Microstructure of free volume in SMA copolymers I. Free volume from Simha-Somcynsky analysis of PVT experiments

The structure of the free volume and its temperature dependence between 25 and 200 °C of copolymers of styrene with maleic anhydride, SMA (0-35 mol% MA), is studied by pressure-volume-temperature (PVT) experiments and positron annihilation lifetime spectroscopy (PALS). In this first part of the work, PVT data are reported which were analysed with the Simha-Somcynsky equation of state to estimate the volume fraction of holes, h, which constitute the excess free volume. The temperature and pressure dependence of the specific volume V, the specific occupied and free volume, Vocc=(1−h)V and Vf=hV, and the corresponding isobaric expansivities and isothermal compressibilities for both the rubbery and glassy state are estimated. We obtained the unexpected results that (i) the occupied volume changes its coefficient of thermal expansion at Tg from αocc,g≈0.5αg≈1×10−4 K−1 below Tg to almost zero (≈0.2×10−4 K−1) above Tg and (ii) the isothermal compressibility of the occupied volume at zero pressure below Tg is rather high, κocc≈2.5×10−4 MPa−1, and decreases only slightly at Tg to about 2×10−4 MPa−1 above Tg. The variation of total, occupied, and free volume parameters with the composition of the SMA copolymers is discussed.