RESUMO
VDF-based polymers, such as poly(vinylidene fluoride) (PVDF) and its copolymers, are well-known ferroelectrics of interest for numerous applications, from energy storage to electrocaloric refrigeration. However, their often complex thermal phase behavior that typically leads to a low phase-stability can drastically affect the long-term dielectric properties of this materials family. Here, we demonstrate on the example of the terpolymer P(VDF-ter-TrFE-ter-CFE) (molar ratio: 64/29/7) that by limiting mass transport/segmental chain motion both during solidification and in the solid state, a drastically smaller "burn-in" in relative permittivity, εr, is observed. Indeed, εr decreases little over time and saturates rapidly at 96-97% of its initial value. Mass transport thereby is limited by using highly entangled systems via the selection of a suitable polymer solution concentration and molecular weight. In addition, rapid solvent extraction assists in reducing unwanted relaxation processes. Accordingly, increased control of the phase stability of P(VDF-ter-TrFE-ter-CFE) is gained. Moreover, pathways are opened to reliably identify processing routes for any given VDF-based polymer, with critical information being obtained from thermal analysis and rheometry data only, enabling rapid feedback to material design, including the prediction of required molecular weights without the need for complex characterization methodologies.
RESUMO
Active thermal control will be a major challenge of the twenty-first century, which has emphasized the need for the development of energy-efficient refrigeration techniques such as electrocaloric (EC) cooling. Highly polar semicrystalline VDF-based polymers are promising organic EC materials, however, their cooling performance, which is highly structurally dependent, needs further improvement to become competitive. Here, we report a simple method to increase the crystalline coherence of P(VDF-ter-TrFE-ter-CFE) terpolymer in the plane including the polar direction. This is achieved by blending P(VDF-ter-TrFE-ter-CFE) with minute amounts of P(VDF-co-TrFE) copolymer with similar VDF/TrFE unit content. This similarity allows for a cocrystallization of the copolymer chains in the terpolymer crystalline lamellae, preferentially extending the lateral coherence without lamellar thickening, as validated with a wide range of structural characterization. This trend results in a significant dielectric and electrocaloric enhancement, with a remarkable electrocaloric effect, ΔTEC = 5.2 K, confirmed by direct measurements for a moderate electric field of 90 MV·m-1 in a blend with 1 wt % of copolymer.
