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 (Tm) and the endothermic enthalpy of melting (ΔHm) were estimated by differential scanning calorimetry (DSC). Tm and the degree of crystallinity (χc) 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, χc 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 (RM) was found to be dependent on the filler type and content. It was found that, RM 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 τ3 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 Vf and the mechanical properties of PDMS composites containing different fillers. A negative correlation was observed between Vf and RM.