Effects of stretching on crystalline phase structure and morphology of hard elastic PVDF fibers

The phase characteristics and morphology of stretched hard elastic poly(vinylidene fluoride) (PVDF) fibers were investigated by X‐ray diffraction (XRD) and wide‐angle and small‐angel X‐ray scattering (WAXS and SAXS). It was indicated that α and β phases coexisted in stretched PVDF fibers, stretching assisted in α to β phase transformation. The β/α ratios of stretched PVDF fibers were affected by stretching temperature, rate, and ratio. The β phase content of stretched PVDF fibers had an abrupt increase when stretched near 70°C, and then it decreased with increasing stretching temperature. Besides, the β/α ratio of PVDF fibers increased with stretching rate and ratio. The total crystallinity of PVDF fibers did not change much even on different stretching conditions. WAXS results indicated that the unstretched and stretched PVDF fibers all exhibited three strong equatorial streaks, with d‐spacing (0.964, 0.488, and 0.439 nm) and (0.946, 0.494, and 0.480 nm), which suggested that PVDF fibers still remained the crystalline reflections of c‐axis orientation even after being stretched. The long periods of stretched PVDF fibers, calculated from SAXS curves, increased from 19.04 to 39.75nm. On the basis of these results, the β transformation mechanism of stretched PVDF fibers was also discussed. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 104: 2254–2259, 2007     

Improving the dielectric performance of poly(vinylidene fluoride)/polyaniline nanorod composites by stretch‐induced crystal transition

The introduction of conductive polyaniline (PANI) can significantly improve the dielectric constant of polymer‐based materials. However, there is a drawback of high dielectric loss. Herein, a simple and efficient stretching process was applied to improve the dielectric performance of poly(vinylidene fluoride)/PANI (PVDF/PANI) nanorod films through the stretch‐induced crystal transition from non‐polar α‐crystal to polar β‐crystal in PVDF and the oriented distribution of PANI nanorods. XRD, DSC and Fourier transform IR analyses indicate that the stretched PVDF and stretched PVDF/PANI films possess a high content of β‐crystal at the stretching temperature of 135 °C under a stretching ratio of 200%–400%. Furthermore, the stretched PVDF/PANI film with 10 wt% PANI displays a high dielectric constant of 338 at 100 Hz, which is increased by 20% compared to non‐stretched PVDF/PANI film (281). More importantly, the corresponding dielectric loss is reduced from 0.31 for the non‐stretched film to 0.17 for the stretched film. © 2018 Society of Chemical Industry     

Towards β-phase formation probability in spin coated PVDF thin films

This work aimed towards the study on variations in the percentage of β-phase in Poly vinylidene fluoride (PVDF) thin films deposited by spin coating technique. PVDF is a semi-crystalline polymer which exhibits α, β, γ and δ polymorphs. Among these polymorphs, generally it crystallizes in a non-polar α-phase, which is of little importance as far as its applications are concerned. However, the β-phase, which exhibits spontaneous polarity created tremendous interest and showed a path towards the devices based on its pyro- and piezoelectric properties. Fourier Transform Infrared (FTIR) spectroscopy and XRD techniques were used to study the percentage of formation of β-phase in spin coated PVDF thin films at different processing conditions viz. spin rotation speed (rpm), solution concentration and annealing temperature. We identified the β-phase percentage in PVDF thin films, which are (i) Deposited with different rotation speeds ranging from 1000 to 9000 rpm, (ii) Annealed at different temperatures viz.; room temperature to 100C, and (iii) Deposited at various solution concentrations. It is identified that percentage of formation of β-phase is high in the films deposited with 15(w/v)% solution concentration which is annealed at 100C. The films deposited at higher rpm is showing low enhancement in the β-phase with annealing temperature.     

Role of poly(lactic acid) in the phase transition of poly(vinylidene fluoride) under uniaxial stretching

The effect of poly (lactic acid) (PLA) on the crystalline phase transition of poly (vinylidene fluoride) (PVDF) from α‐ to β‐phase under uniaxial stretching for immiscible PVDF/ PLA blends was investigated. The typical sea‐island structure in the blends was found to facilitate the necking of PVDF and the transition from α‐ to β‐phase due to the local stress distribution during stretching. The crystalline phase transition of PVDF in the blends is temperature‐dependent and is affected by the content of PLA. The highest content of β‐phase, F(β), was achieved in the samples stretched at 60°C, while the effect of PLA content on the crystalline phase transition of PVDF is more complex. F(β) increases slightly when the sample with a PLA content no more than 15 wt % is stretched at 60, 80, and 100°C, and decreases sharply for the sample containing 20 wt % PLA; in addition, the sample containing 10 wt % PLA exhibits the highest F(β) no matter what the stretching temperature is. The mechanism of the crystalline phase transition of PVDF during the stretching is interpreted from energy barrier of the transition from α‐ to β‐phase and the morphological structures in the blends. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Crystallization behavior of nano-composite based on poly(vinylidene fluoride) and organically modified layered titanate

To understand the effect of the nano-filler particles on the crystallization kinetics and crystalline structure of poly(vinylidene fluoride) (PVDF) upon nano-composite formation, we have prepared PVDF/organically modified layered titanate nano-composite via melt intercalation technique. The layer titanate (HTO) is a new nano-filler having highly surface charge density compared with conventional layered silicates. The detailed crystallization behavior and its kinetics including the conformational changes of the PVDF chain segment during crystallization of neat PVDF and HTO-based nano-composite (PVDF/HTO) have been investigated by using differential scanning calorimetric, wide-angle X-ray diffraction, light scattering, and infrared spectroscopic analyses. The neat PVDF predominantly formed α-phase in the crystallization temperature range of 110-150 °C. On the other hand, PVDF/HTO exhibited mainly α-phase crystal coexisting with γ- and β-phases at low T_c range (110-135 °C). A major γ-phase crystal coexists with β- and α-phases appeared at high T_c (=140-150 °C), owing to the dispersed layer titanate particles as a nucleating agent. The overall crystallization rate and crystalline structure of pure PVDF were strongly influenced in the presence of layered titanate particles.     

Doubly oriented β-form films of poly(vinylidene fluoride)

PVDF sheets, rapidly quenched, were (1) two-step transversely stretched at various temperatures and (2) stretched at various temperatures, rolled at room temperature and then annealed. The orientation patterns of the β-form crystal (which contains the polar b-axis) in these films were analysed on the basis of X-ray diffraction photographs taken with flat and cylindrical cameras. In the case of (1), when both of the two-step transversely stretching temperatures were below 100°C, a doubly oriented film with the plar b-axis oriented parallel to the film surface was obtained. In the case of (2), when the stretching temperature was below 100°C, the sheets then rolled without annealing, another doubly oriented film with the polar b-axis preferentially oriented at 30° to the film surface was obtained. On the other hand, when these films were annealed above 100°C, or the stretching temperatures were above 100°C, orientation patterns in which the polar b-axis was partially rotated through 60° were obtained. The orientation mechanisms of these films are discussed using the measurements of the lattice spacings of the β-form crystal.     

Polymorphism and orientation development in melt spinning,drawing, and annealing of nylon-6 filaments

A new method has been developed for determining the total crystallinity and relative amounts of α- and γ-phases in nylon-6 samples. The procedure is based on a combination of X-ray and density data and does not require complicated analytical procedures to separate overlapping reflections. The technique has been applied to study the structural changes accompanying the melt spinning, annealing, and drawing of nylon-6 filaments. Higher spin draw ratios result in higher crystallinity, greater relative amounts of γ-phase, and higher orientation. Annealing up to 2 h in boiling water or a 20% aqueous formic acid solution decreases the γ-phase content, increases the α-phase content and total crystallinity, but does not eliminate all of the γ-phase in samples spun with high spin draw ratios. Annealing in vacuum also increases the α-phase content when annealing is carried out at temperatures above 120°C, but there is little effect below this temperature. Drawing of as-spun and conditioned filaments at 90°C also increases the α-phase content and decreases the γ-phase content. The total crystalline content increases with draw ratio for samples with low spin draw ratios, but drawing has little effect on the total crystalline content of samples spun with higher spin draw ratios. Drawing also results in substantial increases in orientation, especially for samples spun with low spin draw ratios. The effects of these changes in structure on the mechanical properties are also described.     

Stretching induced phase transformations in melt extruded poly(vinylidene fluoride) cast films: Effect of cast roll temperature and speed

Melt extruded poly(vinylidene fluoride) cast films were prepared at different cast roll temperatures and speeds to study the effect of casting temperature and preorientation of the melt on the α‐ to β‐phase transformation in these films after uniaxial stretching. X‐ray and fourier transform infrared spectroscopy were used to identify the crystalline phases. The unstretched films were characterized using small angle light scattering (SALS). The films were stretched to a stretch ratio of 4.2 and at 80°C. Birefringence of the films and the fraction of β‐phase [F(β)] formed after uniaxial stretching increased with stretch ratio. The films showed increased crystallinity after stretching. For the films prepared at different cast roll temperatures, there was little change in F(β) in the films having a cast roll temperature between 75 and 120°C, but for the film with a cast roll temperature of 130°C F(β) decreased considerably. For the films prepared at different cast roll speeds, the F(β) increased with stretch ratio as well as with cast roll speed for a fixed stretch ratio. The primary effect of changing both the parameters is a change in the average spherulitic radius (R), in the unstretched films measured using SALS. The F(β) obtained correlated well with R and lower spherulitic radii resulted in the higher conversion to the β‐phase. POLYM. ENG. SCI., 47:1992–2004, 2007. © 2007 Society of Plastics Engineers     

Poly(ethylene terephthalate) nucleation as studied by shrinkage of film of low orientation level

Poly(ethylene terephthalate)s of weight average MW 74,000 and 30,000 have been uniaxially stretched, cooled under restraint, reheated, and shrunk unrestrained. Five stretch temperatures between 80 and 120°C and elongations up to 280 percent have been employed. Density and wide-angle X-ray diffraction results indicate conventional crystallization to have occurred only for the highly oriented samples, e.g., stretching above 200 percent at 90°C. The majority of stretching conditions studied produced only nucleated polymer. A sensitive, qualitative measure of nucleation is the degree of stretch imposed. Sufficiently high stretch temperature and low stretch rate lead to negligible nuclei formation. Nucleation in stretched, unshrunk films correlates with relatively high shrinkage, low orientation, low density and the absence of crystallinity until after the film has been shrunk. Crystallization on the other hand correlates with relatively high density, relatively low shrinkage and high orientation.     

Effect of mechanical stretching on electrical conductivity and positive temperature coefficient characteristics of poly(vinylidene fluoride)/carbon nanofiber composites prepared by non-solvent precipitation

Poly(vinylidene fluoride) (PVDF)/carbon nanofiber (CNF) composites with filler content ranging from 0.047 to 4.7 vol.% were prepared with non-solvent precipitation followed by melt compression. The morphology and electrical conductivity of the composites before and after mechanical stretching were examined. The results showed that CNFs were dispersed homogeneously in the PVDF matrix and a low electrical percolation threshold of 0.90 vol.% CNFs was obtained. Mechanical stretching led to a sharp decrease in the electrical conductivity of a composite containing 0.94 vol.% CNF. This was caused by the destruction of a conducting network structure when the fillers aligned along the stretching direction. This did not happen when the filler content was increased to 1.88 vol.%. The percolating composites displayed a positive temperature coefficient (PTC) effect with the effect being larger in stretched composites. This can be attributed to the presence of PVDF β-phase in stretched composites as revealed by X-ray diffraction and Fourier transform infrared spectroscopy.     

Tensile response of polyacrylonitrile fibers during air heating

Axial stresses generated by polyacrylonitrile filaments heated in air at constant length and length changes of filaments heated at constant load were measured. Fibers subjected to loads less than about 0.1 gpd shrank in the temperature range from about 40°C to 160°C. At about 160°C they began to stretch. Fibers that stretched out again to about their original lengths stiffened temporarily before undergoing a further elongation. At a temperature where the oxidation reaction begins to proceed with appreciable rate, elongation was retarded and finally reversed. Shrinkage was recorded during isothermal heating at 270°C, and a final length was approached when the oxygen content approached about 10 wt-%. The tension generated when the fibers were restrained from shrinking increased as temperature increased to 160°C but dropped in the temperature range of 160° to 250°C. Tension again built up during isothermal oxidation at 270°C. In the case of one of the samples, the tension generated below 160°C exceeded the ultimate tensile strength of the fibers above 200°C. This condition leads to tensile failures when the filaments are heated in a steep thermal gradient. The tensile behavior of the filaments is discussed in terms of the helical molecular model.     

Thermal conductivity of PVDF/PANI-nanofiber composite membrane aligned in an electric field

Poly(vinylidene fluoride)(PVDF) is a semi-crystalline thermoplastic polymer with excellent thermal stability,electrochemical stability and corrosion resistance, which has been widely studied and applied in industrial nonmetallic heat exchanger and piezoelectric-film sensor. In this study, polyaniline(PANI) nanofibers were synthesized using dodecylbenzene sulfonic acid as the surfactant. The obtained PANI nanofibers were blended in PVDF matrix to enhance thermal conductivity and tensile strength of composite materials. Electric field was applied for the orientation of membrane structure during membrane formation. Scanning electron microscope(SEM) images exhibited that the PANI nanofibers were well-dispersed in the composite membranes. The structure of composite membranes was more orderly after alignment. X-ray diffraction(XRD) and differential scanning calorimetry(DSC) indicated that the content of PANI nanofibers contributed to the transformation of PVDF from α-phase to β-phase. Both the tensile strength and thermal conductivity of composite membranes were significantly improved. This tendency was further enhanced by the application of electric field. The maximum tensile strength was obtained when the content of PANI nanofibers was 3 wt%, which was 46.44% higher than that of pure PVDF membrane. The maximum thermal conductivity of composite membranes after alignment was 84.5% greater than that of pure PVDF membrane when the content of PANI nanofibers was 50 wt%. The composite membrane is a promising new potential material in heat transfer field and the mechanism explored in this study would be informative for further development of similar thermal conductive polymeric materials.     

The crystalline phase transformation of poly(vinylidene fluoride)/poly(vinyl fluoride) blend films

Poly(vinylidene fluoride) (PVDF), poly(vinyl fluoride) (PVF), and their blends were prepared by solution casting, followed by quenching in ice water after melting to obtain an α-crystalline phase. The films were drawn by solid state extrusion at two different drawing temperatures, 50°C and 110°C. The crystalline phases were analyzed by DSC and FTIR. In the undrawn films, the content of β-crystalline phase in the blend of PVDF/PVF 88.5/11.5 was higher than in the PVDF homopolymer, but it was lower than in the PVDF film with a draw ratio higher than 4. The α-crystalline phase in PVDF/PVF blends was mostly transformed into the β-crystalline phase beyond a draw ratio of 4, regardless of the draw temperature and PVF content. The α-crystalline phase of PVDF systematically transformed into the β-crystalline phase with increasing draw ratio. The crystallinity of PVDF/PVF blend films drawn at 110°C was higher than those drawn at 50°C. In the drawn blend films, characteristic IR bands of the α form were shifted to those of the β form and completely changed into those of β form at draw ratio of 4, regardless of the draw temperature and PVF content.     

The effect of fibre concentration on the α to β-phase transformation, degree of crystallinity and electrical properties of vapour grown carbon nanofibre/poly(vinylidene fluoride) composites

Vapour growth carbon nanofibres/poly(vinilidene fluoride) - VGCNF/PVDF - composites prepared by solution casting were studied. The spherulitic crystallisation morphology of the pure polymer is maintained for the composites. Mechanical stretching of the composite films induces the α to β-phase transformation within the polymer matrix. This phase transition is accompanied by the destruction of the spherulitic microstructure in favour of a microfibrillar one. The incorporation of the VGCNF in the PVDF matrix increases the degree of crystallinity of the polymer composites for concentrations lower than ∼1%, remaining stable for higher VGCNF concentrations. With respect to the electrical properties, the stretching associated to the phase transformation induces a change in the conduction mechanism: the α-phase composite demonstrates a percolative behaviour on the measured conductivity whereas the β-phase demonstrates typical ionic conduction behaviour. Dielectric measurements in conjunction with the the two exponent percolation phenomenological equation demonstrates that in the β-phase an effective reduction in the ratio VGCNF length/domain length could induce the observed percolation behaviour.     

Enhanced electroactive phase,toughness and dielectric properties of poly(vinylidene fluoride) with addition of MMA-BA-IL copolymer

Methyl methacrylate (MMA), butyl acrylate (BA) and 1-butyl-3-vinylimdazolium tetrafluoroborate ([BVIM][BF₄]) copolymer (MMA-BA-IL) was prepared and used to enhance the electroactive phase content, toughness and dielectric properties of poly(vinylidene fluoride) (PVDF). Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and differential scanning calorimetry (DSC) tests indicated that crystal transformation of PVDF from α-phase to β/γ-phase occurred due to ion-dipole interaction between PVDF and [BVIM][BF₄]. Scanning electron microscope (SEM) results showed MMA-BA-IL copolymer dispersed in the PVDF uniformly and the partial replacement of MMA components by [BVIM][BF₄] decreased the miscibility between PVDF and MMA-BA copolymer. MMA-BA-IL copolymer improved the tensile ductility and impact toughness of PVDF. When the content of MMA-BA-IL was beyond 10 wt%, the elongation at break was higher than 400% and the impact strength was higher than 600 J/m. Deformation mechanism researches proved that shear yielding of the PVDF matrix and debonding/cavitation of the MMA-BA-IL copolymer particles were the major toughening mechanisms. The addition of MMA-BA-IL copolymer enhanced the dielectric properties of PVDF significantly. When the MMA-BA-IL content was 15 wt%, the dielectric constant of the PVDF/MMA-BA-IL blend increased to 54.3 at the 100 Hz frequency, which improved by 246% relative to that of the pure PVDF.     

Physical properties and structure of silk: 4. Spherulites grown from aqueous solution of silk fibroin

Spherulite formation in silk fibroin films cast from aqueous solution has been studied for crystallization conditions such as drying temperature, drying rate and pretreatment (freezing). Negatively birefringent spherulites in the α-form are observed in films cast between 0° and 40°C, and with a high drying rate at 20°C; positive β-form spherulites appear at higher temperatures up to 80°C and with a low drying rate at 20°C. Positive β-form spherulites are also obtained by freezing fibroin solution at ?2° to ?18°C and then drying at 20°C. It is found that positive β-form spherulites grow at 20°C on the surface of well-oriented β-form silk fibroin filaments (degummed silk) immersed in fibroin solution.     

Improving ferroelectricity and ferromagnetism of PVDF-CoFe2O4 thick films: Effect of Ethyl acetate and Temperature

PVDF-CoFe₂O₄(CFO) thick membranes were synthesized by solvent casting method. The mixed organic solvents consist of N-methyl-2-pyrrolidone (NMP) and Ethyl acetate (EA), in which the mass ratios of NMP and EA (NMP:EA = 1:0, 1:1, and 1:2) and the temperature of solvent evaporation (range from 50 to 80 °C) has been considered. The phase structure, β-PVDF phase content, and electrical and magnetic properties of these membranes were investigated to reveal the effect of EA addition and temperature on these properties. Results manifest that all samples showed coexistence of ferroelectric and ferromagnetic properties at room temperature. EA addition makes film more compact, which may be responsible for the differences in ferromagnetic properties. Adding EA and enhancing temperature resulted in phase transition of PVDF from α and γ to β, which contributed to membranes’ ferroelectric performances. In all samples, NMP:EA = 1:2 treated at 80 °C exhibits the maximum β-phase fraction and polarization (Pmax) of 87.2% and 2.398 μC cm⁻², respectively. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48345.     

Effect of flow history on poly(vinylidine fluoride) crystalline phase transformation

This study was devoted to the effect of extensional flow during film extrusion on the formation of the β‐crystalline phase and on the piezoelectric properties of the extruded poly(vinylidine fluoride) (PVDF) films after cold drawing. The PVDF films were extruded at different draw ratios with two different dies, a conventional slit die and a two‐channel die, of which the latter was capable of applying high extensional flow to the PVDF melt. The PVDF films prepared with the two‐channel die were drawn at different temperatures, strain rates, and strains. The optimum stretching conditions for the achievement of the maximum β‐phase content were determined as follows: temperature = 90°C, strain = 500%, and strain rate = 0.083 s^?1. The samples prepared from the dies were then drawn under optimum stretching conditions, and their β‐phase content and piezoelectric strain coefficient (d₃₃) values were compared at equal draw ratios. Measured by the Fourier transform infrared technique, a maximum of 82% β‐phase content was obtained for the samples prepared with the two‐channel die, which was 7% higher than that of the samples prepared by the slit die. The d₃₃ value of the two‐channel die was 35 pC/N, which was also 5 pC/N higher than that of the samples prepared with the slit die. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Crystal structure of uniaxially stretched bio-based polyamide 510 films

The crystal structure changes of PA510 films during uniaxially stretching at 80°C, 110°C, 140°C and 170°C had been investigated as a function of stretching ratio and stretching rate. The stress–strain relationship curves showed that the stress of the PA510 films gradually increased when the stretching ratio increased. The wide-angle X-ray diffraction results verified that only one distinct equator reflection of stretched films was clearly identified at 80°C, 110°C and 140°C, namely γ(100) at 2θ = 20.6°. However, when the stretching temperature reached 170°C, the γ(004), γ(006) and γ(008) crystal form appeared in the meridional direction at λ = 12. Combined with differential scanning calorimeter analysis, it was found that the Xc increased from 7% to 40% as a result of the strain induced crystallization phenomenon and the stretching promoted the appearance of γ crystal form. In addition, the increase in the crystallinity and the molecular chain orientation increased the strength of the PA510 films in the tensile direction. And it also found that the microcracks occurred in the stretched films at high stretching ratio (λ = 12).     

Evolution of nanodroplets and fractionated crystallization in thermally annealed electrospun blend fibers of poly(vinylidene fluoride) and polysulfone

In this study, electrospun immiscible blend fibers of poly(vinylidene fluoride) (PVDF)/polysulfone (PSF) were prepared. Due to the strong shearing during electrospinning, cocontinuous fibers of PVDF in the PSF matrix were obtained despite the minor composition of PVDF (5–30 wt.%) in the blend. After annealing these electrospun blend fibers above the melting temperature of PVDF (170 °C) and the glass transition temperature of PSF (185 °C), nanosized droplets (primarily 200–300 nm) of PVDF were developed inside the PSF matrix from the breaking up of PVDF nanofibers because of the Plateau–Rayleigh instability. Fractionated crystallization occurred in these PVDF nanodroplets with the heterogeneously nucleated crystallization in the range of 105–135 °C and the homogeneous nucleation at 55–60 °C. The mechanism of homogeneous nucleation was confirmed by the study of crystallization kinetics using differential scanning calorimetry. Only the nonpolar α-phase was observed by wide-angle X-ray diffraction despite of the homogeneously nucleated crystallization at a high supercooling in these PVDF nanodroplets. This study leads to a conclusion that nanoconfined crystallization at a moderate crystallization rate is less important than the local electric field to induce the ferroelectric phases of PVDF.