Surface structures of thermoplastic and thermoset films after modification by graft copolymerization: Comparative study by x-ray photoelectron spectroscopy and atomic force microscopy

Abstract

Thermal and near ultraviolet-light induced graft copolymerization of pristine, argon-plasma, oxygen-plasma, and ozone-pretreated thermoplastic [high-density polyethylene, poly(tetrafluoro- ethylene), and poly(ethylene terephthalate)], partial thermoset (polyimide), and thermoset (cross-linked polyaniline) films with hydrophilic monomers, such as acrylamide, acrylic acid, and sodium salt of styrene sulfonic acid, were carried out. The microstructures and compositions of the modified surfaces were studied by angle-resolved x-ray photoelectron spectroscopy and atomic force microscopy. In most cases, the density of surface grafting is enhanced by the pretreatment processes. In the case of thermoplastic films, the hydrophilic graft penetrates or becomes partially submerged beneath a thin surface layer which is richer in substrate chains to form a stratified surface structure. In the case of partial thermoset films, the grafted chains form either a surface layer uniformly intermixed with the substrate, or a graft-rich surface layer in a migration that is sterically hindered due to the presence of the grafted chains. In the case of thermoset films, the hydrophilic graft remains predominantly at the outermost surface of the substrate. The surface microstructures are further supported by dynamic water contact angle measurements. The differences in the surface structure and morphology of the pristine, pretreated, and graft copolymerized surfaces are also revealed by atomic force microscopy images.