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

Abstract

AbstractSummary: 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, Vf = hV and Vocc = (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 〈vh〉 and mean dispersion σh were calculated. From a comparison of 〈vh〉 with Vf, the hole density $N'_{\rm h}$ 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 δ. It was found that at 296 K, 〈vh〉 decreases exponentially from 80 Å3 at 0.1 MPa to the extremely low value of 10 Å3 at 1 300 MPa. Starting from a fluctuation approach, the free‐volume fluctuations and the characteristic length scale (ξ) of dynamic heterogeneity of structural relaxation were estimated from σh. It was found that ξ increases from ≈1.5 nm to ≈1.9 nm when the temperature decreases from 293 K to the glass transition temperature Tg = 207 K (at 0 MPa) or the pressure increases from 0.1 MPa to 200 MPa (at 296 K).Fractional free volume and its mean fluctuation in PIB as determined from positron lifetime spectroscopy and PVT experiments analyzed with the Simha‐Somcynsky equation of state.imageFractional free volume and its mean fluctuation in PIB as determined from positron lifetime spectroscopy and PVT experiments analyzed with the Simha‐Somcynsky equation of state.