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Dec 12, 2013 - Implications regarding requirements of performing a successive electrospinning and producing nanoporous polyetherimide (PEI) fibers are...
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Comparative Studies on the Solvent Quality and Atmosphere Humidity for Electrospinning of Nanoporous Polyetherimide Fibers Hossein Fashandi and Mohammad Karimi* Amirkabir University of Technology, Department of Textile Engineering, Hafez Avenue, Tehran, 15914, Iran ABSTRACT: Implications regarding requirements of performing a successive electrospinning and producing nanoporous polyetherimide (PEI) fibers are discussed through electrospinning PEI solutions of three nonvolatile solvents, that is, dimethylforamide (DMF), dimethylacetamide (DMAc), and N-methyl-2-pyrrolidone (NMP), under atmospheres of constant temperature and varying levels of relative humidity (RH). The results demonstrate depending on nature of miscibility area in ternary phase diagram, a minimum RH is necessary to stabilize fiber formation. Furthermore, RH of operating environment affects diameter and both surface and interior morphologies of PEI electrospun fibers through involving the rate of phase demixing and viscoelasticity of solution. Considering fibers produced from NMP solutions because of delayed demixing, solvent drying precedes phase demixing or takes place in a comparable rate in high RH which leads to solid cross-section and texture-less surface with slight porosity. By choosing DMF or DMAc as electrospinning solvent, thicker fibers with rough surface and porous cross-section are expected. breath figure formation which can be considered when volatile solvents are employed to prepare electrospinning solutions or polymer−solvent binary system shows an upper critical solution temperature (UCST) phase diagram,14,16,21,25−28 (2) temperature-induced phase separation (TIPS),29 (3) nonsolventinduced phase separation (NIPS),30−32 (4) vapor-induced phase separation (VIPS),14,16,17,20−22,28,33 (5) polymer−polymer phase separation,34 in which electrospinning is performed with a solution composed of two miscible polymers and followed by leaching process to selectively remove one of components, and (6) interaction of Lewis acid with the Lewis base and subsequent removal of Lewis acid component.35 Among the above-mentioned mechanisms, VIPS and formation of breath figures are the most relevant ones to explain pore formation within electrospun fibers. It was demonstrated by Pai et al.17 that morphology of polystyrene electrospun fibers is determined as a direct consequence of competition between VIPS and two other processes, that is, buckling instability and solvent drying. Evolution of morphologies in electrospun fibers as a result of VIPS mechanism was theoretically investigated by Dayal and Kyu33 in the framework of Cahn−Hilliard equation. They showed it is possible to postulate different morphologies for electrospun fibers regarding the location of concentration of electrospinning solution on nonsolvent/solvent/polymer ternary phase diagram. Recently, Zheng28 emphasized on the occurrence of VIPS process when solutions of nonvolatile solvents are electrospun under humid conditions. They concluded in the case of solutions prepared from volatile solvents, condensation of water vapor due to solvent evaporative cooling of fiber surface causes surface pores to be evolved. In our previous

1. INTRODUCTION Fibers ranging in diameters between few nanometers to few micrometers have been developed for a variety of applications such as controlling surface hydrophobicity and wetting processes,1,2 membranes in sensing materials,3 catalytic systems,4 drug delivery and tissue engineering,5 filtration,6 protective clothing,6,7 and cleanup the oil contaminations through absorption.8,9 A rich literature1−9 demonstrated that electrospinning has been adopted as the dominant technique to produce fibers designed for these applications and attempts have been made to scale-up of this process.6 In this technique, in the presence of an electric field an electrically charged jet is continually formed when the electrostatic forces overcome the surface tension and viscose force of the polymer solution. Traveling of jet in the distance between nozzle and collector, known as working distance, is accompanied by continues stretching, as well as solvent drying, which contribute to morphology evolution of produced fibers.10−13 Controlling the morphology and tailoring the properties of electrospun fibers by altering a variety of factors such as electrospinning parameters,14,15 environmental conditions, that is, temperature (T) as well as relative humidity (RH),14,16−22 vapor concentration of solvent in electrospinning environment,19 and finally solution properties including solvent type,21−23 fluid elasticity,24 polymer concentration,14,15 polymer molecular weight,15,16 surface tension of solution,15 and conductivity15 has enjoyed remarkable attention as a direct consequence of specific requirements imposed by each application. For instance, due to the need for high surface area in some specific areas such as catalytic systems, sensing materials, membranes in solar cells, filtration, hydrogen storage systems, protective clothing and recently oil absorption,3,4,8,9,17 the mechanism of porosity evolution within electrospun fibers is of significant and commercial interest which has been able to attract great attention. In literature mechanisms envisaged to formation of pores during electrospinning process could be classified in six main categories: (1) Rapid solvent drying and © 2013 American Chemical Society

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studies21,22 on polystyrene (PS) and polyethersulfone (PES) fibers, the contribution of phase behavior of nonsolvent/ solvent/polymer ternary system to morphology evolution of electrospun fibers was highlighted. Strictly speaking, it was found that influence of operating temperature and relative humidity on fiber morphology is strongly dictated by phase behavior of polymer solution in the presence of water vapor. Further, in the case of volatile solvent, ambient temperature imparts the electrospun fiber with desired microstructure. At this condition, it is likely that solvent drying precedes phase separation induced by water vapor. PEI is an amorphous, amber-to-transparent polymer that combines high chemical and thermal stability36,37 with good dimensional stability and inherently flame resistance.38 It has been widely used as an appropriate candidate for gas separation processes36,39 especially separation of He or H2 from other gases.36 Fabrication of electrospun mats from PEI solutions has been also considered by some authors. Han et al.40 reported formation of porous electrospun polyetherimide (PEI) fibers through thermal degradation of poly(3-hydroxybutyrate-co-3hydroxyvalerate) (PHBV) in PEI/PHBV blend fibers at 210 °C; They showed porosity varied as a function of composition of PEI/PHBV blend. Production of continuously aligned nanofibers from PEI solution was examined by Moon et al.38 using both a stationary and a rotating grounded target under variable parameters. They investigated the effect of take-up speed and spinning voltage on the molecular orientation and alignment of obtained fibers and reported optimum conditions to produce well-aligned PEI electrospun fibers. Although numerous studies have concentrated on dominant factors influencing the morphology of electrospun fibers, but elaboration of surface and interior structures of fibers prepared using electrospinning approach to afford desirable morphology is still challenging. The present article addresses how solvent quality contributes to phase behavior of fiber-spinning system and solvent physicochemical properties involves to the electrospun PEI fibers with different morphologies. To this end, three nonvolatile solvents, that is, dimethylformamide (DMF), dimethylacetamide (DMAc) and N-methylpyrrolidone (NMP), are used to make electrospinning solutions of PEI. The electrospinning is performed under conditions of various relative humidity and constant temperature. The evolved morphologies are discussed by means of considering the experimentally obtained phase diagram of water/solvent/PEI ternary systems.

Voltage ES 50P-10W) and a syringe pump (JZB 1800D Double Channel Syringe pump from China). 2.3. Preparation of Electrospun Fibers. PEI granules were dissolved in DMF, DMAc and NMP under gentle stirring at room temperature for 24 h to provide electrospinning solutions of concentration 20 wt. %. The prepared solution was loaded in a 10 mL glass syringe with metal needle. Electrospinning was performed at constant temperature of 40 °C and diverse levels of relative humidity (5, 20, 40, and 60%). Flow rate, working distance (nozzle-to-collector distance) and voltage were adjusted to be 1 mL/h, 35 cm, and 15 kV, respectively. At RH = 60%, since phase separation takes place very fast and prevents fibers stretching and traveling, voltage and working distance for PEI/DMF and PEI/DMAc solutions were fixed at 20 kV and 20 cm, respectively, to avoid fiber deposition at the middle of working distance. 2.4. Construction of Ternary Phase Diagram. Phase diagrams of ternary systems H2O/DMF/PEI, H2O/DMAc/PEI and H2O/NMP/PEI, at 40 °C were determined by calculating the binodal, spinodal and tie-lines based on Flory−Huggins (FH) free energy of mixing (ΔGM) extended for a ternary nonsolvent (1), solvent (2), and polymer (3) system, which is referred to as Tompa extension (eq 1).41 ΔGM = n1 ln ϕ1 + n2 ln ϕ2 + n3 ln ϕ3 + n1ϕ2g12(u 2) RT + n2ϕ3χ23 (ϕ3) + χ13 n1ϕ3

(1)

where R is the gas constant and T is the absolute temperature. ni and ϕi stand for the number of moles and volume fraction of component i, respectively. χ23(ϕ3) is assumed as a concentration dependent solvent (2)/polymer (3) interaction function of DMF/PEI, DMAc/PEI and NMP/PEI evaluated using vapor pressure osmometry (VPO) approach according to eq 2 developed by Karimi et al.42 χ23 (ϕ3) = a + bϕ3

(2)

where a and b are constants obtained from VPO experiments.42 g12(u2) in eq 1 is a generalized nonsolvent (1)/solvent (2) interaction function depending on the volume fraction u2 = ϕ2/ (ϕ1+ϕ2) of a pseudo binary mixture.43,44 The data set of binary interaction parameters of H2O/DMF, as well as H2O/NMP binary systems were collected from literatures45,46 and presented based on Koningsveld and Kleintjens model (eq 3). g12(ϕ2) = α0 +

2. EXPERIMENTS AND THEORY 2.1. Materials. Polyetherimide (PEI), type Ultem 1000 (MW = 32800 g/mol) was purchased from General Electric (U.S.A.). The sample was dried before use in an oven for 4 h at 100 °C. The solvents dimethylformamide (DMF), dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), and dichloromethane (DCM) of analytical grade from Sigma Aldrich, Inc., were used as received without further purification. Deionized water was used as nonsolvent. 2.2. Electrospinning Setup. A chamber with controlled temperature, T (±0.1 °C), and relative humidity, RH (±1%), in high accuracy was used to produce electrospun webs at different environmental conditions. Details related to design of chamber have been explained elsewhere.21 The electrospinning setup also contains a high voltage power supply (Gamma High

β0 1 − γ0ϕ2

(3)

where α0, β0, and γ0 are temperature-dependent constants. For the case of H2O/DMAc, the required parameters for eq 3 at 40 °C were calculated according to UNIFAC method. χ13 is the nonsolvent (1)/polymer (3) interaction parameter, often assumed as a constant. Equilibrium water uptake measurement at temperature 40 °C was used as a simple and well-known method to measure χ13 based on GRP model suggested by Karimi et al.47 According to this model χ13 is evaluated using eq 4. Δμ = ln a1 = β[ln(ϕ1) + (1 − ϕ1)] + (1 − β) RT ⎡ ⎛ ϕ ⎞⎤ × ⎢ln⎜⎜ 1 ⎟⎟⎥ + (1 − ϕ1)2 χ13 ⎣⎢ ⎝ ϕ0 ⎠⎥⎦ 236

(4)

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where a1 and ϕ1 are the water activity and volume fraction of absorbed water, respectively. ϕ0 (ϕ0 = ϕFV − ϕ1)represents the volume fraction of unoccupied spaces into the polymer and ϕFV is the fractional free volume of the polymer. Details of the calculation are presented in Appendix A. Water absorption was evaluated for polymer films cast from dichloromethane (DCM) solutions. The procedures relevant to film making and water uptake measurements have been detailed in ref 47. Knowing the interaction parameters between three components at temperature of interest, it is possible to calculate binodal, spinodal and tie lines for ternary systems. More information about the mathematical/numerical treatment may be found, for example, in ref 48. The calculated binodal curves were further verified by measuring cloud points at constant temperatures, that is, 40 °C and superimposing them on the calculated phase diagram. 2.5. SEM Analysis. Scanning electron microscope (SEM) (TESCAN series VEGA 2007 from Czech) evaluated surface morphology along with cross-section of electrospun fibers from captured images at 30 kV acceleration voltages. Before SEM analysis samples were coated with a 30 nm layer of gold. Based on SEM images of produced webs, diameter of electrospun fibers was also determined using an image analysis software (Image J). The mean and standard deviation values were obtained by readying the diameters of 50 randomly selected fibers.

Table 1. Interaction Parameters Required to Calculate Phase Diagram of Ternary Systems: H2O/DMF/PEI, H2O/NMP/ PEI, and H2O/DMAc/PEI ternary system

g12 = α +[β/(1 − γu2)]

H2O/DMF/ PEI H2O/NMP/ PEI H2O/ DMAc/PEI

g12 = 0.425 + [0.062/(1 − 0.949u2)]a g12 = 0.316 +[0.468/(1 − 0.499u2)]b g12 = 0.215 +[0.275/(1 − 0.553u2)]c

χ23 = a + bϕ3d χ23 = 0.498 + 0.008ϕ3 χ23 = 0.387 − 0.11ϕ3 χ23 = 0.449 − 0.024ϕ3

χ13e 2.5 ± 0.2 2.5 ± 0.2 2.5 ± 0.2

a

These data were taken from ref 45. bThese data were taken from ref 46. cThese data were calculated by UNIFAC method. dCalculated by VPO. eCalculated according to GRP model suggested by Karimi et al. 47

Included in Figure 1 are calculated ternary phase diagrams for H2O/DMF/PEI, H2O/DMAc/PEI and H2O/NMP/PEI systems at 40 °C. As illustrated, in addition of homogeneous domain (I), phase diagrams contains two other distinct regions known as metastable (II) and unstable (III). The mechanism through which phase separation proceeds is different for these regions. In metastable section, liquid−liquid phase demixing is governed by nucleation and growth whereas spinodal is dominant mechanism in unstable region. Well representation of experimentally determined cloud points with binodal curves as evidenced by phase diagrams included in Figure 1, denotes to predominant role of interaction parameters to predict phase behavior of polymer solutions destabilized by nonsolvent. Comparison of phase diagrams of H2O/DMF/PEI and H2O/DMAc/PEI ternary systems reveals that their phase diagrams are similar, while both of them exhibit significant difference in miscibility area (region I) with water/NMP/PEI system. On the other hand, the homogeneous region for ternary system composed of NMP is larger than that of two other systems based on DMF and DMAc, corresponding to higher amount of nonsolvent (water) required inducing phase separation. For the sake of more clarity, the miscibility area of these systems has been compared in Figure 2; it verifies the similarity in phase behavior when DMF and DMAc are used as solvent and highlights a discrepancy once NMP is considered as solvent for making PEI solution. This difference in the nature of miscibility area can be attributed to solvent/polymer and probably nonsolvent/solvent interaction. The variation of these parameters with composition has been depicted in Figure 3. As evident from this figure, there exist remarkable differences between solvent/PEI as well as water/solvent interaction parameters when the performance of DMF or DMAc is compared with NMP. However, these parameters include approximately the same value when DMF or DMAc are employed to make PEI solutions which leads to similar phase behavior for water/DMF/PEI and water/DMAc/PEI systems, as shown in Figures 1a,b and 2. Relatively more favorable interactions between NMP and PEI as expressed by lower values of interaction parameters over the whole composition range (Figure 3b), makes more capable of dissolving nonsolvent in PEI solution at which phase demixing could occur. This factor, that is, lower solvent/ polymer interaction parameter dominantly favors the mixing of ternary system which corresponds to larger miscibility area for water/NMP/PEI rather than water/DMF/PEI or water/ DMAc/PEI as presented in Figures 1 and 2. Besides, the higher values of interaction parameter for water/NMP

3. RESULTS AND DISCUSSION During electrospinning, traveling jet experiences continues stretching from nozzle to collector which is implemented by solvent drying, as well as uptake of water vapor molecules from surrounding atmosphere. Both of these phenomena would contribute to increase the free energy of spinning solution. As a matter of fact, the increment of free energy is not thermodynamically favorable and makes the system instable. This instability can be greatly diminished by separation into two phases, that is, polymer-rich and solvent-rich phases. The former makes the polymeric matrix in which the solvent-rich domains have been dispersed. Solutions composed of amorphous polymers are known to phase separate via liquid− liquid demixing while in the case of semicrystalline polymers phase separation from a homogeneous solution can proceed via both liquid−liquid and solid−liquid demixing mechanisms.49 Depending on the experimental circumstances two mechanisms are possible for phase separation, that is, nucleation and growth (NG) and spinodal decomposition (SD). The former leads to isolated pores while interconnected network of pores are created as a result of spinodal mechanism.50 Phase behavior of homogeneous polymer solution exposed to different concentrations of nonsolvent vapor, known as vapor-induce phase separation (VIPS),51 can be discussed in terms of thermodynamic interactions of dope components visualized by ternary phase diagram. 3.1. Ternary phase diagrams. Three sets of interaction parameters for H2O/DMF/PEI, H2O/DMAc/PEI, and H2O/ NMP/PEI system at 40 °C required for constructing the ternary phase diagram have been tabulated in Table 1. Note that our previous works on determination of solvent/polymer interaction parameter using VPO approach, revealed no remarkable temperature-dependency exists.42 Additionally, although H2O/NMP interaction parameter obtained from literature46 has not been reported for 40 °C, but reasonable agreement with experimental cloud points was established. 237

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Figure 1. Calculated ternary phase diagram for (a) water/DMF/PEI, (b) water/DMAc/PEI, and (c) water/NMP/PEI at 40 °C in comparison with experimental data. Included in the diagrams are the experimentally measured cloud points (full circle), the calculated binodal curves (full line), spinodal curves (dashed line) and tie lines (thin dotted lines). Three regions in diagrams are homogeneous (I), metastable (II) and unstable (III) regions.

3.2. Surface and Interior Morphologies of PEI Electrospun Fibers. Recent studies17,21,28 have emphasized on the occurrence of VIPS process during electrospinning of dopes composed of nonvolatile solvents. In our previous research work21 on morphology evolution in polystyrene fibers during electrospinning, we qualitatively demonstrated that mechanism, through which morphology of electrospun fibers evolves, depends on the path under which the composition of spinning dope changes in homogeneous region. Pai et al.17 could successfully rationalize the evolved morphology in fibers electrospun from 30 wt.% PS/DMF solutions under humid atmosphere using calculated mass transfer pathways. Polyetherimide used in this work is an amorphous polymer from which prepared solutions in different solvents are capable of undergoing liquid−liquid demixing in the presence of nonsolvent. Once electrospinning jet comes into contact with surrounding atmosphere, mass exchange of solvent and nonsolvent vapor brings the electrospinning solution into either metastable or unstable states in which its morphology can be controlled by phase separation mechanisms. In other words, liquid−liquid demixing plays the pivotal role on morphology evolution of fibers electrospun from solutions of amorphous polymers. Hence, it seems reasonable to postulate that water vapor concentration adjacent the electrospinning jet along with phase behavior of polymer solution affects the morphology of resultant fibers. Figure 4 shows SEM images captured from surface of fibers electrospun from PEI/DMF, PEI/DMAc and PEI/NMP solutions under atmospheres of constant temperature, i.e., 40 °C and varying RHs.

Figure 2. Comparison of miscibility area in the phase diagram of water/DMF/PEI, water/DMAc/PEI, and water/NMP/PEI ternary systems.

compared to water/DMF or water/DMAc over the entire concentration range (Figure 3a) indicates lower tendency of NMP to mix with water and may also contribute to enlarge the miscibility area in water/NMP/PEI system. Our investigations on the qualitative as well as quantitative influence of solvent/polymer along with nonsolvent/solvent interaction parameters on phase diagram are in good accordance with theoretical calculations argued extensively in literature.43,44 238

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Figure 3. Variation of (a) nonsolvent/solvent, as well as (b) solvent/polymer interaction parameters with composition for different systems.

Figure 4. Surface morphology of fibers electrospun from 20 wt.% PEI/DMF (a, b, c, d), PEI/DMAc (e, f, g, h), and PEI/NMP (i, j, k, l) solutions under environmental conditions of 40 °C and (a, e, i) 5%, (b, f, j) 20%, (c, g, k) 40%, and (d, h, l) 60% RH.

persists to continue at RH of 20% and even 40% for fibers produced from NMP solutions (Figure 4j, k) and further increasing of RH to value of 60% causes to slight surface roughness embedding shallow small pores. In the case of two other solvents, DMF and DMAc, surface development goes in a different manner. On the other hand, by changing solvent from NMP to DMF or DMAc, in electrospun fiber smooth surface is

From Figure 4, one can witness the impression of solvent on surface structure as well as fiber diameter of electrospun PEI fibers. Additionally, for a given solvent surface features as well as fiber diameters vary with operating RH. Changing fiber diameter with solvent and RH will be discussed in section 3.3. Regardless of used solvent, at RH of 5% electrospinning leads to fibers of texture-less surfaces (Figures 4a, e, i). This feature 239

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missed. This is why formation of pores is restricted to the surface of these fibers. In contrast to NMP, fibers electrospun from DMF solutions at RH = 20% and higher values of RH experience high asperities on their surfaces resembling bicontinuous structure which is characteristic feature of SD mechanism. Considering short times required for SD and NG as evident from small miscibility areas (Figures 1 and 2), appearing phase separated structure on the surface of these fibers is not wondering. Furthermore, it can be said that the track through which composition of these fibers has changed, passes probably from the metastable region and ends in the unstable area of phase diagram. Thus isolated pores are replaced by cocontinuous interconnected network of pores on the surface of fibers produced from PEI/DMF solutions under humid atmosphere. Strictly speaking, morphology evolution associated with these fibers may be explained by SD mechanism but more evidence is still needed to clarify phase separation mechanism. SEM images captured from cross-section of fibers electrospun from PEI/DMF solutions (Figure 6) reveals once electrospinning is performed under RH = 40% and RH = 60%, developing the bicontinuous morphology is not limited to the interfacial surface layer and continues to form the interior skeleton of fibers. While at RH = 20%, electrospinning leads to fibers with solid cross-section (Figure 6a) indicating no phase inversion is experienced into these fibers. Contrarily, as shown in Figure 4b surfaces of these fibers benefit from rough morphology which is an indicative of precipitation. This contrast between surface and interior morphology can be presumably discussed considering high concentration of water near the surface layer which balances the long time required for phase demixing. It is likely such a high concentration of water is not available for the solution below the surface layer and hence, precipitation becomes unlikely phenomena. As stated earlier, fibers produced from DMF and DMAc solutions can be investigated in a same class as a consequence of high similarity disclosed for their phase behaviors in the presence of water as nonsolvent and illustrated in Figures 1a, b. Physical properties of these solvents as included in Table 2 are also approximately similar. Thus, there is no reason to observe different morphologies for these fibers. But comparison SEM images obtained from surfaces of these fibers (Figure 4) demonstrates that surface roughness reduces when DMF is replaced by DMAc. In the case of interior morphology, fibers produced from PEI/DMAc solutions display solid cross-section at RH=20% and porous morphology containing cocontinuous networks of pores at RH = 40% and 60% (Figure 7) which is qualitatively the same as observed for fibers resulted from PEI/ DMF solutions at corresponding RHs (Figure 6). The difference between surface morphologies of PEI fibers electrospun from DMF and DMAc-based solutions can be rationalized in terms of principles of spinodal decomposition. Several authors studied54−57 occurrence and evolution of SD during membrane formation. They found SD leads to cocontinuous network of interconnected pores which its retainment is threatened with coarsening and coalescence of phase separated domains. Additionally, the evolution of nascent bicontinuous structure in the air−solution interface and crosssection of membrane goes in different manners. At crosssection only reduction of interfacial tension between polymerrich and polymer-lean domains acts as driving force for coarsening while at air−solution interface the surface tension of casting solution becomes also important and further improves

replaced by irregular surface morphology which is an increasing function of RH and varies in roughness with used solvent, DMF or DMAc. Hence, according to obtained experimental results, evolving surface morphology in PEI fibers at varying levels of RH can be investigated in two different categories depending on solvent type: fibers produced from NMP solution contain the first category, while the second category is filled with those electrospun from DMF or DMAc solutions. Considering the comparable physical properties of employed solvents (Table 2) Table 2. Physical properties of DMF, NMP, and DMAc23,52,53 physical property

molecular weight (g/mol)

boiling point (°C)

.density (g/cm3)

dipole moment (debye)

dielectric constant

DMF NMP DMAc

73.1 99.1 87.1

153.0 202.0 165.0

0.95 1.03 0.94

3.8 4.1 3.7

37.06 33 37.78

and different phase behavior of ternary systems (Figure 1), such classification for evolution of surface morphology can be discussed in terms of occurrence of liquid−liquid phase inversion. For both classes, when electrospinning is performed under RH of 5%, phase demixing does not contribute to morphology of resultant fibers and leads to smooth surfaces (Figures 4a, e, i). This corresponds to mass transfer path for RH = 5% does not cross the binodal curve. Fibers obtained from NMP-based solutions exhibit smooth surfaces even at RH = 40%. It seems reasonable to postulate that time of phase demixing for PEI solutions made of NMP, is too long due to large miscibility area in ternary phase diagram, making it impossible to impart the fiber with phase-separated morphology prior to fixing its structure. At this condition, the morphology of fibers is dominantly determined by solvent drying phenomena. Further increasing of RH to 60% reduces time of phase separation and creates some small isolated pores on the fiber surface. Here, it can be said that solvent drying and phase demixing take place at comparable rates. The observation of the interior morphology of these fibers (Figure 5) verifies

Figure 5. Cross-sectional SEM images of fibers electrospun from 20 wt.% PEI/NMP solution under environmental conditions of 40 °C and (a) 40% and (b) 60% RH.

our findings; solid cross-section of fibers electrospun at RH = 40% and 60% excludes the possibility of phase separation in the interior structure of fibers. Strictly speaking, at RH = 60%, the concentration of water at the surface reaches the value needed for precipitation but beneath the surface layer the required amount of water is not available and phase demixing is probably 240

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Figure 6. Cross-sectional SEM images of fibers electrospun from 20 wt. % PEI/DMF solution under environmental conditions of 40 °C and (a) 20%, (b) 40%, and (c) 60% RH.

Figure 7. Cross-sectional SEM images of fibers electrospun from 20 wt.% PEI/DMAc solution under environmental conditions of 40 °C and (a) 20%, (b) 40%, and (c) 60% RH.

different solvents under RH ranging from 5% to 60% are shown in Figure 8. Electrospinning of PEI/DMF, as well as PEI/ DMAc solutions at low-humidity environment (RH = 5%) leads elongated irregularly shaped beads as well as very fine fibers to be produced (see Figure 8a, e). By further increasing RH to values 20%, 40% and 60%, beadless fibers are generated which differ in diameter. By changing solvent to NMP, analogous to PEI/DMF and PEI/DMAc solutions beads and too fine fibers form dominant morphologies when electrospinning is performed under RH = 5% and even continue to exist despite raising RH to 20% (Figure 8i, j). Additionally, comparing Figure 8a, e with Figures 8i, j demonstrates that obtained beads are different in both size and shape. Such that beads produced from PEI/NMP solutions are collapsed spheres whereas those belong to webs of PEI/DMF and PEI/DMAc have irregularly shaped structure and are very large in size. Another feature to note is the diameter of fibers electrospun from these solutions under various humid circumstances. As depicted in Figure 9, fibers obtained from PEI/DMF solutions are evaluated larger in diameter than those produced from PEI/ DMAc and PEI/NMP solutions regardless of operating RH. Exceptionally, when RH increases to 60%, both DMF and DMAc lead to fibers which are the same in diameter. Besides, without paying attention to the used solvent, average diameters

the driving force for evolution which eventually leads to a skin with isolated pores.57,58 Tsai et al.57 showed that growth of phase separated domains can be effectively prevented by increasing viscoelasticity of polymer-rich phase. Therefore, increased viscosity of polymer-rich domains in water/DMF/ PEI system compared to that of water/DMAc/PEI system retards the growth and coarsening of separated areas prior to fixing of fiber structure and presumably accounts for difference in surface morphologies of PEI fibers electrospun from DMF and DMAc-based solutions. In other words, nascent lacy surface structure in fibers produced from PEI/DMAc solution can experience coarsening for longer times and consequently less roughness on the surface of resultant fiber is expected. Our previous research20,21 on morphology of PS fibers electrospun from DMF solutions would further verify this consequence. Developing of smooth skin populated with small isolated pores in the surface of PS fibers electrospun at humid atmosphere can be attributed to coarsening of nascent bicontinuous structure. Here, it may be said that viscoelasticity of polymer-rich domains developed after occurrence of phase separation in water/DMF/PS system is less than those of both water/DMF/ PEI and water/DMAc/PEI ternary systems. 3.3. Properties of PEI Electrospun Webs. SEM images captured from webs electrospun from solutions of PEI in 241

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Figure 8. SEM micrographs of webs electrospun from 20 wt.% PEI/DMF, PEI/DMAc and PEI/NMP solutions under environmental conditions of 40 °C and (a, e, i) 5%, (b, f, j) 20%, (c, g, k) 40%, and (d, h, l) 60% relative humidity.

operating RH is more pronounced for the case of DMF solutions compared to DMAc and NMP solutions. Since spinning parameters including applied voltage, working distance and feed rate have been chosen to be the same for all solutions, the resultant differences in morphology of fibers and beads can be assigned to either solvent physical properties or phase behavior of ternary systems. The former was compared in Table 2. As evident from Table 2, employed solvents, DMF, DMAc and NMP, show no significant difference in their physical properties and their dipole moments and dielectric constants are in acceptable range for electrospinning. However, as illustrated in Figure 3, the interactions of these solvents with other components, PEI and water, are different. These differences could account for the different phase behavior of ternary systems. In other words, constructed phase diagrams provide a powerful tool to get insight how environmental conditions can be optimized to obtain uniform bead-less fibers with specific morphology. Regarding correlation between our observations and bead formation, one can conclude that bead development can be controlled by rate of phase separation which is governed by solvent type and ambient RH. In literature,59 capillary

Figure 9. Variation of average (a) as well as standard deviation (b) of measured diameters of fibers electrospun from PEI/DMF, PEI/DMAc and PEI/NMP solutions as a function of operating RH.

of fibers, as well as corresponding standard deviations, show ascending orders when RH goes up, as illustrated in Figure 9. However, one can witness that impressibility of average and standard deviation of fiber diameter and its distribution from 242

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polymer-rich domains as studied by Tsai et al.57 However, further evidence is still needed to prove this correlation.

instability has been introduced as the main parameter dominating bead evolution and morphology. This event emerges from surface tension of polymer solutions and must be prevented to stabilize fiber formation. Electrospinning parameters such as solution concentration, polymer molecular weight, solution viscosity as well as solvent volatility have been discussed as dominant factors influencing capillary instability.60,61 To be exact, it has been demonstrated that this type of instability can be effectively suppressed by viscoelastic stresses.62 These forces may reach an optimum value when electrospinning jet gradually becomes solidified and, hence, bead formation is prevented. Solvent drying and liquid−liquid demixing are two exclusive approaches involved in electrospinning process through which solidification can be brought about. In the case of nonvolatile solvents considered here, DMF, DMAc, and NMP, the former can not contribute to solidify jetted solution and the latter phenomena, phase demixing, will find great importance. As expected for systems made of DMF and NMP at RH = 5% composition path settles on miscibility area and do not intersect binodal curve; hence, bead formation is inevitable (Figure 8a, i). By increasing RH to 20%, phase separation precedes capillary instability and beads may cease to form (Figure 8b). However, increased size of miscibility area in H2O/NMP/PEI systems (Figures 1c) and delayed phase demixing, causes the composition path and binodal not to meet at 20% RH and so raises RH value at which stabilized fiber formation is established. For both systems, electrospinning at 40% RH and higher is followed by uniform beadless fibers. It seems fair to postulate that minimum RH under which uniform beadless fibers can be electrospun is a case-sensitive factor depending upon phase behavior of nonsolvent/solvent/polymer ternary system. Experimental results (Figure 8) as well as similar phase behavior of solutions made of DMF and DMAc (Figures 1a, b) demonstrate that bead formation follows from the same rules in these solutions. Similar results were obtained during electrospinning of PS/ DMF solutions at various environmental conditions.21 Interpretation regarding increment of fiber diameter with RH can also be provided by considering rate of phase demixing. Higher RH causes solidification to accelerate and thereby, further stretching of electrospinning jet as a consequence of whipping instability is prevented and thicker fibers are produced. In the case of PEI/DMF solutions, solidification at a given RH takes place very rapidly compared to PEI/NMP solutions and fibers of larger diameters as well as broader distribution are expected (see Figure 9). In other words, during electrospinning with PEI/NMP solution, the solution jet has more time to be stretched after leaving the nozzle but in the case of PEI/DMF solutions, solidification occurs shortly after nozzle and further stretching becomes impossible. This can be best described by nature of miscibility area evaluated for ternary systems under investigation. The remarkable differences in mean and standard deviation values of diameters of fibers electrospun from DMF and DMAc solutions can be probably assigned to the reduced viscosity of polymer-rich phase in water/DMAc/PEI system in comparison with water/DMF/PEI which was investigated in preceding section to rationalize dissimilarity in surface morphology of fibers despite of alike phase behavior. As evident from Figure 9, these differences vanish as RH approaches to 60% and nonsolvent concentration in the jet increases. This highlights the contribution of nonsolvent concentration to the viscoelastic properties of

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CONCLUSION Phase diagram of nonsolvent/solvent/polymer system was considered as a powerful and promising tool to provide insight into morphology evolution during electrospinning. To this end, PEI was electrospun from nonvolatile solvents, DMF, DMAc and NMP, under environments of 40 °C and various RHs ranging from 5% to 60%. The results presented in this contribution demonstrate that morphology of electrospun fibers can be designed by altering two factors which are dominantly determined by solvent type: size of miscibility area and kinetics of phase separation. The former is a direct consequence of thermodynamic interaction of solvent with other components of spinning dope, polymer and nonsolvent, and the latter is a strong function of operating RH and goes to be quick when RH increases. The above-mentioned factors contribute to electrospun fiber morphology by affecting time of phase inversion at a given RH. It was shown it is possible to obtain porosity within electrospun PEI fibers when DMF or DMAc is used as solvent and electrospinning is performed at RH = 40% or higher because of induction of phase inversion while fibers produced from NMP-based solutions exhibited solid cross-section with shallow surface pores although the electrospinning was conducted at high RH, i.e., 60%. The occurrence of phase separation in fibers was discussed based on competition between solvent drying and liquid−liquid phase demixing which will find great importance for fibers of small diameters. Evolution of surface morphology was interpreted by the fact that retainment of nascent lacy bicontinuous structure formed after occurrence of SD can be threatened by coarsening and growth phenomena of polymer-rich domains. Finally, it was emphasized that low extreme of required RH to stabilize fiber formation and prevent bead appearing reduces once homogeneous region gets smaller and fiber diameter is affected by operating RH depending on solvent type.

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APPENDIX A Calculation of water/PEI interaction parameter based on GRP model In GRP model, φFV can be taken from ref 47. In this model, β, plasticization factor, is a function of temperature defined as eq A.1. β=

TgM − Tg3 T − Tg3

(A.1)

where TgM and Tg3 are glass transition temperatures of mixture and polymer. T is the operation temperature. The eq A.1 is only valid for TgM > T. For TgM ≤ T, β = 1.On the other hand β ranges between 0 (for a completely glassy like state) and 1 (for a completely rubber like state). TgM can be calculated according to eq A.2. TgM =

x1ΔCp1Tg1 + x3ΔCp3Tg3 x1ΔCp1 + x3ΔCp3

(A.2)

where Tgi and ΔCpi are the glass transition temperature of component i and the increment change in heat capacity at Tgi, respectively. xi = Ni/(N1 + N3) represents the mole fraction of component i (N1 and N3 are number of absorbed nonsolvent molecules and polymer segments, respectively). The parame243

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ters required to estimate TgM and β have been listed in Table A1.

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Table A1. Required Parameters to Calculate Water/PEI Interaction Parameters water63

Polyetherimide (PEI) Tg3 (K) 488 a

−1

ΔCp3(Tg3) (J mol

a

85.6

−1

K )

Tg1 (K)

ΔCp1(Tg1) (J mol−1 K−1)

138

34.9

47

Measured by differential scanning calorimetry (DSC).

The calculated values of GRP model for water/PEI system at 40 °C have been reported in Table A2. Table A2. Calculated Values of GRP Model for Water/PEI System 40 °C temperature (°C)

water uptake (mgwater/mgpolymer)

TgM (K)

β

φFV

40

0.0154

441.49

0.26

0.130

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +98-21-6454-2600. Fax: +9821-66400245. Notes

The authors declare no competing financial interest.

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