Correlation between the free volume and thermal conductivity of porous poly(vinyl alcohol)/reduced graphene oxide composites studied by positron spectroscopy

Light-weight porous poly(vinyl alcohol) (PVA)/reduced graphene oxide (rGO) composites were prepared by freeze-drying method. The effects of doping rGO nanosheets on the free volume and the thermal conductivity were systematically investigated. Experimental results indicated that the free volume decreased after addition of rGO due to the interfacial interaction between rGO and PVA, such as hydrogen bond. The thermal conductivity increased nearly linearly with rGO content. In order to understand the mechanism of thermal conductivity enhancement, the thermal conductivity was calculated using different kinds of models, including parallel model, series model and Maxwell-Garnett effective medium approximation (EMA), which were then compared with experimental result. The difference value between the experimentally measured thermal conductivity and the EMA theoretically calculated result was closely associated with interfacial interaction, revealing that the better the dispersion of rGO, the stronger the interfacial interaction, and the larger the thermal resistance and the lower the relative thermal conductivity. A direct relationship between the fractional free volume and the thermal conductivity has been obtained, suggesting that the interfacial free volume plays an important role in determining phonon scattering and the thermal conductivity of composites.