Vinylidene Fluoride Copolymers Research Articles

Polarization switching at the nanoscale in ferroelectric copolymer thin films

The polarization switching kinetics were measured at the nanoscale in continuous thin films of a ferroelectric copolymer of vinylidene fluoride and trifluoroethylene. The dependence of the switching rate on voltage for a 54-nm thick film exhibits extrinsic nucleation and domain-growth type kinetics with no true threshold coercive field, and is qualitatively different from the behavior of an 18-nm thick film, which exhibits intrinsic switching kinetics, and a true threshold field. The results are consistent with studies of thin film capacitors of much larger area and with a recent refinement of the theory of the critical size for intrinsic switching.

Detection of obscured glass transitions by QiDSC

Determination of compatibility in the amorphous phase for a two component blend is usually accomplished by analyzing for whether one notes one or two glass transitions. This can be complicated when one of the components is semicrystalline and its melting peak obscures the second glass transition. Quasi-isothermal differential scanning calorimetry (QiDSC) can be used to detect an obscured glass transition by allowing the semicrystalline component to melt and relax revealing the underlying glass transition of the other component. QiDSC is accomplished by performing a modulated temperature DSC experiment at a particular temperature and step ramping through the transitions of interest. For this study two systems are investigated. The first system is a model system based on a blend of polystyrene (PS) and a copolymer of vinylidene fluoride and hexafluoropropylene, P(VF2/HFP). The glass transition for the PS occurs at the same temperature as the melting point for the fluoro-copolymer. The second system is a fluoro-copolymer/acrylic dried latex. In both cases the hidden glass transition can be noted in the reversing heat capacity of the QiDSC analysis.

Polarization fatigue in ferroelectric vinylidene fluoride and trifluoroethylene copolymer thin films

In recent years polarization fatigue in ferroelectric polymers has attracted much attention due to their potential use in nonvolatile memories, though the mechanism is still an open question. Here we mainly focus on the experimental observations on two physical phenomena occurring during fatigue process: The frequency dependence of the fatigue rate and the change in the coercive field with increased fatigue cycles. The frequency dependence of fatigue rate shows that at lower fatigue frequencies the rate obeys a universal scaling behavior with respect to N/f, where N is the fatigue cycles and f is the fatigue frequency, while at much higher frequencies this scaling behavior is broken and the fatigue rate is N/f dependent. In addition, both the increase and the decrease of the coercive field are observed, which indicates that the change in the coercive field should be dominated by different mechanisms. Based on the understanding of the crucial influence of space charges on electrical fatigue, we give a detailed discussion on both phenomena.

Ferroelectric Nanocrystals from Copolymer P(VDF-TrFE), Their Switching and Properties

Ferroelectric polymers show interesting peculiarities in spite of some constraints, which limit the number of possible materials. These constraints are compact crystal structure, conformational flexibility and minimal steric hindrance. One of the important properties of the ferroelectric polymers is the simple method of the nanostructures preparation. The present paper is devoted to the review of switching and ferroelectric properties of ultrathin copolymer Langmuir-Blodgett films and nanocrystals from vinylidene fluoride copolymer.

Poly(vinylidene fluoride)-graft-poly(2-hydroxyethyl methacrylate): a novel material for high energy density capacitors

We report the graft copolymerization synthesis of novel high dielectric constant HEMA (2-hydroxyethylmethacrylate)-graft-poly(vinylidene fluoride) (PVDF) copolymers for potential applications in high energy density capacitors. In the present study the graft copolymerization reaction was introduced by irradiation (electron beam) treatment of the PVDF polymer powder. The chemical changes in the PVDF polymers after HEMA grafting were monitored and the results were evaluated by FTIR, DSC, TGA and contact angle measurement. An analysis is presented on the effectiveness of this approach as a function of electron beam operating variables including radiation dose, reaction time, reaction temperature, monomer concentration and effect of solvent. The PHEMA-g-PVDF copolymers demonstrate improved dielectric properties such as high dielectric constant, lower leakage and low dielectric loss as compared to pristine PVDF. Correlation of dielectric properties with the graft copolymerization reaction mechanism was discussed.

Functional poly(vinylidene fluoride) copolymer membranes via surface-initiated thiol–ene click reactions

Thermally induced graft copolymerization of allyl methacrylate (AMA) from ozone-preactivated poly(vinylidene fluoride) (PVDF) chains was first carried out in N-methyl-2-pyrrolidone (NMP) solution to produce the PVDF-g-PAMA copolymers. The PVDF-g-PAMA copolymers were cast, by phase inversion in an aqueous medium, into microporous membranes with enriched allyl groups on the membrane and pore surfaces. The pendant allyl groups in the PAMA side chains allowed the thiol–ene click grafting of thiol-functionalized moieties on the membrane surface. Thermally initiated thiol–ene click reaction with 3-mercaptopropionic acid (MPA) produced the PVDF-g-P[AMA-comb-MPA] membrane which exhibited a pH-responsive permeation behavior, with the most drastic change in permeation rate of the aqueous media being observed between pH 2 and 4. The PVDF-g-P[AMA-comb-DMAPS] membrane was synthesized via UV-initiated thiol–ene click reaction of 1,6-hexanedithiol (HDT) with the PVDF-g-PAMA membranes, using 2,2-dimethoxy-2-phenylacetophenone (DMPA) as the photoinitiator, followed by thiol-Michael addition reaction of the zwitterionic molecules, N,N′-dimethyl-(methylmethacryloyl ethyl) ammonium propanesulfonate (DMAPS). A significant reduction in microbial adhesion was observed on the PVDF-g-P[AMA-comb-DMAPS] membrane in a flow chamber, in comparison to the pristine hydrophobic PVDF membrane.

Growth and Switching of Ferroelectric Nanocrystals from Ultrathin Film of Copolymer of Vinylidene Fluoride and Trifluoroethylene

The ferroelectric nanocrystals of the copolymer of vinylidene fluoride and trifluoroethylene P(VDF-TrFE) were grown from ultrathin Langmuir-Blodgett (LB) films on Si substrate. The annealing of ultrathin LB films with thickness of 3 monolayers (5 nm) in air in paraelectric phase at temperature 125∘C was performed. The self-assembly leads to the growth of nanocrystals of ferroelectric copolymer 15–25 nm thick and 100–200 nm in diameter. The nanocrystals presumably belong to orthorhombic space group, where axis 2 is the direction of spontaneous polarization (and normal to substrate). By means of atomic force microscopy (AFM), the kinetics of ferroelectric nanocrystals growth and their switching were investigated. The obtained results confirm the conclusions that copolymer nanocrystals are candidates for high-density nonvolatile storage media devices.

Copolymerization of VDF and HFP in Supercritical Carbon Dioxide: Experimental Analysis of the Reaction Loci

Free radical copolymerization reactions of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) were carried out in supercritical carbon dioxide at T = 50 °C. When ammonium carboxylate perfluoro...

Ferroelectric Polymer Gate Transistor as a Model System for Exploring the Mechanisms of the Retention Loss

An attractive combination of properties of the ferroelectric copolymer of vinylidene fluoride and trifluoroethylene (P(VDF-TrFE)), including a relatively high spontaneous polarization and low dielectric constant as well as low processing temperature makes this material useful for studying the ferroelectric gate operation for 1T nonvolatile ferroelecetric memory applications. Here we explore a silicon-based ferroelectric field effect transistor with P(VDF-TrFE) gate showing a persistent switching of the drain current with the “on”/“off” current ratio of 103–102 and retention exceeding 5 days. The physical mechanism of the retention loss has been addressed by monitoring the drain current relaxation in combination with the time-resolved piezo-force scanning probe microscopy. The results suggest that the retention loss is controlled by the polarization screening due to the charge injection into the interface-adjacent layer rather than the polarization loss due to the depolarization effect.

Structure and electrical properties of PMNSZT/PVDF pyroelectric composites doped with BiCl3

Lead manganese niobate stibium zirconate titanate (PMNSZT) ceramic powder fabricated by the conventional method has been incorporated into a poly(vinylidene fluoride) (PVDF) copolymer matrix to form the pyroelectric composites. Furthermore, the composites doped with BiCl3 were fabricated by solvent casting method, and their microstructure was investigated by scanning electron microscopy and X-ray diffraction. The dielectric, piezoelectric, and pyroelectric properties of the composites with different mass fractions of BiCl3 were studied. It was found that the electric properties were enhanced with the addition of BiCl3. In addition, when 2% BiCl3 was added, the composites exhibited the best electric properties, especially the pyroelectric properties that can satisfy the need for their applications in pyroelectric devices.

Effect of recrystallization on the molecular mobility of a copolymer of vinylidene fluoride and hexafluoropropylene

AbstractRelaxation processes in copolymers of vinylidene fluoride with hexafluoropropylene (93/7) were studied by means of a dielectric method. The initial extruded film was recrystallized by free heating to temperatures above the melting point and by subsequent cooling. This increased both the perfection of the crystal phase and the degree of crystallinity. The impact of recrystallization on both the relaxation times (τ's) and the activation parameters of the local mobility (β process) and micro‐Brownian cooperative mobility in amorphous phase (αa process) was almost negligible, whereas the τ's of the αc relaxation were an order of magnitude higher after recrystallization. Qualitatively, it was predicted by the soliton model for the αc relaxation. The recrystallization affected characteristics of the transition at the highest temperature (α) registered in the region of the melting of the crystals even more. The process is related to the relaxation of the space charge formed by ionogenic impurities, which migrate through the amorphous phase with high free volume. It was shown that the dynamics in the amorphous phase controlled the drift mobility of free charge carriers and, by that, determined the τ of the space charge relaxation process. The structuring processes during the recrystallization also affected the parameters of the order–disorder transition in the low‐perfect ferroelectric (antiferroelectric) phase. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

Symmetric Ferroelectric Switching in Ferroelectric Vinylidene Fluoride and Trifluoroethylene Copolymer Films

We report the observation of asymmetric switching dual peaks in ferroelectric copolymer films. These dual peaks occurs when the poling electric field is just below the coercive field and can be removed by continuous application of high enough switching voltage. Our experimental observations can be explained by the injection and the redistribution of space charges in ferroelectric films.

Imaging liquid crystals dispersed in a ferroelectric polymer matrix by means of thermal-pulse tomography

A new arrangement of the optical elements in a Thermal-Pulse-Tomography (TPT) setup allows to scan micrometer structures in composite and heterogeneous samples such as polymer-dispersed liquid crystals (PDLCs). The non-destructive TPT technique allows the determination of three-dimensional profiles of polarization and space charge in dielectrics. The samples under study were 12 μm thick films of a copolymer of vinylidene fluoride with trifluoroethylene P(VDF-TrFE) (65/35) with embedded liquid-crystal droplets. The poling process was performed in direct contact well above the coercive field of the copolymer. The 3D map obtained from scanning with a 10 μm wide spot shows elliptically shaped areas with liquid-crystal droplets. Considering the droplets as oblate spheroids, their major axis lies in the x-y plane, while their minor axis in the z direction measures 0.5 μm or more. This result is in good agreement with scanning electron micrographs. It is believed that the major axis is overestimated due to imaging of liquid-crystal clusters.

Combustion of Silicon/Teflon/Viton and Aluminum/Teflon/Viton Energetic Composites

DOI: 10.2514/1.46182 The combustion of Si- and Al-based systems using polytetrafluoroethylene (PTFE) as the oxidizer and Fluorel FC 2175 (a copolymer of hexafluoropropylene and vinylidene fluoride) as a binder has been studied. Experimental data were obtained using two methods: 1) instrumented tube burns and 2) pressed pellets inside a windowed pressure vessel. Loose-powder burning rates were seen to optimize at slightly-fuel-rich mixture ratios for Si/PTFE/FC-2175 (SiTV). Al/PTFE/FC-2175 (AlTV) burning rates optimized near a stoichiometric ratio. Pressures calculated by assuming constant-volume combustion equilibrium were seen to match experimental values from burn-tube experiments when burning rates were at or near peak values. The pressure dependence of SiTV and AlTV pellet burning rates was also characterized and compared with reported Mg/PTFE/Viton (MTV) results. SiTV showed power-law dependence with a constant-pressure exponent over the experimental range of pressures. AlTV was showntoexhibitnonconstant-pressureexponentbehavior.SiTVburningratesoptimizedatmixtureratiossimilarto that of the tube burns. AlTV burning rates increased well past a stoichiometric ratio and decreased at a fuel-rich ratio, which is a similar trend to MTV burning rates.

The Strong Dependence of Polarization Fatigue on Poling-Voltage Conditions in Ferroelectric Vinylidene Fluoride and Trifluoroethylene Copolymer Films

Recently, polarization fatigue in ferroelectric polymers has attracted much attention due to their potentials in nonvolatile memory devices, although the mechanism behind is still an open question. In this letter, we report the strong dependence of polarization fatigue on poling voltages, which indicates that the application of a poling electric field slightly below the coercive field can greatly degrade the fatigue endurance. This phenomenon may be directly related to the fatigue mechanism, and its origin is discussed based on the understanding of the injection and redistribution of space charges in ferroelectric films.

Comparative investigation of the structure and properties of ferroelectric poly(vinylidene fluoride) and poly(vinylidene fluoride–trifluoroethylene) thin films crystallized on substrates

AbstractPoly(vinylidene fluoride) (PVDF) and copolymers of vinylidene fluoride and trifluoroethylene [P(VDF/TrFE)s] were deposited on silicon substrates, and their structure and properties were comparatively investigated. Compared to P(VDF/TrFE), which is the polymer material currently most dominant for ferroelectric thin‐film devices applications, our β‐phase PVDF homopolymer thin film demonstrated several advantages, such as higher dielectric breakdown strength, improved polarization fatigue endurance, and larger ferroelectric polarization at elevated temperatures with improved thermal stability. The reasons for the observed different characteristics between the PVDF and P(VDF/TrFE) thin films were analyzed on the basis of their different structures and morphologies. The results indicate that the low‐cost β‐phase PVDF homopolymer thin films have great potential as an alternative to P(VDF/TrFE) for ferroelectric and piezoelectric thin‐film‐device applications. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Preparation of microporous poly(VDF-co-HFP) membranes by template-leaching method

The copolymer of vinylidene fluoride and hexafluoro propylene, poly(VDF–HFP), and the terpolymer of vinylidene fluoride, tetrafluoro ethylene, and hexafluoro propylene, poly(VDF–TEF–HFP), were solution blended at first to form dense composite films. Then, 90% acetone aqueous solution was used to leach out the terpolymer in the film at 5 °C. By varying the fraction of terpolymer in the blend and the leaching condition, microporous membranes with different porous morphology were produced, as revealed by SEM imaging. When the terpolymer content in the blend was lower than 30 wt%, low porosity (e.g., 24%) asymmetric membranes were obtained. On the other hand, when the terpolymer content approached 50%, skinless membranes with uniform pore distribution (pore size, ca. 1 μm) were produced. These skinless membranes demonstrate high tensile strengths (∼6 MPa) and water fluxes (3000–8000 LMH) that are potentially applicable to micro-filtration operations. Furthermore, crystallinities of the membranes were determined both by X-ray diffractometry and differential scanning calorimetry. Both methods indicated crystallinity near 24%, typical of poly(VDF–HFP) polymer in different forms.

Highly selective proton conductive networks based on chain-end functionalized polymers with perfluorosulfonate side groups

The copolymers of vinylidene fluoride and perfluoro(4-methyl-3,6-dioxane-7-ene) sulfonyl fluoride containing amino end-groups were synthesized for the first time. The prepared amino-terminated polymers underwent cross-linking reactions with 1,3,5-benzene triisocyanate to form proton conductive networks. The prepared membranes exhibited excellent thermal, hydrolytic and oxidative stabilities. The ion exchange capacity, water uptake, the state of absorbed water, and transport properties of the membranes were found to be highly dependent upon the chemical composition of the copolymers. The cross-linked membranes showed extremely low methanol permeability, while maintaining high proton conductivity at the same order of magnitude as Nafion. This unique transport feature gave rise to exceedingly higher electrochemical selectivity in relation to Nafion. The selectivity characteristics have been rationalized based on the formation of restrained ionic domains and the state of the absorbed water within the membranes.

Features of structure formation and electrophysical properties of poly(vinylidene fluoride) crystalline ferroelectric polymers

AbstractThe influence of intramolecular dipole–dipole interaction changes on structure formation peculiarities and some electrophysical properties were investigated with example of copolymers of vinylidene fluoride with tetrafluoroethylene and hexafluoropropylene with different compositions. The decrease of such dipole–dipole interactions in vinylidene fluoride/tetrafluoroethylene copolymers leads to an increase of the a and b parameters of the ferroelectric phase lattice and were accompanied by a shift of the Curie point to lower temperatures. The presence of peak‐halo at angles near 2θ = 18° were attributed to a paraelectric phase localized in the interfacial domains at the crystal–amorphous phase boundaries. Similar peak‐halos for vinylidene fluoride/hexafluoropropylene copolymers crystallizing into the nonpolar α phase were associated with the presence of an antiferroelectric phase formed by the chains in the planar zigzag conformation. The temperature range where dielectric anomalies were detected was characterized by conformational changes at which the decrease in planar zigzag conformation isomers took place. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

The effect of side chain architectures on the properties and proton conductivities of poly(styrene sulfonic acid) graft poly(vinylidene fluoride) copolymer membranes for direct methanol fuel cells

The effect of side chain architectures on the properties and proton conductivities of graft copolymer membranes for direct methanol fuel cells (DMFCs) are studied. Poly(vinylidene fluoride)-g-poly(styrene sulfonic acid) (PVDF-g-PSSA) copolymers with either linear or arborescent PSSA side chains are prepared and examined. For the copolymers with similar ion exchange values, both graft copolymers show similar water uptakes and bound water contents. Meanwhile, the arborescent samples exhibit higher proton conductivity, lower methanol permeability, and higher selectivity compared to the linear analogues. Incorporation of highly branched side chains effectively increases the properties of the PVDF-g-PSSA-based PEMs for DMFCs because of formation of agglomerate PSSA domains. The PSSA domains promote proton conduction and depress methanol permeation through the PEMs, consequently significantly increase the selectivities (proton conductivity/methanol permeability) of the PEMs.