The structure of the free volume in poly(styrene-co-acrylonitrile) from positron lifetime and pressure volume temperature (PVT) experiments - II. Local free volume from positron annihilation lifetime spectroscopy (PALS)

The structure of the free volume and its temperature dependence between 25 and 190 degreesC 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 anal sed. The mean hole volume, v(h), varies in PS between 105 Angstrom(3) (27 degreesC) and 220Angstrom(3) (190 degreesC), and shows a systematic decrease with increasing AN content, at T-g from 121 Angstrom(3) for PS to 102 (+/-2) Angstrom(3) for SAN50. The specific number of holes, N-h('), estimated from a comparison of PVT and PALS results, is independent on the temperature and the composition of SAN, N-h(') = (0.56 +/- 0.02) x 10(21) g(-1) which corresponds to N-h(T-g) = N-h(')/V(T-g) = 0.59 nm(-3) and 1/N-h(T-g) = 1.7 mn(3) 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 V-h above and below T-g.