Nano-graphene /polymer composites can functionas pressure induced electro-switches, at concentrations around their conductivity percolation threshold. Close to the critical point, the pressure dependence of the electron tunneling through the polymer barrier separating nanon-graphenes results from thecompetition among shorteningof the tunneling length and the increase of the polymer's polarizability. Such switching behaviorwas recentlyobserved inpolyvinyl alcohol (PVA) loaded withnano-graphene platelets (NGPs). In this work, PVA is blended withh alpha-poly(vinylidene fluoride) (PVdF)and NGPs. Coaxial mechanical stress and electric field render the nano-composite piezoelectric. We investigate the influence of heterogeneity, thermal properties, phase transitions and kinetic processes occurring in the polymer matrix on the macroscopicelectrical conductivity and interfacial polarization in casted specimens. Furthermore, the effect of electro-activity of PVdF grains on the electric and thermal properties are comparatively studied. Broadband Dielectricspectroscopy is employed to resolve and inspect electron transport and trapping with respect to thermal transitions and kineticprocessestraced via Differential Scanning Calorimetry. The harmonic electric field applied during a BDS sweep induces volume modifications of the electro-active PVdF grains, while, electro-activity of PVdF grains can disturb the internal electric field that free (or bound) electric. The dc conductivity and dielectric relaxation was found to exhibit weakdependencies.
The relationship between elastic fatigue and electrical cyclic loading in alpha-phase poly(vinylidene fluoride) films has been investigated. Our experimental studies have shown that the electric-field-induced fatigue behavior can be described by a stress relaxation, which belongs to the Kohlrausch function group, and the corresponding exponent is a modified two-parameter Weibull distribution function.
A density-functional study of the alternatively-deflected zigzag structure for the beta-phase of poly(vinylidene fluoride) or (-CH2-CF2-)n is presented. Structure, polarization and effective charges have been determined. Using lattice constants and atomic positions transformed from experiment, the space group for the alternatively-deflected zigzag structure was found to be Cc2m (C2v 16), in agreement with previous studies. We find a polarization for the alternatively-deflected structure identical to that of the planar-zigzag structure with small changes in the accompanying effective charges.
The structures and electronic states in all polymorphs of poly(vinylidene fluoride) (PVDF) were calculated in various levels using the CRYSTAL software. The calculated lattice constants with PBE0/cc-pVTZ agreed well with experimental values. The molecular structure of PVDF was clarified. Derived electronic and phonon dispersions correspond closely with the experimental valence X-ray photoelectron and infrared (IR)/Raman spectra, respectively. The amount of spontaneous polarization in polar crystal forms was determined and the effect of long-range Coulomb interactions were discussed. The calcul…
The Poly(vinylidene fluoride)/CaCu3Ti4O12 (CCTO) nanocrystal composite films (85nm) with relatively high dielectric permittivity (90 at 100Hz) were prepared by the solution casting followed by spin coating technique. The structural, the microstructural and the dielectric properties of the composites were studied using X-ray diffraction, Scanning Electron Microscope, and Impedance analyzer respectively. The effective dielectric permittivity of the composite increased with increase in the volume fraction of CCTO at all the frequencies (100 Hz to 1MHz) under investigation. The room temperature di…
We present the computation of structural and polarization for a ferroelectric polymer, specifically, the beta-phase of poly(vinylidene fluoride) or (-CH$_2$-CF$_2$-)$_n$. Two structures have been described in the literature: one containing an undistorted carbon backbone (planar-zigzag) and the other containing alternatively deflected -CF$_2$- groups. Previous studies have indicated that these deflections have no significant effect on calculated elastic properties. We employ density functional theory within the generalized gradient approximation to find relaxed atomic positions for the planar-z…
Two structures have been proposed in the literature for the b-phase of the ferroelectric polymer, poly(vinylidene fluoride) (b-PVDF); planar-zigzag and alternatively-deflected forms. Using density-functional theory, we have found the planar-zigzag structure is the preferred form and upon atomic relaxation, the alternatively-deflected structure attains a structure very similar to the planar-zigzag structure. In order to better understand the atomic origin of the ferroelectricity in b-PVDF, we have for the first time determined the dynamic Born effective charges (Z*) for the planar-zigzag struct…
Phonon engineering focuses on heat transport modulation on atomic-scale. Different from reported methods, it is shown that electric field can also modulate heat transport in ferroelectric polymers, poly(vinylidene fluoride), by both simulation and measurement. Interestingly, thermal conductivities of poly(vinylidene fluoride) array can be enhanced by a factor of 3.25 along the polarization direction by simulation. The semi-crystalline poly(vinylidene fluoride) film can be also enhanced by a factor of 1.5 which is found by both simulation and measurement. The morphology and phonon property anal…
The possibility of obtaining relatively high dielectric constant polymer-ceramic composite by incorporating the giant dielectric constant material, CaCu3Ti4O12 (CCTO) in a Poly(vinylidene fluoride) (PVDF) polymer matrix by melt mixing and hot pressing process was demonstrated. The structure, morphology and dielectric properties of the composites were characterized using X ray diffraction, Thermal analysis, Scanning Electron Microscope, and Impedance analyzer. The effective dielectric constant of the composite increased with increase in the volume fraction of CCTO at all the frequencies (100 Hz…
In literature it has been reported that the gamma-phase PVDF film is formed from dimethyl sulfoxide (DMSO) solvents regardless of preparation temperature. In this report the crystallization of both alpha and gamma phase from DMSO solvent by varying preparation temperature has been described. This paper also describes the conversion of gamma-phase to beta- phase and alpha-phase at different annealing condition. When thin films were annealed at 90C for 5 hours, then maximum beta-phase content (greater than 95%) is present in PVDF thin film. The PVDF films completely converted to alpha-phase, whe…
Semi-empirical quantum chemistry methods offer a very interesting compromise between accuracy and computational load. In order to assess the performance of NDDO methods in the interpretation of terahertz spectra, the low frequency vibration modes of three crystalline materials, namely, polyethylene, poly(vinylidene fluoride) form II and $α$-D-glucose have been studied using the PM6 and PM7 Hamiltonians and the results have been compared with the experimental data and former calculations. The results show good qualitative or semi-quantitative agreement with the experimentally observed terahertz…
Herein, we elucidate the impact of tubular confinement on the structure and relaxation behaviour of poly(vinylidene difluoride) (PVDF) and how these affect the para-/ferroelectric behavior of this polymer. We use PVDF nanotubes that were solidified in anodic aluminum oxide (AAO) templates. Dielectric spectroscopy measurements evidence a bimodal relaxation process for PVDF nanotubes: besides the bulk-like -relaxation, we detect a notably slower relaxation that is associated with the PVDF regions of restricted dynamics at the interface with the AAO pore. Strickingly, both the bulk-like and the i…
For pyroelectric detector application materials should have low dielectric constant, high pyroelectric coefficient, large non volatile polarization at small applied electric field and low specific heat. Large field (greater than 1200kV/cm) is need to pole ferroelectric polymer poly (vinylidene fluoride) (PVDF) and it has low sensitivity compared to other pyroelectric materials. To increase non volatile polarization at low poling field and to increase pyroelectric coefficient, LiTaO3 (LT) nano particles were added to PVDF matrix to make LT/PVDF composite. It is important to study the dielectric…
For pyroelectric sensor application, materials having large ferroelectric polarization at low applied field and high pyroelectric sensitivity are required. PVDF is one of the ferroelectric polymers, which is being used for pyroelectric detector application. Following properties of PVDF such as low density, low acoustic impedance, wide bandwidth, flexibility, toughness, and ease of fabrication into complex patterns and arrays make it suitable for large area detector applications. But the PVDF has low value of pyroelectric coefficient (p). High field (above 1200kV/cm) is required to pole the PVD…
Poly(vinylidene fluoride), PVDF nanofibers were prepared using electrospinning method and the influence of electrospinning parameters such as PVDF concentration, DMF:Acetone ratio on the formation of different phases (α, \b{eta}, and γ) and desired morphology were investigated. A detail analysis of X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectrometer data by deconvoluting the peaks were carried out for assigning and accurately identifying the peaks associated to a particular PVDF phase. SEM micrographs further helped in understanding the mechanism of desired \b{eta}-phase…
In this paper, we present some properties of Lead Zirconate Titanate - Poly Vinylidene Flouride composites. The obtained results indicate that PZT-PVDF fibers involved firms were achieved using electrospining method. In other hand, the effect of grain size, content and other factors under the purview of Young's modulus and ferroelectric properties.
We report on the fabrication and electrical characterization of few-layer graphene (FLG) devices coated with a ferroelectric polymer layer of poly(vinylidene fluoride/trifluoroethylene) [P(VDF/TrFE)]. Highly stable and reliable resistance changes were observed under floating conditions, which were dependent on the back gate voltage applied beforehand. Nonvolatile memory functionality in the hybrid FLG-P(VDF/TrFE) devices is attributed to a remanent electric field induced by the ferroelectric polarization of the P(VDF/TrFE) layer.
We report phonon renormalization induced by an external electric field E in ferroelectric poly(vinylidene fluoride-trifluoroethylene) [P(VDF-TrFE)] nanofibers through measuring the E-dependent thermal conductivity. Our experimental results are in excellent agreement with the theoretical ones derived from the lattice dynamics. The renormalization is attributed to the anharmonicity that modifies the phonon spectrum when the atoms are pulled away from their equilibrium positions by the electric field. Our finding provides an efficient way to manipulate the thermal conductivity by tuning external …
We demonstrate the direct dry transfer of large area Chemical Vapor Deposition graphene to several polymers (low density polyethylene, high density polyethylene, polystyrene, polylactide acid and poly(vinylidenefluoride-co-trifluoroethylene) by means of only moderate heat and pressure, and the later mechanical peeling of the original graphene substrate. Simulations of the graphene-polymer interactions, rheological tests and graphene transfer at various experimental conditions show that controlling the graphene-polymer interface is the key to controlling graphene transfer. Raman spectroscopy an…
The recently developed evolutionary algorithm USPEX proved to be a tool that enables accurate and reliable prediction of structures for a given chemical composition. Here we extend this method to predict the crystal structure of polymers by performing constrained evolutionary search, where each monomeric unit is treated as one or several building blocks with fixed connectivity. This greatly reduces the search space and allows the initial structure generation with different sequences and packings using these blocks. The new constrained evolutionary algorithm is successfully tested and validated…
Nanogenerators based on piezoelectric materials convert ever-present mechanical vibrations into electrical power for energetically autonomous wireless and electronic devices. Nanowires of piezoelectric polymers are particularly attractive for harvesting mechanical energy in this way, as they are flexible, lightweight and sensitive to small vibrations. Previous studies have focused exclusively on nanowires grown by electrospinning, but this involves complex equipment, and high voltages of $\approx$ 10 kV that electrically pole the nanowires and thus render them piezoelectric. Here we demonstrat…
Lattice thermal conductivities (LTC) for a subset of polymer crystals from the Polymer Genome Library were investigated to explore high LTC polymer systems. We employed a first-principles approach to evaluating phonon lifetimes within the third-order perturbation theory combined with density functional theory, and then solved the linearized Boltzmann transport equation with single-mode relaxation time approximated by the computed lifetime. Typical high LTC polymer systems, polyethylene (PE) crystal and fiber, were benchmarked, which is reasonably consistent with previous references, validating…
In the present work, the morphology and the electrical and thermal conduction properties of co-continuous poly(vinylidene fluoride) (PVDF), maleated polypropylene (PPgMA) and multiwall carbon nanotubes (CNT) nanostructured blends are investigated. CNT preferentially locates in the PPgMA phase and clearly causes a refinement in the co-continuous structure. Electrical conductivity experiments show that nanocomposites are well above the percolation threshold and evidence for one order of magnitude enhancement in conductivity for the co-continuous nanocomposites compared to the monophasic nanocomp…
We use first principles methods to study static and dynamical mechanical properties of the ferroelectric polymer Poly(vinylidene fluoride) (PVDF) and its copolymer with trifluoro ethylene (TrFE). We use density functional theory [within the generalized gradient approximation (DFT-GGA)] to calculate structures and energetics for various crystalline phases for PVDF and P(VDF-TrFE). We find that the lowest energy phase for PVDF is a non-polar crystal with a combination of trans (T) and gauche (G) bonds; in the case of the copolymer the role of the extra (bulkier) F atoms is to stabilize T bonds. …
