Cite this: RSC Advances, 2013, 3,13851

Polymer fibers with hierarchically porous structure:combination of high temperature electrospinning andthermally induced phase separation3

Received 19th March 2013,Accepted 21st May 2013

DOI: 10.1039/c3ra41315b

www.rsc.org/advances

Xiang-Yu Ye,a Fu-Wen Lin,b Xiao-Jun Huang,a Hong-Qing Lianga and Zhi-Kang Xu*a

Isotactic polypropylene (iPP) fibers with hierarchically porous structure were successfully prepared by

electrospinning at 200 uC combined with thermally induced phase separation (TIPS). Dioctyl phthalate

(DOP) and dibutyl phthalate (DBP) are typical diluents for iPP in TIPS and are used as solvents for

electrospinning. An ionic liquid was added to increase the solution conductivity, facilitate the

electrospinning process, and maintain a stable cone-jet electrospinning mode. Theoretical calculation

demonstrates that the jet cools rapidly, and phase separation takes place in the jet during its travelling

path, as the system traverses across the phase diagram from the single phase region to the metastable

region. For the iPP/DOP system, the surface morphology of fibers changes from aligned microvoids

bridged by fibrils to a wrinkled structure with the addition of ionic liquid, as the ionic liquid inhibits iPP

crystallization. The pore morphology can also be modulated by varying co-diluent composition. Open

pores appear on the fiber surface and the cross-section varies from closed cellular pores to a bi-continuous

structure with the increase of DBP content in the co-diluent, which clearly demonstrates the phase

separation mechanism changes from solid–liquid to liquid–liquid phase separation. The as-spun porous

fibers show more than a 100-fold increase in specific surface area compared with the non-porous ones.

The main advantages of this method are the pore formation process has a precise mechanism and the

pore morphology is well correlated with the phase diagram. Furthermore, it is readily extended to other

polymers with TIPS. Highly porous poly(vinylidene fluoride) (PVDF) fibers can be easily prepared from

PVDF/DBP solution with a 157-fold increase in specific surface area.

Introduction

Nature has witnessed the evolution of elaborate porousstructures over the ages, like bamboos, bird feathers, animalfurs and human bones. These amazing porous structures havebeen envied by many researchers.1 One prominent advantageof porous structures is to make the most economical use ofmaterials to achieve optimized performances. It is especiallyinteresting for electrospun nonwoven meshes to have porousstructures. Fine fibers with micro- or nano-textures/pores willhave further increased specific surface area which greatlyenhances their application prospects. For instance, one cantake advantage of porous fiber topologies to modulate thesurface wetting properties and to fabricate superhydrophobicmats,2 which show high oil adsorption capacity and oil/water

selectivity.3,4 Porous fibers are also useful for protein filtra-tion5 and construction of ultrasensitive sensors.6 Moreover,porous structures are beneficial for promoting cell attachmentand proliferation, making them excellent tissue scaffoldcandidates.7 Therefore, preparation of porous fibers hasattracted much attention in the electrospinning field nowa-days.

Various processing technologies have been developed so farand can be mainly classified into three categories: one-stepelectrospinning,8 selective dissolution of one component(including salt,9 nanoparticles10 and immiscible polymer11)from the electrospun fibers, and thermal calcination.12 Thelast two approaches require additional post-treatments usuallyaccompanied by the loss of mechanical strength and in somecases the extracted component is very difficult to removecompletely. The former one is much more convenient anddirect without any post-treatment. One-step electrospinningoften involves the use of volatile solvents and high environ-mental relative humidity,13 or the adoption of a non-solvent.14,15 It is generally granted that phase separationbehavior plays a pivotal role here and the pore morphologydepends on the competition between the rate of solvent

aMOE Key Laboratory of Macromolecular Synthesis and Functionalization,

Department of Polymer Science and Engineering, Zhejiang University, Hangzhou,

310027, China. E-mail: xuzk@zju.edu.cn; Fax: +86 571 8795 1773bDepartment of Chemistry, Zhejiang University, Hangzhou, 310027, China

3 Electronic supplementary information (ESI) available: Physical properties ofiPP and diluents, and complementary pictures of electrospun porous fibers. SeeDOI: 10.1039/c3ra41315b

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evaporation and the dynamics of phase separation.16

Accordingly, four main phase separation mechanisms havebeen proposed to explain the formation of surface pores andinner pores: breath figures (BFs),8,17,18 thermally inducedphase separation (TIPS) by evaporation cooling,16,19 vaporinduced phase separation (VIPS),13,20,21 and non-solventinduced phase separation (NIPS).14,15 However, the exact poreformation mechanism is still unsettled in electrospun fibersand different phase separation processes may interfere witheach other. A theoretical Maxwell bead-strand model was builtto study the phase separation dynamics, and spatiotemporalgrowth of the porous morphology was calculated in theframework of the Cahn–Hilliard time-evolution equation.19 Itwas found that the pore formation was driven by theconcentration sweep across the UCST envelope. Nevertheless,this calculation was based on a quasi steady state assumption,and the practical electrospinning process was much morecomplicated. Electrospinning faces excessive processing dis-turbances including solvent diffusion/evaporation, solventevaporation cooling, heat transfer, water condensing, andmutual diffusion of polymer/solvent/water molecules besidesoperation variables. Therefore, it is still very hard to predictthe phase separation path in the phase diagram and further tomodulate the pore morphology at present, sometimes evendifferent research groups observed different results fromsimilar experiments.17,18,20,21

TIPS is an effective strategy to obtain porous structures andhas been extensively applied to microporous membranepreparation from lots of polymers, such as polypropylene,22

polyethylene,23 poly (vinylidene fluoride),24 and polyacrylo-nitrile.25 Basically, it is a very simple process and consists offive steps: dissolution in diluents with high boiling tempera-ture at elevated temperature, casting the homogeneoussolution into the desired shape, programmed cooling toinduce phase separation, diluent extraction, and drying.22

TIPS is driven by a heat transfer rather than a complex multi-component mass exchange, therefore, it features severaladvantages relative to the conventional casting processincluding greater flexibility, a low tendency for defect forma-tion, good mechanical properties, ease of control, highporosity, effective control of the final pore size, and moreimportantly the ability to generate both isotropic andanisotropic structures.26

Here, our goal is to integrate TIPS into a high temperatureelectrospinning process, and to the best of our knowledge, thisis the first time that such technique has been used to fabricatehierarchically porous fibers. Compared with previous work,the most essential advantages of this process are that the poreformation mechanism is precise and the pore formationprocess has much fewer processing disturbances. The poremorphology is also well correlated with the phase diagram andcan be easily modulated by adjusting the TIPS processvariables. What’s more, this technique is universal andespecially beneficial for polymers having no common solventsat room temperature like isotactic polypropylene.

Experimental

Materials

Isotactic polypropylene resin (iPP, H110-02N) was supplied byDow Chemical Company, and its melt index was 2.0 g/10 min.Poly(vinylidene fluoride) powder (PVDF 6010, Mn = 1.51 6 105

g mol21) was bought from SOLEF International Ltd. ofBelgium. Dioctyl phthalate (DOP, Tb = 386 uC) and dibutylphthalate (DBP, Tb = 340 uC) were commercial products ofSinopharm Chemical and used as diluents. Isopropanol andethanol were used as extractants. 1-Methyl-3-methylimidazo-lium bis((trifluoromethyl)sulfonyl)imide ([CMim][NTf2], a typi-cal ionic liquid, purity .95%) was obtained from LanzhouGreenchem ILS, LICP, CAS. [CMim][NTf2] was selected herebecause it has high thermal stability and suitable conductivity.All the chemicals were of analytical grade and used withoutfurther purification.

High temperature electrospinning

Weighted amounts of polymer, DOP, DBP and ionic liquidwere mixed for 2 h at 180 uC with mechanical agitation to gethomogeneous polymer/diluent solutions. The mass fraction ofDOP in co-diluent was assigned as a, for example, a = 1.0means pure DOP was used. The ionic liquid content wasrelative to the total mass of solution. Different polymer/diluents/ionic liquid solid samples were formed by quenchingthe solutions. High temperature electrospinning was per-formed in a homemade laboratory apparatus. Fig. 1 shows theschematic representation of the setup. An integral stainlesssyringe/needle system was embedded in an Al2O3 tube, and thetightly twisted electric heating wires were able to maintain thepolymer solution at a desirable temperature as high as 300 uC.Further, the whole heating system was closely surrounded by aPTFE tube to ensure safety and heat preservation. A rotarydrum was adopted to mimic the industrial take-up process.One advantage of the integral system is that it could suppress

Fig. 1 Schematic representation of the high temperature electrospinning setup.The solution temperature can be maintained as high as 300 uC by electricheating without high voltage leakage. During the electrospinning process, thejet cools rapidly and thermally induced phase separation occurs. After extractingthe residue diluents in the solid fiber, a microporous structure forms.

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the temperature difference between the syringe and the needleto prevent a possible gelling problem, and so the needle lengthwas designed to be 4 mm with an inner diameter 1.2 mm(#12). The ingenious usage of Al2O3 tube shows efficient heattransfer and excellent insulation performance. Therefore, highvoltage could be directly connected to the needle without anyelectrical breakdown risk. In the high temperature electro-spinning experiment, the setup was preheated to a predeter-mined temperature, and then a solid sample of polymer/diluent/ionic liquid was put into the stainless syringe andheated until a homogeneous solution was formed.

All the electrospinning experiments were conducted at roomtemperature and 50% relative humidity. The optimum opera-tion conditions were carefully tested. The syringe temperaturewas set to 200 uC to maintain the polymer solution in liquidstate. The polymer concentration of iPP and PVDF was chosento be 15% (w/w) and 30% (w/w), respectively, to ensuresufficient chain entanglement density or viscosity while lowconcentration led to a small fiber diameter and high porosity.The feeding rate was controlled by a microinfusion pump, andwhen the feeding rate was 4 mL h21, voltage was 10.5 kV,collection distance was 14 cm, and drum speed was 200 rpm, astable cone-jet electrospinning mode could be maintained forour apparatus. During its travelling path from the needle tip tothe rotary drum, the initial hot jet cooled rapidly with asimultaneous thermally induced phase separation (TIPS), andfinally solid polymer fibers were collected. The residuediluents were extracted by isopropanol27 and ethanol28 foriPP and PVDF. After being dried under vacuum at 60 uC, theporous fibers were then subjected to further characterization.The variables in the electrospinning process were co-diluentcomposition and ionic liquid content, and their effects on themorphology of the as-spun fibers were thoroughly studied.

Some solid fibers electrospun from iPP/DOP with 1.33%ionic liquid were further subjected to a stretch process. The as-prepared fibers were first annealed at 120 uC for 30 min atfixed length, and then the residue diluents were extracted anddried as described above. Subsequently, the fibers were cold-stretched at 30 cm min21 to 200% extension ratio. Finally, thefibers were heat-set at 120 uC for 20 min and compared withunstretched ones.

Phase diagram determination

It is essential to establish the phase diagram of the polymer/solvent system to guide the phase separation path of the jetacross various coexistence regions of the phase diagram drivenby heat transfer. This diagram can be modeled in theframework of Flory–Huggins (FH) free energy of mixing, wherethe FH interaction parameter (x) between the polymer and thesolvent mainly determines the phase separation mechanism(solid–liquid or liquid–liquid). The value of the FH interactionparameter is given by eqn (1):

x = (V0/RT)(dP 2 dS)2 (1)

where V0 is molar volume of solvent, dP and dS are thesolubility parameters of polymer and solvent, respectively. Thelower the x value, the better the compatibility between thepolymer and the solvent. Clearly, DOP has better compatibility

with iPP than DBP (Table S1 in ESI3). Good agreement can alsobe achieved between the calculated phase diagram of iPP/DOP/DBP and the one generated from experiments.27

Differential scanning calorimetry (DSC7, Perkin-Elmer, USA)was used to determine the crystallization temperature (Tcry) ofdifferent polymer/diluent/ionic liquid mixtures. A sample of 6–8 mg solid was sealed in an aluminum DSC pan, melted at 453K and kept for 5 min to eliminate thermal history, then cooledat 10 K min21 to 323 K. The crystallization temperature wasdetermined from the exothermic peak, with the correspondingexothermic enthalpy (DHcry) used to evaluate the crystallinity(Xc). The crystallinity of iPP samples was calculated by thefollowing eqn (2):

Xc~DHcry

DHcryo|0:15

|100% (2)

where DHcryu is the fusion enthalpy of iPP with 100%crystallinity, taken as 209.2 J g21, and 0.15 is the mass fractionof iPP in solid samples.

The cloud point was determined visually by observing theappearance of turbidity during the cooling process under anoptical microscope (BX51, Olympus, Japan). A small piece ofsolid sample was sandwiched between a pair of glassmicroscope cover slips with Teflon taper around to preventdiluents evaporation, placed on a hot stage (THMSE600,Linkam, UK) at 453 K for 5 min, and then cooled to 323 K ata constant rate of 10 K min21. The temperature of the hotstage could be controlled precisely by a Linksys 32 system.

Characterization

The conductivity of ionic liquid/diluent solutions was mea-sured with a conductivity meter (FE30, Mettler Toledo,Switzerland). The morphology of the porous fibers wasobserved using a field-emission scanning electron microscope(FESEM, Hitachi S4800, Japan) after the sample was sputtercoated with gold. At least 10 fibers were selected and measuredby Image Tool to determine the average fiber diameter. Tocharacterize the fiber cross-section, the sample had to beimmersed in liquid nitrogen and fractured. The specificsurface area of different porous fibers was measured at 77 Kby a Chemisorption–Physisorption Analyzer (Autosorb-1-C,Quantachrome, USA). All samples were first dehydrated at373 K for 24 h before analysis. The actual specific surface areawas calculated from the nitrogen isotherms using theBrunauer–Emmet–Teller (BET) equation, while the theoreticalspecific surface area value was calculated from non-porouscolumned fibers with the same average diameter.

Results and discussion

Theoretical temperature profile of jet during electrospinning

Pore formation in the jet is actually a thermally driveninstability process, therefore, we first study the temperaturereducing procedure of the jet during its travelling path byapproximate calculation. The physical properties are listed inTable S2 (ESI3) for the materials used in the current study. Forsimplicity, the properties of the jet are set as follows: density r

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= 0.95 (g cm23), heat capacity Cp = 1.80 (J g21 K), thermalconductivity l = 0.20 (W m21 K), and the diameter is 20 mm.There are three factors influencing the jet temperature: 1) heattransfer with the surrounding environment, 2) solventevaporation cooling, and 3) iPP exothermic crystallization.We found that the solid fiber composition is only slightlyhigher than the raw solid sample (from 15% to 16%) due to thehigh boiling temperature and low vapor pressure of thediluents, so the evaporation cooling can be eliminated. Thetemperature change caused by iPP exothermic crystallization isless than 10 K as the mass fraction of iPP is only 15%. In short,jet cooling mainly depends on the heat transfer rate.

Considering the volume conservation during the electro-spinning process:

Q = pD2m/4 (3)

where Q is the feeding rate 4 mL h21 and D is the fiberdiameter 20 mm, and the jet velocity m is resolved to be 3.5 ms21. Accordingly, the heat convection-radiation transfer coeffi-cient (h) satisfies

h = 6.2 + 4.2m (4)

and h is 20.9 W m22 K.The Biot number (Biv) is a dimensionless number used in

heat transfer calculations, and gives a simple index of the ratioof the heat transfer resistances inside of and at the surface of abody. Biv can be calculated by

Biv = hV/Al (5)

where h is the heat convection-radiation transfer coefficient, Vis the volume, A is the surface area and l is the thermalconductivity. The calculated value of Biv = 0.000522 % 0.1 M(for a cylinder M = 0.5), which means uniform temperaturefields inside the body. Therefore, the temperature changecaused by heat transfer can be represented by the followingequation:

rV Cp dT = hA (T 2 T‘) dt (6)

where T is the instant jet temperature and T‘ is the roomtemperature 298 K. If h = T 2 T‘, the equation can betransformed into

rV Cp dh = hA h dt (7)

By integral operation, we can get h/h0 = exp(2hA t/rV Cp),where h0 (h0 = T0 2 T‘) is the heat transfer boundary conditionat t = 0 s. T0 is the initial solution temperature 473 K. Thetemperature profile of the jet can be written as follows:

T = (T0 2 T‘) exp(2hA t/rV Cp) + T‘ (8)

After calculation, the jet temperature is T = 175 exp(22.44t)+ 298 K. It drops rapidly, as can be seen from Fig. 2, and thenreaches a phase separation point within 0.35 s. This result isinteresting as the model studies of the electrospinning process

show that the whipping instability of the jet happened at thesame time range.19,29 Therefore, it means that jet thinning andthermally induced phase separation happen simultaneouslyafter jet initiation.

Effects of ionic liquid content on fiber morphology

Isotactic polypropylene (iPP) is the first polymer for thepreparation of porous membranes by the TIPS method, andthe effects of processing parameters were systematicallyinvestigated to modulate membrane morphologies.30 Varioussolvents were demonstrated as diluents for iPP, whichincluded phthalates,31 diphenyl ether,32 and diphenylmethane.33 Here we chose the iPP/DOP/DBP system to verifythe validity of our proposed method because the boilingtemperature of DOP and DBP are very high and diluentsevaporation was neglectable even after high temperatureelectrospinning as mentioned above. This means that unlikeconventional electrospinning experiments, the disturbancessuch as the diluent evaporation caused polymer concentrationsweep and jet cooling could be eliminated from determiningthe phase separation trajectory. However, the poor conductiv-ity of both iPP and diluents precludes smooth electrospinning.Overcoming this hurdle requires the addition of a conductivesalt,34,35 and an ionic liquid with almost zero vapor pressurewas reported to be very effective for polyolefin electrospinningin our previous work.36 Here, DOP and DBP are below thedetection limit of a conductivity meter, but the solutionconductivity can be obviously increased after adding 1% (w/w)[CMim][NTf2] into the diluents (Fig. S1 in ESI3).

Fig. 3 shows the effects of ionic liquid content on the fibermorphology of iPP fiber from iPP/DOP systems. The averagediameter decreases significantly in sequence (Table 1), yetfurther addition of ionic liquid has no obvious effect inreducing the fiber diameter. The increase in conductivityenhances the whipping instability of the charged jet; as aconsequence it depresses the orientation degree of thecollected fibers (Fig. S2 in ESI3). The cross-section of sampleselectrospun from the iPP/DOP system all show a closed cellularstructure, which results from a solid–liquid phase separationmechanism. However, it is very interesting to see that thesurface features of iPP fibers vary from aligned microvoidsbridged by fibrils to a wrinkled structure with grooves.

Fig. 2 Theoretical temperature profile of the jet during electrospinning byapproximate calculation.

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Generally, breath figures (BFs) was used to explain theformation mechanism of the surface pores on the electrospunfibers.17,18,21 BFs occurs as the jet cools, and moisture in theair condenses and grows in the form of closely packed dropletson the jet. The droplets act as hard spheres leaving imprintson the fiber surface in the form of pores as the jet solidifies.The high relative humidity plays a dominant role in the BFsmechanism. However, BFs is inapplicable to this case, as evenwhen we electrospun in below 20% relative humidity thesurface microvoids of iPP/DOP without ionic liquid do notvanish.

It can be seen that the aligned microvoids bridged by fibrilsare similar to the stretched pores prepared by the melt-spinning and cold-stretching methods (MSCS), where thepores are generated by the deformation of aligned chainfolding row lamella sandwiched by amorphous regions.37

Meanwhile, iPP/DOP systems only undergo solid–liquid phaseseparation (S–L) by iPP crystallization. Therefore, we speculatethat the crystallization behavior is a critical factor here. Asshown in Fig. 4A, iPP crystallizes very rapidly at 112.9 uC withthe highest crystallinity of 53.1% for iPP/DOP without ionicliquid. What’s more, the electrospinning process is character-ized by a high stretching rate,38 and the substantial elonga-tional flow of the jet usually results in high degrees of

crystalline domain and molecular orientation.39,40 To sum up,the formation mechanism of the aligned microvoids bridgedby fibrils on the surface of the as-spun fibers can be describedas follows: during the electrospinning procedure, the iPPcrystalline domain (row lamella) aligns along the jet axis dueto rapid crystallization and simultaneous strong electricalstretching; then the stacked lamellas are subjected to furtherstretching and deformation which ultimately leads to theformation of microvoids. As iPP is a semicrystalline polymer,some amorphous regions become the bridged fibrils. However,the addition of only a little ionic liquid remarkably reduces thecrystallization temperature (102.9 uC) and crystallinity (41.5%),and thus restrains the formation of row lamella which is theprerequisite condition for stretched pores. Therefore, electricalstretching only results in the appearance of grooves ratherthan microvoids on the fiber surface. To further clarify the

Table 1 Characteristics of the iPP solutions and the as-spun iPP porous fibers

Co-diluentcomposition

Ionic liquidcontent (w/w) Tcry (uC) Xc Tcloud (uC)

Phase separationmechanism

Average fiberdiameter (mm) Surface morphology

a = 1.0 0 112.9 53.1% — S–L 36.6 ¡ 2.9 Aligned microvoids bridged by fibrilsa = 1.0 0.67% 102.9 41.5% — S–L 19.6 ¡ 0.7 Wrinkled structure with groovesa = 1.0 1.33% 99.2 37.3% — S–L 13.3 ¡ 1.3 Wrinkled structure with groovesa = 1.0 2.00% 99.9 37.3% — S–L 11.7 ¡ 1.76 Wrinkled structure with groovesa = 0.9 1.33% 101.3 42.8% Tcry S–L and L–L 14.2 ¡ 1.6 Wrinkled structure with groovesa = 0.8 1.33% 101.2 43.4% 127 L–L prior to S–L 12.3 ¡ 0.8 Wrinkled structure with circular poresa = 0.6 1.33% 102.2 40.5% 135 L–L prior to S–L 14.5 ¡ 1.8 Wrinkled structure with oval pores

Fig. 4 DSC cooling curves of (A) iPP/DOP solutions with different [CMim][NTf2]contents and (B) iPP solutions of different co-diluent composition with[CMim][NTf2] content set to 1.33% (w/w).

Fig. 3 Surface morphology and cross-section of the iPP fibers electrospun fromiPP/DOP solutions with different [CMim][NTf2] contents: (A) 0; (B) 0.67; (C) 1.33;(D) 2.00% (w/w).

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rationality of the proposed formation mechanism, an addi-tional MSCS process was introduced to the fibers electrospunfrom iPP/DOP with 1.33% ionic liquid. Clearly, microvoidsappear after the annealing and stretching processes (Fig. S3 inESI3). Besides grooves, we can also observe obvious wrinkledsurfaces from ionic liquid doped fibers, which is common forelectrospun fibers with typical diameters larger than 1 mm. Theformation of such wrinkled structures can be ascribed to awell-known buckling instability.41,42 The key to understandthis buckling instability is the formation of a glassy skin on thejet surface. For TIPS, the formation of a dense skin layer on theas-prepared membranes due to fast solvent evaporation nearthe very surface at high temperature is a usual phenom-enon.26,43 Moreover, thinner fibers electrospun from ionicliquid doped solutions promote the solvent evaporationprocess due to increased surface area. Then the initiallyformed thin skin layer on the jet surface tends to collapseunder compressive hoop stress (buckling), which results inwrinkled structures. Similar rough surface features were alsoreported for low-density polyethylene (LDPE) electrospunfibers, where the authors also found the crystallizationbehavior and thick diameter are two critical factors.34,44

Effects of co-diluent composition on fiber morphology

It is widely acknowledged that the selection of diluents playsan important role in the TIPS process, and one cansuccessfully control the phase diagram and the as-preparedmembrane morphology by varying co-diluent composition.27,28

Here, we also studied the effects of co-diluent composition onthe morphology of iPP fiber. Table 1 summarizes theparameters of the experiment design and the characterizationresults. As mentioned previously, DBP has poorer compat-ibility with iPP than DOP. Therefore, the crystallizationtemperature rises slightly with the increase of DBP composi-tion. However, the crystallization curve is hardly influenced bya (Fig. 4B). The most distinct change happens at a = 0.8 whereliquid–liquid phase separation proceeds solid–liquid phaseseparation, and we speculate the fiber morphology will alsovary greatly at this point. Further reducing a shifts the cloud-point to a higher temperature, and the attempts to electrospiniPP solution with a , 0.5 fail as phase separation happens soquickly that the needle is blocked frequently and seriously.Fig. 5 shows the surface morphology and cross-section of iPPfibers electrospun from solutions with different co-diluentcomposition. A similar wrinkled structure with grooves canalso be found on the surface of the iPP fibers electrospun froma = 0.9 just as a = 1.0 because they have the samecrystallization behavior, and the detailed formation mechan-ism is elaborated above. As speculated, distinct open poresappear on the surface of iPP fibers electrospun from a = 0.8and a = 0.6 which can be explained by the different phaseseparation behavior. Along with the liquid–liquid phaseseparation occurring, polymer-rich and polymer-lean phasesform in the jet, and after polymer vitrification and diluentextraction, the polymer-lean phases will leave circular pores onthe fiber surface. For iPP fibers electrospun from a = 0.6, thesystem is so unstable that the jet undergoes liquid–liquidphase separation even before whipping instability develops,thus stretched oval pores form. Accordingly, the cross-sections

of iPP fibers vary from a closed cellular structure for a = 1.0and a = 0.9 to a bi-continuous structure for a = 0.8 and a = 0.6,which definitely demonstrates the solid–liquid and liquid–liquid phase separation mechanism.45 It is worthwhile to notesome large and deep pores located around the cross-sectioncenter of fibers electrospun from a = 1.0 and a = 0.9, which canbe ascribed to their longer stable single phase time for thedevelopment of solutocapillary-driven convection caused by asurface diluent evaporation induced concentration gradient.46

Altogether, we can clearly see that the fiber morphology is wellcorrelated with the phase diagram (Fig. 5E), and is mainlydetermined by the polymer crystallization behavior and phaseseparation mechanism.

A nitrogen adsorption experiment was used to determine thespecific surface area of the porous fibers (Fig. 6). According tothe International Union of Pure and Applied Chemistry(IUPAC) classification,47 the isotherm is akin to a type IIisotherm with a type H3 hysteresis loop. The reversible type IIisotherm is the normal form of isotherm obtained with amacroporous (.50 nm pore width) adsorbent and the type H3loop represents slit-shaped pores, which are in good agree-ment with the SEM results. The actual specific surface area ofiPP porous fibers was calculated by using the Brunauer–Emmet–Teller (BET) equation. As expected, the introduction ofa porous structure results in 107 and 67-fold increases inspecific surface area compared with non-porous fibers

Fig. 5 Effect of co-diluent composition on the morphology of as-spun fibers: (A)a = 1.0; (B) a = 0.9; (C) a = 0.8; (D) a = 0.6. The insets are the cross-section of asingle fiber. (E) A plausible electrospinning jet path from the needle tip to therotating drum with reference to the phase diagram estimated by experimentalresults. The [CMim][NTf2] content was fixed at 1.33% (w/w).

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electrospun from a = 1 and a = 0.8, respectively. Furthermore, astable cone-jet and continuous electrospinning mode ismaintained here, and a large iPP electrospun mesh can beobtained (Fig. S4 in ESI3). Given the similarity between ourhomemade high temperature electrospinning apparatus andan industrial melt-blow machine, it has high prospects for themass production of highly porous iPP non-woven meshes.

High temperature electrospinning of PVDF

We tested another widely used TIPS membrane material-PVDFto further verify the universality of our proposed hightemperature electrospinning/TIPS method. For comparison,DBP was chosen as the electrospinning solvent and the PVDF/DBP system only undergoes solid–liquid phase separation byPVDF crystallization.34 PVDF porous fibers with an averagediameter of 13.4 ¡ 0.7 mm were readily electrospun withoutthe addition of ionic liquid as PVDF is a polar polymer (Fig. 7).Numerous stretched pores and fibrils appear on the surface ofthe electrospun fibers as PVDF crystallizes rapidly at hightemperature. Further, the cross-section morphology has aparticulate structure (Fig. 7C) which is a typical poremorphology resulting from the solid–liquid phase separationmechanism.45 We can conclude that the morphology of theelectrospun PVDF fibers is also well correlated with the phasediagram just like iPP fibers. More interestingly, the PVDFporous fibers have a 157-fold increase in specific surface area.It is reasonable to combine the unique interconnectedstructure of a non-woven mesh and the high porosity of asingle fiber, therefore, our electrospun PVDF porous mem-

brane has high potential to be used as a battery separator orsuper-capacitor.

Conclusions

In summary, we found a universal method to fabricatehierarchically porous fibers that is high temperature electro-spinning combined with thermally induced phase separation.iPP and PVDF porous fibers were successfully prepared as twoof the most commonly used membrane materials. The as-spunporous fibers have high porosity and more than a 100-foldincrease in specific surface area compared with non-porousfibers, which will dramatically improve their in-serviceperformance as separation media. The essential advantagesof this method are that the pore formation mechanism isprecise and the pore formation process is controllable with fewinfluencing factors compared with previous work, which canbe easily modulated by adjusting processing variables. Thefiber morphology is well correlated with the phase diagramand is mainly determined by the crystallization and phaseseparation behavior during the high temperature electrospin-ning procedure. Furthermore, the whole electrospinningprocedure is stable and continuous. We believe our workopens a new avenue for the preparation of porous fibers in thelaboratory and industry.

Acknowledgements

The authors are grateful to the financial support from theNational Natural Science Foundation of China (Grant no.50933006), Zhejiang Provincial Innovative Research Team(Grant no. 2009R50004) and the Fundamental ResearchFunds for the Central Universities (Grant no.588040*172210321/001).

Fig. 6 Nitrogen adsorption–desorption isothermal curves of iPP porous fiberselectrospun from (A) a = 1.0 and (B) a = 0.8.

Fig. 7 (A) SEM images of PVDF porous fibers; (B) cross-section of PVDF porousfibers; (C) closed-up view of (B); (D) nitrogen adsorption–desorption experimentresults.

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13858 | RSC Adv., 2013, 3, 13851–13858 This journal is � The Royal Society of Chemistry 2013

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