Proton Modification of Ultra High Molecular Weight Polyethylene to Promote Crosslinking for Enhanced Chemical and Physic
- PDF / 447,630 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 17 Downloads / 234 Views
observed UHMW-PE contained dissolved oxygen and it was impossible to eliminate oxidation completely during irradiation.[8,9] These studies noted that irradiation environment was extremely important for effective crosslinking with gamma rays and that presence of oxygen decreased the extent of crosslinking. As ion implantation is done in vacuum, one would expect better crosslinking. Nonetheless, it is practically impossible to exclude all absorbed and adsorbed oxygen from the polymer. The greater problem is post oxidation of any remaining free radicals. Linear energy transfer(LET) is another important parameter for successful polymer modification. Early studies used gamma radiation or fast electrons which have lower cross sections for ionization than these or heavier, high energy ions. Gamma radiation and electron beams penetrate to greater depths than most high energy ions and linear energy transfer is much less.[10J The higher LET for ion implantation can lead to excessive radiation damage and carbonization. Lee et al studied 200 keV to 2 MeV ion implantation for various polymers.[1 1] Rao et al studied 4 4 implantation of polycarbonate and PEEK with 2 MeV B , O,2 and 1 MeV Ar to fluences as high as 8
2
5x10' ions/cm . At highest dose observed, 5x1018 ions/cm , both polycarbonate and PEEK turned shiny black and hardness increased with dose. Ion ranges were 2-4 ulm, and considerable energy was
deposited in near surface region(high LET).[12,13] The improvement in hardness observed by Rao et al at highest fluence was probably related to carbonization and not crosslinking. Puglisi et al observed crosslinking in polystyrene of narrow molecular weight distribution at much lower implanted doses(100 keV He+ ions at 1012_1013 ions/cm 2).[14] Crosslinking in the absence of chain scission increases the molecular weight and is effectively measured by gel permeation chromatography(GPC) until crosslinking reaches the point where the polymer is no longer soluble in suitable solvents. The GPC columns are filled with a high porosity filler(2 gm pores) which allows lower molecular weight fragments to pass and excludes high molecular weight constituents. The high molecular weight fragments leave the column first(short elution times) followed by lower molecular weight fragments. The distribution is obtained as a plot of the refractive index of UHMW-PE in solution. As the concentration of UHMW-PE in solution changes, the refractive index changes. This change in refractive index is directly related to the molecular weight distribution. GPC measures both relative molecular weight and molecular weight distribution. FTIR can be used to determine radiation damage and to evaluate the final products. Increases in methyl and butyl end groups are a measure of chain scission during irradiation. Typical products show 1 up as reduced CH2 peak intensity at 1368 cm- [15], carbonyl at 1720 cm [8,9], and double carbon 1 bond at 1640 cm .[6,15] Carbonyl was observed by Streicher and Ishitani et al for gamma irradiation and high energy ions, respective
Data Loading...
