Transmission in irradiated hydroxyethyl methacrylate copolymer at elevated temperatures
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C.C. Han Polymer Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received 11 February 2002; accepted 3 June 2002)
Transmission losses were monitored in the ultraviolet-visible spectra of irradiated hydroxyethyl methacrylate (HEMA) copolymer at elevated temperatures. The transmission in irradiated HEMA in the ultraviolet and visible wave length range was almost the same for doses 400 kGy 艋 ⌽ 艋 1000 kGy, but was smaller than that of the nonirradiated HEMA copolymer. The reduction in transmission in the irradiated specimens was attributed to the presence of color centers. The concentration of color centers was enhanced by thermal annealing. The transmission data (or absorption data) at 467 nm was found in good agreement with the theoretical model in which the color center production followed a first-order kinetic process. The rate constant satisfies the Arrhenius equation, and the corresponding activation energy is 17.37 kJ/mol and is independent of the dosage. The results were compared with those reported in the literature.
I. INTRODUCTION
The hydroxyethyl methacrylate (HEMA) copolymer is one of the most useful bio-polymers because it exhibits a high-equilibrium water content and is bio-compatible.1–4 The hydroxyl group and carbonyl group on each repeat unit make the HEMA compatible with water; the hydrophobic ␣-methyl group and backbone provide hydrolytic stability to the HEMA and impart mechanical strength to the polymer matrix.5–7 The “structure” of the water in crosslinked HEMA gels, mass transport, and equilibrium swelling have been investigated by many researchers.8–16 When polymeric materials are exposed to gamma rays, their properties are altered in different ways. For example, the backbones may undergo scission or crosslinking.17–22 Scission leads to a reduction in molecular weight, while crosslinking increases the molecular weight. Fujisawa et al. observed that the hardness and wearability of ultrahigh molecular weight polyethylene increased with gamma-ray dose.23 Chou et al. observed transmission losses in the ultraviolet-visible (UV-vis) spectrum that increase with increasing gamma-ray dose.14,15 Todd24 and David et al.25 investigated the volatile products in the irradiated poly(methyl methacrylate). Ohnishi and Nitta measured the formation rate of free radicals in irradiated poly(methyl methacrylate),26 and Kusy and Katz determined the fracture surface energy as a function of viscosity average molecular weight in the irradiated poly(methyl methacrylate) (PMMA).27 2260
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J. Mater. Res., Vol. 17, No. 9, Sep 2002 Downloaded: 17 Mar 2015
Color centers have drawn attention for several decades. LiF single crystals with color centers may absorb nonlinearly high-power optical radiation and have been used as passive laser switches.28 These color centers are very stable at room temperature. Lin et al. studied transmission in irradiated LiF single crystals at high temperatures as a function of annealing time, but they did not fully explore th
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