Temperature Dependence of Free Volume in Pure and Silica‐Filled Poly(dimethyl siloxane) from Positron Lifetime and PVT Experiments

Summary: 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 °C, P  = 10–200 MPa) and positron annihilation lifetime spectroscopy (PALS, T  = −173–100 °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, σ i , in all three of the lifetimes: the para‐positronium ( p ‐Ps, τ 1 ), positron (e + , τ 2 ), and ortho‐positronium ( o ‐Ps) lifetime ( τ 3 ). This kind of analysis delivered correct p ‐Ps lifetime parameters, τ 1 , σ 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, σ h , were calculated from the o ‐Ps lifetimes. A comparison of 〈 v h 〉 with V f was used to estimate the specific hole number, $N'_{\rm h}$ . During melting of the semicrystalline samples at−38 °C ( T m ), 〈 v h 〉 increased abruptly, and σ 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 σ 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.