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Sorption of CO2 in polymers observed by positron annihilation technique
Authors:Y Ito  H F M Mohamed  K Tanaka  K Okamoto  K Lee
Institution:(1) Research Center for Nuclear Science and Technology, The University of Tokyo, 319-11 Tokai Ibaraki, (Japan);(2) Faculty of Engineering, Yamaguchi University, 755 Ube, Yamaguchi, (Japan);(3) Institute of Advanced Engineering (IAE), Yongin, P.O. Box 25, 449-800 Kyonggi-Do, (Korea);(4) Present address: El-Minia University, Egypt
Abstract:Positron annihilation lifetimes were measured for several polymers in the atmosphere of high pressure CO2 gas. At low CO2 pressured both tau3 andI 3 decreased due to the Langmuir-type sorption, and at higher pressures their values recovered because the Henry-type sorption takes over. The amount of sorbed CO2 and dilation of the bulk volume were measured simultaneously, and the free volume fraction was determined at each CO2 pressure. The free volume fraction became smaller (for polyimide and polycarbonate) or slightly larger (for polyethylene) with the progress of sorption. However, the size of the o-Ps hole estimated from the tau3 value increased regardless of the change of the free volume fraction. It appears that o-Ps is selectively looking at larger holes or expanding the holes in which it is accommodated. For polycarbonate, which remains to be glassy even at the largest CO2 sorption attained in the experiment, the o-Ps hole size became larger than that before sorption. This implies that, even if the polymer is glassy as bulk, the sorption site is strongly prone to molecular displacement by the pressure of the penetrating Ps. Cautious consideration is evoked about directly correlating the o-Ps lifetime and intensity with the free volume in general.
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