Influence of Local Phase Separation on Dielectric Properties of PVDF/PMMA Blends During Thermal Aging

High energy storage performance is commonly obtained in polymer composites rather than in pure substances. The thermal aging behaviors of these composites, however, remain puzzling and limit their long-term reliability. Here we report the influence of local phase separation of a typical energy storage polymer blend polyvinylidene fluoride (PVDF)/polymethyl methacrylate (PMMA) during thermal aging. The specimens were subjected to accelerated aging at temperatures of 90 °C, 110 °C, and 130 °C for 10-30 days, and their dielectric properties were subsequently measured. Surprisingly, we observed a reduction in dielectric permittivity and energy density of PVDF/PMMA during aging, contrary to the behaviors exhibited by linear dielectrics such as polypropylene (PP) and polyethylene. Fourier transform infrared spectrometry measurements indicate that thermal oxidation played a minor role. Microstructure investigations, further, reveal that local phase separation of PVDF and PMMA in amorphous regions due to the recrystallization should be responsible for the variation of dielectric properties. Our findings uncover the thermal aging mechanism of PVDF/PMMA blends and might provide a guideline for developing energy storage polymer blends with enhanced long-term reliability.