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The glass transition temperature (T_g) of the Nafion 117 membrane was traced by DSC step by step during the preparation of the membrane electrode assembly (MEA). Wide-angle x-ray diffraction and frequency response analysis were used for the determination of the crystallinity and proton conductivity of the membrane. As-received Nafion 117 membrane showed two glass transition temperatures in the DSC thermogram. The first T_g, caused by the mobility of the main chain in the polymer matrix, was 125 °C; the second T_g, derived from the side chain due to the strong interaction between the sulfonic acid functional groups, was 195 °C. During the pretreatment of the membrane, the T_g of the Nafion 117 membrane drastically decreased because of the plasticizer effect of water. In the hot-pressing process, the T_g of the Nafion 117 membrane gradually increased due to the loss of water. When the Nafion 117 was …