Oxygen diffusion in glassy propylene carbonate: Energetics and spatial correlation of jump rates

Quenching of phenanthrene phosphorescence by molecular oxygen was used as a tool to determine the distribution function of the activation free energy of oxygen jumps in glassy propylene carbonate. The distribution width is shown to increase linearly with temperature within the range from 135 to 156 K. The dispersion of the activation free energy is caused by the spatial fluctuations of the activation entropy of oxygen jumps but not the energy barriers; the spatial dispersion of energy barriers is found to be zero. A characteristic length of the spatial correlation of the activation entropy is about 1.3 nm. The apparent activation energy linearly correlates with the macroscopic high-frequency shear modulus . The jump rates of oxygen molecules lie in the range of Johari-Goldstein relaxation rates. We suggest that an oxygen molecule overcomes the barrier between two neighboring interstitial sites due to low-frequency localized vibrational excitations of string-like groups of host molecules which draw the oxygen molecule in the motion.