Structure and Deformational Behavior of Poly(vinylidene fluoride) Hard Elastic Films (original) (raw)
Journal of Polymer Research, 2011
The phase transformation from the α-to the electroactive β-phase of poly(vinylidene fluoride) (PVDF) extruded filaments submitted to different stretching conditions was investigated. Sample filaments of α-PVDF thermoplastic were extruded and stretched uniaxially at different temperatures (80 ºC to 120 ºC) and stretch ratios (1 to 6). The stretched samples were studied and characterised by x-ray diffraction and quasi-static mechanical experiments. High β-phase contents (~ 80%) are achieved using a stretch ratio of 5 independently of the stretching temperature, between 80 ºC and 120 ºC. Subsequently, in order to obtain filament geometries and material configurations suitable for application, a two layer filament with coaxial layers was produced by coextrusion. The inner layer consisted of a commercially available grade of a conductive thermoplastic with a polypropylene (PP) matrix. For the outer layer the same grade of PVDF was employed. The double-layer filament was also stretched under the same conditions of the PVDF filaments and the results obtained shows that the inner layer material, acting as an electrode, does not have any influence in the PVDF 2 crystallization process: PVDF crystallizes in the α-phase for stretch ratios of 1 and the α− to β-phase transformation occurs for higher stretch ratios.
Fracture analysis of annealed PVDF films
Polymer Testing, 2008
The effect of annealing 50 mm thick, extruded poly(vinylidene fluoride) films in a form (a-PVDF) was investigated by differential scanning calorimetry and mechanical testing on unnotched and double-edge notched tensile specimens. As to the fracture behavior, micromechanisms of cavitation, spherulite breakdown, fiber bundle structure formation and ato b-phase transformation were detected. The progressive structural evolution taking place during annealing affected the deformation and fracture behavior significantly. While the annealing-induced subtle distinctions in microstructure are slightly reflected by the determined mechanical and essential work of fracture (EWF) properties, a distinct differentiation was possible by analysis of the process zone. A correlation between characteristic values of the process zone at necking and the endothermic transitions due to secondary crystallization determined by differential scanning calorimetry (DSC) was established. Annealing of PVDF films facilitates the micromechanism of cavitation, which is presumably related to perfection of morphological superstructures (spherulites) and, thus, interspherulitic stress concentration and failure.
Polymer, 2001
The present work focuses on the effects of thermomechanical history on the structure and mechanical behaviour of PVDF. In the ®rst part of our investigation differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) are used to follow the evolution of the structure of PVDF after different annealing treatments or deformation. Processing has a signi®cant impact on the quantities of the amorphous and crystalline phases and their interphase. Subsequent annealing affects these phases in a different proportion. This in¯uence depends on the position of the annealing temperature in comparison with the upper glass transition temperature of PVDF. Deformation induces conformational change in the injection moulded samples. Thus, the a conformation is transformed to the b conformation. The b conformation also has a noticeable in¯uence on the mechanical behaviour of the material, which will be discussed in the second part of our study.
Poly (vinylidene fluoride) Thin Film Prepared by Roll Hot Press
Various methods have been developed in the manufacture of Poly (vinylidene fluoride) (PVDF) thin films such as coating, spreading, evaporation and calendaring. The method of making this film certainly has many advantages and disadvantages as in the case of operational processes and also the cost of the required treatment. In this paper, an alternative method to preparation of PVDF thin films has been done through the development calendaring method by using roll hot press. The advantages of roll hot press are simple in terms of operation and relative low cost. PVDF thin film has been produced for several temperatures of roll hot press with several different thicknesses. The PVDF thin films are characterized using X-Ray Diffraction and IR spectra. In addition to determine the surface resistivity are caried out using I-V meter. We found that an increase in β fraction of PVDF thin films with increasing temperature at fixed film thickness, while the surface resistivity of PVDF film are decreased. This shown that the piezoelectric property of PVDF films has been improved.
Properties of PVDF films stretched in machine direction
Polymers and Polymer Composites, 2020
Polyvinylidene fluoride (PVDF) films possess superior piezoelectric properties due to the β-phase obtained by methods, such as addition of nanofillers, application of high electric field, use of polar solvents and mechanical stretching. Simultaneous stretching and heating of the films can reduce porosity, increase transformation from α-phase to β-phase, and hence, improve their piezoelectric properties. This article presents the effects of stretching PVDF films on the β-phase formation and the resulting mechanical properties. A custom-designed stretching unit with roller mechanism and heating provision was employed for the purpose. The 200% stretched films at 100°C showed 86.79% β-phase, which is in correlation with X-ray diffraction peaks at 2 θ = 20.3–20.6°. Transmission electron microscopy and scanning electron microscopy of the stretched films revealed spherulitic to lamellar transformation and decrease in porosity. Stretching increased crystallinity from 32.99% to 44.84%. Nanoi...
Influence of the Crystallisation Kinetics on the Microstructural Properties of α-PVDF
MRS Proceedings, 2006
ABSTRACTThe kinetics of the isothermal crystallization from the melt at different crystallisation temperatures and the melting behaviour of Poly(vinylidene fluoride) (PVDF) in the alpha phase has been investigated. The variation of the microstructure of the samples crystallized at different temperatures was monitored with time by Optical Microscopy. The correlation between microstructure and kinetic parameters allows the tailoring of the microstructure by choosing the crystallisation conditions of the samples. Raman and Infrared Transmission Spectroscopy also show the appearance of the γ-phase for higher crystallisation temperatures. The influence of the crystallisation kinetics on the degree of crystallinity of the samples will be also presented and discussed.
α to β Phase Transformation and Microestructural Changes of PVDF Films Induced by Uniaxial Stretch
Journal of Macromolecular Science, Part B, 2009
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 βphase 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 αto β-phase transformation.
Crystallization kinetics and PVT behavior of poly (vinylidene fluoride) in process conditions
2003
The so-called fluoropolymers have gained, in recent years, considerable industrial success, and the increasing industrial interest in this class of materials has caused a need for better characterization of the properties of interest for processability, for instance, for injection molding or extrusion. In this work, the pressure–volume–temperature (PVT) relationship of a poly(vinylidene fluoride) is described by combining specific volumes of amorphous and crystalline phases present in the material. The volumes of the two phases are described simply by thermal expansion and compressibility coefficients drawn from standard PVT data below and above the crystallization range. Within the crystallization range, the material volume is assumed to change from amorphous to crystalline according to the evolution of an overall crystallinity degree, which is described by the Nakamura nonisothermal formulation of an Avrami crystallization kinetic model. Model parameters are identified by comparison with standard calorimetric results, PVT data, and final densities of thin samples solidified during quenches conducted with cooling rates of several hundreds of Kelvins/second. The resulting model allows the description of the PVT behavior of PVDF in the pressure -and cooling-rate ranges of interest for processing. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 3396–3403, 2003
α- and γ-PVDF: Crystallization kinetics, microstructural variations and thermal behaviour
Materials Chemistry and Physics, 2010
The kinetics of the isothermal crystallization from the melt of ␣and ␥-poly(vinylidene fluoride), PVDF, and the corresponding melting behaviour has been investigated. The crystallization kinetics was evaluated on the basis of the theory of Avrami. The variation of the microstructure of the samples crystallized at different temperatures was monitored along time by optical microscopy with polarized light. The correlation between microstructure and kinetic parameters allows tailoring the microstructure of the polymer by choosing specific crystallization conditions. Infrared spectroscopy and Raman spectroscopy show the appearance of the ␥-phase for higher crystallization temperatures.
Effect of thermal processing conditions on the structure and dielectric properties of PVDF films
The effect of annealing and quenching temperatures on the crystallinity, β phase fraction and dielectric behavior of poly (vinylidene fluoride) (PVDF) have been studied. The crystallinity and β phase fraction of these films were evaluated using X-ray diffraction and FTIR techniques for different annealing and quenching temperatures. It is seen that the thermal processing conditions play a crucial role in determining the dominant phase in PVDF. The β phase PVDF is the most desired phase for device applications such as sensors and actuators. Hence, the thermal processing conditions are optimized for obtaining β rich PVDF films. The β rich phase of PVDF is obtained for films which are annealed at 80 °C and quenched at 20 °C. The as-synthesized films for the optimized processing conditions was studied for their dielectric behavior and was found to exhibit dielectric constant as high as ~60.