Mechanistic studies of plasma polymerization
Plasma polymerization is a solventless method for depositing polymeric layers onto any substrate at room temperature. This technique comprises excitation, fragmentation, and polymerization of precursor molecules by an electrical discharge. Although widely used, the fundamental molecular processes associated with plasma polymerization are not fully understood. Basic plasma / polymer interactions were studied by investigating the surface treatment of polytetrafluoroethylene (PTFE) using inert and reactive gas discharges. Depending upon the feed gas employed, chemical, UV, or ion beam modification of the PTFE surface were found to be important. Argon glow discharge treatment was found to result in similar physicochemical phenomena at the PTFE surface to that observed during argon ion beam studies, thereby supporting the relative importance of ion bombardment during noble gas plasma modification. In high power discharges it has been shown that extensive ion bombardment of PTFE can lead to the simultaneous sputtering and plasma polymerization of ejected species onto an adjacent substrate. The chemical nature of the resultant fluorocarbon deposits for various gases was found to correlate to the earlier surface treatment studies. Another way of carrying out plasma polymerization is to use pulsed plasmas; these offer the advantage of greater retention of monomer structure within the plasma polymer matrix. In the case of maleic anhydride less fragmentation of the precursor, reduced beam damage of the polymer, and radically initiated polymerization was observed by increasing the off-period of the pulse. Similarly the structural characteristics of 2- iodothiophene plasma polymers were found to be influenced by the electrical discharge power and pulsing parameters leading to a gradual destruction of the aromatic ring structure.