Dry-jet Wet Spinning Process Research Articles

The impact of additives and dope composition on hollow fiber ultrafiltration membrane for pure water permeability

Abstract In the current work, a dry-jet wet spinning process was used for preparing the polysulfone (PS) polymer hollow fiber ultrafiltration membrane. The polysulfone (PS) polymer was prepared by phase inversion technique with two distinct additives: glycerol and polyethylene glycol (PEG). The HF membranes were characterized and their performance was compared for pure water permeability (PWP). While characterizing, scanning electron microscopy (SEM) analysis on the internal section and cross-sectional area of membranes exhibited thin finger-like structures and thick sponge-like structures, respectively. With aid of an atomic force microscopic (AFM), the average surface roughness (Ra) of extruded HF membranes was found to be 61.253 nm and 81.086 nm, respectively for the membranes prepared using glycerol and PEG as additives Wettability studies revealed that the both membranes were hydrophilic. Further, they subjected to ascertained for the average pore size, surface porosity, stretch length, and breakage load. In addition, an investigation through X-ray diffraction (XRD) analysis exhibited that the acquired fibers were amorphous. The results of these findings showed that an increase in pressure caused a rise in water flow. Though PEG-supported HF membrane was an additive that has been shown to provide a greater water flux than glycerol additive HF, its structural stability suggests that it might be employed for higher pressure applications to satisfy the necessary demand for water processing.

Fabrication of high performance polyimide fibers by dry-jet wet spinning technology

High-performance co-polyimide (co-PI) fibers derived from 3,3`4,4`-biphenyldianhydride (BPDA), p-phenylenediamine (p-PDA) and 2-(4-aminophenyl)-5-amino-benzimidazole (BIA) were prepared via the wet spinning and dry-jet wet spinning method, respectively. The distinctions between wet spinning and dry-jet wet spinning processes on the structures and properties of co-PI fibers were systematically investigated. The co-PI fibers prepared by dry-jet wet spinning exhibited smoother surface morphology and higher mechanical properties than those of co-PI prepared by wet spinning. Wide-angle X-ray diffraction (WAXD) results showed that the co-PI fibers prepared by dry-jet wet spinning exhibited highly oriented structure along the fiber direction along with a low degree of lateral packing order in the transverse direction. Small-angle X-ray scattering (SAXS) revealed that the radius, length, misorientation and internal surface roughness of the microvoids in the co-PI fibers prepared by the dry-jet wet spinning are lower than those in co-PI fibers prepared by wet spinning. Moreover, the effect of air layer height, as well as the heat treatment on the structure and properties of fibers during the dry-jet wet spinning process, was also studied. The tensile strength and modulus of co-PI fibers prepared by dry-jet wet spinning reached 2.72 GPa and 114.29 GPa, respectively, with an air layer height of 10 mm and a heat stretching temperature of 440°C.

Optimization of Dry-Jet Wet Spinning of Regenerated Cellulose Fibers Using [mTBDH][OAc] as a Solvent.

Superbase-based ionic liquids (ILs) have demonstrated excellent dissolution capability for cellulose, and employing the dry-jet wet spinning process, high-tenacity regenerated textile fibers have been made. Among a range of superbase-based ILs, [mTBDH][OAc] exhibited not only good spinnability but also exceptional recyclability, making it highly suitable for a closed-loop production of regenerated cellulose fibers. To further optimize the spinning process, we investigated the influence of the cellulosic raw materials and the IL with residual water on spinnability and fiber properties. In addition, single-filament spinning and multifilament spinning using spinnerets with different hole densities were investigated to reveal the upscaling challenges of the dry-jet wet spinning process. The air gap conditions, for example, temperature and moisture concentration were simulated using COMSOL multiphysics. The results indicate that the presence of a small amount of water (3 wt%) in the IL has a positive effect on spinnability, while the mechanical properties of the fibers remain unchanged.

Application of a Twin-Screw Extruder with an Ultra-High Length-to-Diameter Ratio in the Dry-Jet Wet Spinning Process of Polyacrylonitrile Nascent Fibers

The quality of nascent fiber is the primary factor determining the performance of polyacrylonitrile (PAN)-based carbon fibers. This work presented a novel twin-screw extruder (TSE) with an ultra-high screw length-to-diameter (L/D) ratio for the dry-jet wet spinning process of high-quality PAN nascent fibers. The TSE with an ultra-high L/D ratio of up to 150 enabled the homogeneous mixing of PAN powder and dimethyl sulfoxide (DMSO), resulting in improved dope concentration of up to 30 wt %, improved molecular chain disentanglement, and reduced void formation. Orthogonal experiments were conducted to analyze the influence of dope concentration, barrel temperature, and screw speed on the properties of the PAN nascent fibers. Especially, three L/D ratios of 64, 96, and 150 were established to clarify the influence of the L/D ratio. It was found that with the increase in the L/D ratio from 64 to 150, the average tensile strength and modulus of the PAN nascent fibers were improved by 42.1 and 67.2%, respectively, and the crystallinity was increased by 45.5%. The PAN nascent fibers produced using the TSE with the ultra-high L/D ratio were found to have dense and smooth surfaces and fewer microvoid defects. It demonstrated the innovativeness and potential industrial application of the ultra-high L/D technique in the manufacturing of high-quality PAN-based carbon fibers.

Fabrication of phase change microcapsules with controllable size via regenerated nanochitin stabilized pickering and their applications for lyocell fiber

Lyocell fiber has attracted much attention in the textile fields as a greener alternative to viscose. Thermo-regulating Lyocell fiber, which composited with phase change materials (PCMs) and Lyocell fibers, would be desirable for making smart clothing with improved comfort. Herein, microencapsulated phase change materials (MPCMs) with polyurea shell and n-octadecane as core material were fabricated using regenerated nanochitin (RCh) as stabilizer via Pickering emulsion strategy. The size of MPCMs can be controlled by a tunable oil/water mass ratio (O/W). When the O/W ratio was above 1/25, the MPCMs exhibited melting enthalpy as high as 189.92 J/g with average size of less than 5 µm, which was suitable for spinning thermo-regulating Lyocell fibers. The thermo-regulating Lyocell fibers that were fabricated by a dry-jet wet spinning process showed high heat capacity (21.07 J/g) with a diameter of about 30 µm at 20 % loading microcapsules. They demonstrate good durability, where 95 % of the original heat capacity were retained after 100 heating/cooling cycles.

Braid-reinforced PVDF hollow fiber membranes for high-efficiency separation of oily wastewater

Several efforts to produce mechanically robust polymeric hollow fiber membranes have led to the use of polyethylene terephthalate (PET) hollow braided tubes, which offer rigidity to hollow fiber membranes. However, a compensation of flux exists with increased filtration pathway occasioned by braid substrate and thick polymer coating. It is often difficult to control the fabrication parameters to achieve uniformly coated and thin separation layer. In this work, we report a modification of the erstwhile dry-jet wet spinning process for braided hollow fiber by introducing a loop-system to increase resident time in the coagulation bath for effective solvent exchange and leaching of pore former additive. A combination of air gap (5, 15 and 25 cm), polymer concentration (15%, 17% and 19%) and take-up speed (1.2, 2.4 and 3.6 m/min) were varied to produce membranes with optimum filtration performance against crude oil emulsion feed. BRP-HM8 spun with 15 wt% PVDF, 15 cm air gap and take-up speed of 2 rpm exhibited thin and uniform coating outside of braid, well-developed pore structure resulting in high flux up to 620 L/m2h, good hydrophilic/oleophobic characteristics with > 98% rejection of crude oil as well high mechanical properties against harsh feed and elevated pressure.

Polyelectrolyte Complex Hollow Fiber Membranes Prepared via Aqueous Phase Separation.

Hollow fiber (HF) membrane geometry is the preferred choice for most commercial membrane operations. Such fibers are conventionally prepared via the non-solvent-induced phase separation technique, which heavily relies on hazardous and reprotoxic organic solvents such as N-methyl pyrrolidone. A more sustainable alternative, i.e., aqueous phase separation (APS), was introduced recently that utilizes water as a solvent and non-solvent for the production of polymeric membranes. Herein, for the first time, we demonstrate the preparation of sustainable and functional HF membranes via the APS technique in a dry-jet wet spinning process. The dope solution comprising poly(sodium 4-styrenesulfonate) (PSS) and polyethyleneimine (PEI) at high pH along with an aqueous bore liquid is pushed through a single orifice spinneret into a low pH acetate buffer coagulation bath. Here, PEI becomes charged resulting in the formation of a polyelectrolyte complex with PSS. The compositions of the bore liquid and coagulation bath were systematically varied to study their effect on the structure and performance of the HF membranes. The microfiltration-type membranes (permeability ∼500 to 800 L·m–2·h–1·bar–1) with complete retention of emulsion droplets were obtained when the precipitation rate was slow. Increasing the concentration of the acetate buffer in the bath led to the increase in precipitation rate resulting in ultrafiltration-type membranes (permeability ∼12 to 15 L·m–2·h–1·bar–1) having molecular weight cut-offs in the range of ∼7.8–11.6 kDa. The research presented in this work confirms the versatility of APS and moves it another step closer to large-scale use.

TIO2-ENABLED POLYVYNYLIDENE FLUORIDE FOR PALM OIL MILL EFFLUENT TREATMENT: EFFECTS OF MEMBRANE MORPHOLOGY AND AERATION ON FLUX AND SUSPENDED SOLID REMOVAL

A polyvinylidene fluoride-based mixed matrix membrane (PVDF- MMM) has been developed to treat palm oil mill effluent (POME). The addition of TiO2 into PVDF membrane was conducted. Hollow fibers were spun from a dope solution containing PVDF/PVP 30K/DMAc/additives by using a dry-jet wet spinning process at different air gaps. AFM image demonstrated that wet spun hollow fiber had a rougher outer surface than that of dry-jet wet spun fibers and exhibited symmetric cross-section structure. Experimental results showed that hydrophilicity of membranes increased with adding of TiO2 particles and the varied air gap length influenced the characteristic of membrane pore size and outer membrane surface roughness. In addition, aeration could increase the turbulence and flux and reduce membrane fouling. The values of flux and suspended solids removal were 92.04 L/m2.hr and 94.86 %, respectively, with the varied aeration flow rate of 2.0, 3.0 and 4.0 mL/min and bubble size distribution of 4.0 µm. Overall, this study has proven that PVDF-based MMM could achieve expected performance for POME treatment.

Effect of bore fluid composition on poly(lactic-co-glycolic acid) hollow fiber membranes fabricated by dry-jet wet spinning

Poly(lactic-co-glycolic acid) (PLGA) is a widely used polymeric biomaterial due to its excellent biocompatibility and biodegradation. Asymmetric PLGA hollow fiber membranes (HFMs) have been extensively used as porous tubular scaffolds for tissue engineering, e.g. for nerve conduits and artificial blood vessels. PLGA HFMs are generally fabricated through a dry-jet wet spinning process, and the porous morphology of PLGA HFMs induced by polymer/solvent/nonsolvent ternary phase inversion plays a key role in their biomedical performance. In this study, PLGA was dissolved in dimethyl sulfoxide (DMSO) as the dope fluid, and deionized water with different DMSO contents was used as the bore fluid. The influence of bore fluid composition on the HFM morphology and performance was investigated by measuring phase separation time, porosity, mechanical properties, and in vitro hydrolytic degradation. The results indicated that with increased bore fluid DMSO content, the inner and outer diameters, as well as the wall thickness of the HFMs, decreased significantly; also, the inner surface became more porous and the inner surface pore size and porosity increased. Cross-sectional SEM images showed that the middle layer of the five-layered microstructure gradually changed from large voids to finger-like microvoids when the DMSO content in bore fluid increased from 0% to 60%. Furthermore, the HFMs resulting from higher DMSO content in bore fluid had a lower porosity, a faster degradation rate, a higher Young's modulus, and a lower suture retention strength. More importantly, the in vitro cell culture on the inner surface of PLGA HFMs showed that PC12 cells cultured on HFMs prepared with 60% DMSO bore fluid exhibited better cell adhesion, longer neurite length, and alignment along the HFM longitudinal axis. A ternary phase diagram and the Flory-Huggins theory were applied to analyze HFM morphological formation. The study contributes to understanding of the mechanism of HFM porous morphology formation and should guide potential application of PLGA HFMs as tissue engineering scaffolds.

Comparison of high-performance polyimide copolymer fibers containing pyrimidine moieties based on coplanar structures

The effects of molecular coplanar structure on crystallinity, orientation, and overall performances of polyimide (PI) copolymer fibers, especially mechanical properties, were investigated. In detail, three diamines with similar structures, including 4,4′-diaminobenzophenone (DABP), 4,4′-diaminobenzanilide (DABA), and benzidine (Bz), were introduced as third monomers into the classical heterocyclic 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA)/2,5-bis(4-aminophenyl)pyrimidine (PRM) system, eventually the high-performance PI fibers being fabricated successfully via dry-jet wet spinning process. The conformation simulations illustrated the different coplanar levels found in three similar diamines: DABP (worst), DABA (better), and Bz (best). Thus, the BPDA/Bz dimer showed excellent coplanar structures, and the dihedral angle of the farthest benzene rings was only 4.42°. The wide-angle X-ray diffraction patterns indicated that the crystallinity and orientation of fibers increased with the increase of the coplanarity degree and molecular packing. Interestingly, the Bz-PI fiber possessed the strongest mechanical properties, i.e., fracture strength of 4.01 GPa and initial modulus of 137.34 GPa, which was primarily contributed by optimal molecular coplanarity and highest orientation.

Novel collagen/GO-MWNT hybrid fibers with improved strength and toughness by dry-jet wet spinning

ABSTRACT Considering the poor mechanical properties of regenerated collagen materials, the possibility of creating stronger and tougher collagen composite materials attracts particular research interests. In this study, we report that GO (Graphene oxide)-multiwalled carbon nanotube (MWNT) nanomaterials were constructed by using GO dispersed MWNT, and the strength and toughness of collagen fibers were significantly improved by introducing GO-MWNT nanomaterials into the collagen matrix during dry-jet wet spinning that exhibits the advantage of positive stretching to the extruded fiber before coagulation. We found that the combination of GO and MWNT can enhance the dispersion of both in the collagen matrix. Due to their strong interactions, they can not only spontaneously form the GO-MWNT network but also mutually induce collagen molecules to self-assemble and form aligned fibrils microstructure in dry-jet wet spinning process and resulting hybrid fibers. The interconnected networks of orderly aligned GO and MWNT can greatly enhance the interfacial interactions by forming hydrogen bonds with collagen molecules, resulting in the synergistic enhancement of the tensile strength, toughness and thermal stability of collagen hybrid fibers. This study provided asimple and environmentally friendly pathway to generate collagen-based fiber materials with improved performances and could be suitable for amyriad of biomedical applications.

Preparation and characterization of nanoclay loaded polyacrylonitrile hollow fiber membranes

New Polyacrylonitrile (PAN) nanoclay hollow fiber membranes were successfully fabricated through the dry-jet wet spinning process. The effects of adding the nanoclay on the hollow fiber mechanical properties and permeation performance and dye removal from water were investigated. Direct red 80, Direct orange 26 and Direct brown 2 dyes were used for dye removal experiments. Field emission scanning electron microscopy (FESEM) was used to analyze the cross-sectional and surface morphology and porosity. X-ray diffraction (XRD) has been used to compare crystalline structure of the fabricated hollow fiber membranes. The results showed that permeation is increased by adding nanoclay to the spinning dope. In addition, hollow fibers spun with lower nanoclay content showed lower hydraulic permeability and higher dye removal efficiency. Tensile strength increased from 1.09 to 2.06 with 3 wt% nanoclay loading.

Preparation and properties of high-performance polyimide copolymer fibers containing rigid pyrimidine and benzoxazole moieties with hydrogen bonding

The utilization of 5-amino-2-(2′-hydroxy-4′-aminophenyl)-benzoxazole (HBOA) as third monomer was beneficial for the introduction of OH-type H-bonding into polyimide (PI) matrix through the copolymerization with 2,5-bis(4-aminophenyl)pyrimidine (PRM) and 3,3′,4,4′-biphenyltetracarboxylic dianhydride (BPDA). And high-performance PI fibers were successfully fabricated via the dry-jet wet spinning process. The FTIR spectra confirmed that the strong interactions of intramolecular and intermolecular H-bonding existed simultaneously in PI fibers. Moreover, the glass transition temperatures (Tg) of all fibers were within 276–318 °C, and the temperatures at 5% weight loss (T5%) ranged from 574 °C to 604 °C, revealing the remarkable thermal stability of PI fibers. The two-dimensional wide-angle X-ray diffraction (2D WAXD) results illustrated a U-shaped change in the crystallinity (from 32.1% to 12.2% until 33.4%) of PI fibers with increasing HBOA content, which was also reflected in the density variation of fibers. The two-dimensional small-angle X-ray scattering (2D SAXS) patterns demonstrated that the micro-defects in all co-PI fibers profitably decreased in comparison with those in homo-PI fibers. Furthermore, the orientation factor of fibers presented an improvement from 0.843 to 0.899 after incorporating HBOA moiety. When the molar ratio of PRM/HBOA was 7/3, the co-PI fiber exhibited optimum mechanical properties with tensile strength and modulus as high as 4.15 GPa and 125.93 GPa, respectively. Interestingly, the variation in fiber orientation was in line with the same trend as that of mechanical properties of PI fibers, indicating orientation was also a crucial factor to improve the mechanical properties. The relationship between structure and mechanical property was also investigated.

Delicacy Management on Kiloton Dry Wet Spinning Bath Liquid

During the dry-jet wet spinning process of polyacrylonitrile carbon fiber precursor, the fluctuation of the coagulation bath liquid level affects the stability of the nozzle directly. The motion trajectory and the motion intensity in all directions of the fluid during the movement of the fiber in the coagulation bath fluid field were studied. A three-dimensional model of the cross flow and jet collision motion trajectory was established, and the impact of fluids with different strengths on the surface of the coagulation bath was analyzed. Solidification of the liquid surface in the fitting strength of the peak effect of the return wave overflow trough coupled superimposed to determine the coagulation bath surface to eliminate interfering fluctuations affecting factors. Based on the above analysis, a smart device for real-time monitoring of the coagulation bath air layer has been developed by using the damping equipment in the coagulation bath, where the impact of various fluid waves on the liquid surface can be eliminated on fixed point or position. According to the understanding of kiloton dry-jet wet spinning precursor fiber production line, in-depth exploration has been made to control the high dynamic coagulation bath level effectively from the perspective of technology and delicacy management.

Superbase-based protic ionic liquids for cellulose filament spinning

Lyocell fibers have received increased attention during the recent years. This is due to their high potential to satisfy the rising market demand for cellulose-based textiles in a sustainable way. Typically, this technology adopts a dry-jet wet spinning process, which offers regenerated cellulose fibers of excellent mechanical properties. Compared to the widely exploited viscose process, the lyocell technology fosters an eco-friendly process employing green direct solvents that can be fully recovered with low environmental impact. N-methylmorpholine N-oxide (NMMO) is a widely known direct solvent that has proven its success in commercializing the lyocell process. Its regenerated cellulose fibers exhibit higher tenacities and chain orientation compared to viscose fibers. Recently, protic superbase-based ionic liquids (ILs) have also been found to be suitable solvents for lyocell-type fiber spinning. Similar to NMMO, fibers of high mechanical properties can be spun from the cellulose-IL solutions at lower spinning temperatures. In this article, we study the different aspects of producing regenerated cellulose fibers using NMMO and relevant superbase-based ILs. The selected ILs are 1,5-diazabicyclo[4.3.0]non-5-ene-1-ium acetate ([DBNH]OAc), 7-methyl-1,5,7-triazabicyclo[4.4.0] dec-5-enium acetate ([mTBDH]OAc) and 1,8-diazabicyclo[5.4.0]undec-7-enium acetate ([DBUH]OAc). All ILs were used to dissolve a 13 wt% (PHK) cellulose pulp. The study covers the fiber spinning process, including the rheological characterization of the various cellulose solutions. Moreover, we discuss the properties of the produced fibers such as mechanical performance, macromolecular properties and morphology.Graphic abstract

Employing a green cross-linking method to fabricate polybenzimidazole (PBI) hollow fiber membranes for organic solvent nanofiltration (OSN)

A solvent resistant polybenzimidazole (PBI) hollow fiber membrane has been designed and fabricated by a dry-jet wet-spinning process and subsequently cross-linked with a K2S2O8 aqueous solution at 35 °C. This green cross-linking process not only dramatically improves the membrane stability in harsh solvents including N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF) and dimethylsulfoxide (DMSO), but also enhances the mechanical properties of the PBI hollow fibers. A cross-linking duration of 14 h is found to produce PBI hollow fibers with most balanced separation performance. The newly developed cross-linked PBI hollow fiber membrane shows a rejection of Rose Bengal (RB) more than 99% in acetone, exhibiting very good prospects for solvent resistant nanofiltration (SRNF) applications. To effectively mitigate hollow fibers from damage due to membrane swelling during organic filtration, it is recommended that the module configuration for SRNF has one end of hollow fibers free from the module housing. A simple backwash cleaning process has also been developed to restore the membrane performance of a fouled membrane.

Close Packing of Cellulose and Chitosan in Regenerated Cellulose Fibers Improves Carbon Yield and Structural Properties of Respective Carbon Fibers.

A low carbon yield is a major limitation for the use of cellulose-based filaments as carbon fiber precursors. The present study aims to investigate the use of an abundant biopolymer chitosan as a natural charring agent particularly on enhancing the carbon yield of the cellulose-derived carbon fiber. The ionic liquid 1,5-diazabicyclo[4.3.0]non-5-enium acetate ([DBNH]OAc) was used for direct dissolution of cellulose and chitosan and to spin cellulose–chitosan composite fibers through a dry-jet wet spinning process (Ioncell). The homogenous distribution and tight packing of cellulose and chitosan revealed by X-ray scattering experiments enable a synergistic interaction between the two polymers during the pyrolysis reaction, resulting in a substantial increase of the carbon yield and preservation of mechanical properties of cellulose fiber compared to other cobiopolymers such as lignin and xylan.

Preparation of hydrophobic zeolitic imidazolate framework-71 (ZIF-71)/PVDF hollow fiber composite membrane for membrane distillation through dilute solution coating

Mixed matrix membranes (MMMs) combine the advantages of different materials and hopefully break through the limitation of high mass transfer resistance and low permeability in membrane distillation (MD) process. In this study, hydrophobic zeolitic imidazolate framework-71 (ZIF-71)/polyvinylidene fluoride (PVDF) hollow fiber composite membrane was prepared by the dilute solution coating method. ZIF-71 was incorporated into the PVDF dilute solution and a macroporous hydrophobic ZIF-71/PVDF coating layer was formed on the outer surface of PVDF hollow fiber support membrane through the nonsolvent induced phase separation (NIPS) process. PVDF hollow fiber support membrane was fabricated by the dry-jet wet spinning process. Membrane surface morphologies were observed through Field emission scanning electron microscope (FESEM). The effects of zeolitic imidazolate frameworks (ZIFs) content in PVDF coating layer on membrane surface hydrophobicity and desalination performance were investigated through Dynamic water contact angle (WCA) measurements and the vacuum membrane distillation (VMD) experiment, respectively. The VMD experiment was carried out at feed temperature 50 °C and permeation side vacuum 31.3 kPa using 3.5 wt% NaCl aqueous solution as the feed solution. Compared with the uncoated PVDF hollow fiber membrane, both the surface hydrophobicity and the water permeability of the ZIFs-incorporated PVDF hollow fiber composite membranes are improved. The surface WCA and permeation water flux of ZIFs-PVDF composite membrane increase from 94.5 to 136.5° and 13.5 to 27.1 kg.m−2.h−1 meanwhile the NaCl rejection could maintain at 99.99%. No obvious pore wetting phenomenon was observed for ZIFs-PVDF hollow fiber composite membrane during the continuous VMD experiment for 60 h. The dilute solution coating is a promising method to prepare high-performance hollow fiber composite membrane for the desalination in MD process.

Effects of Dry-Jet Wet Spinning Parameters on Properties of Polysulfone Hollow Fiber Membranes

The influence of parameters of the dry-jet wet spinning process was studied on properties of porous polysulfone hollow fiber membranes. The air gap distance, the dope extrusion pressure, bore fluid pressure and temperature were chosen as the studied parameters. Their effect on the geometric and morphological properties of membranes was investigated. The optimal parameters were revealed for the formation of polysulfone hollow fiber membranes, promising for use as porous supports of composite membranes. High gas permeability were achieved with pore sizes providing the Knudsen flow regime: carbon dioxide permeability P/l(CO2) = 48.3 m3 m–2 h–1 atm–1, and values of ideal selectivity α = 2.63 and 0.87 for He/CO2 and CO2/N2 gas pairs, respectively.

Microfibril alignment induced by stretching fields during the dry-jet wet spinning process: Reinforcement on polyacrylonitrile fiber mechanical properties

The high-quality polyacrylonitrile (PAN) precursor fibers are indispensable for the manufacture of high-performance carbon fibers. For the purpose of regulating microfibril structures and improving fiber quality, scanning electron microscopy, high-resolution transmission electron microscopy and the nitrogen adsorption measurement were carried out to investigate the microfibril alignment evolution in PAN fibers, which was obtained by different stretching fields during dry-jet wet spinning process. The interconnected microfibrillar network forming in coagulation bath was plastically elongated and gradually developed into the oriented microfibrils while the transverse lamellae formed. Then break-reorganization of the lamellae brought the remarkable enhancement of fiber crystallinity during the hot-treatment process. Finally, the well-aligned and regular microfibrils appeared due to the deep plastic deformation. There were three stages in the total evolution of microfibril alignment: the plastic deformation of microfibrillar network, the fragmentation of the original lamella, and the formation and development of regular microfibrils. The microfibrils in PAN fibers were stacked by crystal layers in an order and tightly manner. Served as excellent reinforcing elements, aligned microfibrils could efficiently improve the fiber mechanical properties. Compared with nascent fiber, the tensile strength of PAN precursor fiber increased by 1492% and its tensile modulus enhanced by 574%.