Beta Phase Crystallization and Ferro- and Piezoelectric Performances of Melt-Processed Poly(vinylidene difluoride) Blends with Poly(methyl methacrylate) Copolymers Containing Ionizable Moieties

Poly(methyl methacrylate-co-methacrylic acid) (PMMA-co-MAA) copolymer containing ionizable moieties is here investigated as a melt processing additive for poly(vinylidene difluoride) (PVDF) to develop high-quality ferro- and piezoelectric polymer films by extrusion-calendering. The PVDF/PMMA-co-MAA miscibility and the β-phase crystallization from the melt state at high cooling rates were first explored by flash differential scanning calorimetry (FDSC). Transposition to the melt-processing of thin films by extrusion-calendering was attempted, and direct production of β-crystals in high amounts was confirmed at a specific content of 5 wt % PMMA-co-MAA. Ferro- and piezoelectric properties were subsequently investigated, and classical ferroelectric-type hysteresis loops clearly appeared at room temperature for AC electric fields higher than 900–1200 kV/cm. Enhanced remanent polarizations (Pr) were observed with only 5 wt % PMMA-based additives, and the best ferroelectric performances were identified for PVDF/PMMA-co-MAA blends, in agreement with a higher β-phase content. Stable piezoelectric properties are also highlighted with maximal piezoelectric coefficient (d33) of 11 pC/N for these formulations. A linear relationship is found between d33 and Pr in accordance with several models, and in this respect, the origin and optimization of the remanent polarization was investigated. Crystal transformations were revealed during high-voltage AC poling, and high-quality ferroelectric behaviors with high Pr values up to 7 μC/cm2 were obtained at elevated poling temperatures for PVDF/PMMA-co-MAA blends (theoretical d33 up to 16 pC/N) approaching the theoretical limit value for perfectly poled β-crystals. This study clearly opens up interesting perspectives in the development of cost-effective electroactive polymer films using industrially relevant processes and demonstrates that PVDF-based blends with miscible functional PMMA copolymers represent an interesting approach for this purpose.