Influence of SiO2 fillers on the irradiation ageing of silicone rubbers

Effects of irradiation on the viscoelastic properties of peroxide-crosslinked PDMS elastomers (EVC) without filler (1), filled with 30% untreated nanoscopic SiO2 (2) and with 30% surface modified silica (3) were investigated. Electron irradiation in air using a high dose rate allowing homogeneous irradiation at different doses ranging from 25 to 500 kGy have led to additional crosslinking of these three types of silicone rubbers making them become more rigid and brittle. The apparent crosslink density is higher for filled elastomers since links at the polymer–silica interfaces have been created as a result of irradiation. A further crosslink density enhancement is obtained for the surface-treated silica fillers. High resolution low frequency dynamic mechanical spectroscopy shows that the magnitude of the crystallisation peak located at approximately −40°C decreases with the irradiation dose. As a result, the amplitude of the relaxation process associated with the glass transition near −125°C increases, since a larger amount of amorphous phase is present due to the hindrance of the crystallisation behaviour. DSC measurements confirm that crystallisation is limited by the formation of a higher number of crosslinks, and that this effect is amplified for filled PDMS samples. A study of mechanical hysteresis at high strains (Mullins effect) shows an increase of the associated dissipative phenomena due to the induced polymer/silica bonds.