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     

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     

Investigations of the preparation technology for polyglycolic acid fiber with perfect mechanical performance

Polyglycolic acid (PGA) fibers were prepared by melt‐spinning process in this report. The effects of spinning parameters, such as windup rates and drawn ratio, on the mechanical properties of the fibers were discussed by analyzing the internal stress of as‐spun fibers, axial sound velocity, fiber tenacity, etc. The results showed that windup rate had a slight effect on the macromolecular orientation degree of the as‐spun fibers, which was quite unusual for melt spinning, whereas, the subsequent drawing process effectively increased the macromolecular orientation degree of the PGA fibers and consequently increased the tensile strength of the fibers. Low internal stress of as‐spun fibers obtained at lower windup rate led to higher drawing ratio, and the drawn fibers possessed relatively excellent mechanical properties. As a contrast, higher windup rate resulted in the strong internal stress of the as‐spun fibers, which had a negative influence on the drawing process, and so the tensile strength of the drawn fibers was relatively poor. Therefore, PGA fiber with perfect mechanical performance could be prepared at the technical parameters of lower windup rate and higher drawing multiples as well as slow drawing rate. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Solid‐state polymerization of melt‐spun poly(ethylene terephthalate) fibers and their tensile properties

The production of high modulus and high strength poly(ethylene terephthalate) fibers was examined by using commercially available melt‐spun fibers with normal molecular weight (intrinsic viscosity = 0.6 dL/g). First, molecular weight of as‐spun fibers was increased up to 2.20 dL/g by a solid‐state polymerization, keeping the original shape of as‐spun fibers. Second, the polymerized as‐spun fibers were drawn by a conventional tensile drawing. The achieved tensile modulus and strength of as‐drawn fibers (without heat setting) were 20.0 and 1.1 GPa, respectively. A heat setting was carried out for the as‐drawn fibers. Tensile properties of the treated fibers were greatly affected by the condition of the heat setting. This was related to the increase of sample crystallinity and molecular degradation during the treatments. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 103: 1791–1797, 2007     

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     

Preparation of conductive polyaniline fibers by a continuous forming‐drawn processing routine

Polyaniline fibers were prepared with a continuous forming‐drawn processing routine that better met practical production requirements. The continuous forming drawing of the fibers was conducted successfully with the following methods. A reducing agent was added to a polymer solution during the dissolution of a polyaniline emeraldine base in N‐methyl‐2‐pyrrolidinone (NMP). After the entire wet‐spinning process was finished, the fibers were reoxidized and doped to obtain electric conductivity. The as‐spun fibers were predrawn at a low drawing ratio in a warm water bath before a plasticization drawing process on a hot plate. After the fibers were predrawn, some solvent was still kept in the fibers and used as a plasticizer of the fibers so that the plasticization drawing process would be performed successfully. The spinning conditions that affected the mechanical properties and conductivity of the fibers were the content of NMP in the coagulation bath, the coagulation‐bath temperature, the warm‐water‐bath temperature, the predrawing ratio, the hot‐plate temperature, the plasticization drawing ratio, and the reoxidation and protonation treatment time. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 956–960, 2004     

Processing,structure, and properties of gel spun PAN and PAN/CNT fibers and gel spun PAN based carbon fibers

Polyacrylonitrile (PAN) and PAN/carbon nanotube (PAN/CNT) fibers were manufactured through dry‐jet wet spinning and gel spinning. Fiber coagulation occurred in a solvent‐free or solvent/nonsolvent coagulation bath mixture with temperatures ranging from ?50 to 25°C. The effect of fiber processing conditions was studied to understand their effect on the as‐spun fiber cross‐sectional shape, as well as the as‐spun fiber morphology. Increased coagulation bath temperature and a higher concentration of solvent in the coagulation bath medium resulted in more circular fibers and smoother fiber surface. as‐spun fibers were then drawn to investigate the relationship between as‐spun fiber processing conditions and the drawn precursor fiber structure and mechanical properties. PAN precursor fiber tows were then stabilized and carbonized in a continuous process for the manufacture of PAN based carbon fibers. Carbon fibers with tensile strengths as high as 5.8 GPa and tensile modulus as high as 375 GPa were produced. The highest strength PAN based carbon fibers were manufactured from as‐spun fibers with an irregular cross‐sectional shape produced using a ?50°C methanol coagulation bath, and exhibited a 61% increase in carbon fiber tensile strength as compared to the carbon fibers manufactured with a circular cross‐section. POLYM. ENG. SCI., 55:2603–2614, 2015. © 2015 Society of Plastics Engineers     

Structure and property changes of polyamide 6 during the gel‐spinning process

Polyamide 6 (PA6) gels were prepared by the dissolution of PA6 powder in formic acid with CaCl₂ as a complexing agent. The concentration of the polymer was 16% w/v. PA6 fibers were obtained through gel‐spinning, drawing, decomplexation, and heat‐setting processes. The structure and properties of the fibers at different stages were characterized with differential scanning calorimetry, thermogravimetric analysis, X‐ray diffraction, Fourier transform infrared spectroscopy, and scanning electron microscopy. The experiment results indicate that the melting transition of the as‐spun fibers obtained by the extrusion of the PA6/CaCl₂/HCOOH solution into a coagulation bath through a die disappeared. A porous structure existed in the as‐spun fibers, which led to poor mechanical properties. Compared with the as‐spun fibers, the melting and glass‐transition temperatures of the decomplexed and drawn fibers retained their original values from PA6, the degree of crystallinity increased, the porous structure disappeared, and the mechanical properties were improved. The maximum modulus and tensile strength obtained from the drawn fibers in this study were 32.3 GPa and 530.5 MPa, respectively, at the maximum draw ratio of 10. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 4449–4456, 2013     

Effect of drawing on the molecular orientation and polymorphism of melt‐spun polyvinylidene fluoride fibers: Toward the development of piezoelectric force sensors

Thin piezoelectric polyvinylidene fluoride fibers containing a high piezoelectric β‐phase content of up to 80% were developed in this work using a melt‐spinning process. After crystallization from the melt, the fibers were subsequently stretched unidirectionally at 120°C between 25 and 75% of their original length. The effects on the molecular orientation, polymorphism and tensile properties of the fibers were investigated. Polarized infra‐red spectroscopy and X‐ray diffraction results show that the conversion of α‐phase to β‐phase occurred during the stretching process as a result of molecular alignment and creation of a dipole induced by the CF₂ groups normal to the fiber direction. These fibers were then integrated into various weave architectures in order to design flexible two‐dimensional textile‐based piezoelectric force sensors. The piezoelectric responsiveness of these materials, tested under impact (70 Newton force, 1 Hz frequency) was very promising, with a maximum output voltage of up to 6 V and an average sensitivity of up to 55 mV/N measured. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Structure and properties development in poly(phenylene sulfide) fibers,part I: Effect of material and melt spinning process variables

The detailed research study of manufacturing PPS fibers using melt spinning and further enhancement of tensile properties by drawing and annealing experiments, a study lacking as of today in open scientific literature, was the focus of this research. This article discusses the effect of polymer molecular weight (MW) and melt spinning process variables on the structure and properties development in melt spun fibers manufactured from proprietary Fortron® linear PPS resins. Structure‐properties relationship was studied using several characterization tools like tensile testing, differential scanning calorimetry, polarized light optical microscopy, and wide‐angle x‐ray scattering. Changes in dynamic mechanical behavior of as‐spun fibers manufactured from resins of varying MW and different melt spinning take‐up speeds were also studied. The study showed that by a combination of higher MW of the polymer and spinning at higher take‐up speeds, tensile properties of as‐spun PPS fibers can be improved. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Preparation of poly(lactic acid) fiber by dry‐jet‐wet‐spinning. I. Influence of draw ratio on fiber properties

Poly(lactic acid) fiber was prepared by dry‐jet‐wet spinning of the polymer from chloroform solution and with methanol as the precipitating medium. The as‐spun fiber was subsequently made into high strength fiber by two‐step process of drawing at a temperature of 90°C and subsequent heat setting in the temperature range of 120°C. The draw ratio had significant influence on the crystallinity and the tensile strength of the fiber. The fiber with the tenacity of 0.6 GPa and modulus of 8.2 GPa was achieved at a draw ratio of 8. The differential scanning calorimetry revealed an increase in the glass‐transition temperature with the increase in the draw ratio, which suggests the orientation of chains during the drawing process. The surface morphology of the filament as revealed by scanning electron microscopy shows that fibers are porous in nature, but a significant reduction in the porosity and pore size of the fiber was observed with the increase in the draw ratio. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1239–1246, 2006     

Preparation and properties of conductive polyaniline/poly‐ω‐aminoundecanoyle fibers

Historically, polyaniline (PANI) had been considered an intractable material, but it can be dissolved in some solvents. Therefore, it could be processed into films or fibers. A process of preparing a blend of conductive fibers of PANI/poly‐ω‐aminoundecanoyle (PA11) is described in this paper. PANI in the emeraldine base was blended with PA11 in concentrated sulfuric acid (c‐H₂SO₄) to form a spinning dope solution. This solution was used to spin conductive PANI / PA11 fibers by wet‐spinning technology. As‐spun fibers were obtained by spinning the dopes into coagulation bath water or diluted acid and drawn fibers were obtained by drawing the as‐spun fibers in warm drawing bath water. A scanning electron microscope was employed to study the effect of the acid concentration in the coagulation bath on the microstructure of as‐spun fibers. The results showed that the coagulating rate of as‐spun fibers was reduced and the size of pore shrank with an increase in the acid concentration in the coagulation bath. The weight fraction of PANI in the dope solution also had an influence on the microstructure of as‐spun fibers. The microstructure of as‐spun fibers had an influence on the drawing process and on the mechanical properties of the drawn fibers. Meanwhile, the electrically conductive property of the drawn fibers with different percentage of PANI was measured. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 85: 1458–1464, 2002     

Morphology and structure changes of aromatic copolysulfonamide fibers heat‐drawn at various temperatures

Pre‐drawn aromatic copolysulfonamide (co‐PSA) fibers were prepared by wet spinning and then heat drawing at temperatures varying from 350 to 390 °C, which are below the decomposition temperature. The fibers were then characterized using tensile testing, dynamic mechanical analysis, wide‐angle X‐ray diffraction and small‐angle X‐ray scattering. The relationship between structure and properties of the co‐PSA fibers drawn at different temperatures was investigated. The heat‐drawn co‐PSA fibers displayed similar glass transition temperature of about 355 °C, which was higher than that of pre‐drawn co‐PSA fibers of 345 °C. The crystal orientation was high as a crystalline structure formed during heat drawing and the crystallinity increased with the heat‐drawing temperature. However, the tenacity of the co‐PSA fibers did not increase linearly with the draw temperature. When the drawing temperature was higher than the glass transition temperature, a decrease in tenacity was observed, which could be attributed to an increase of crystallite size of the (100) plane and a decrease of the long period of the lamellar structure. © 2014 Society of Chemical Industry     

超高相对分子质量PPTA树脂及其高模量芳纶的研究

对高相对分子质量聚对苯二甲酰对苯二胺(PPTA)树脂进行了表征,开展了添加超高相对分子质量PPTA树脂与普通相对分子质量PPTA树脂共混进行液晶纺丝得到高强度和高模量芳纶的结构表征与性能试验,同时对芳纶的力学性能与其PPTA树脂相对分子质量的关系进行了研究。结果表明,芳纶的力学性能与其PPTA聚合体的相对分子质量紧密相关,如果PPTA树脂的相对分子质量不够高,加上液晶纺丝和高模量热处理过程分子链的进一步降解,高模量芳纶的制备就无法实现。在系统研究PPTA聚合反应规律,特别是聚合诱导相互转变规律及其影响因素研究基础上,通过调控连续聚合的反应条件,在1 000 t/a连续聚合生产线上制备出比浓对数粘度高达9.2 dl/g的超高相对分子质量PPTA树脂;用超高相对分子质量PPTA树脂与通用级PPTA树脂(比浓对数粘度6.8 dl/g)混合进行纺丝,制备出高强度的芳纶,并进一步热处理得到高强度和高模量的芳纶。     

Polyacrylonitrile based carbon fibers: Spinning technology dependent precursor fiber structure and its successive transformation

In this study, the effects of different spinning methods including traditional wet and dry-jet wet spinning, and newly developed dry-jet gel spinning, on the structures and performances of polyacrylonitrile fibers, as well as the structural evolution during stabilization and carbonization, are compared in detail. The structural differences along radial direction, surface roughness, and chain orientation of carbon fibers are inherited from their precursor fibers, and these factors are determined by spinning technologies and processing conditions. Among all spinning methods, dry-jet gel spinning could prepare fibers with the best chain orientation, the highest tensile properties, and the lowest surface roughness, which would be favorable for achieving higher mechanical performance. Additionally, for the resultant carbon fibers, the surface modification of dry-jet gel spun carbon fibers is easier than dry-jet wet spun carbon fibers, and comparable to wet spun carbon fibers. Overall, dry-jet gel spinning is promising to make carbon fibers with both excellent tensile properties and good interfacial adhesion with epoxy matrix.     

Melt spinning of PVDF fibers with enhanced β phase structure

Polyvinylidene fluoride (PVDF) fibers with a high amount of β phase crystal structure were prepared by melt spinning. With this technique, the cold drawing process is critical and efficient when aiming for a high amount of β phase. During the cold drawing process, more than 80% of the originally formed α phase crystal structure was converted into the β phase structure. In addition, the incorporation of 0.01 wt % of amino‐modified double wall carbon nanotube (NH₂‐DWCNT) could further enhance the β phase content in the PVDF fibers. FTIR and DSC studies showed that the addition of NH₂‐DWCNT to PVDF fibers could increase both the total crystallinity and β phase fraction in PVDF. The addition of nanoclay was found to be less efficient in this respect. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2603–2609, 2013     

3D Printing of BaTiO3/PVDF Composites with Electric In Situ Poling for Pressure Sensor Applications

This paper presents 3D printing of piezoelectric sensors using BaTiO₃ (BTO) filler in a poly(vinylidene) fluoride (PVDF) matrix through electric in situ poling during the 3D printing process. Several conventional methods require complicated and time‐consuming procedures. Recently developed electric poling‐assisted additive manufacturing (EPAM) process paves the way for printing of piezoelectric filaments by incorporating polarizing processes that include mechanical stretching, heat press, and electric field poling simultaneously. However, this process is limited to fabrication of a single PVDF layer and quantitative material characterizations such as piezoelectric coefficient and β‐phase percentage are not investigated. In this paper, an enhanced EPAM process is proposed that applies a higher electric field during 3D printing. To further increase piezoelectric response, BTO ceramic filler is used in the PVDF matrix. It is found that a 55.91% PVDF β‐phase content is nucleated at 15 wt% of BTO. The output current and β‐phase content gradually increase as the BTO weight percentage increases. Scanning electron microscopy analysis demonstrates that larger agglomerates are formulated as the increase of BTO filler contents and results in increase of toughness and decrease of tensile strength. The highest fatigue strength is observed at 3 wt% BTO and the fatigue strength gradually decreases as the BTO filler contents increases.     

Preparation of poly(lactic acid) fiber by dry–jet–wet spinning. II. Effect of process parameters on fiber properties

The dry–jet–wet spinning process was employed to spin poly(lactic acid)(PLA) fiber by the phase inversion technique using chloroform and methanol as solvent and nonsolvent, respectively, for PLA. The as spun fiber was subjected to two‐stage hot drawing to study the effect of various process parameters, such as take‐up speed, drawing temperature, and heat‐setting temperature on the fiber structural properties. The take‐up speed had a pronounced influence on the maximum draw ratio of the fiber. The optimum drawing temperature was observed to be 90°C to get a fiber with the tenacity of 0.6 GPa for the draw ratio of 8. The heat‐setting temperature had a pronounced effect on fiber properties. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3774–3780, 2006     

Gel spinning of polyacrylonitrile fibers with medium molecular weight

A new gel‐spinning method was employed to prepare polyacrylonitrile (PAN) fibers from a PAN spinning solution with dimethylsulfoxide and water as a mixed solvent. Aging at 25 °C for 120 min brought the spinning solution to the sol–gel transition and a three‐dimensional gel formed before entering the coagulation bath. The as‐spun fibers from the solution at the sol–gel transition and in the gel state possess a circular cross‐section. Compared with dry‐jet wet‐spun fibers, the gel‐spun fibers have a more compact structure, fewer voids and better mechanical properties after a three‐stage drawing. Moreover, the gel‐spun fibers obtained from the extraction bath have a more homogeneous microstructure and better packed supermolecular structure. The physical properties of the extracted gel‐spun fibers are also better than those of coagulated gel‐spun fibers. Copyright © 2010 Society of Chemical Industry     

Preparation and characterization of high‐performance poly(ether ether ketone) fibers with improved spinnability based on thermotropic liquid crystalline poly(aryl ether ketone) copolymer

High‐performance poly(ether ether ketone) (PEEK) fibers were prepared by melt‐spinning in the presence of thermotropic liquid crystalline poly(aryl ether ketone) copolymer (FPAEKLCP). The rheological and mechanical properties, birefringence, orientation, and crystallization of the resulting PEEK/FPAEKLCP fibers were characterized by using a melt flow indexer, capillary rheometer, single fiber electronic tensile strength tester, polarized light microscopy (PLM), and wide‐angle X‐ray diffraction (WAXD), respectively. The results indicate that the melt viscosity of PEEK significantly reduced by introducing FPAEKLCP, followed by the improvements in the spinnability and the quality of as‐spun fibers. The tensile properties of PEEK/FPAEKLCP fibers mainly depend on the content of FPAEKLCP, drawing temperature, drawing ratio, and annealing processes. Moreover, the tensile strength and modulus of PEEK/FPAEKLCP fibers are obviously higher than those of neat PEEK fibers under the same processing conditions. This should be attributed to an enhancement in the orientation and crystallization of PEEK compounded with FPAEKLCP. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 1406‐1414, 2013