Amplitude of spatial density fluctuations in glassy propylene carbonate probed by oxygen diffusion

Oxygen translational mobility is very sensitive to structural heterogeneities in molecular glass formers. The heterogeneities manifest themselves as the regions, within which the oxygen jump rates have either the same or similar values. The length of spatial correlation for the oxygen jump rates in glassy propylene carbonate has been determined to be about 1.5 nm [Syutkin et al., J. Chem. Phys. 133 (2010) 074501]. The heterogeneities detected by oxygen diffusion are likely the static density fluctuations associated with a spatially nonuniform distribution of free volume. In the present paper the isothermal dependence of oxygen diffusion coefficient on glass density was measured in propylene carbonate using the technique of phenanthrene phosphorescence quenching by oxygen molecules. Glasses of different densities were produced by annealing samples to the equilibrium state at various temperatures. The amplitude of spatial density fluctuations was calculated assuming that (i) a heterogeneous diffusion is due to density fluctuations, and (ii) the dependence of jump rates on region density is the same as that of oxygen diffusion coefficient on the average glass density. The value for ( Δ ρ ) 2 / ρ 2 was found to be 0.008 ± 0.003.