Temperature dependence of free volume in pure and silica-filled poly(dimethyl siloxane) from positron lifetime and PVT experiments

The microstructure of the free volume and its temperature dependence in two poly(dimethyl siloxane)s (PDMS), one in the pure state and the other filled with 35 wt.-% of an in situ hydrophobized fumed silica with a specific surface area of 200-300 m(2) - g(-1), were studied by pressure volume-temperature experiments (PVT, T = 22-156 degrees C, P = 10-200 MPa) and positron annihilation lifetime spectroscopy (PALS, T = -173-100 degrees C, P = 10(-5) MPa). The Simha-Somcynsky equation of state was used to estimate the hole free volume fraction, h, and free and occupied volumes, V-f = hV and V-occ = (1 - h)V, from the specific total volume, V. The PALS spectra were analyzed with the routine LT9.0, which allowed for a dispersion, sigma(i), in all three of the lifetimes: the para-positronium, (p-Ps, tau(1)), positron (e(+), tau(2)) and orthopositronium (o-Ps) lifetime This kind of analysis delivered correct p-Ps lifetime parameters, tau(1), sigma(1), and I-1. It was speculated that e+, like o-Ps, undergoes Anderson localization at empty sites of the, static or dynamic, disordered structure. The hole size distribution, its mean value, < v(h)>, and dispersion, at,, were calculated from the o-Ps lifetimes. A comparison of < v(h)> with Vf was used to estimate the specific hole number, N-h('). During melting of the semicrystalline samples at-38 degrees C (T-m), < v(h)> increased abruptly, and sigma(h) suddenly decreased. Both effects are explained by the disappearance of the rigid-amorphous fraction (RAF) and, thus, a reduction in the dynamic heterogeneity. The following leveling-off in (v(h)) and the low value of sigma(h) are attributed to the fast segmental relaxation in the PDMS melts which leads to a smearing of the molecule density distribution around a hole during the o-Ps lifetime.