Effects of electronic and recoil processes in polymers during ion implantation
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It has been shown that ion implantation produces remarkable improvements in surface-sensitive mechanical properties, as well as other physical and chemical properties in polymers. To understand mechanisms underlying such property changes, various polymeric materials were subjected to bombardment by energetic ions in the range of 200 keV to 2 MeV. The magnitude of property changes is strongly dependent upon ion species, energy, and dose. Analysis indicated that hardness and electrical conductivity increased by employing ion species with larger electronic cross sections and with increasing ion energy and dose. The results showed that electronic stopping or linear energy transfer (LET, energy deposited per unit track length per ion) for ionization was the most important factor for the enhancement of hardness, while nuclear stopping or linear energy transfer for displacement generally appeared to reduce hardness.
I. INTRODUCTION In 1839, Charles and Nelson Goodyear succeeded in transforming a sticky, fairly useless thermoplastic elastomer (Hevea rubber) into a useful elastomer (Vulcanized rubber) or a hard thermoset plastic (Ebonite) by heating with sulfur, thereby introducing many crosslinks between the neighboring polyisoprene chains. Such a vulcanization process needed a crosslinking agent such as sulfur and the presence of double bonds in the pristine polymer chains. In our recent work,1"4 when polymeric materials were subjected to irradiation by energetic particles, in general, a much higher degree of hardening was observed even in the absence of either a crosslinking agent or unsaturated vinyl group as in vulcanization. Some polymers, however, suffered radiation-induced degradation depending on their structure. Examples of uncrosslinkable polymers include PTFE ( - C F 2 - C F 2 - ) n and acetal ( - C H 2 - O - ) n . It is believed that the unstable nature of the PTFE under irradiation is due to the Coulombic repulsive forces among the large electron clouds of the pendant fluorine atoms which leads to the disintegration of the molecules during irradiation which is also responsible for the low friction coefficient of the material. During irradiation of acetal, monomers, dimers, and a small amount of higher molecular units are emitted, indicating the weak C - 0 bonds under irradiation. The hardness of most unimplanted polymers was found to be less than 0.5 GPa by a nanoindentation method, while that of ebonite was measured to be about 1 GPa. On the other hand, the hardness of crosslinkable polymers that were subjected to ion beam treatments often exceeded 10 GPa. This can be compared to the hardness of stainless steels which is about 3 GPa. Such
hardness increases were also observed previously in ion-implanted organo-silicon polymers5 and PEEK,6 but precise hardness values have not been measured in the previous work because of the unavailability of instruments with nanoindentation capabilities to measure hardness of very thin surface layers. In the past, most radiation studies on polymers were carried out using uv
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