The Structure of the Free Volume in Poly(styrene-co-acrylonitrile) from Positron Lifetime and Pressure Volume Temperature (PVT) Experiments

Abstract

Summary: The structure of the free volume and its temperature dependence between 25 and 190 °C of copolymers of styrene with acrylonitrile, SAN (0 to 50 mol-% AN), is studied by pressure volume temperature (PVT) experiments and positron annihilation lifetime spectroscopy (PALS). In this second part of the work, PALS data are reported from which the temperature dependence of the mean size and size distribution of local free volumes (subnanometer size holes) is analysed. The mean hole volume, vh, varies in PS between 105 Å3 (27 °C) and 220 Å3 (190 °C), and shows a systematic decrease with increasing AN content, at Tg from 121 Å3 for PS to 102 (±2) Å3 for SAN50. The specific number of holes, equation image, estimated from a comparison of PVT and PALS results, is independent on the temperature and the composition of SAN, equation image = (0.56 ± 0.02) × 1021 g−1 which corresponds to Nh(Tg) = equation image/V(Tg) = 0.59 nm−3 and 1/Nh(Tg) = 1.7 nm3 for the volume which contains one hole. The analysed size distributions of the holes above Tg follow the compressibility of the free volume as it is predicted by the theory of thermal volume fluctuation. Moreover, we showed that the bulk elasticity modulus follows mastercurves as a function of vh above and below Tg.