The phase transformation from α to β poly(vinylidene fluoride) (PVDF) through a stretching process at different temperatures was investigated. Samples of originally α-PVDF were stretched uniaxially at different temperatures at draw ratios from 1 to 5. The stretched samples were studied and characterized by infrared spectroscopy, scanning electron microscopy, and differential scanning calorimetry. The maximum β-phase content was achieved at 80°C and a stretch ratio of 5, but the samples still showed 20% of the original α-phase. Accompanying the phase transformation, an orientation of the polymer chains was observed. The stretching process also influenced the degree of crystallinity of the polymer. Poling of the samples also improves the α- to β-phase transformation.

α to β Phase Transformation and Microestructural Changes of PVDF Films Induced by Uniaxial Stretch

The ferroelectric switching behaviour and piezoelectric response of poly(vinylidene fluoride) (PVDF) prepared by drawing at stretching ratios from 1 to 5 and temperatures from 80 to 140 °C has been studied. Stretching ratio and temperature deeply influence the α (non-ferroelectric) to β (ferroelectric) phase transformation. The variations in the phase content are accompanied by changes in the degree of crystallinity and the microstructure, all of them influencing the macroscopic piezoelectric and ferroelectric response of the material. This work shows how the piezo- and ferroelectric behaviour of PVDF depends on the aforementioned parameters and, in particular, on the crystalline β-phase content. Coercive electric field, remnant polarization and saturation polarization increase with increasing ferroelectric β-phase content in the sample. In a similar way, samples with higher β-phase content show higher d33 piezoelectric coefficients.

https://iopscience.iop.org/article/10.1088/0964-1726/19/6/065010/pdf

Influence of the β-phase content and degree of crystallinity on the piezo- and ferroelectric properties of poly(vinylidene fluoride)

Polyvinylidene fluoride (PVDF) and its copolymers with a particular semi-crystalline conformation (β) have evolved as lead (Pb)-free alternatives to electroactive perovskite piezo-, pyro-, and ferroelectric materials. In particular, semi-crystalline ferroelectric polymers are suitable for a large variety of flexible devices and have attracted tremendous research interest. Realizing the application potential of this polar form, there have been numerous attempts to enhance and stabilize this β crystalline phase. A completely pure (100%) β crystalline phase has been achieved in a nanocomposite of PVDF with carbon nanotubes (CNTs) with proper functionalization. With an emphasis on pure ferroelectric β-phase formation and related issues, this paper presents a timely review of the literature on recent sequential development and understanding of PVDF-CNT composites.

https://iopscience.iop.org/article/10.1088/1361-6463/aa5f85/pdf

Pure β-phase formation in polyvinylidene fluoride (PVDF)-carbon nanotube composites

A series of ferroelectric switching measurements are performed on the 75/25mol% vinylidene fluoride/trifluoroethylene copolymer thin films with various structures to elucidate the factors governing the switching characteristics of such films. It is shown that the spin-coated and well-annealed samples exhibit a remanent polarization Pr of 85 mC/m2. Unlike that in previous reports, the switching time taus of these samples is independent of thickness down to 50 nm if Au electrodes are used instead of Al electrodes. Thin films with different metal electrodes exhibit anisotropic taus due to a potential difference between metal electrodes produced by their work function difference. A train of unipolar short on-off electric field pulses induce a slow but full polarization reversal whose time evolution supports the nucleation and growth mechanism. Continuous switching under bipolar on-off electric field pulses reveals an accelerated polarization reversal with decreasing pulse width. This means that taus is a function of not only applied electric field but also the preceding poling conditions. Fatigue is significantly improved using Au electrodes instead of Al electrodes. The results are discussed in relation to the microscopic features of the VDF/TrFE copolymer and the underlying nucleation-growth mechanism

Factors governing ferroelectric switching characteristics of thin VDF/TrFE copolymer films

From scientific and technological points of view, poly(vinylidene fluoride), PVDF, is one of the most exciting polymers due to its overall physicochemical characteristics. This polymer can crystalize into five crystalline phases and can be processed in the form of films, fibers, membranes, and specific microstructures, being the physical properties controllable over a wide range through appropriate chemical modifications. Moreover, PVDF-based materials are characterized by excellent chemical, mechanical, thermal, and radiation resistance, and for their outstanding electroactive properties, including high dielectric, piezoelectric, pyroelectric, and ferroelectric response, being the best among polymer systems and thus noteworthy for an increasing number of technologies. This review summarizes and critically discusses the latest advances in PVDF and its copolymers, composites, and blends, including their main characteristics and processability, together with their tailorability and implementation in areas including sensors, actuators, energy harvesting and storage devices, environmental membranes, microfluidic, tissue engineering, and antimicrobial applications. The main conclusions, challenges and future trends concerning materials and application areas are also presented.

pdf/smart-and-multifunctional-materials-based-on-electroactive-vf2wrky5so.pdf

Smart and Multifunctional Materials Based on Electroactive Poly(vinylidene fluoride): Recent Advances and Opportunities in Sensors, Actuators, Energy, Environmental, and Biomedical Applications.

Oriented poly(vinylidene fluoride) (PVDF) films consisting of β crystals were prepared by the solid-state coextrusion (SC) of a gel film near the melting temperature (Tm) and by conventional cold tensile drawing (TD) of a melt-quenched film. These films were annealed over the temperature range of 75–190 °C (below and above the static Tm) while the sample length was kept constant or constant loads were applied. After annealing with the sample length kept constant, the dynamic Young's modulus markedly decreased because of the relaxation of oriented amorphous chains, as shown by infrared spectroscopy. In contrast, annealing under a constant load improved the chain orientation in both the crystalline and amorphous regions, resulting in an increase in the modulus from an initial 10.5 to 14.3 GPa for the SC and from an initial 3.3 to 4.8 GPa for the TD. The SC, annealed at 190 °C with a constant load corresponding to an initial tension of 200 MPa, exhibited an extreme crystalline-chain orientation of 0.998 and a modulus of 14.3 GPa, among the highest values ever reported for PVDF. Although the remanent polarization (Pr) of the TD increased slightly from the initial 62 to 68 mC/m2, Pr of the SC stayed constant at 100 mC/m2 independently of the annealing conditions. This suggests that the Pr value of 100 mC/m2 approached the equilibrium value for this PVDF sample containing 3.5 mol % structural defects. Therefore, although the modulus and Pr of the TD increased slightly with annealing, the maximum values achieved by annealing were markedly lower than those of the SC and annealed SC. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 1701–1712, 2003

Effect of annealing on the structure and properties of poly(vinylidene fluoride) β-form films

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Effect of annealing on the structure and properties of poly(vinylidene fluoride) β-form films