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 σ_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-ΔV), where ΔV 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. [ABSTRACT FROM AUTHOR]