What is porous fiber?

Porous fiber consists of bonded fibrous strands which create two-dimensional cross-sections that can be extruded to create three-dimensional shapes. Between the directionally aligned fibrous strands are voids, or pores, which can be independently controlled for fluid management applications that require faster flow or greater absorption of liquids.

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These bicomponent fiber materials have an inner core and outer sheath that provide an ideal capillary structure for liquid transfer, filtration, and diffusion applications where wicking speed, wicking distance, filtration efficiency, and flow resistance are key drivers of the device&#;s performance.

Made from a variety of thermoplastic materials with varying properties, our materials are moldable for high-volume applications and customizable for easy fabrication and conversions. We provide large sheets, rolls, or other material formats specific to your needs.

There are many ways for the pore generation in fibers. Sacrificial particle etching, phase separation, fiber fusion, gas foaming, melt blending and spinning, drawing and extrusion, laser blazing, etc. are some of the common examples. In this section, the emphasis will be place on spinning including electrospinning and melt spinning techniques because they are simple and easy to implement. Electrospinning will be discussed first. Then the development in melt spinning will be presented. Other techniques such as self-assembling, hydrothermal oxidation, chemical vapor deposition, cladding, and electrochemical etching will be briefly introduced in the last part of this section.

2.1. Electrospinning

Electrospinning or simply called E-spinning is a commonly used fiber production method. Under the action of an electric force, charged polymer solutions or polymer melts were drawn into continuous fibers with radial dimensions in the range from nm to µm. Electrospinning shows the characteristics of both viscous fluid electrospraying and traditional solution dry spinning of fibers. Many polymers show good E-spinning capability. Polyacrylonitrile (PAN) is one of them. As a linear polymer, polyacrylonitrile (PAN) is readily dissolvable in various polar solvents and shows good spinning ability. In addition, PAN is a high carbon yielding precursor for manufacturing carbon fibers. Solid carbon fiber can be readily prepared through the electrospinning of PAN followed by oxidation and pyrolysis as illustrated in Figure 1 A,D,E [ 36 ]. Figure 1 reveals the results from both classical (A and B) and current new (C) methods for producing carbon fibers from many polymer-based precursors. By adding a space-holding or pore generating polymer, for example poly-(methyl methacrylate) (PMMA) into PAN, highly porous microstructural features can be obtained. The later removal of PMMA from the spun fibers through the thermal decomposition creates micro- and nanoscale pores depending on the processing conditions. Zhou et al. [ 36 ] also showed that direct mixing PAN and PMMA together into a solution resulted in micro- and nanoporous carbon-based fibers with random distributed pores of different sizes as illustrated in Figure 1 B,F,G [ 36 ]. However, bonding PAN and PMMA to create a block copolymer (PAN-b-PMMA) led to porous carbon nanofibers with uniform pore sizes. After synthesizing the block copolymer of PAN-b-PMMA, a viscous solution can be made for processing porous carbon fibers. There are several steps to generated porous carbon nanofibers. First of all, electrospinning the copolymer was conducted to create fibrous strands. Next, the electrospun polymers went through an oxidation heat treatment process or thermal annealing. During this heat treatment, the thermally induced separation of two phases led to the generation of PMMA domains in the PAN fibers. The self-assembling of PMMA in the PAN allowed rather uniform distribution of PMMA. In the last step of pyrolysis, the heat treated polymer fibers were exposed to an even higher temperature under inert gas protection. This high temperature process converted the PAN into carbon and decomposed PMMA. The removal of PMMA left behind interconnected pores throughout the carbon fibers as illustrated in Figure 1 C,H,I [ 36 ]. The high temperature annealing treatment followed by pyrolysis converted the PAN-b-PMMA copolymer fiber into porous carbon fiber. The size and center-to-center spacing of pores were observed increasing largely with the increase of the annealing temperature. The size and morphology of pores in the PAN derived carbon could also be controlled by solvent annealing of PAN-b-PMMA in dimethyl sulfoxide, dimethylformamide, toluene, and chloroform [ 37 ].

Lin et al. [ 38 ] proposed a method for direct making highly porous polystyrene fibers through the electrospinning approach. High porosity in polystyrene polymer fibers was generated by using a high vapor pressure solvent, tetrahydrofuran (THF). The polystyrene polymer was dissolved into THF to make the 20 wt.% solution for spinning. The THF solvent was subsequently removed during the electrospinning process to directly form pores as shown in Figure 2 A [ 38 ].

From Figure 2 A, we can see that the shape of the fibers spun from the solution with THF as the only solvent is ribbon-like. A large number of surface nanopores were observed. Using a mixed solvent with 25% DMF, the nanopores disappeared from the surface. Only the wrinkled surface feature was found as illustrated by Figure 2 B. The fiber electrospun from the solution with equal amount of THF and DMF mixture displayed surface roughness as shown in Figure 2 C. The further increase in the concentration of DMF in the solvent promoted the formation of surface smooth fiber as can be found from both Figure 2 D,E. The surface nanopores only formed on the fibers E-spun from the high vapor pressure THF. With the addition of DMF, the decrease of the vapor pressure could significantly slow down the solvent evaporation. Consequently, the rate of solvent evaporation from the jets was reduced, which allowed the charged jets to keep in the fluidic state for longer time. Therefore, the continued stretching of the ejected polymer favored the formation of smooth fibers. It is well known that the highly volatile solvents including acetone, THF, and dichloromethane used in electrospinning can easily generate surface nanopores on the E-spun fibers. The surface wrinkles were caused by the buckling instability in processing based on the electrospinning studies of PS in THF and PAN in DMF [ 39 ], and poly-L-lactide (PLLA) in dichloromethane [ 40 ]. It must be also noted that groove-like pores could sometimes form from the same 20% polystyrene solution with a mixed solvent containing 25% DMF + 75% THF and the porous polystyrene fiber is highly adsorptive to oil as shown in [ 41 ]. The surface modification of the electrospun nanofibrous polystyrene fiber was performed by Zhang et al. [ 42 ]. The polystyrene nanoporous fibers was coated with a polyethyleneimine (PEI) layer to make a fibrous PS-PEI sensor. The sensor was tested and the enhanced formaldehyde sensing performance was achieved.

In addition to the surface nanopore generation, electrospinning is also an effective method to make nanoporous hollow fibers. As shown by Zhang et al. in [ 43 ], coaxial electrospinning was used to make core/shell hollow fibers. The polycaprolactone (PCL) polymer with the biodegradable property was used as the shell of the nanofiber. While polyvinylpyrrolidone (PVP) was taken as a sacrificial agent to form the core of the hollow porous fiber. The PVP core was dissolved by water instantly so that the nanoporous PCL hollow fiber can be generated. It is reported that the polycaprolactone (PCL) polymer has unique properties of biocompatibility, thermal stability, and chemical inertness [ 44 ]. Therefore, the nanoporous hollow PCL fiber may be used for drug delivery. Coaxial electrospinning is considered as an effective way for establishing a nanoscale confinement environment to control the crystallinity of the fiber core [ 45 ].

2O3 nanoparticles were embedded in the PAN derived carbon nanofibers (CNFs) for rechargeable Li-ion battery application. The composite nanofibers show the high conductivity of continuous carbon nanofiber networks, while the incorporated nanoporous iron oxide particles provide a lot of active sites and significantly enhance the charge capacity of the nanofibers. The nanoporous Fe2O3 are uniformly distributed in CNFs by the electrospinning. The conductive behavior of PAN derived carbon porous nanofiber can be further improved by silver incorporation based on the comparative studies of PAN-derived pure CNFs and Ag-incorporated CNFs [

Traditionally, polyacrylonitrile (PAN) derived carbon fibers are applied as the reinforcements for structural composite materials [ 46 ]. Recently, the electrospun PAN fiber has been studied as the support for nanoporous particles. In the paper published by Xu et al. [ 47 ], nanoporous Fenanoparticles were embedded in the PAN derived carbon nanofibers (CNFs) for rechargeable Li-ion battery application. The composite nanofibers show the high conductivity of continuous carbon nanofiber networks, while the incorporated nanoporous iron oxide particles provide a lot of active sites and significantly enhance the charge capacity of the nanofibers. The nanoporous Feare uniformly distributed in CNFs by the electrospinning. The conductive behavior of PAN derived carbon porous nanofiber can be further improved by silver incorporation based on the comparative studies of PAN-derived pure CNFs and Ag-incorporated CNFs [ 48 ]. The porous carbon and metal/carbon conducting nanofiber were made into enzymatic biosensors. They showed high sensitivity in the detection of triglyceride. Guo et al. [ 49 ] performed electrospinning a PAN-DMF solution added with reduced graphene oxide (GO) sheets to make porous composite carbon nanofibers. The benzene and butanone adsorption performance of the prepared GO/CNFs with well-developed porous structure were investigated. The incorporation of GO in the porous carbon fiber was found enhancing the adsorption performance and increasing the affinity of volatile organic compounds to the nanofibers.

51,3 (Er:NKN) nanofibers by sol-gel electrospinning PVP followed by calcination. Ferroelectric properties of the Er:NKN oxide nanofiber were tested. More details in sol-gel electrospinning process may be found in the research work performed by Aykut et al. [2) nanofiber was prepared by this method as shown in 3) and the PVP polymer were added in water to obtain one of the solutions. Referring to the left part of 3COO)2&#;4H2O, lithium acetate: Li(CH3COO)&#;2H2O, and polyvinyl alcohol (PVA) in DI (deionized) water as well. Before the electrospinning, the as-prepared two solutions were sufficiently mixed. Silver nanoparticles were generated by the reaction between silver nitrate and PVP. In the reaction, silver nitrate was reduced to silver by PVP to generate very fine silver precipitates. While the sol-gel reactions in the second solution converted lithium acetate (designated as LiAc) and cobalt acetate, simply designated as Co(Ac)2, sols into lithium hydroxide and cobalt hydroxide gels. After the sol-gel electrospinning as shown in the middle part of 2. The fiber product showed surface roughness and porosity as revealed by the SEM and TEM images in the far right-hand side of

Porous oxide nanofibers can be made through electrospinning metal compound containing polyvinylpyrrolidone (PVP) solutions followed by calcination in air [ 50 52 ]. Grishin and Markova [ 50 ] made bead-free and continuous Er-doped (Na,K)NbO(Er:NKN) nanofibers by sol-gel electrospinning PVP followed by calcination. Ferroelectric properties of the Er:NKN oxide nanofiber were tested. More details in sol-gel electrospinning process may be found in the research work performed by Aykut et al. [ 51 ]. A silver/lithium cobalt oxide (Ag/LiCoO) nanofiber was prepared by this method as shown in Figure 3 . Silver nitrate (AgNO) and the PVP polymer were added in water to obtain one of the solutions. Referring to the left part of Figure 3 , the second solution was made by adding cobalt acetate: Co(CHCOO)&#;4HO, lithium acetate: Li(CHCOO)&#;2HO, and polyvinyl alcohol (PVA) in DI (deionized) water as well. Before the electrospinning, the as-prepared two solutions were sufficiently mixed. Silver nanoparticles were generated by the reaction between silver nitrate and PVP. In the reaction, silver nitrate was reduced to silver by PVP to generate very fine silver precipitates. While the sol-gel reactions in the second solution converted lithium acetate (designated as LiAc) and cobalt acetate, simply designated as Co(Ac), sols into lithium hydroxide and cobalt hydroxide gels. After the sol-gel electrospinning as shown in the middle part of Figure 3 , polymer based composite nanofibers were obtained and dried. Next, calcination of the fibers was carried out in air for 2 h to remove the polymers. During this process the hydroxides were converted into LiCoO. The fiber product showed surface roughness and porosity as revealed by the SEM and TEM images in the far right-hand side of Figure 3

xSr1-xNiO3) based porous nanofiber was made by the similar sol-gel electrospinning method [3)2, Gd(NO3)3, and Ni(NO3)2 powders were added and stirred in DMF first. Then, the polymer binder, PVP, was dispersed into the solution for E-spinning. The spun fibers were dried at 60 °C for 8 h to get rid of the DMF solvent and subsequently annealed at 650 °C for 2 h to degrade PVP and convert the hydroxide gels into the complex oxide. Electron microscopic analysis of the prepared nanofibers with different Gd:Sr:Ni ratios was performed and the images are shown in 3 and Gd0.9Sr0.1NiO3, respectively, are featured by densely packed uniform nanocrystals. The surface of the two types of fiber were found relatively smooth. The mean size of the GdNiO3 fibers in diameter was found less than 100 nm. While the Gd0.9Sr0.1NiO3, nanofiber has a larger average diameter in the range from 100 to 200 nm. With the decrease in relative amount of Gd or increase of Sr, the Gd0.7Sr0.3NiO3 fiber shows high porosity as revealed in 0.7Sr0.3NiO3 reveals shallow pores with different sizes. The surface roughness of the fiber became observable. The diameter of the fiber was increased to larger than 200 nm. Such a porous feature of the fiber can offer many electrochemically active sites, which is an advantage for electrolyte storage in some applications such metal-ion batteries and supercapacitors. With the further decreasing in the Gd content or increasing in Sr, the Gd0.5Sr0.5NiO3 fiber started collapsing in structure and piled up together. The fiber also changed its surface morphology slightly. From 0.5Sr0.5NiO3 fiber is easily seen especially when compared with the image in 0.3Sr0.7NiO3 fiber changed its structure dramatically. Many tiny spikes are attached to the surface as from by the over growing of the pores with irregular sizes. Consequently, the nominal size of the fiber was increased to over 1 µm and the roughness of the fiber surface increased significantly. The fiber is thorn-like due to numerous tiny spikes attached to the surface of the Gd0.3Sr0.7NiO3 fiber. The average size of these spikes is around 200 nm, which can be estimated from 0.3Sr0.7NiO3 fiber allowed the formation of the robust fibrous thorns. The crystal structure of Gd0.3Sr0.7NiO3 transformed into that of the SrNiO3 with high crystallinity with the increase of Sr content in the oxide fiber [

The complex oxide (GdSrNiO) based porous nanofiber was made by the similar sol-gel electrospinning method [ 52 ]. Briefly, Sr(NO, Gd(NO, and Ni(NOpowders were added and stirred in DMF first. Then, the polymer binder, PVP, was dispersed into the solution for E-spinning. The spun fibers were dried at 60 °C for 8 h to get rid of the DMF solvent and subsequently annealed at 650 °C for 2 h to degrade PVP and convert the hydroxide gels into the complex oxide. Electron microscopic analysis of the prepared nanofibers with different Gd:Sr:Ni ratios was performed and the images are shown in Figure 4 52 ]. It was found that the morphology of the fiber was composition dependent. The scanning electron microscopic (SEM) images in Figure 4 a,b for the fibers with the compositions of GdNiOand GdSrNiO, respectively, are featured by densely packed uniform nanocrystals. The surface of the two types of fiber were found relatively smooth. The mean size of the GdNiOfibers in diameter was found less than 100 nm. While the GdSrNiO, nanofiber has a larger average diameter in the range from 100 to 200 nm. With the decrease in relative amount of Gd or increase of Sr, the GdSrNiOfiber shows high porosity as revealed in Figure 4 c. The SEM image with a higher magnification in Figure 4 d for a selected single fiber strand with the composition of GdSrNiOreveals shallow pores with different sizes. The surface roughness of the fiber became observable. The diameter of the fiber was increased to larger than 200 nm. Such a porous feature of the fiber can offer many electrochemically active sites, which is an advantage for electrolyte storage in some applications such metal-ion batteries and supercapacitors. With the further decreasing in the Gd content or increasing in Sr, the GdSrNiOfiber started collapsing in structure and piled up together. The fiber also changed its surface morphology slightly. From Figure 4 e, with the decrease in Gd concentration or increase in Sr, the increasing in the pore size of the GdSrNiOfiber is easily seen especially when compared with the image in Figure 4 c for the fiber with a higher Gd (or lower Sr) content. With even lower concentration of Gd or even higher content of Sr, the GdSrNiOfiber changed its structure dramatically. Many tiny spikes are attached to the surface as from by the over growing of the pores with irregular sizes. Consequently, the nominal size of the fiber was increased to over 1 µm and the roughness of the fiber surface increased significantly. The fiber is thorn-like due to numerous tiny spikes attached to the surface of the GdSrNiOfiber. The average size of these spikes is around 200 nm, which can be estimated from Figure 4 f. Structure analysis also revealed that as the atomic ratio of Gd was reduced from 1 to 0.3, the crystal lattices were increasingly distorted. This promoted the pore expansion and amorphous particle growth at the surface, leading to the surface coarsening of the fiber. The over growth of the pores and the shrinking in size of the GdSrNiOfiber allowed the formation of the robust fibrous thorns. The crystal structure of GdSrNiOtransformed into that of the SrNiOwith high crystallinity with the increase of Sr content in the oxide fiber [ 52 ].

2 nanofibers wrapped by reduced graphene oxide sheets (rGO) via sol-gel electrospinning with the potential application for improved Li-ion storage. Titanium (IV) isopropoxide was used as the source for TiO2. PVP was the binder, and DMF was the solvent. The graphene oxide (GO) used for wrapping the TiO2 nanofibers was made from chemical peeling of graphite using potassium permanganate, sulfuric acid, and other related substances. Two solutions were made for the sol-gel electrospinning. Titanium (IV) isopropoxide was dropped into a mixture of 2 mL of ethanol and 2 mL of acetic acid to form the first solution. The solution became transparent and in pale yellow color under vigorous stirring for about an hour. The second solution was made by dissolving 0.5 g of PVP into a mixed solvent containing 10 mL ethanol and 2 mL DMF under the stirring of a magnetic rod for 30 min. Then, the first solution was added into the second one. With vigorous stirring for 10 min, a yellowish solution was obtained for electrospinning. The solution was transferred into a syringe attached to a metallic needle with a nominal diameter of 0.6 mm. The feed rate from pumping was 0.3 mL/h. A DC voltage of 20 kV was imposed between the end of the needle and an aluminum paper wrapped plate collector. The collector was located about 16 cm away from the end of the metallic spinneret. The collected nanofibers were annealed at 450 °C in air for 2 h to get rid of the PVP polymer. The resulted TiO2 nanofibers showed the anatase crystalline phase structure [2 nanofiber, 2 mg of graphite oxide (GO) sheets was uniformly dispersed into 60 mL of DI (deionized) water via sonication for 1 h. After that, 5 mg of the TiO2 nanofiber was added into the GO solution. After vigorous stirring at the room temperature, a homogeneous mixture formed. The mixture was gradually dried on a hot plate through the removal of water under continuous stirring. As a result, a brown-colored sample was obtained. The sample was subsequently annealed at 400 °C for 2 h in Ar gas. This resulted in the reduction of the GO sheets to rGO at the surface of the TiO2 nanofiber. Eventually, the rGO wrapped TiO2 nanofiber product was obtained [

Thirugunanam et al. [ 53 ] synthesized nanoporous TiOnanofibers wrapped by reduced graphene oxide sheets (rGO) via sol-gel electrospinning with the potential application for improved Li-ion storage. Titanium (IV) isopropoxide was used as the source for TiO. PVP was the binder, and DMF was the solvent. The graphene oxide (GO) used for wrapping the TiOnanofibers was made from chemical peeling of graphite using potassium permanganate, sulfuric acid, and other related substances. Two solutions were made for the sol-gel electrospinning. Titanium (IV) isopropoxide was dropped into a mixture of 2 mL of ethanol and 2 mL of acetic acid to form the first solution. The solution became transparent and in pale yellow color under vigorous stirring for about an hour. The second solution was made by dissolving 0.5 g of PVP into a mixed solvent containing 10 mL ethanol and 2 mL DMF under the stirring of a magnetic rod for 30 min. Then, the first solution was added into the second one. With vigorous stirring for 10 min, a yellowish solution was obtained for electrospinning. The solution was transferred into a syringe attached to a metallic needle with a nominal diameter of 0.6 mm. The feed rate from pumping was 0.3 mL/h. A DC voltage of 20 kV was imposed between the end of the needle and an aluminum paper wrapped plate collector. The collector was located about 16 cm away from the end of the metallic spinneret. The collected nanofibers were annealed at 450 °C in air for 2 h to get rid of the PVP polymer. The resulted TiOnanofibers showed the anatase crystalline phase structure [ 53 ]. To make the rGO wrapped composite TiOnanofiber, 2 mg of graphite oxide (GO) sheets was uniformly dispersed into 60 mL of DI (deionized) water via sonication for 1 h. After that, 5 mg of the TiOnanofiber was added into the GO solution. After vigorous stirring at the room temperature, a homogeneous mixture formed. The mixture was gradually dried on a hot plate through the removal of water under continuous stirring. As a result, a brown-colored sample was obtained. The sample was subsequently annealed at 400 °C for 2 h in Ar gas. This resulted in the reduction of the GO sheets to rGO at the surface of the TiOnanofiber. Eventually, the rGO wrapped TiOnanofiber product was obtained [ 53 ].

2/ZnO heteronanostructures using the approach of hydrothermal growth of nanorods on the surface of spun and decorated fibers. The resulted final product is a composite nanofiber. It consists of a 3D-nanorod arrangement of the single-phase, hexagonal ZnO on the surface of a TiO2-decorated nanoporous polymer fiber. The composite fiber was characterized for the potential application as a photochemical catalyst. Since it has large surface area and can produce high concentration of charge carriers, the recombination of electrons and holes during photocatalytic processes can be minimized.

In [ 54 ], Tolosa et al., showed their work on electrospinning a vanadium oxide-carbon hybrid fiber mat for making cathodes in Li and Na-ion batteries. Vanadium (V) oxytripropoxide (VITP) was used as the metal oxide source and polyvinyl acetate (PVAc) as the polymer binder and the carbon source. After electrospinning, the fiber was annealed in a graphite heating furnace at 700 °C for 3 h under Ar protection. It has been also demonstrated that graphitic carbon nanohorns can be added into the spinning dope resulting in a reduction of 85% in the fiber diameter. Araújo et al. [ 55 ] synthesized hierarchical TiO/ZnO heteronanostructures using the approach of hydrothermal growth of nanorods on the surface of spun and decorated fibers. The resulted final product is a composite nanofiber. It consists of a 3D-nanorod arrangement of the single-phase, hexagonal ZnO on the surface of a TiO-decorated nanoporous polymer fiber. The composite fiber was characterized for the potential application as a photochemical catalyst. Since it has large surface area and can produce high concentration of charge carriers, the recombination of electrons and holes during photocatalytic processes can be minimized.

3, boiling point 62 °C), dichloromethane (CH2Cl2, boiling point 40 °C), dichloroethane (CH2ClCH2Cl, boiling point 82&#;84 °C), dioxane (C4H8O2 boiling point 101 °C). Also used are two mixtures of solvents. One is a mixture of chloroform with hexafluoroisopropanol (HFIP, boiling point 58 °C) (78/22%

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and 90/10%

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). The other is a mixture of chloroform with methanol (CH3OH, boiling point 65 °C) (96/4%

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and 92/8%

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). The concentration of the polymer solutions changed from 20% to 14% for chloroform, from 18% to 14% for dichloromethane, while the polymer concentration was set to 16% for dichloroethane, to 10% for dioxane, and to 18% and 20% for the mixture solvents. The scanning electron microscopic images of PEOT/PBT fibers without pore and with pore are shown in

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for making a 20% polymer solution, there is no pore formation observed in

To understand the pore formation mechanism on electrospun fibers, copolymer scaffolds consisting of poly(ethylene oxide terephthalate)-poly(butylene terephthalate) (PEOT/PBT) were prepared [ 56 ]. The copolymer with a PEOT to PBT ratio of 55 to 45 was added into different solvents to study the influence of solvents on the formation, size and shape of pore in fibers. Also studied is the effect of concentration on dimension of the fibers and porosity of the scaffolds. The solvents used include chloroform (CHCl, boiling point 62 °C), dichloromethane (CHCl, boiling point 40 °C), dichloroethane (CHClCHCl, boiling point 82&#;84 °C), dioxane (Cboiling point 101 °C). Also used are two mixtures of solvents. One is a mixture of chloroform with hexafluoroisopropanol (HFIP, boiling point 58 °C) (78/22%and 90/10%). The other is a mixture of chloroform with methanol (CHOH, boiling point 65 °C) (96/4%and 92/8%). The concentration of the polymer solutions changed from 20% to 14% for chloroform, from 18% to 14% for dichloromethane, while the polymer concentration was set to 16% for dichloroethane, to 10% for dioxane, and to 18% and 20% for the mixture solvents. The scanning electron microscopic images of PEOT/PBT fibers without pore and with pore are shown in Figure 5 56 ]. Figure 5 a is a relatively low magnification image providing a global view of the scaffold. The images in Figure 5 b through e reveal that by reducing the boiling point of the solvents used to make the polymer solutions, the morphology of the pore changed into irregular shape. The influence of the boiling point on the pore size was also found. With the increase of the boiling point of the solvent, the size of the pore was reduced. For fibers obtained from a 16% solution, the pore size changed from 1.2 µm for dichloromethane (with a boiling point of 40 °C), to 800 nm for chloroform (It has a boiling point of 62 °C), and to 350 nm for dichloroethane (It boils in the temperature range of 82&#;84 °C.). When dioxane was used as the solvent, elongated pores were seen with a size of 800 nm along the fiber direction. The dimension is 70 nm along the transverse direction. In a typical case, the polymer solution has a concentration of 10%. Increasing the polymer concentration resulted in a gel with high viscosity that could not initiate spinning. The polymer solutions with dioxane and with dichloroethane became gels at ambient temperature. Typically, the gels were needed to keep warm at 50 and 60 °C, respectively, to maintain the fluidity, before and during electrospinning. When HFIP was used together with chloroform in a mixture of 22/78%for making a 20% polymer solution, there is no pore formation observed in Figure 5 f. It is believed that the better solvent properties of HFIP for PEOT/PBT copolymers than other solvents causes such a phenomena [ 56 ].

3O4 nanoparticles. It was shown that the pore size was around 2 nm. The porous fiber demonstrated a high specific surface area of 964 m2/g. And a relatively high specific capacitance of 137 F/g was reached [

Up to now our discussion about electrospinning porous fibers mainly focuses on using organic solvents and synthetic polymers as the starting materials. In fact, some natural grown biopolymers and substances were also studied for electrospinning fibers to develop the so-called sustainable processing technology. Typical examples include the uses of starches [ 57 ], dextrins [ 58 ], and lignin [ 59 ]. The spun fibers could serve as the precursors of carbon fibers. As known, starches belong to polymeric carbohydrates which consist of numerous glucose units connected by glycosidic bonds. They are generated by most of the green plants. Starches are the most commonly consumed carbohydrates in human diets. Kebabsa et al. [ 57 ] reported their work on using corn starch to make nanofibers via electrospinning. Dip-coating the fibers was performed to attach a cobalt compound to the surface. The fibers were heat treated at high temperatures and converted into highly porous carbon nanofibers consisting of Conanoparticles. It was shown that the pore size was around 2 nm. The porous fiber demonstrated a high specific surface area of 964 m/g. And a relatively high specific capacitance of 137 F/g was reached [ 57 ].

2/g.

The feasibility and versatility in processing and converting a readily available renewable carbon precursor, dextrin, into highly porous carbon fibers were also shown [ 58 ]. The hydrolysis of starches generates dextrins consisting of multiple low-molecular-weight carbohydrate polymers. Electrospinning a cyclodextrin as the precursor for carbon fiber sponges has been demonstrated by Cecone et al. [ 58 ]. The carbon fibers with a diameter distribution of 1.3 ± 0.5 µm were successfully made upon the pyrolysis of the spun polymeric mat. The pore size of the fiber was in the range of 5&#;12 Å. The specific surface area was found to be 692m/g.

2. It was found that adding a polyethylene oxide (PEO) carrier with a small portion (only up to one ninth) of lignin allowed effective electrospinning the mixture into fibers and retained the fibrous structures during heating. Therefore, a separate thermal stabilization step is not needed. It was also found that impregnating alkali hydroxide activating reagents in-situ enabled the simultaneous carbonization and activation in a single heating cycle. The obtained microporous and mesoporous activated carbon fibers showed the superior high specific surface area of m2/g.

Lignin can form a family of complex organic polymers. Lignin establishes the essential framework in the support tissues of vascular plants and some algae. Lignin are essential for the formation of rigid cell walls, especially in wood and bark. Since lignin are cross-linked phenolic copolymers, they yield high percentage of carbon after the pyrolysis in inert gases. An earlier work performed by Hu and Hsieh [ 59 ] used lignin as the source for carbon. Activated porous carbon fibers with the diameters of 100&#;500 nm were made in two steps. First, electrospinning the aqueous solutions consisting of predominantly alkali lignin (low sulfonate content) was carried out. Then, the carbonization and activation were done simultaneously at 850 °C under N. It was found that adding a polyethylene oxide (PEO) carrier with a small portion (only up to one ninth) of lignin allowed effective electrospinning the mixture into fibers and retained the fibrous structures during heating. Therefore, a separate thermal stabilization step is not needed. It was also found that impregnating alkali hydroxide activating reagents in-situ enabled the simultaneous carbonization and activation in a single heating cycle. The obtained microporous and mesoporous activated carbon fibers showed the superior high specific surface area of m/g.

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