Uncovering the glass-transition temperature and temperature-dependent storage modulus of graphene-polymer nanocomposites through irreversible thermodynamic processes

Several recent experiments have shown that the glass-transition temperature and temperature-dependent storage modulus of graphene-polymer nanocomposites are dependent on the graphene loading, but at present no theory exists to explain these observations. In this paper, we take the view that both issues are closely tied to the principle of irreversible thermodynamics, and that, by considering the phase transition from the glassy to the rubbery state in the polymer, and the temperature-affected degradation of the interphase, two independent state variables can be chosen and implemented into a two-scale homogenization scheme. In this approach, we also adopt the temperature-dependent complex modulus in the reduced frequency scale as the homogenization parameter. The developed theory is highlighted with a direct comparison with experiments. It is demonstrated that, with the addition of graphene fillers, both theory and experiments show an increase of glass-transition temperature and effective storage and loss moduli, but that, within the glass-transition range, the storage and loss moduli decrease drastically. The present research could provide the directions to tune the glass-transition temperature and storage modulus of graphene-polymer nanocomposite through graphene loading and temperature.