Temperature and Pressure Dependence of the Free Volume in Polyisobutylene from Positron Lifetime and Pressure-Volume-Temperature Experiments

The microstructure of the free volume and its temperature and pressure dependence in PIB were studied by PVT experiments and PALS. Employing the Simha-Somcynsky equation of state, the hole-free volume fraction h and the specific free and occupied volumes, V-f=hV and V-occ=(1-h)V, respectively, and their expansivity and compressibility were calculated. From the PALS spectra analyzed using the LT9.0 software, the hole size distribution, the mean hole size < v(h)> and mean dispersion sigma(h) were calculated. From a comparison of < v(h)> with V-f, the hole density N'(h) was obtained. It was found that PIB shows an unusual thermal expansion with glass-like transitions at 210 K and 290 K (at 0 MPa). A similar behavior is observed for tan delta. It was found that at 296 K, < v(h)> decreases exponentially from 80 angstrom(3) at 0.1 MPa to the extremely low value of 10 angstrom(3) at 1300 MPa. Starting from a fluctuation approach, the free-volume fluctuations and the characteristic length scale (xi) of dynamic heterogeneity of structural relaxation were estimated from sigma(h). It was found that xi increases from approximate to 1.5 nm to approximate to 1.9 nm when the temperature decreases from 293 K to the glass transition temperature T-g=207 K (at 0 MPa) or the pressure increases from 0.1 MPa to 200 MPa (at 296 K).