Both the Xe sorption isotherms and 129 Xe NMR spectra of poly(2,6-dimethyl-1,4-phenylene oxide), PPO, were obtained across a temperature range of -55 1C to þ 80 1C to investigate the temperature dependence of the gas sorption and the unrelaxed volume of PPO. All of the obtained sorption isotherms were analyzed based on the dual-mode sorption model, and the Langmuir saturation constant, C H 0 , which corresponds to the unrelaxed volume, was determined by curve fitting. The values of C H 0 increased linearly with decreasing temperature, and the obtained temperature was almost identical to the glass transition temperature, T g , of PPO when the C H 0 value was extrapolated to 0. The 129 Xe NMR chemical shift of 129 Xe in the PPO showed a nonlinear, low-field shift with increasing Xe pressure at temperatures below the T g . The mean volumes of individual microvoids were determined at each temperature based on the 129 Xe NMR chemical shift analysis, assuming a fast exchange of the Xe atoms between the Henry and Langmuir sorption sites. The temperature dependence of the individual microvoid volumes was similar to that of C H 0 .

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