Positron annihilation study in inorganic-polymer nano-composites

Positron annihilation lifetime experiments in a series of polystyrene (PS) nano-composites of different wt% of SiO₂ as a function of temperature have been performed and the glass transition temperatures are determined. Previously, in PS/CNF(carbon nanofiber), a system with strong interfacial bonding, we found single glass transition temperature (T_g), which increases with wt % of CNF. Currently, in PS/SiO₂ nano-composites, a weak interfacial interaction system, we found that there are two T_gs, one at 57 °C, which is lower than the PS bulk T_g (= 91 °C). The lower T_g is interpreted from the free volume in the weak bonding interface between PS and SiO₂. This result is consistent with our previous slow positron beam results of PS on Si substrate, which shows a significant lower T_g in the interface than in the bulk PS.