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     

Determination of the crystalline phases of poly(vinylidene fluoride) under different preparation conditions using differential scanning calorimetry and infrared spectroscopy

A method with good precision has been developed to quantitatively measure the degree of α‐, β‐, and γ crystallinity in poly(vinylidene fluoride) (PVDF) by means of infrared spectroscopy. The phase composition of solution‐deposited PVDF films was found to be strongly influenced by the presence of hydrophilic residues on the silicon substrate, the relative humidity present at film deposition, the spatial position on the substrate, and the thermal treatment of the deposited film. Films produced on pristine surfaces gave predominantly α‐phase PVDF, but when a layer of polar solvent (acetone or methanol) remained on the surface, the films produced were predominantly γ phase. Higher humidity promoted a higher fraction of γ crystallinity in the solution‐deposited PVDF films. Solution‐cast films had highly variable composition across the substrate, whereas spin‐cast films were uniform. High‐temperature annealing of PVDF films normally converts the polymer to the γ phase, but annealing the film while still attached to the silicon substrate inhibited this phase transformation. Low‐temperature annealing of freestanding films led to a previously unreported thermal event in the DSC, a premelting process that is a kinetic event, assigned to a crystalline relaxation. Higher‐temperature annealing gave a double endotherm, assigned to melting of different‐sized crystalline domains. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1093–1100, 2003     

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.     

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     

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     

Pre‐thermal treatment in binary solvent systems promoting β crystalline phase of electrospun poly(vinylidene fluoride) nanofibers

Poly(vinylidene fluoride) (PVDF) nanofibers were fabricated via electrospinning with an investigation of various ratios of binary solvents at different temperatures. The amount of acetone influenced the morphology. Scanning electron microscopy showed a PVDF membrane composed of smooth and unblemished fibers without beads and dark spots with small diameters of 201 ± 54 nm at a dimethylformamide‐to‐acetone ratio of 4:6. The temperature of pre‐thermal treatment from room temperature to 120 °C was investigated to promote the β crystalline phase in electrospun PVDF nanofibers. The result was characterized using Fourier transform infrared (FTIR) spectroscopy and X‐ray diffraction (XRD). PVDF solution prepared at 80 °C was used to increase the β crystalline phase of the electrospun PVDF nanofibers due to the transformation of α to β phase occurring during the spinning process and also bead‐free PVDF nanofibers were obtained. Differential scanning calorimetry revealed crystallization behavior corresponding with that determined using FTIR spectroscopy and XRD. Therefore, the solvent proportion and pretreatment temperature were observed to affect ultrafine nanofiber and crystalline structure of PVDF, respectively. © 2020 Society of Chemical Industry     

Formation and morphologies of ethylcyanoethyl cellulose/poly(ε‐caprolactone) blending films

The texture of ethylcyanoethyl cellulose [(ECE)C]/poly(ε‐caprolactone) (PCL)/dichloroacetic acid (DCA) solutions and the morphology of the (ECE)C/PCL blending films were studied by polarizing optical microscope. The critical concentration of the (ECE)C/PCL/DCA solutions, at which the mesophase began to appear, was not influenced by the addition of PCL molecules, and the evolution of the cholesteric texture varied with different contents of PCL in the blends. Spherulites with a ringed extinction pattern in the (ECE)C/PCL blending films were formed below an (ECE)C concentration of 25 wt %. Moreover, the bright band distance of the alternative ringed extinction pattern decreased with increasing (ECE)C concentration. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 550–554, 2004     

Effect of molecular weight on spherulitic growth rate of poly(ϵ‐caprolactone) and poly(ϵ‐caprolactone)‐b‐poly(ethylene glycol)

The spherulitic growth rates of a series poly (?‐caprolactone) homopolymers and poly(?‐caprolactone)‐b‐ poly(ethylene glycol) (PCL‐b‐PEG) block copolymers with different molecular weights but narrow polydispersity were studied. The results show that for both PCL homopolymers and PCL‐b‐PEG block copolymers, the spherulitic growth rate first increases with molecular weight and reaches a maximum, then decreases as molecular weight increases. Crystallization temperature has greater influence on the spherulitic growth rate of polymers with higher molecular weight. Hoffman–Lauritzen theory was used to analyze spherulitic growth kinetics and the free energy of the folding surface (σ_e) was derived. It is found that the values of σ_e decrease with molecular weight at low molecular weight level and become constant for high molecular weight polymers. The chemically linked PEG block does not change the values of σ_e significantly. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

聚碳酸酯对聚偏氟乙烯结构和结晶行为的影响

通过熔融共混法制备聚偏氟乙烯/聚碳酸酯（PVDF/PC）共混物,采用X线衍射仪（XRD）和差示扫描量热仪（DSC）表征共混物的结构、熔融和结晶行为.考察不同聚碳酸酯含量对聚偏氟乙烯晶体结构、熔点以及晶体完善程度等的影响.同时通过Avram i方程和结晶速率系数的研究,探讨PC对PVDF非等温结晶动力学的影响.研究结果表明：PC的掺杂没有改变PVDF的晶体结构,但是高PC质量分数（70%以上）却不利于PVDF晶体的生成;随着PC质量分数的增加,生成的PVDF晶体完善程度逐渐降低;当PC质量分数在70%以下时,PC起到类似成核剂作用,提高PVDF结晶速率.     

Studies on α to β phase transformations in mechanically deformed PVDF films

PVDF cast films were drawn at different temperatures to different draw ratios at constant draw rate to understand the mechanism of α to β phase transformation during mechanical deformation. WAXD and FTIR studies were carried out to determine the formation and content of β phase in the drawn films. Lower stretch temperatures gave higher fractions of β phase. The cast PVDF films were also drawn at suitable temperatures below the PVDF ambient melting point to the draw ratio of 6.4. The highest fraction of β phase obtained in these ultra drawn films was 0.98. SALS studies carried out for films at different stretch ratios show the change in spherulitic structure with the stretching parameters and give information for the understanding of phase transformation during stretching of PVDF films. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

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.     

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

The 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     

Morphology and nonisothermal crystallization of a polyacetal/poly(ε‐caprolactone) reactive blend prepared via a chain‐transfer reaction

A polyacetal (POM)/poly(ε‐caprolactone) (PCL) reactive blend prepared via a chain‐transfer reaction was investigated with respect to its morphology and nonisothermal crystallization, and the results were compared with those of a simple POM/PCL blend. The reactive blend had a microscopically phase‐separated morphology in which the diameter of the PCL microphase was below 100 nm, and it clearly yielded ring‐banded spherulites, whereas between the two blends, there were no significant differences in the diameters and polygonal edges of the spherulites and in the long period of the POM phases. The PCL part of the reactive blend crystallized within the confined microspace with about 10% lower crystallinity than that of the corresponding simple blend. A lower Avrami exponent and crystallization rate parameter of the PCL part were observed in the primary crystallization process of the reactive blend. In contrast, the crystallinity of the POM component and the nonisothermal crystallization kinetic parameters of the POM part showed no noticeable differences between the two blends at any given cooling rate. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Crystallization kinetics and crystalline morphology of poly(ethylene naphthalate) and poly(ethylene terephthalate‐co‐bibenzoate)

Copolymers of ethylene glycol with 4,4′‐bibenzoic acid and terephthalic acid are known to crystallize rapidly to surprisingly high levels of crystallinity. To understand this unusual behavior, the isothermal crystallization of poly(ethylene bibenzoate‐co‐terephthalate) in the molar ratio 55:45 (PETBB55) was studied. Poly(ethylene naphthalate) (PEN) was included in the study for comparison. The kinetics of isothermal crystallization from the melt and from the amorphous glass was determined using differential thermal analysis. The results were correlated with the crystalline morphology as observed with atomic force microscopy (AFM). Crystallization of PEN exhibited similar kinetics and spherulitic morphology regardless of whether it was cooled from the melt or heated from the glass to the crystallization temperature. The Avrami coefficient was close to 3 for heterogeneous nucleation with 3‐dimensional crystal growth. The copolymer PETBB55 crystallized much faster than did PEN and demonstrated different crystallization habits from the melt and from the glass. From the melt, PETBB55 crystallized in the “normal” way with spherulitic growth and an Avrami coefficient of 3. However, crystallization from the glass produced a granular crystalline morphology with an Avrami coefficient of 2. A quasi‐ordered melt state, close to liquid crystalline but lacking the order of a recognizable mesophase, was proposed to explain the unusual crystallization characteristics of PETBB55. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 98–115, 2002     

The effect of lanthanum oxide (La2O3) on the structure and crystallization of poly(vinylidene fluoride)

Rare earth polymers, due to their excellent luminescence, fluorescence, laser protective, optical and magnetic properties, have attracted much research attention in recent years. However, little attention has been paid to the effect of rare earths on the structure and crystallization of polymers, which is of important significance in the development of functional polymers. X‐ray diffraction and differential scanning calorimetry were used to investigate the structure and crystallization behavior of a poly(vinylidene fluoride) (PVDF)/lanthanum oxide (La₂O₃) composite. The results showed that the degree of perfection, crystal size, crystallization rate and isothermal crystallization activation energy of PVDF in the composite decreased, compared with pure PVDF. The spherulite nucleation and growth for PVDF and PVDF composite were analyzed in detail using the Lauritzen‐Hoffman equation. The modified Avrami equation and the Mo equation were used to study the non‐isothermal crystallization kinetics. The addition of La₂O₃ did not change the crystal structure and nucleation process for PVDF, but it decreased markedly the crystal growth rate and led to the formation of unstable crystals. This was attributed to the fact that too much La₂O₃ prevented PVDF molecular chains from moving and arranging in an orderly manner into crystals. Copyright © 2010 Society of Chemical Industry     

Soft nanoconfinement effects on the crystallization behavior of asymmetric poly(ethylene oxide)‐block‐poly(ε‐ caprolactone) diblock copolymers

The time‐ and temperature‐related crystallization process for the structure transitions of asymmetric crystalline‐crystalline diblock copolymers from the melt to crystallites was investigated with synchrotron simultaneous small‐angle/wide‐angle X‐ray scattering. Two asymmetric poly(ethylene oxide)‐poly(ε‐caprolactone) diblock copolymers were chosen. It is found in the course of the copolymer crystallization that the shorter blocks are uncrystallizable in both of the asymmetric diblock copolymers and final lamellar structures are formed in both of them. The final lamellar structure was confirmed from atomic force microscopy observations. The small‐angle X‐ray scattering data collected were analyzed with different methods for the early stage of crystallization. Guinier and Debye‐Bueche plots indicate that there are neither isolated domains nor correlated domains formed before the formation of lamellae in the asymmetric diblock copolymers during the crystallization process. The structure evolution was calculated according to the correlation function, and the soft nanoconfined crystallization behavior is discussed. Copyright © 2012 Society of Chemical Industry     

Polymer precursor to diamondlike carbon prepared by the polymerization of α,α,α‐trichlorotoluene and acetonitrile

A novel liquid/solid two‐phase reaction of α,α,α‐trichlorotoluene (PhCCl₃) and acetonitrile (AN) has been discovered that produces a carbon‐based polymer with diamondlike structure at the atomic level. The solid phase is potassium hydroxide particles, and the liquid phase is a tetrahydrofuran solution of PhCCl₃ and AN containing a certain amount of tetrabutylammonium bromide as a phase‐transfer catalyst. The structure of the carbon‐based polymer has been characterized by elemental analysis, Fourier transform infrared spectroscopy, and ¹H‐ and ¹³C‐NMR spectroscopy. The pyrolysis of the carbon‐based polymer at 800°C under a nitrogen atmosphere leads to the formation of a diamondlike carbonaceous material according to the results of X‐ray photoelectron spectroscopy and Raman spectroscopy. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 16–23, 2003     

Synthesis of star‐shaped poly(ε‐caprolactone)‐b‐poly(L‐lactide) copolymers: From star architectures to crystalline morphologies

Hexa‐armed star‐shaped poly(ε‐caprolactone)‐block‐poly(L ‐lactide) (6sPCL‐b‐PLLA) with dipentaerythritol core were synthesized by a two‐step ring‐opening polymerization. GPC and ¹H NMR data demonstrate that the polymerization courses are under control. The molecular weight of 6sPCLs and 6sPCL‐b‐PLLAs increases with increasing molar ratio of monomer to initiator, and the molecular weight distribution is in the range of 1.03–1.10. The investigation of the melting and crystallization demonstrated that the values of crystallization temperature (T_c), melting temperature (T_m), and the degree of crystallinity (X_c) of PLLA blocks are increased with the chain length increase of PLLA in the 6sPCL‐b‐PLLA copolymers. On the contrary, the crystallization of PCL blocks dominates when the chain length of PLLA is too short. According to the results of polarized optical micrographs, both the spherulitic growth rate (G) and the spherulitic morphology are affected by the macromolecular architecture and the length of the block chains. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Electron beam‐induced piezoelectric phase in poly(vinylidene fluoride) nanohybrid: effect at the molecular level

A nanohybrid has been synthesized by incorporating organically modified layered silicate in a poly(vinylidene fluoride) (PVDF) matrix. Molecular‐level phenomena have been explored after exposing PVDF and its nanohybrid to an electron beam of varying doses. The electron beam interacts with polymer chains and thereby generates different free radicals, the number of which is quite high in nanohybrid as compared to pure PVDF. The stability of free radicals has been confirmed through density functional theory energy minimization, predicting stable β‐phase free radicals in the nanohybrid. Quantitative analyses of chain scission, crosslinking and double bond formation are reported and compared after irradiation for both PVDF and its nanohybrid using UV‐visible and Fourier transform infrared spectroscopies, sol–gel analyses and gel permeation chromatography, revealing both chain scission and crosslinking phenomena in irradiated PVDF and its nanohybrid, but at higher dose (>90 Mrad) crosslinking dominates in the nanohybrid due to more free radicals and proximity of radical chains on top of templated system in the nanohybrid as compared to pure PVDF. The enhanced crosslinking alters the nanostructure causing disappearance of the peak at 2θ ≈ 3°. Moreover, the electron beam induces significant piezoelectric β‐phase in the nanohybrid against only α‐phase in pure PVDF at a similar dose and raises the possibility for the use of electron‐irradiated nanohybrid as an electromechanical device. β‐Phase formation is also supported through solid‐state NMR, scanning electron microscopy and differential scanning calorimetry studies. The thermal properties in terms of heat of fusion and degradation temperature have been verified indicating steady decrease of melting point and heat of fusion for pure PVDF while considerably less effect is observed for the nanohybrid. The combined effect of chain scission and crosslinking makes both PVDF and its nanohybrid brittle, but with greater stiffness with respect to unirradiated specimens. © 2014 Society of Chemical Industry     

Studies on poly(vinylidene fluoride)-clay nanocomposites: Effect of different clay modifiers

Montmorillonite clay based poly(vinylidene fluoride) nanocomposites were prepared by melt-mixing. The clays used included unmodified clay, a commercially available ammonium based clay, and two organically modified clays prepared by cation exchange with hexadecylpyridinium chloride and with octadecyltriphenylphosphonium bromide. PVDF-clay nanocomposites were processed in a mini twin-screw extruder. The structure of nanocomposites, analyzed using WAXD and TEM, indicated different extents of the clay dispersion depending on the modifier. PVDF formed β-phase crystals in the presence of organically modified clay when crystallized from its melt; in contrast, α-crystals were formed in the absence of clay and with unmodified clay. SAXS analysis indicated that the long period and crystalline lamella thickness decreased with the addition of clay. The melting and crystallization temperatures increased around 10 and 13 °C, respectively, with 5 wt% of phosphonium modified clay, which was the highest among the clays used. Further, the clay served as a nucleating agent for PVDF matrix, as observed by hot-stage polarized optical microscopy. The average spherulitic radius, determined from small angle light scattering, decreased with clay content. The elongation at break increased around 200% with the addition of only 5 wt% of ammonium clay. The storage and loss moduli of the nanocomposites were significantly higher than those of PVDF throughout the temperature range. Dielectric measurements showed a maximum increase of about 8 units of dielectric constant at 1 Hz frequency with 5 wt% organoclay.