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 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)-DeltaV), where DeltaV 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.