ m(-2). We have used capillary electrophoresis ion analysis (CIA) to identify the acidic low molecular weight fragments formed at the plasma-treated surface and found that they are mainly oxalic, malonic and fumaric acids, a result which is confirmed by ATR-IR analyses.