Structure and Dose Effects on Ion Beam Modification of Polymers
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STRUCTURE AND DOSE EFFECTS ON ION BEAM MODIFICATION
OF POLYMERS
G. R. Rao*, E. H. Lee and L. K. Mansur Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831 * On Assignment from Auburn University, Auburn, AL 36849. ABSTRACT Polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyethersulfone (PES) were implanted with 200 keV boron ions at three different doses. PS was also implanted with 100 keV boron. A nanoindentation technique and reciprocating sliding wear tests were used to characterize mechanical properties of the implanted polymers. The results showed that hardness increased with increasing dose as well as increasing energy. The percentage increase in hardness was reduced with increasing complexity of the side groups although absolute hardness values were higher. With increasing complexity of the main backbone. pclymer chain, the percentage increase in hardness as well as numerical hardness values were lower. These effects were attributed to the relative contributions of cross-linking and chain scission which were viewed as competing processes. Wear tests showed that in general, a lower or intermediate implant dose yielded the best improvement in wear resistance. No specific trends were observed for relating friction coefficient values and improved wear resistance. The results indicate that wear is a complex phenomenon which has a sensitive dependence on hardness and elastic properties of the mating surfaces. INTRODUCTION Recent studies at Oak Ridge National Laboratory (ORNL) have shown that ion implantation can significantly improve surface mechanical properties of polymers [1]. Until now, most studies have focussed on improving optical and electrical properties of ion implanted polymers (for example ref [2-4]). However, implantation of energetic ions can also have significant effects on properties such as hardness and wear in polymers. While ion implantation, in general, improves the near surface hardness of polymers, one of our recent studies showed that improvement in wear properties have a sensitive dependence on the implant dose. For boron implantation in polycarbonate at energies of 100 and 200 keV, it was observed that an intermediate dose gave the best improvement in wear and a corresponding 40% decrease in friction coefficient value [5]. The present study was undertaken to explore the effects of polymer structure, ion dose and ion energy on modification of surface mechanical properties of polymers by ion implantation recognizing that these could be significant factors for technological applications of ion implanted polymers in the future. EXPERIMENTAL Four polymers were selected for this investigation: polyethylene (PE), polypropylene (PP), polystyrene (PS) and polyethersulfone (PES). These polymers were chosen for the increasing complexity of the side groups as well as the type of backbone chain (in the case of PES) as shown in figure 1. The polymers were in the form of 76 lim thick films except PP which was obtained from 1 mm thick sheets. The polymers were im
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