Effect of conductive fillers on the cyclic stress-strain and nano-scale free volume properties of silicone rubber

The effect of carbon black (CB) and graphite (G) powders on the macroscopic and nano-scale free volume properties of silicone rubber based on poly(di-methylsiloxane) (PDMS) was studied through thermal and cyclic mechanical measurements, as well as with positron annihilation lifetime spectroscopy (PALS). The melting temperature of the composites (T _m) and the endothermic enthalpy of melting (ΔH _m) were estimated by differential scanning calorimetry (DSC). T _m and the degree of crystallinity (χ _c) of PDMS composites were found to decrease with increasing the CB content. This can be explained due to the increase in physical cross-linking which results in a decrease in the crystallite thickness. Besides, χ _c was found to be dependent on the filler type. Cyclic stress-strain behavior of PDMS loaded with different contents of filler has been studied. Mullins ratio (R _M) was found to be dependent on the filler type and content. It was found that, R _M increases with increasing the filler content due to the increase in physical cross-linking which results in a decrease in the size of free volume, as observed through a decrease of the o-Ps lifetime τ ³ measured by PALS. Moreover, the hysteresis in PDMS-CB composites was more pronounced than in PDMS-G composites. Furthermore, a correlation was established between the free volume V _f and the mechanical properties of PDMS composites containing different fillers. A negative correlation was observed between V _f and R _M.