Poly(vinylidene difluoride) (PVDF) displays attractive ferroelectric/piezoelectric properties and its polar β-crystals are specifically targeted for achieving electroactive applications. However, their direct crystallization from the melt state represents a challenging task that has never been addressed using melt-state processes. The use of poly(methyl methacrylate) (PMMA) is herein investigated to promote the PVDF polar β-phase using melt-blending and extrusion-calendering technologies. The presence of the β-phase is here confirmed by ATR-FTIR and WAXS experiments with blends at a PMMA content as low as 5 wt%. The key role of PMMA for the β-phase crystallization from the melt state was unambiguously highlighted with the help of Flash DSC experiments in non-isothermal cooling mode from the melt state. PMMA is able to efficiently shift the α-to-β crystal transition to lower cooling rates (>100-200 °C s-1), making the achievement of the PVDF polar β-phase for these blends compatible with conventional processing tools. A crystal phase diagram is proposed for the PVDF/PMMA blends to highlight the dual effects of both PMMA and cooling rate on the PVDF crystallization during melt-processing. Ferroelectric properties were even observed for the blends containing PMMA up to 10 wt% with the highest remanent polarization obtained at 5 wt% PMMA. After 10 wt% PMMA, a progressive transition from ferroelectric to pseudo-linear dielectric behavior is observed more likely due to the presence of PMMA in the interlamellar amorphous phase of the polar PVDF spherulites as shown by SAXS experiments. In this work, we successfully demonstrated that PMMA plays a key role in the crystallization of PVDF polar crystals from the melt state, enabling large-scale and continuous extrusion processing of PVDF-based materials with attractive dielectric properties for sensing and harvesting applications.

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 (d 33) of 11 pC/N for these formulations. A linear relationship is found between d 33 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 d 33 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.