Free volume from molecular dynamics simulations and its relationships to the positron annihilation lifetime spectroscopy

The free volume micro-structural properties of propylene glycol obtained by means of molecular dynamics simulations have been investigated and compared with the experimental data from positron annihilation lifetime spectroscopy (PALS). The results are also compared to those recently obtained on glycerol. The bulk microstructures of the samples have been analyzed in the temperature range 100-350 K with a probe-based procedure for exploring the free volume cavities of the microstructures. The basic free volume property, i.e., mean cavity volume, is compared with the hole volume data from PALS. A comparison between calculated and experimental data suggests the existence of a threshold volume for the smallest cavity detectable by PALS, which may be ascribed to fast local motions of the matrix constituents. At high temperatures the cavity analysis reveals the formation of an infinite cavity, i.e., percolation phenomenon. The onset temperatures of the percolation process in propylene glycol and glycerol are found to be close to the characteristic PALS temperature T-b2(L), where a pronounced change in the PALS response occurs, as well as to the characteristic dynamic Schnhals temperature T-B(SCH), and Stickel's temperature T-B(ST), marking a dramatic change in the primary alpha-relaxation properties.