Thermal Development of Free Volumes in Nafion Membrane

In this work we employed positron lifetime spectroscopy for investigation of the thermal development of free volumes in H⁺ Nafion membrane over a broad range of temperatures from -150 to 150°C. Positron lifetime studies were combined with differential scanning calorimetry. Size distribution of free volumes was determined from ortho-positronium contribution to positron lifetime spectra. Our investigations revealed that the mean size of free volume holes strongly increases with temperature. On the other hand, the width of the size distribution and the positronium yield decreases with temperature. Transition temperatures corresponding to a change in the slope of the temperature dependence of ortho-positronium lifetime were identified. Results of positron lifetime spectroscopy agree well with the curve obtained by differential scanning calorimetry.