Modeling the impact of glass transition on the frequency-dependent complex conductivity of CNT-polymer nanocomposites

Abstract

While it is well known that glass-transition process tends to lead to significant reduction of Young's modulus in polymers, it is not commonly known how it affects the electrical conductivity and dielectric permittivity of carbon nanotube (CNT)-polymer nanocomposites, especially under AC loading. In this paper, a novel two-scale effective-medium approximation with complex conductivity is presented to answer these questions. The theory is developed by considering the irreversible thermodynamics of phase transition from the glassy to rubbery state and the thermal degradation of the interphase layer. In addition, the temperature-affected interfacial connection, electronic tunneling, Maxwell-Wagner-Sillars polarization, and the frequency-dependent electron hopping and dielectric relaxation, are also considered. It is demonstrated that the predicted results are in close agreement with the experimental data of MWCNT/polyester nanocomposites across the glass-transition range from 240 K to 380 K and the frequency spectrum from 102 Hz to 106 Hz. It is also found that both conductivity and permittivity decrease and then increase nonlinearly with temperature, with the minimum detected around the glass-transition point. Several other novel features of the theory are also presented.