Surface structures and adhesion characteristics of poly(tetrafluoroethylene) films after modification by graft copolymerization

Abstract

Surface modifications of Ar plasma-pretreated poly(tetrafluoroethylene) (PTFE) film were carried out via near-UV light-induced graft copolymerization with acrylic acid (AAc), Na salt of styrenesulfonic acid (NaSS) and N,N-dimethylacrylamide (DMAA). The structure and chemical composition of each copolymer surface and interface were studied by angle-resolved X-ray photoelectron spectroscopy (XPS). For each PTFE substrate with a substantial amount of grafting, the hydrophilic graft penetrated or became partially submerged beneath a thin surface layer of dense substrate chains to form a stratified microstructure. This surface microstructure was further confirmed by the water contact angle results. In all cases, the concentration of surface grafted polymer increased with the plasma pretreatment time and monomer concentration. Adhesive-free adhesion studies revealed that the AAc graft copolymerized PTFE surface adhered strongly to another similarly modified surface (homo-interface) or to a DMAA graft copolymerized surface (hetero-interface) when brought into direct contact in the presence of water and subsequently dried. The lap shear strengths for both types of interface exceeded the yield strength of the PTFE film. The development of the adhesion strength depends on the concentration of the surface graft, the microstructure of the grafted surfaces, the nature of the interfacial interaction, and the adhesion (drying) time.