Ion Beam Modification of PVDC and Pe Polymers
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A. L. EVELYN*, D. ILA*, J. FISHER* AND D. B. POKER** *Center for Irradiation of Materials, Alabama A&M University, Normal AL **Solid State Division, Oak Ridge National Laboratory, Oak Ridge, TN.
ABSTRACT The electronic and nuclear stopping effects produced by MeV ion bombardment in polyvinylidine chloride (PVDC) and polyethylene (PE) are separated by stacking thin films of the polymers. The resulting multi-layer laminates of each polymer were bombarded with 3.5 MeV alpha particles. The energy of the incident ions was selected, using TRIM, such that the first layers experienced most of the effects of the electronic energy deposited and the last layers received most of the effects of the nuclear stopping power. The changes in the conductance and the chemical structure of each layer were measured by direct resistivity measurements and Raman microprobe analysis. INTRODUCTION Polymers are becoming increasingly useful and, as such, their importance has increased very rapidly over the past years. Although they do not exhibit the wide range of electrical and optical properties exhibited by inorganic materials, their ease of production and relatively low cost make them suitable materials for modification by ion beams. It is known that ion-beam irradiation of certain polymer materials leads to the formation of new carbonaceous materials which exhibit enhanced electrical conductivity and an increase in the optical absorption [1-10]. Changes in the appearance of the polymers after bombardment is attributed to the modification of the chemical bonds. It has been shown that the darkening is due to the increase in the absorption in the near UV and shifts gradually toward the red region of the visible spectrum [11 ]. Ion bombardment causes physical damage which includes breaking covalent bonds, scissoring polymer chains, cross-linking and releasing of volatile species. Studies have shown that ion-modified regions can be produced with enhanced mechanical and electrical properties as much as six orders of magnitude, owing to the rapid change in the structure of the polymers chosen [2, 12]. The formation of these regions is shown to be due to the electronic process during ion bombardment rather than the recoil process by nuclear stopping power [13]. The effects caused by the electronic and nuclear stopping cross-sections are inherent in ion-beam irradiation and are related to the energy loss of the incident ions as they traverse their target [14]. We decided to investigate the separate effects of these two stopping powers on the electrical and optical properties of two commercially available, relatively cheap polymers: polyvinylidine chloride (PVDC) and polyethylene (PE). EXPERIMENTAL PROCEDURE We selected the energy of the incident ions using the TRIM 93 [14] computer simulation code, such that the first layer experienced most of the effects of the electronic energy deposited and the last layer received most of the effects of the nuclear stopping power, illustrated in Figure 1. The dotted line separates the first layer from the seco
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