Regulating Dielectric and Ferroelectric Properties of Poly(vinylidene fluoride-trifluoroethylene) with Inner CH=CH Bonds

Abstract

Poly(vinylidene fluoride) (PVDF) based ferroelectric polymers have attracted considerable attention both academically and industrially due to their tunable ferroelectric properties. By pinning the conformation of the polymer chain and the ferroelectric phase physically or chemically, the ferroelectric behaviors of PVDF based polymers could be finely turned from normal ferroelectric into relaxor ferroelectric, anti-ferroelectric like, and even linear dielectric. Besides high energy electron irradiation and chemical copolymerization with the bulky monomers, in this work, an alternative strategy is presented to regulate the dielectric and ferroelectric performances of PVDF based ferroelectric polymer for the first time. CH=CH bonds with the desired content are inserted by a controlled dehydrofluorination reaction into a poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) copolymer (TrFE refers to trifluoroethylene) synthesized from the hydrogenation of P(VDF-CTFE) (CTFE refers to chlorothrifluoroethylene). The influence of the CH=CH bonds along with the fabrication conditions on the crystallization and ferroelectric relaxation of the resultant copolymers (referred to P(VDF-TrFE-DB)) was carefully characterized and discussed. The nonrotatable CH=CH bonds result in depressed dielectric and ferroelectric performances in the as-cast films by confining the orientation of ferroelectric grains in P(VDF-TrFE). The normal ferroelectric performance of P(VDF-TrFE) is turned into anti-ferroelectric like behavior in the resultant P(VDF-TrFE-DB). The cleavage of CH=CH bonds is responsible for the recovery of the ferroelectric behavior in the annealed samples. Uniaxial stretching favors the alignment of the polymer chain and ferroelectric domains, which may address the further regulated ferroelectric characters in the stretched samples.

Enhanced Electrocaloric Response of Vinylidene Fluoride–Based Polymers via One‐Step Molecular Engineering

“…The polarizability is also enhanced because of a decrease in the crystalline dimensions along the polymer backbone direction, lowering the energy barrier for dipole flipping, while the crystal dimensions parallel to the polarization direction are extended at the same time. The degree of crystallinity stays thereby essentially unchanged; however, the density of crystalline regions increases, indicating an improvement in the crystalline quality -in strong contrast to previous de-hydrochlorination approaches where relatively large numbers of double bonds were introduced to enable mechanical stretching and crosslinking, generally lowering the overall crystallinity [24][25][26][27] . Further progress seems possible, especially if formation of conjugated double bonds units can be prevented.…”

“…Finally, we would like to stress that the chemical modification does not negatively affect the overall degree of crystallinity, c, of DB-modified ter-polymers compared to the pristine material, in contrast to previous approaches introducing larger fractions of double bonds enabling crosslinking and mechanical stretching [24][25][26][27] . We qualitatively deduced this from the differential scanning calorimetry thermograms displayed in Figure 3c, where we record very similar enthalpies of fusion, Hm, for the different materials, with Hm generally being directly linked with c.…”

“…In this paper, we demonstrate a very simple strategy for ECE enhancement in VDF-based terpolymers through the introduction of a small fraction of double bonds (DB) in their backbones, achieved via a simple de-hydrochlorination step of P(VDF-ter-TrFE-ter-CTFE) that leads to an increase of the polarization at low field and an increase in their polarizability. Dehydrochlorination of fluorinated polymers has been used before, to enable crosslinking and facilitate uniaxial stretching, however, relatively high amounts of double-bond defects were generally introduced often leading to a drastic reduction in crystallinity [24][25][26][27] . In contrast, we introduced double bonds into the polymer backbones (up to 5.8%) to extend the coherence of the molecular order along the a-b planes of the material's crystalline moieties (i.e.…”

“…De-hydrochlorination of fluorinated polymers has been used before, to enable crosslinking and facilitate uniaxial stretching, however, relatively high amounts of double-bond defects were generally introduced often leading to a drastic reduction in crystallinity. [24][25][26][27] In contrast, we introduced double bonds into the polymer backbones (up to 5.8%) to extend the coherence of the molecular order along the a-b planes of the material's crystalline moieties (i.e., along the polar direction) [28,29] via, for example, favorable π−π interactions between macromolecular segments that can nucleate ordering and packing. This should increase the net dipole of the crystalline regions.…”

Organosilicon – Organic Hybrid Polymers and Materials

“…The reaction between hydrido-and allyl functional ladder-like silsesquioxanes has led to obtaining very interesting polysiloxanes containing nanoscale tubular structure [123], according to (Eq. 8.47): Considerable effort was made to use the POSS cubes as building blocks, in order to prepare organic-inorganic nanocomposite materials and precursors [124,125].…”