Fourteen-year-old aging study of the effect of thickness on methanol transport in crosslinked poly(methyl methacrylate)
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Fourteen-year-old aging study of the effect of thickness on methanol transport in crosslinked poly(methyl methacrylate) Sanboh Lee Department of Materials Science, National Tsing Hua University, Hsinchu, Taiwan, Republic of China (Received 8 June 1995; accepted 20 May 1996)
The effect of thickness on methanol transport in fourteen-year-old crosslinked poly(methyl methacrylate) was investigated. The samples studied here are from the same primary source of those used by a study made fourteen years earlier. The sample was encapsulated by a plastic bag and maintained in a desiccator at room temperature. Four thicknesses, 0.6, 1.0, 1.5, and 1.9 mm, were examined. Methanol sorption data were fit to a model in which the mass sorption is a combination of case I, case II, and anomalous sorption. The diffusion coefficient for case I transport increases with increasing thickness, but the velocity for case II transport does the opposite. The diffusion coefficient for case I transport and the velocity for case II transport exhibit the Arrhenius behavior. The activation energies for case II transport are 18.9, 16.3, 14.6, and 13.4 kcalymole, corresponding to the thicknesses 0.6, 1.0, 1.5, and 1.9 mm, respectively. The activation energies for case I transport are 24.7, 24.2, 21.7, and 21.9 kcalymole, corresponding to the thicknesses 0.6, 1.0, 1.5, and 1.9 mm, respectively. For thickness 1.5 mm the activation energies for case I and case II transport are 21.7 and 14.6 kcalymole for this study and 24.9 and 17.3 kcalymole obtained fourteen years ago. Organic solvent transport in a polymer is commonly classified as case I, case II, and anomalous transport. Case I and case II are controlled by the concentration gradient and swelling (or stress relaxation), respectively. Anomalous transport is a combination of case I and case II behavior. Kwei and co-workers1–5 proposed an equation for anomalous transport which reduces to case I and case II, respectively. They applied this equation to the methanol-poly(methyl methacrylate) system. Harmon et al.6–8 modified Kwei’s equation to account for specimens of finite size. They studied the effect of deformation on transport of methanol in poly(methyl methacrylate). In this communication the effect of thickness on the methanol transport in fourteen-year-old crosslinked poly(methyl methacrylate) is investigated. The results are compared with those in the literature. Crosslinked PMMA (Electroglas) contact lens buttons were obtained from Glasflex in Sterling, NJ, fourteen years ago. These buttons were encapsulated by a plastic bag and put into a desiccator at room temperature. The samples were polished by grid carbimet paper and then followed with aluminum slurries. Final dimensions of the specimens were 12.5 mm in diameter and 0.6, 1.0, 1.5, and 1.9 mm in thickness, respectively. Next, the samples were annealed in a vacuum chamber at 130 ±C for 24 h and furnace cooled to approximately 25 ±C. Each specimen was immersed in J. Mater. Res., Vol. 11, No. 10,
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