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 (P_r) 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 P_r 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 P_r 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.