SURFACE MODIFICATION OF POLYMERS : GRAFT COPOLYMERIZATION AND SURFACE CHARACTERIZATION

Abstract

Angle-resolved X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), water contact angle hysteresis and atomic force microscopy (AFM) were used to study the surface composition, microstructure and morphology of the pristine, Arplasma-, Oz-plasma- and 03-pretreated polymeric films before and after graft copolymerization. The polymeric films used include thermoplastics, such as, low-density and high-density polyethylene (LDPE and HDPE), polypropylene (PP), polystyrene (PS), polycarbonate (PC), polyethylene terephthalate (PET) and polytetrafluoroethylene (PTFE), partial-thermostats, such as, polyimide (PI) and thermoset, such as, epoxy and cross-linked polyaniline (PAN). Surface modification via near-UV-light induced graft copolymerization of these polymeric films were carried out using hydrophilic monomers including acrylamide (AAm), acrylic acid (AAc) and sodium-salt of 4-styrene sulfonic acid (NaSS). With the exception of PTFE film which requires Ar-plasma pretreatment, graft copolymerization of all the pristine polymer films were successfully carried out. Surface pretreatment of the polymeric films by Ar-plasma, O2-plasma and O3 were shown to enhance substantially the efficiency of surface grafting. This phenomenon is consistent with the peroxide-initiated mechanism for near-UV light-induced graft copolymerization. For the thermoplastic polymer film surfaces graft copolymerized with AAm and AAc polymers, the polymer grafts were found to penetrate into the substrate polymer to form a subsurface layer (the complete penetration model). The evidence of this stratified structure is provided by angle-resolved XPS, SIMS and contact-angle hysteresis data. The surface layer which is richer in the substrate polymer is estimated, from mean-free-path calculations, to be of the order of 2-3 nm thick. The presence of the graft in the subsurface layer could be attributed to the migration and counter-migration of the substrate polymer and the graft chains. In the graft, copolymerization with NaSS polymer, the polymer graft is shown to only partially penetrate the substrate polymer (the partial penetration mode]). This less efficient penetration of the grafted NaSS polymer at the surface must have resulted from the sterically hindered migration of the graft chains arising from bulky substituents. For the partial-thermoset PI films graft copolymerized with AAm and AAc polymers, the grafted polymers are shown to be retained on a surface layer arising from the uniform intermixing of the graft and the substrate polymer chains (the intermixing model). For the NaSS grafted polymer with its bulky substituent, a similar surface and interfacial structure as that of the NaSS graft copolymerized thermoplastic substrates was observed.