SURFACE MODIFICATION AND FUNCTIONALIZATION OF POLYMERS VIA GRAFT COPOLYMERIZATION AND ITS RELEVANCE TO ADHESION

Abstract

Surface modifications of Ar plasma pretreated poly(tetrafluoroethylene) (PTFE) film were carried out via near-UV-light induced graft copolymerization with glycidyl methacrylate (GMA). For PTFE substrate with a substantial amount of grafting, the grafted GMA polymer penetrates or becomes partially submerged beneath a thin surface layer of dense substrate chains to form a stratified surface microstructure. The GMA graft copolymerized PTFE surfaces adhere strongly to one another when brought into direct contact. In the presence of diamine alone, failure occurs in the interfacial region. For epoxy adhesive-promoted adhesion, the failure mode is cohesive, i.e. takes place in the bulk of one of the delaminated PTFE films. The interactions between evaporated Al and AAc and GMA graft copolymerized, pristine and Ar plasma treated PTFE surfaces were investigated by in-situ XPS analysis. As evaporation proceeds, aluminum oxide and fluoride, expressed in AlOxFy, aluminum carbide and metallic aluminum are formed. The functional groups arising from graft copolymerization are more reactive toward Al, in comparison with that of fluorine and oxygen already existing on the surfaces. In addition, graft copolymerization can significantly activate the fluorine species and increase their reactivity toward Al. Lap shear adhesion strength of about 130 N/cm2 can be achieved between two PTFE surfaces modified by graft copolymerization with a combination of GMA and REA, and a combination of GMA and HMMAAm. GMA was found to have stronger reactivity in comparison with HEA. HMMAAm showed stronger curing reactivity since it contains both hydroxyl and amine groups which can participate in the curing reaction. Surface modification of ozone-pretreated low density polyethylene (LDPE) films were also carried out via an all-new technique of UV-induced graft copolymerization with concurrent lamination under atmospheric condition and in the complete absence of an added initiator or oxygen scavenger. For LDPE films graft copolymerized with AAm, DMAA and DMAEMA, surface chain rearrangement to form a stratified surface structure with a higher substrate to graft chain ratio at the outermost surface than in the sub-surface layer was observed. However, for those graft copolymerized with DMAPS and NaSS, the stratified microstructure was not pronounced due to the relatively lower graft yields and the steric hindrance of bulky substituents in the two monomers. The surface of the epoxy-based BGA substrate was modified via graft copolymerization with OMA to improve the adhesion with epoxy molding compound. The surface of the BOA substrate became completely covered with OMA polymer after 2 h of near-UV-light induced surface graft copolymerization. Test coupons with surface modified BGA substrate exhibited a minimum of three-fold increase in adhesion strength to the epoxy overmold.