The temperature dependence of free volume in phenyl salicylate and its relation to structural dynamics: A positron annihilation lifetime and pressure-volume-temperature study

Positron annihilation lifetime spectroscopy (PALS) and pressure-volume-temperature (PVT) experiments were performed to characterize the temperature dependent microstructure of the hole free volume in the low molecular weight glass-former phenyl salicylate (salol). The PALS spectra were analyzed with the new routine LT9.0 and the volume distribution of subnanometer size holes characterized by its mean < v(h)> and standard deviation sigma(h) was calculated. Crystallization of the amorphous sample was observed in the temperature range above 250 K, which leads to a vanishing of the positronium formation. The positronium signal recovered after melting at 303 K. A combination of PALS with PVT data enabled us to calculate the specific density N-h('), the specific volume V-f, and the fraction of holes f(h) in the amorphous state. From comparison with dielectric measurements in the temperature range above T-B=265 K, it was found that the primary structural relaxation slows down with temperature, faster than the shrinkage of the hole free volume V-f would predict, on the basis of the Cohen-Turnbull (CT) free volume theory. CT plots can be linearized by replacing V-f of the CT theory by (V-f-Delta V), where Delta V is a volume correction term. This was interpreted as indication that the lower wing of the hole size distribution contains holes too small to show a liquidlike behavior in their surroundings. Peculiarities of the relaxation behavior below T-B=265 K and the possible validity of the Cohen-Grest free volume model are discussed.