Study of the effects of MeV Ions on PS and PES
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EXPERIMENTAL PROCEDURE Self supporting thin films of PES were fabricated from pellets of Radel A 200 polyethersulfone, obtained from Amoco Performance Products of Alpharetta, GA. The pellets were dissolved in methylene chloride and spun coated to produce films with thicknesses on the order of 4 microns. The PS samples were prepared by spin coating a solution of PS onto silicon wafers creating - 1.5 pxm thick supported films. PES films were stacked and bombarded with 5 MeV alpha particles. The SRIM (1996 version of TRIM) computer simulation code [I] was used to predict the penetration depth of the bombarding ions, This allowed a determination to be made of the required number of layers needed to stop the ions in the PES, to ensure that the ion tracks would not extend beyond the last laminae of the film stacks. SRIM predicted an ion penetration depth of 30 pm in the PES for the 5.0 MeV alpha particles. This required eight layers of the thin polymer films. The film stacking technique resulted in the first layers experiencing most of the effects of the deposited electronic energy and the last layer receiving almost all of the effects of the nuclear stopping power. A rastered incident ion beam delivered 5.0 MeV alpha particles, at fluences of 10"3, 10"4, 10'5 and 10"6 ions/cm 2 , to the target. The beam current density was kept below 35 nA/cm 2 to allow enough time for volatile gases, produced by the decomposing polymer, to diffuse out of the films and to allow shrinking without cracking. The films were attached to aluminum holders using silicone grease which allowed the films to slide along the surface of the holders during shrinking. Ion ranges for the bombarding alpha particles extended to 7 layers. Changes in the chemical composition and structures of the PES and PS were analyzed by Raman microprobe analysis, Fourier transform infrared spectroscopy (FTIR), residual gas analysis (RGA) and Rutherford backscattering spectrometry (RBS). RESULTS AND DISCUSSION Raman spectra from the irradiated PES films show the presence of two peaks around 1595 cm' (G-line) which is attributed to graphene structure formation and around 1350 cm' (D-line) which is attributed to amorphous structure or "disorder" in the material. Figure 1 shows the Raman spectra for three layers of the 5 MeV alpha irradiated PES films to a fluence of 1 x 1016 ions/cm 2. It shows that the D/G ratio decreases from the first layer to the fourth then increases in the last layer. The electronic stopping power which dominates in the uppermost layers and which is effected by the greater ion energies is responsible for causes the sulfone group in the PES polymer chain to enhance the crosslinking of polymers. As the ions lose energy in the last layers the nuclear stopping, which accounts for most of the molecular dissociation and bond breakage, results in increasing D/G ratios as the crosslink enhancing group (sulfone) becomes disrupted. This effect was also seen in the films irradiated to 5x10'5 cm 2 .Figure 2 compares the Raman spectra from the first layers of the
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