Phase behavior of poly(ε-caprolactone)/ poly (vinylidene fluoride) blends

The miscibility of blends of poly (ε-caprolactone) (PCL)/poly(vinylidene fluoride) (PVDF) was studied by measuring the cloud point, melting point depression and crystallization kinetics. Lower critical solution temperature (LCST) behavior was observed at PCL-rich compositions, whilst it was not observed at high compositions of PVDF. However it is possible that an LCST could exist below the melting point of PVDF. From analysis of the melting point depression, the Flory interaction parameter x₁₂, was calculated from the Nishi-Wang equation and the value was found to be-1.5. The crystallization rate of PCL increased with increasing amount of PVDF in the blend. The spinodal curve for PCL/PVDF blends was simulated by using the lattice-fluid theory.     

γ→β Phase transformation induced in poly(vinylidene fluoride) by stretching

The purpose of this work was to show in a conclusive way the γ→β phase transformation induced by uniaxial stretching in poly(vinylidene fluoride). Poly(vinylidene fluoride) films were melted and crystallized at 163°C for 36 h. Under these conditions, a mixture of α and γ phases was formed, with a predominance of the latter. These films were uniaxially stretched at 130°C at a draw ratio of 4. Fourier transform infrared and differential scanning calorimetry analyses showed a γ→β phase transition in the solid state, whereas orientation of the α phase without any transition was observed. Optical microscopy analysis permitted the observation of the transformation of spherulitic structures into oriented lamellae during stretching. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Friction‐induced electroactive β polymorph of poly(vinylidene fluoride)

Poly(vinylidene fluoride) (PVDF) has been widely used in electric devices due to electroactive β polymorph. In this article, we probe the formation of β phase under friction by spectroscopy and thermal analysis. During continuous friction, entire sliding of PVDF is identified with two regimes, i.e., running‐in and steady‐state. At initial running‐in period, friction surfaces are dominated by plastic strain, which leads to striking formation of β phase from α polymorph (α→β). Subsequently, melting‐flow domains almost cover friction surfaces at steady‐state. Thus, formation of β crystal is correspondingly induced by shear crystallization. Nevertheless, β‐crystal content at steady‐state is lower than that at running‐in. With sliding proceeding, moreover, β‐crystal content exhibits a gradually decreasing tendency, attributed to rising surface temperature. Besides, the friction‐induced β phase is further confirmed by evaluation of wear debris. Overall, friction plays a crucial role as to the formation of β phase. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46395.     

Network of γ-irradiated poly(vinyl alcohol) beads

The crosslinking of poly(vinyl alcohol) beads was performed by γ-ray irradiation. The maximum molecular size, M_lim, which can permeate through beads, was obtained by eluting poly(ethylene glycol) through a column packed with beads. The root-mean-square of the end-to-end distance $\text{(}\text{r}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ and that of the hydrodynamic radius $\text{(}\text{s}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ were also calculated. Meanwhile, the average size of the network of the beads, r_c, was estimated from the average molecular weight between crosslinkages, M_c, which was derived from the glass transition temperatures of crosslinked and non-crosslinked poly(vinyl alcohol) beads. r_c values of poly(vinyl alcohol) beads decreased as the total dose of γ-irradiation increased when dry poly(vinyl alcohol) beads were γ-irradiated, while the values were almost constant when poly(vinyl alcohol) beads swollen in water were γ-irradiated. Further, linear relations were obtained among r_c, $\text{(}\text{r}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$, and $\text{(}\text{s}{}^2\text{)}{}^{\mathrm{<mtext>1</mtext><mtext>2</mtext>}}$ covering the wide range of total dose.     

Determination of the α, β, and γ crystalline phases of poly(vinylidene fluoride) films prepared at different conditions

Samples containing the three crystalline phases of poly(vinylidene fluoride), α, β, and γ, have been obtained under distinct crystallization conditions. Samples containing exclusively unoriented β phase have been obtained by crystallization from dimethylformamide (DMF) solution at 60°C. Oriented β phase has been obtained by uniaxial drawing, at 80°C, of an originally α phase sample. Samples containing exclusively α phase have been obtained by melting and posterior cooling at room temperature. Samples containing both α and γ phases have been obtained by melt crystallization at 164 °C for 16 and 36 h. Presence of the crystalline phases in each sample were confirmed by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), wide‐angle X‐ray scattering (WAXD), polarized light optical microscopy (PLOM), and scanning electron microscopy (SEM). Infrared absorption bands identifying unequivocally the presence of β and γ phases in a sample are presented. It is shown that solution crystallization at T < 70°C always results in the β phase, regardless of the solvent used. Melt temperatures of the respective phases have also been determined. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 3272–3279, 2006     

Increasing solvent polarity and addition of salts promote β‐phase poly(vinylidene fluoride) formation

This article investigates the effects of solvent polarity and salt addition on β‐phase poly(vinylidene fluoride) (PVDF) formation. Films were solvent cast in aprotic solvents of varying polarities with or without salt addition. Characterization was done by Fourier transformed infra‐red spectroscopy, differential scanning calorimetry, and scanning electron microscopy. Decreasing fractions of β‐phase PVDF was observed with increasing drying temperature when less polar solvents were used. The most polar solvent (hexamethylphosphoramide) consistently produced films with at least 90.0% β‐phase PVDF within the crystalline regions. Melting temperatures increased in correlation to absolute proportions of β‐phase PVDF. Salt addition increased the formation of β‐phase PVDF by 30%, with salts of higher valencies and smaller ionic radii resulting in more significant increases. Taken collectively, using solvents of higher polarities and addition of salts with high cationic valencies and small ionic radii will maximize β‐phase formation in solvent cast PVDF films. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Viscoelastic and pressure–volume–temperature properties of poly(vinylidene fluoride) and poly(vinylidene fluoride)–hexafluoropropylene copolymers

The relationship between the pressure, volume, and temperature (PVT) of poly(vinylidene fluoride) homopolymers (PVDF) and poly(vinylidene fluoride)–hexafluoropropylene (PVDF–HFP) copolymers was determined in the pressure range of 200–1200 bar and in the temperature range of 40°C–230°C. The specific volume was measured for two homopolymers having a molecular weight (M_w) of 160,000–400,000 Da and three copolymers containing between 3 and 11 wt % HFP with a molecular weight range of 320,000–480,000 Da. Differential scanning calorimetry (DSC) was used to simulate the cooling process of the PVT experiments and to determine the crystallization temperature at atmospheric pressure. The obtained results were compared to the transitions observed during the PVT measurements, which were found to be pressure dependent. The results showed that the specific volume of PVDF varies between 0.57 and 0.69 cm³/g at atmospheric pressure, while at high pressure (1200 bar) it varies between 0.55 and 0.64 cm³/g. For the copolymers, the addition of HFP lowered its melting point, while the specific volume did not show a significant change. The TAIT state equation describing the dependence of specific volume on the zero‐pressure volume (V₀,T), pressure, and temperature has been used to predict the specific volume of PVDF and PVDF–HFP copolymers. The experimental data was fitted with the state equation by varying the parameters in the equation. The use of the universal constant, C (0.0894), and as a variable did not affect the predictions significantly. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 80: 230–241, 2001     

Colloidal Crystalline Arrays of β‐Polymorphic Poly(vinylidene fluoride)

Stable layers of nearly monodisperse spheres of β‐polymorphic poly(vinylidene fluoride) with iridescent properties are prepared. The colloidal crystalline arrays (CCAs) were characterized by optical microscopy, differential scanning calorimetry (DSC), and FT‐IR spectroscopy. FT‐IR spectroscopic and wide‐angle X‐ray scattering (WAXS) studies revealed a β‐polymorphic PVF₂ structure, the DSC study showed that the level of crystallinity in the CCA was much higher than that in the melt‐crystallized sample, and UV‐visible spectroscopy showed extinction peaks at 323 and 510 nm in the CCAs. The β‐polymorphic PVF₂ structure, along with the optical extinction properties of these CCAs, raises the prospect of their application in optical filters and/or piezoelectric sensors.

Optical micrograph of PVF₂ CCA films cast on glass substrates.     

Miscibility and morphology of poly(vinylidene fluoride)/poly[(vinylidene fluoride)‐ran‐trifluorethylene] blends

This study presents an investigation of the effect of the different crystalline phases of each blend component on miscibility when blending poly(vinylidene fluoride) (PVDF) and its copolymer poly[(vinylidene fluoride)‐ran‐trifluorethylene] [P(VDF–TrFE)] containing 72 mol % of VDF. It was found that, when both components crystallized in their ferroelectric phase, the PVDF showed a strong effect on the crystallinity and phase‐transition temperature of the copolymer, indicating partial miscibility in the crystalline state. On the other hand, immiscibility was observed when both components, after melting, were crystallized in their paraelectric phase. In this case, however, a decrease in crystallization temperatures suggested a strong interaction between monomers in the liquid state. Blend morphologies indicated that, in spite of the lack of miscibility in the crystalline state, there is at least miscibility between PVDF and P(VDF–TrFE) in the liquid state, and that a very intimate mixture of the two phases on the lamellar level can be maintained upon crystallization. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1362–1369, 2002     

Melt spinning of β‐phase poly(vinylidene fluoride) yarns with and without a conductive core

When poly(vinylidene fluoride) (PVDF) is to be used as a piezoelectric material, the processing must include the formation of polar β‐phase crystallites, as well as the application of electrically conducting charge collectors, that is, electrodes. In this article, results from the melt spinning of PVDF yarns and a novel bicomponent PVDF‐yarn with a conductive carbon black/polypropylene (CB/PP) core are presented. Melt spinning has been done under conditions typical for industrial large‐scale fiber production. The effects on the resulting crystalline structure of varying the spinning velocity, draw rate, and draw temperature are discussed. The results show that, for maximum α‐to‐β phase transformation, cold drawing should take place at a temperature between 70 and 90°C, and both the draw ratio and the draw rate should be as high as possible. It was observed that the cold drawing necessary to form β‐phase crystallinity simultaneously leads to a decrease in the core conductivity of the bicomponent yarns. In this work, the melt spinning of bicomponent fibers with high‐β‐phase PVDF in the sheath and a CB/PP core was successfully accomplished. The core material remained electrically conductive, paving the way for the use of a CB‐polymer compound as inner electrode in the melt spinning of piezoelectric bicomponent fibers. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

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.     

Transition behavior of poly(vinylidene fluoride)/poly(ethyl methacrylate) blends

Previous work has shown evidence that PMMA and PEMA are miscible with PVF₂. The present paper examines in detail the behavior of PEMA/PVF₂ blends by thermal analysis and dynamic mechanical testing. All transitions and relaxations are affected by blond composition but in a complex manner owing to the crystallization of PVF₂ from blends rich in this component. Inadequacies of the simple two-phase picture of semi-crystalline polymers is believed responsible for some of the transitional behavior observed here. The melting point depression observed for PVF₂ was found to be consistent with an exothermic heat of mixing for this pair comparable in value to that found for PPMA/PVF₂/All evidence here are consistent with the previous conclusion of miscibility for these systems.     

Melt spinning of poly(vinylidene fluoride) fibers and the influence of spinning parameters on β‐phase crystallinity

The effects of melt‐spinning and cold‐drawing parameters on the formation of β‐phase crystallinity in poly(vinylidene fluoride) (PVDF) fibers and ways of increasing such crystallinity were studied. Fibers were melt‐spun with four different melt draw ratios (MDRs) and were subsequently cold‐drawn at different draw ratios (λ). The maximum λ value in cold drawing was dependent on the MDR used in melt spinning. The crystalline structure of the fibers was studied mainly with differential scanning calorimetry (DSC) and X‐ray diffraction (XRD). The results showed that the degree of crystallinity in the fibers was determined by the MDR and that before cold drawing the crystalline structure of the fibers was predominantly in the α form. By cold drawing, α‐phase crystallites could be transformed into the β phase. It was established that, under certain conditions of melt spinning and cold drawing, PVDF fibers of up to 80% crystallinity, mainly in the β form, could be prepared. It was further proposed that fibers spun at a sufficiently high MDR consist to a large extent of extended‐chain crystals, and this greatly affects the melting point of PVDF. Thus, DSC melting‐point data were shown to be insufficient for determining the crystalline phase of PVDF. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Study of crystallization behavior of neat poly(vinylidene fluoride) and transcrystallization in carbon fiber/poly(vinylidene fluoride) composite under a temperature gradient

The crystallization behavior of poly(vinylidene fluoride) (PVDF) and transcrystallization in carbon fiber (CF)/PVDF composite were investigated under a temperature gradient. The crystallization temperature (T_c) was controlled in the range of 110–180 °C. For neat PVDF, the results showed that exclusive γ phase formed at T_c above 164 °C, but coexisted with α phase at T_c ranging from 137 to 160 °C. The promotion of γ phase to nucleation of α phase at low T_c was observed for the first time. For CF/PVDF composite, a cylindrical transcrystalline (TC) layer formed on the surface of CF when T_c was between 137 and 172 °C. The TC layer was exclusively composed of γ phase at T_c above 164 °C. The hybrid nucleation was dominated by γ phase though some α phase nuclei emerged on the surface of CF when T_c was in the range of 144–160 °C. As T_c decreased, competition between the hybrid nucleation of α and γ phase became more intense. The γ phase nuclei was soon circumscribed by the rapidly developed α phase when T_c was below 144 °C. Furthermore, some α phase nuclei were induced at the surface of the γ phase TC layer, and developed into α phase TC layer when T_c was in the range of 146–156 °C, which resulted in a doubled TC layer of α and γ phase at the interface of the composite. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43605.     

The crystallization and transition behavior of poly(vinylidene fluoride)/copoly(chlorotriofluorethylene-vinylidene fluoride) blends

Thermal analysis of solution precipitated blends of two crystallizable polymers, poly(vinylidene fluoride) (PVDF) and copoly(chlorotrifluorethylene-vinylidene fluoride) (copoly(CTFE-VDF)), has been carried out to study the transition temperatures, crystallinity, and crystallization rates. PVDF crystallizes over the whole blend composition either during precipitation from solution or upon cooling from the melt. The high degree of crystallinity attained, higher than in PVDF by itself, suggests the occurrence of partial PVDF-copolymer cocrystallization. The melt crystallization temperature, decreasing with cooling rate, is lower in PVDF-rich blends than for lean blends. However, the heat of crystallization increases with cooling rate, suggesting that the crystal composition depends on crystallization rate. No significant melting temperature depression due to blending was observed. However, the blends glass transition (T_g) changes linearly with composition, but less than expected by any mixing rule applicable to compatible systems. Annealing of the blends above T_g results in an additional crystalline phase consisting mainly of the copolymer. The amount of these crystals increases with PVDF content, due to partial cocrystallization and kinetic effects. The addition of the copolymer to PVDF results in a volume-filling spherulitic structure consisting of spherulites which decrease in size with increasing copolymer content.     

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.     

Radiational hardening of poly(vinylidene fluoride)–poly(methyl methacrylate)–polystyrene ternary blends

Specimens of poly(vinylidene fluoride) (PVDF)–poly(methyl methacrylate) (PMMA)–polystyrene (PS) polyblends with different weight percentage ratios of the three polymers were prepared with the solution cast technique. The effect of γ irradiation on the Vicker's microhardness was studied. Among the three pure polymers, PVDF, PMMA, and PS, the γ irradiation imparted crosslinking in PVDF, thereby causing radiational hardening. In the cases of PMMA and PS, the effect of irradiation exhibited a predominance of both the scissioning and crosslinking processes in different ranges of doses. Moreover, at a dose of 5 Mrad, in both PMMA and PS, maximum radiational crosslinking was observed. The effect of γ irradiation seemed to stabilize beyond 15 Mrad in PVDF and beyond 20 Mrad in PMMA and PS. Microhardness measurements on ternary blends of PVDF, PMMA, and PS revealed that the blend with low contents of PMMA, that is, up to 5 wt %, yielded softening, whereas increasing the content of PMMA beyond 5 wt % produced a hardened material because of radiational crosslinking, and a higher content of PMMA in the blend facilitated this crosslinking. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3107–3111, 2004     

Crystallization behavior of poly(vinylidene fluoride) composites containing zinc phenylphosphonate

Poly(vinylidene fluoride) (PVDF) composites with different ratios of zinc phenylphosphonate (ZPP) were prepared by mixing in a kneader. The crystallization behavior of the composites from the melt was investigated by polarized optical microscopy (POM), differential scanning calorimetry (DSC), and wide‐angle X‐ray diffraction (WAXD). The POM observations directly showed that nucleation of the PVDF crystal was promoted by adding ZPP. The data obtained from DSC measurements also indicated that the crystallization temperature of the composites became higher than that of pure PVDF. These results suggested that ZPP acted as a good nucleating agent for PVDF. Nonisothermal crystallization behavior was analyzed by Avrami equation adopted by Jeziorny. The results of the analysis were found to be consistent with the POM observations for the composites. WAXD analysis suggested that the PVDF crystal structure was influenced by ZPP addition. These results proved that as a nucleating agent, ZPP increased the crystallization rate and temperature of PVDF. POLYM. ENG. SCI., 2013. © 2012 Society of Plastics Engineers     

Dielectric behavior of polymer blends based on poly(vinylidene fluoride)

Dielectric properties of polymer blends based on PVDF and two amorphous polymers, i.e. poly(vinyl acetate) and poly(methyl methacrylate), have been studied at different frequencies in the temperature range between –150 and +150°C. The novelty of this work lies in the way of planning the polymer blends compositions, which have been prepared according to a statistical design proposed by Scheffé. The purpose is to obtain an empirical equation capable of reproducing the behavior of these systems. From the results obtained after applying the model to the values of the glass transition temperature of PVDF in these polymer blends, it is possible to draw conclusions about their compatibility.     

Swelling behaviour of poly(α‐hydroxyacrylic acid) gel

Poly(α‐hydroxy acrylic acid) (PHA) and poly(acrylicacid) (PAA) gels were prepared by irradiating the respective 15 wt% aqueous solutions with γ‐rays. Swelling ratios for PHA gel were measured as a function of pH and divalent cation (Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺) concentration C₂ in the external solution to provide a comparison with the results for PAA gels. It was found that the swelling ratio of PHA gel steeply increases between pH 2 and 4, followed by a gradual swelling in the higher pH region. The corresponding steep swelling of PAA gel was observed at pH 3–6. Cation specificity in the equilibrium swelling ratio at a lower C₂ value (1.0 × 10⁻³ M) was approximately consistent with the binding selectivity in the solution system. Typically, the swelling ratio of PHA gel in the presence of Ca²⁺ was significantly lower than in the Mg²⁺ system, while the difference was slight for PAA gel. The response of the swelling ratio to changes in pH and C₂ was analysed as a first order relaxation to estimate the time constants. The (de)swelling kinetics measured by both the pH and C₂ jump were qualitatively interpreted in terms of main‐chain stiffness and intermolecular hydrogen bonding in the respective polymers. © 2000 Society of Chemical Industry