Structure and piezoelectric properties of poly(vinylidene fluoride) studied by density functional theory

Enhanced crystallization behaviors and dielectric performance of poly(vinylidene fluoride) film induced by polyamide-1

Poly(vinylidene fluoride) (PVDF)-based composites attract tremendous attention as dielectric materials. However, their development has been limited due to the raised problem in the in-homogeneous polymer composites. In this work, a novel PVDF-based film incorporated with polyamide-1, containing the highest density of dipole among all polyamides, was prepared to improve the crystallization behaviors and dielectric properties. The results showed that the optimal concentration of polyamide-1 in PVDF was 6 wt.%. The crystallization rate of PVDF was improved in the presence of polyamide-1. Interestingly, the polyamide-1 was conductive to the formation of β form crystal of PVDF, which exhibited great electric performance. The dielectric constant of PVDF increased sharply and loss tangent still kept at a low level of 0.03@100 Hz when the concentration of polyamide-1 was 6 wt.%. This work may provide a new direction to design dielectric materials for PVDF blends.

Kinetics of thermal degradation and lifetime study of poly(vinylidene fluoride) (PVDF) subjected to bioethanol fuel accelerated aging

PVDF was prepared by compression molding, and its phase content/structure was assessed by WAXD, DSC, and FTIR-ATR spectroscopy. Next, PVDF samples were aged in bioethanol fuel at 60 °C or annealed in the same temperature by 30 ─ 180 days. Then, the influence of aging/annealing on thermal stability, thermal degradation kinetics, and lifetime of the PVDF was investigated by thermogravimetric analysis (TGA/DTG), as well as the structure was again examined. The crystallinity of ~41% (from WAXD) or ~49% (from DSC) were identified for unaged PVDF, without significant changes after aging or annealing. This PVDF presented not only one phase, but a mixture of α-, β- and γ-phases, α- and β-phases with more highlighted vibrational bands. Thermal degradation kinetics was evaluated using the non-isothermal Ozawa-Flynn-Wall method. The activation energy (E _a ) of thermal degradation was calculated for conversion levels of α = 5 ─ 50% at constant heating rates (5, 10, 20, and 40 °C min^─1), α = 10% was fixed for lifetime estimation. The results indicated that temperature alone does not affect the material, but its combination with bioethanol reduced the onset temperature and E _a of primary thermal degradation. Additionally, the material lifetime decreased until about five decades (T _f = 25 °C and 90 days of exposition) due to the fluid effect after aging.

Development of a Piezoelectric PVDF-TrFE Fibrous Scaffold to Guide Cell Adhesion, Proliferation, and Alignment

Severe peripheral nervous system injuries currently hold limited therapeutic solutions. Existing clinical techniques such as autografts, allografts, and newer nerve guidance conduits have shown variable outcomes in functional recovery, adverse immune responses, and in some cases low or minimal availability. This can be attributed in part to the lack of chemical, physical, and electrical cues directing both nerve guidance and regeneration. To address this pressing clinical issue, electrospun nanofibers and microfibers composed of piezoelectric polyvinylidene flouride-triflouroethylene (PVDF-TrFE) have been introduced as an alternative template for tissue engineered biomaterials, specifically as it pertains to their relevance in soft tissue and nerve repair. Here, biocompatible scaffolds of PVDF-TrFE are fabricated and their ability to generate an electrical response to mechanical deformations and produce a suitable regenerative microenvironment is examined. It is determined that 20% (w/v) PVDF-TrFE in (6:4) dimethyl formamide (DMF):acetone solvent maintains a desirable piezoelectric coefficient and the proper physical and electrical characteristics for tissue regeneration. Further, it is concluded that scaffolds of varying thickness promoted the adhesion and alignment of Schwann cells and fibroblasts. This work offers a prelude to further advancements in nanofibrous technology and a promising outlook for alternative, autologous remedies to peripheral nerve damage.

Unprecedented dipole alignment in α-phase nylon-11 nanowires for high-performance energy-harvesting applications

Dipole alignment in ferroelectric polymers is routinely exploited for applications in charge-based applications. Here, we present the first experimental realization of ideally ordered dipole alignment in α-phase nylon-11 nanowires. This is an unprecedented discovery as dipole alignment is typically only ever achieved in ferroelectric polymers using an applied electric field, whereas here, we achieve dipole alignment in as-fabricated nanowires of 'non-ferroelectric' α-phase nylon-11, an overlooked polymorph of nylon proposed 30 years ago but never practically realized. We show that the strong hydrogen bonding in α-phase nylon-11 serves to enhance the molecular ordering, resulting in exceptional intensity and thermal stability of surface potential. This discovery has profound implications for the field of triboelectric energy harvesting, as the presence of an enhanced surface potential leads to higher mechanical energy harvesting performance. Our approach therefore paves the way towards achieving robust, high-performance mechanical energy harvesters based on this unusual ordered phase of nylon-11.

Characterization and Application of PVDF and Its Copolymer Films Prepared by Spin-Coating and Langmuir-Blodgett Method

Poly(vinylidene fluoride) (PVDF) and its copolymers are key polymers, displaying properties such as flexibility and electroactive responses, including piezoelectricity, pyroelectricity, and ferroelectricity. In the past several years, they have been applied in numerous applications, such as memory, transducers, actuators, and energy harvesting and have shown thriving prospects in the ongoing research and commercialization process. The crystalline polymorphs of PVDF can present nonpolar α, ε phase and polar β, γ, and δ phases with different processing methods. The copolymers, such as poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)), can crystallize directly into a phase analogous to the β phase of PVDF. Since the β phase shows the highest dipole moment among polar phases, many reproducible and efficient methods producing β-phase PVDF and its copolymer have been proposed. In this review, PVDF and its copolymer films prepared by spin-coating and Langmuir-Blodgett (LB) method are introduced, and relevant characterization techniques are highlighted. Finally, the development of memory, artificial synapses, and medical applications based on PVDF and its copolymers is elaborated.

Nonbonding interaction analyses on PVDF/[BMIM][BF4] complex system in gas and solution phase

The present study provides a detailed quantum chemical description of the physicochemical interactions between poly-vinylidene fluoride (PVDF) and 1-butyl-3-methyl-imidazolium tetrafluoro borate ([BMIM][BF₄]) ionic liquid (IL). Geometry optimization and frequency calculations are carried out for four monomer units of α- and β-PVDF, [BMIM][BF₄], and PVDF/[BMIM][BF₄] using dispersion corrected density functional theory. The effects of solvation on the systems under study are demonstrated for three polar aprotic solvents, namely tetra-hydrofuran (THF), acetone, and n,n-dimethyl formamide (DMF) using the integral equation formalism polarizable continuum model (IEFPCM). Calculated negative solvation free energy values suggest solution phase stability of the systems under study. Binding and interaction energies for β-PVDF/IL are found higher in magnitude than those for α-PVDF/IL. The nonbonding interaction phenomenon of β-PVDF/[BMIM][BF₄] is elucidated on the basis of natural bond orbital (NBO), Bader's quantum theory of atoms in molecules (QTAIM), delocalization indices, Hirshfeld surface, and reduced density gradient (RDG) analyses. Both anions and cations of ionic liquids are found to show weak van der Waals interaction with PVDF molecule but the anion ([BF₄]^-)/PVDF interaction is found to be stronger than cation ([BMIM]⁺)/PVDF interaction. Inter-unit C-H⋯F type hydrogen bonds are found to show improper (causing blue shifts in vibrational frequencies) nature. Frontier molecular orbital analysis is carried out, and different chemical parameters like electronegativity, chemical potential, chemical hardness and softness, and electrophilicity index are calculated using Koopmans' theorem. Thermochemical calculations are also performed, and the variation in different standard thermodynamic parameters with temperature is formulated. Graphical abstract (a) Hirshfeld surface mapped onto electron density and (b) NCI isosurfaces showing inter-unit interactions of β-PVDF/[BMIM][BF₄].

Properties and Applications of the β Phase Poly(vinylidene fluoride)

Poly(vinylidene fluoride), PVDF, as one of important polymeric materials with extensively scientific interests and technological applications, shows five crystalline polymorphs with α, β, γ, δ and ε phases obtained by different processing methods. Among them, β phase PVDF presents outstanding electrical characteristics including piezo-, pyro-and ferroelectric properties. These electroactive properties are increasingly important in applications such as energy storage, spin valve devices, biomedicine, sensors and smart scaffolds. This article discusses the basic knowledge and character methods for PVDF fabrication and provides an overview of recent advances on the phase modification and recent applications of the β phase PVDF are reported. This study may provide an insight for the development and utilization for β phase PVDF nanofilms in future electronics.

Energy barriers for dipole moment flipping in PVDF-related ferroelectric polymers

Energy barriers for flipping the transverse dipole moments in poly(vinylidene fluoride) (PVDF) and related copolymers and terpolymers are predicted using the nudged elastic band method. The dipole moments flip individually along the chain, with an order and energy barrier magnitudes (0.1-1.2 eV) that depend on the chain composition and environment. Trifluoroethylene (TrFE) and chlorofluoroethylene (CFE) monomers have larger energy barriers than VDF monomers, while a chain in an amorphous environment has a similar transition pathway as that of an isolated molecule. In a crystalline environment, TrFE and CFE monomers expand the lattice and lower the energy barriers for flipping VDF monomers. This finding is consistent with experimental observations of a large electrocaloric effect in P(VDF-TrFE-CFE) terpolymers.

Towards new strategies for the synthesis of functional vinylidene fluoride-based copolymers with tunable wettability

–COOH functionalized poly(vinylidene fluoride) prepared using water-based non-fluorinated solvents displays tunable wettability suitable for potential application in coating, oil recovery and water purification.

Conformational studies of poly(vinylidene fluoride), poly(trifluoroethylene) and poly(vinylidene fluoride-co-trifluoroethylene) using density functional theory

Different conformations of systems consisting of poly(vinylidene fluoride) (PVDF), poly(trifluoroethylene) (PTrFE) and poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) were investigated using density functional theory with dispersion correction. It was found that the trans-gauche-trans-gauche' (TGTG') conformation of a single PVDF chain is the lowest energy conformer. Crystals of PVDF were modelled using between two to five chains with up to 12 repeat units in each chain and, in agreement with experiment, structures comprised partly or completely of chains with the TGTG' conformation are more stable than structures built up from chains with all-trans (TTTT) conformation. This indicates that an all-trans segment or chain will not induce the growth of a larger crystal with the same chain conformations. In contrast, the energetically preferred structure of PTrFE chains is an all-trans (TTTT) conformation, and the results indicate that copolymerization of vinylidene fluoride with trifluoroethylene can facilitate the formation of the all-trans PVDF conformations. This is probably due to increased intramolecular repulsion between the fluorine atoms and an increased intermolecular attraction in the crystal structure.

Near-field electrospinning enhances the energy harvesting of hollow PVDF piezoelectric fibers

The power generation of the piezoelectric PVDF fiber tubes was 2.46 times higher than that of the solid fibers, which not only displayed mechanical stiffness but also produced a greater power output.

Revisiting the δ-phase of poly(vinylidene fluoride) for solution-processed ferroelectric thin films

Ferroelectric poly(vinylidene-fluoride) (PVDF) has, in the past, been proposed as an ideal candidate for data storage applications as it exhibits a bistable, remanent, polarization that can repeatedly be switched by an electric field. However, fabrication of smooth ferroelectric PVDF thin films, as required for microelectronic applications, is a long-standing problem. At present, the copolymer of PVDF with trifluoroethylene P(VDF-TrFE) is used, but the stack integrity and the limited thermal stability of its remanent polarization hamper large-scale integration. Here we show that smooth neat PVDF films can be made at elevated substrate temperature. On applying a short electrical pulse the ferroelectric polar δ-phase is formed, an overlooked polymorph of PVDF proposed 30 years ago, but never experimentally verified. The remanent polarization and coercive field are comparable to those of the copolymer. The enhanced thermal stability of the polarization is directly related to the high Curie temperature, whereas the ferroelectric properties are related to the molecular packing as derived from the refined crystal structure. The replacement of P(VDF-TrFE) by the commodity polymer PVDF may boost large-scale industrial applications.

Nano-Scale Characterization of a Piezoelectric Polymer (Polyvinylidene Difluoride, PVDF)

The polymer polyvinylidene difluoride (PVDF) has unique piezoelectric properties favorable for Micro-Electro-Mechanical Systems (MEMS) and Nano-Electro-Mechanical Systems (NEMS) applications. In the present research, we conducted nanometer-length scale characterization of this material using several high-resolution techniques. Specifically, we used an atomic force microscope (AFM) to study the nanoand microstructures of the PVDF under stress and to measure their nanoscale conductivity and piezoelectricity. We found that the surface morphology, electronic structure, and microstructure are profoundly affected under electrical potential. Such a behavior is important for the properties and performance of MEMS and NEMS.