Nanoscopic properties of silica filled polydimethylsiloxane by means of positron annihilation lifetime spectroscopy

Positron annihilation lifetime spectroscopy (PALS) was performed on a series of polydimethylsiloxane (PDMS)/fumed silicon dioxide (SiO2) composites at temperatures between −185 and 100 °C to study the effect of filler content and filler particle size on the free volume properties and the positron annihilation characteristics. The glass transition behavior of the PDMS/SiO2 composites was determined with differential scanning calorimetry. A clear influence on the o-Ps lifetime (τ3) in the polymer upon addition of nano-sized fumed SiO2 was observed at all temperatures. The observed o-Ps lifetime behavior was related to filler content and filler particle size. A transition in the temperature dependence of the o-Ps lifetime was observed close to −35 °C above which temperature PDMS exhibits long o-Ps lifetimes. A relationship between τ3 and the surface tension, equivalent to the behavior of ordinary molecular liquids was observed in this temperature region. The o-Ps yield was strongly reduced in the crystallization region and by addition of SiO2. The reduction due to filler addition did, however, in the case of nano-sized SiO2 not follow a linear relationship with filler weight, which was observed for micron-sized fillers. The nonlinear relationship between filler weight and o-Ps yield could be due to out-diffusion of positrons and/or o-Ps from the filler particles to the matrix.