Influence of annealing temperature on the existence of polar domain in uniaxially stretched polyvinylidene-co-hexafluoropropylene for energy harvesting applications

The structural and electroactive properties of the as-prepared random copolymer polyvinylidene-co-hexafluoropropylene thin film are explored as a function of thermal treatment at various temperature regions. The thermal treatment of the polymer thin film not only changes the structural conformations that is very natural but establishes a polar domain in the non-polar α-phase. Here, we discover an anomalous temperature-dependent crossover behavior from the non-polar α-phase to an appreciable enhancement in ferroelectric and piezoelectric responses. The maximum unipolar strain $(Smax=−5.01%)$⁠, an ultrahigh value of normalized piezoelectric coefficient $(d33∗=−556pm/V)$⁠, high electromechanical coupling factor $(Kp=0.78)$ factor including the high dielectric constant $(ϵ′=23at100Hz)$ at a relatively low electric field of 900 kV/cm may, therefore, be an effect of the established polar domain for the sample annealed at 105 °C. The direct piezoelectric charge coefficient (d₃₃), a key factor for the performance of a prepared polymer thin film system as an energy harvester, lies in the range of $−10±2pC/N$. Also, the annealed sample exhibited a persistent polarization after several cumulative cycles of applied stress.