On the impact of functionalization and thermal treatment on dielectric behavior of low content TiO₂ PVDF nanocomposites

Owing to the high surface to volume ratio of nanoparticles, nanoparticle-modified polymers promise to exhibit properties exceeding bounds predicted by effective media approaches, where the effective response is not solely dependent on inherent nanoparticle properties but rather dominated by nature and volume content of interface. This study is focused on 1) investigating impact of surface treatment and thermal processing of nanoTiO2-modified-PVDF on dielectric and mechanical properties of resulting nanocomposites, and 2) highlighting a unique dielectric behavior of nanoparticle-reinforced polymers where an initial increase in dielectric permittivity is seen at very low content followed by a decrease resulting from nanoparticle-polymer interaction. X-Ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared spectroscopy (FT-IR) confirm efficient particle surface modification by showing the presence of atoms from coupling agent molecules on the particle's surface. Scanning Electron Microscopy (SEM) of TiO2/PVDF nanocomposites verifies that surface porosity was removed by a post annealing process. Dynamic Mechanical Analysis (DMA) results demonstrate an increase in storage modulus in the glassy region from 67 to 108% as compared to the pristine PVDF for TiO2 contents varying from 5 to 20 wt% (2.3 to 10 vol%) as a result of adding functionalization and thermal treatment. Based on Dielectric Spectroscopy measurements, a 74% increase in permittivity is observed at 0.1 Hz and a 30% increase at 1 kHz, both at relatively low volume content (2.3 vol%), owing to the good dispersion, high interfacial volume content and strong interaction. A further increase of TiO2 content decreases the dielectric permittivity as a result of dipolar confinement. Contributions of the study are two-fold: firstly, compared to the current literature, this increase in the value of dielectric permittivity at such a low volume content using TiO2 nanoparticles is unprecedented and has not been reported so far; and secondly, the study brings to light a unique nanodielectric behavior where the initial increase in permittivity is followed by a decrease owing to dipolar confinement resulting from particle/polymer interaction.