Chlorinated poly(vinylidene fluoride)

Chlorinated poly(vinylidene fluoride) (PVF₂) was prepared by introducing chlorine gas into a CCI₄ suspension of PVF₂ at reflux temperature. Polymer crystallinity and softening point decrease, while solubility and adhesion increase with the degree of chlorination. In contrast to PVF₂, the chlorinated polymer is soluble in low-boiling common organic solvents, such as acetone, methyl ethyl ketone, and 1,2-dimethoxyethane. Chlorinated PVF₂ is resistant to dehydrochlorination and is thermally more stable than PVF, chlorinated PVF, PVC, or chlorinated PVC. Chlorinated PVF₂ coatings on wood, prepared by solution casting at room temperature, show outstanding weathering resistance.     

Properties of poly(vinylidene fluoride)

The properties of fluorocarbon plastics have made them desirable for numerous new applications. Poly(vinylidene fluoride) (PVF₂) can be easily extruded into films or fibers or injection molded into a variety of shapes. It was found that crystallinity, molecular weight distribution and polymer structure are important in establishing resin properties. Highly crystalline PVF₂ with a narrow Gaussian molecular weight distribution gives specimens with optimum physical and chemical properties.     

Heat capacity of copolymers of chlorotrifluoroethylene and vinylidene fluoride

The heat capacities of two copolymers of chlorotrifluorethylene and vinylidene fluoride, one 30:70 mol % and the other 44:56 mol % in composition, were measured in an adiabatic calorimeter from 80 to 340 K. The glass-transition points T_g observed at 256 and 269 K for the 30:70 and 44:56 samples, respectively, closely agree with the prediction of the theory of Flory and Fox; the accompanying jump of heat capacities at these points was found to be 2.2 cal/K ‘per bead’ for both samples, in fair agreement with the ‘constant ΔC_p rule’. A small peak was observed immediately above T_g, and interpreted as a time-dependent effect arising from the lack of equilibrium in the region. The data of the heat capacities were analysed in detail in the framework of the Tarasov model and the principle of additivity, and were found to agree well ( 1–3%) with theory up to the respective glass-transition points in all cases, thus giving support to the validity of the previously published segmental values of heat capacity and of characteristic temperature θ₁.     

Crystallization kinetics of poly(vinylidene fluoride)

Specimens of poly(vinylidene fluoride) were crystallized isothermally at a series of temperatures in the vicinity of the melting point. The -form was the only crystalline polymorph present in the crystallized samples. Crystallization rates have been measured by differential scanning calorimetry. The results were analysed in terms of the Avrami equation. The rates of crystallization depend upon the undercooling and the data agree with a process of growth of spherulites controlled by a secondary surface coherent nucleation mechanism. The equilibrium melting temperature, the Avrami exponent, the free energy of formation of a nucleus of critical dimensions and the surfaces free energy of the lamellar crystallites were determined.     

Failure modes in piezoelectric poly(vinylidene fluoride)

This article describes a number of potential failure mechanisms for piezoelectric poly(vinylidene fluoride) (PVDF), including decay of piezoelectric properties, film shrinkage at elevated temperatures, electrode erosion due to water, curling and fibrillation of the highly oriented PVDF films, and abrasion or impact damage. Piezoelectric aging and shrinkage are found to be strongly correlated; neither process occurs below 60°C, and both show logarithmic time dependences above that temperature. Thus, shrinkage and piezoelectric aging likely both result from similar mechanisms associated with micro-structural annealing effects at elevated temperatures. The degradation of piezoelectric response observed when PVDF films are exposed to moisture is found to be due primarily to water-induced erosion of the vapor-deposited aluminum electrodes rather than to enhanced piezoelectric decay. Observations of mechanical damage suggest that the susceptibility of PVDF films may be partially due to imperfections introduced during the manufacturing process. One conclusion of this study is that biaxial PVDF offers advantages over uniaxial film, including reduced shrinkage and piezoelectric decay, superior resistance to curling and fibrillation, and lower susceptibility to formation of pinholes and other localized defects.     

Infra-red analysis of poly(vinylidene fluoride) thermoelectrets

The copolymer poly(vinylidene fluoride)—poly(tetrafluoroethylene) studied by infra-red spectroscopy during its transformation into electrets, shows structural modifications of the copolymer due to the electrostatic field action. The dielectric used is a copolymer monoaxially oriented by stretching and the paper reports on the structural changes in the molecular arrangements, relative to the induced electrical properties. These results complete and confirm data obtained previously by Raman spectroscopy^1,7.     

Synthesis of a poly(vinylidene fluoride) macromonomer

Summary Methacrylate-type poly(vinylidene fluoride) macromonomer was synthesized from vinylidene fluoride via hydroxy-ended poly(vinylidene fluoride). The macromonomer copolymerized easily with methyl methacrylate, but homopolymerization did not proceed completely.     

FTIR-microspectroscopy and DSC studies of poly(vinylidene fluoride)

Poly(vinylidene fluoride) (PVDF) samples, obtained by casting from tetrahydrofuran solutions and submitted to various thermal treatments, have been examined by Fourier transform–infrared microspectroscopy (FTIR-M) and differential scanning calorimetry (DSC). This kind of analysis allowed us to examine microdomains of samples with different morphological characteristics and to obtain an indication of the polymorphism of PVDF. In some cases the simultaneous presence of two or three forms has been evidenced thanks to the comparison of FTIR-M spectra and DSC traces. Vibrational spectra of single crystalline forms can be recorded by FTIR-M on phase homogeneous microdomains.     

Thermal behaviour and annealing of poly(vinylidene fluoride)

In this study, we report the melting behavior of poly(vinylidene fluoride) (PVF₂) annealed in a differential scanning calorimeter. PVF₂ annealed under isothermal conditions often shows double or triple melting endotherms depending on the annealing temperature (T_a) and the heating rate. The lower melting peak temperature increases as T_a increases. When the annealing time is varied, there is a systematic increase in the size of the lower endotherms. This suggests that a portion of the main endothermic response is due to reorganization during the scan. Annealing PVF₂ not only increases the degree of crystallinity, but also improves the crystal perfection. The ability of an annealing sample to reorganize decreases as the annealing time increases. However, an additional third melting peak appears when PVF₂ is annealed at 140°C for a sufficiently long time. The existence of this peak suggests that more than one kind of distribution of crystal perfection may occur when PVF₂ is quenched from the melt into liquid nitrogen and subsequently annealed.     

Electrospinning induced ferroelectricity in poly(vinylidene fluoride) fibers

Poly(vinylidene fluoride) (PVDF) fibers with diameters ranging from 70 to 400 nm are produced by electrospinning and the effect of fiber size on the ferroelectric β-crystalline phase is determined. Domain switching and associated ferro-/piezo-electric properties of the electrospun PVDF fibers were also determined. The fibers showed well-defined ferroelectric and piezoelectric properties.     

Crystalline transformations in spherulites of poly(vinylidene fluoride)

Parts of poly(vinylidene fluoride) spherulites of the α-phase undergo a transformation to the higher-melting γ-form when crystallized at high temperatures. As a rule, this transformation originates at the periphery of α-spherulites where their lamellae are in contact with, and oppositely directed to, lamellae of γ-spherulites; the latter are formed only at high temperatures. The transformation on then retreats towards the nuclei of α-spherulites at a slow (～10^?4μm s^?1), linear rate, which increases with temperature. At very high temperatures, this transformation is also initiated at some α-nuclei; a few of the α-spherulites also exhibit areas of the high-melting phase irregularly dispersed within their interiors. In samples crystallized below ～154°C, the transformation is of very limited extent, even after prolonged annealing at higher temperatures.     

Tensile,stress-strain and creep rupture properties of poly (vinyl chloride)/poly(neopentyl glycol adipate)/poly(vinylidene fluoride) blends

Poly (vinyl chloride), PVC, and poly(vinylidene fluoride), PVDF, are incompatible polymers. Poly(neopentyl glycol adipate), PDPA, is miscible with both PVC and PVDF. With PDPA acting as a compatibilizer between PVC and PVDF. compatible PVC/PDPA/PVDF blends can be formed at PVDF content of about less than 50wt%. Above 50wt% PVDF the ternary blends exist in two phases exhibiting two glass transition temperatures, T_g, PVC is the main contributor to the mechanical strength while PDPA and PVDF contribute to the elastic properties of these blends. A compatible blend of 55/22.5/22.5 wt% PVC/PDPA/PVDF exhibiting one single T_g appears to show an interesting balance of the properties of the blend components.     

Surfactant-free emulsion polymerization of chlorotrifluoroethylene with vinylacetate or vinylidene fluoride

A surfactant-free emulsion process has been developed for the preparation of copolymers of chlorotrifluoroethylene with vinylacetate or vinylidene fluoride. A redox initiator system, consisting of sodium-meta-bisulfite, t-butylhydroperoxide, and ferrous sulfate heptahydrate, has been found to be effective in preparing self-emulsifying fluoropolymers with a monodisperse particle size distribution, having up to 45% polymer solids in water. Over the range studied in this investigation, the particle number and the ultimate particle size is linearly related to the quantity of initially charged redox catalyst. Under conditions of optimal catalyst concentrations, a greater number of particles is produced in the surfactant-free process than that which can be obtained using conventional fluorosurfactants. Particle number is defined at the earliest stage of polymerization and remains constant throughout the polymerization, unless surfactant is postadded to the surfactant-free latex at a very early stage in the polymerization. The aqueous phases of various latices have been purified by ion-exchange and dialysis, enabling the sulfonic acid-terminated fluoropolymer end groups to be quantified. The highest level of bound sulfonic acid is obtained at elevated temperatures. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 70: 2211–2225, 1998     

Interactions in poly(vinylidene fluoride)/poly(methyl methacrylate-co-ethyl methacrylate) blends

Summary The interaction parameters B for blends of poly(vinylidene fluoride) (PVDF) with poly(methyl methacrylate) (PMMA), poly(ethyl methacrylate) (PEMA) and five methyl methacrylate/ ethyl methacrylate copolymers (PMEMA) were determined by measurements of melting point depression of PVDF. The B values are negative, indicating an attractive intermolecular interaction. The intramolecular interaction parameter between MMA and EMA segments in PMEMA was found to be +3.25 cal/cm³, indicating a repulsive interaction between different monomer segments in the copolymer.     

Miscibility of poly(methoxymethyl methacrylate) and poly(methylthiomethyl methacrylate) with poly(vinylidene fluoride)

Summary The miscibility behaviour of poly(methoxymethyl methacrylate) (PMOMA) and poly(methylthiomethyl methacrylate) (PMTMA) with poly(vinylidene fluoride) (PVDF) was examined by differential scanning calorimetry. PMOMA/PVDF blend system was judged to be miscible on the bases of the presence of a single, composition-dependent glass transition for the blend and a pronounced melting point depression of the PVDF component. Furthermore, lower critical solution temperature (LCST) behaviour was observed for all PMOMA/PVDF blends. PMTMA/PVDF blends were found to be immiscible. Based on the melting point depression of PVDF in PMOMA/PVDF blends, the interaction parameter B was found to be -14.5 J/cm³.     

聚偏氟乙烯功能膜研究进展

聚偏氟乙烯(PVDF)机械性能、耐老化及耐化学腐蚀等性能优良,广泛应用于膜分离与膜过程领域,但其亲水性能差、易受污染使其在膜分离方面的应用受到限制,对其亲水性和抗污染性是目前研究的重要课题;其β晶型的特殊结构使PVDF具有优异的介电性、压电性和铁电性等,在介电材料、压电材料等领域应用愈来愈广,但晶型的多样性及聚集状态使得其电学性能不稳定,因此控制β晶相的含量及其有序聚集状态是能量存储和转换材料领域研究的前沿趋势。     

Calculation of piezoelectric constants of poly(vinylidene fluoride)

Summary Piezoelectric constants of poly(vinylidene fluoride) form I crystal have been calculated to give a good agreement with observed values; d₃₃ = –2.5 × 10^–11 C/N (observed: –2 × 10^–11 C/N) and d₃₃ = –2.5 × 10^–13 C/N (observed: ca. 10^–13 C/N). The macroscopic piezoelectric constants have also been calculated using such a model that the piezoelectric crystal form I is embedded in the non-piezoelectric amorphous matrix to be d ₃₁ ^M 0.6 × 10^–11 C/N (at room temperature) and 4.6 × 10^–13 C/N (below glass transition temperature), and d ₃₃ ^M –1.4 × 10^–11 C/N and –0.5 × 10^–11 C/N, respectively, reproducing well the observed temperature dependence of these constants.     

Crystallization of poly(vinylidene fluoride) by freeze-extracting method

Summary High crystallinity poly(vinylidene fluoride) (PVF₂) has been achieved by freezing its very dilute solution and followed by freeze-extracting the frozen solvent. FT-IR, WAXD, and DSC studies indicate that the freeze-extracted PVF₂ has higher crystallinity than those samples prepared by solution crystallization or by thermal annealing techniques.     

Poly(vinylidene fluoride) - poly(methyl methacrylate) blends

A dynamic mechanical study of four blends of PVdF and PMMA plus the constituent homopolymers indicates that these two polymers are partly compatible, but that a crystalline PVdF phase also exists. The pure PVdF sample showed four transitions at ?95°, ?25° 75° and 100°C which are ascribed to a restricted chain motion of the Schatzki type, to a crystalline phase transition, to the glass transition and to premelting of poorly formed crystallites respectively. A longitudinal sonic velocity versus composition plot is interpreted in terms of the overall morphology of the blends whilst differential scanning calorimetry measurements are used to study crystalline melting.     

Concentrated solution and melt rheology of poly(vinylidene fluoride)

The concentrated solution and melt rheology of poly(vinylidene fluoride) [PVDF] were studied by using a falling needle solution viscometer, a Brookfield viscometer, and a Kayeness capillary rheometer. It was found that the concentrated solution (15 wt% in N-dimethyl acetamide) rheology exhibited a different behavior for various grades of PVDF produced by different types of polymerization. While Newtonian behavior was found in one type of PVDF, shear thinning was found in another type. The power law model was used to describe the general solution behavior of these materials. Zero shear rate viscosity correlated well with the molecular weight (M_w) of the material. Melt viscosity of PVDF exhibited continuous shear thinning behavior throughout the whole range of shear rates. The data were best fitted by a second-degree polynomial curve. Correlations were established between the molecular weight, molecular weight distribution, and the parameters of the polynomial curve. These correlations are useful for the prediction of various grades of PVDF designated for specific engineering applications. The correlations obtained from solution provided better and more accurate correlations to M_w parameters than those of melt rheology.