Interplay of Phase Separation and Physical Gelation in Morphology

Article pubs.acs.org/IECR

Interplay of Phase Separation and Physical Gelation in Morphology Evolution within Nanoporous Fibers Electrospun at High Humidity Atmosphere Hossein Fashandi* and Amirreza Ghomi Department of Textile Engineering, Isfahan University of Technology, Isfahan, 84156-83111, Iran ABSTRACT: Electrospinning of polymer solutions under a high humidity environment is studied. It is shown that the porous morphology formed after phase separation can be preserved or collapsed depending on three factors: phase demixing time, physical gelation, and viscoelastic properties of the polymer-rich phase. Fibers with rough surfaces and nonporous cross sections are produced when poly(ether imide)/dimethylformamide is electrospun. This is due to accelerated vitrification-related gelation of the water/dimethylformamide/poly(ether imide) system on the fiber surface. Similar phase behavior can be expected for a ternary system based on polystyrene. However, the new system results in fibers with smooth surfaces and porous cross sections. This discrepancy can be resolved by considering delayed gelation as well as lower elastic and loss moduli of the polymer-rich phase in the latter system. Further evidence is also provided by poly(ether sulfone) and polysulfone. Crystallization-induced gelation observed for poly(vinylidene fluoride) fibers can well account for the obtained morphology. However, crystallizationinduced gelation cannot lock in the fiber morphology similar to the vitrification-related gelation.

1. INTRODUCTION Nowadays, producing nanoporous fibers with diameters ranging between a few nanometers and a few micrometers is an area of constant development. These new and advanced nanoscale materials provide extremely high specific surface areas to meet the requirements of a variety of scientific and industrial applications including sensing materials,1 catalytic systems,2 filtration,3,4 hydrogen storage systems,5 protective clothing,6 drug delivery and tissue engineering,7 oil absorption,8,9 and recently sportswear, activewear, and workwear.10 Electrospinning provides a facile and versatile route to obtain fine nanoporous fibers. In this process, a high voltage electrical field is applied to a polymer solution ejected from a nozzle with specific polarity. When the electrostatic forces exceed the surface tension and viscose force of the polymer solution, the charged solution jet is elongated and accelerated toward the collector. In the path from nozzle to collector, the solution jet undergoes continuous stretching accompanied by solvent drying.4,11,12 Liquid−liquid (L−L) phase separation has been undoubtedly accepted as a well-known technique to prepare porous polymeric structures. In this process, a thermodynamically unstable polymer solution separates into two coexisting liquid phases in thermodynamic equilibrium. One of these phases, i.e., the solvent-rich phase, eventually leads to pores distributed in a matrix made of the other phase, viz., the polymer-rich phase. Temperature variations and absorption of nonsolvent are two events which are mainly held responsible for the thermodynamic instability of a polymer solution. The former is discussed as temperature-induced phase separation (TIPS), while the latter is known as nonsolvent-induced phase separation (NIPS). Depending on the physical form of nonsolvent, NIPS can in turn be categorized into two classes: VIPS (vapor-induced phase separation) and LIPS (liquid-induced phase separation).13−15 © 2014 American Chemical Society

Different L−L phase separation mechanisms involved in electrospinning have been the subject of tremendous research,16−19 among which a considerable effort has been devoted to gaining insight into how VIPS can contribute to porosity evolution within electrospun fibers.16,19−22 VIPS is triggered by absorption of nonsolvent molecules from the vapor phase into the polymer solution. The process can only be observed for solutions made of low-volatility solvents.15 For solutions composed of volatile solvents, nonsolvent droplets are condensed on the air−solution interface due to evaporative cooling of polymeric surface. The condensed droplets leave some imprints known as breath figures on the cooled surface.16,23 Tailoring the morphology of membranes prepared by the VIPS process has been extensively explored by several authors. A theoretical model for predicting the mass transfer pathway of three components during membrane formation precipitated from the vapor phase was developed by Matsuyama et al.24 and Yip and McHugh.25 Recently, Bouyer et al.26 used a model similar to the one presented by Yip and McHugh to tune the morphology of membranes prepared using VIPS from the poly(ether imide) (PEI)/N-methyl-2-pyrrolidone (NMP)/ water system. Membrane formation via the VIPS process was also argued by Su et al.15 They considered experimentally calculated mass transfer paths to support the occurrence of spinodal decomposition. In another work,27 collapsing the nascent lacy (bicontinuous) structure into cellular structure was explained regarding competition between the growth rate of the polymer-rich phase and its gelation rate. The lacy structure could be preserved when gelation surpasses the domain growth. Received: Revised: Accepted: Published: 240

September 29, 2014 December 17, 2014 December 19, 2014 December 19, 2014 DOI: 10.1021/ie503848v Ind. Eng. Chem. Res. 2015, 54, 240−253

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Industrial & Engineering Chemistry Research According to our previous investigations,16,20,22 the morphology of fibers electrospun at humid atmosphere is chiefly controlled by competition between solvent drying and L−L phase inversion, whose contribution to the morphology evolution of fibers varies based on the following factors: • solvent volatility • temperature (T) and relative humidity (RH) of operating environment • size of miscibility area in the phase diagram of nonsolvent/ solvent/polymer ternary system In the case of volatile solvents, regardless of RH, solvent drying precedes L−L phase demixing and the composition path settles on the homogeneous region of the phase diagram. Hence, fibers with nonporous surface and cross section are produced.16 Here, as demonstrated by Pai et al.,28 solvent drying competes against buckling instability to develop the morphology of electrospun fibers. The onset of L−L phase separation is expedited at higher temperatures. This can be well explained based on faster exchange of nonsolvent/solvent from the air−electrospinning jet interface as well as the enlarged miscibility area in the ternary phase diagram. A higher value of RH also speeds up L− L phase inversion more effectively than T.16,20,22,28 The size of the miscibility area affects both fiber diameter and bead formation. A larger homogeneous region allows further stretching of the electrospinning jet due to delayed L−L demixing, and hence, thinner fibers are obtained. From another point of view, capillary instability which is responsible for bead formation is more likely to act when L−L phase inversion is retarded.16,20,22 Literature review reveals, prior to L−L phase demixing, the morphology as well as properties of electrospun fibers can be tailored by controlling various factors as summarized below: • electrospinning parameters including working distance, voltage, and feed rate of polymer solution29,30 • solution properties including solvent type,31 fluid elasticity,32 polymer concentration,29,30 polymer molecular weight,30,33 surface tension of solution,30 and conductivity30 • environmental conditions including T as well as RH16,20,22,28,29,34−36 and vapor concentration of solvent in electrospinning environment35 Following L−L phase demixing, the solution composed of amorphous polymer separates into two distinct domains, i.e., solvent rich and polymer rich, with specific concentration and rheological properties. The latter benefits from much higher polymer concentration and viscosity, both of which slow down the movement of polymer chains and make the polymer-rich phase more susceptible to vitrification. The vitrified phase arrests the solvent-rich phase and causes physical gelation, known as vitrification-related gelation.37,38 In the case of solutions made of semicrystalline polymers, physical gelation may also happen as a result of polymer crystallization. Polymer crystals act as physical junction points to form a threedimensional network. This type of gelation, also called solid− liquid (S−L) phase separation, takes place when enough concentration of nonsolvent is added to the polymer solution.39 Therefore, physical gelation can be induced either after L−L phase demixing or as an independent mechanism, i.e., S−L phase inversion, competing with L−L demixing. Overall, it has been repeatedly demonstrated, regardless of polymer type, that the occurrence of L−L phase separation is necessary to obtain nanoporous bead-free electrospun fibers. However, at high humidity environment (for example, RH

60%) which L−L phase separation happening is inevitable, fibers of various porous morphologies can be electrospun from solutions composed of different polymers dissolved in the same solvent.16,20,22 This is the center of interest in the present contribution which is crucial to affording electrospun fibers the desired morphologies and is still controversial. To this end, solutions of four glassy polymers including polystyrene (PS), poly(ether imide) (PEI), poly(ether sulfone) (PES), and polysulfone (PSf) as well as a semicrystalline polymer, poly(vinylidene fluoride) (PVDF), in a nonvolatile solvent, dimethylformamide (DMF), are exposed to electrospinning under constant temperature (T = 20 °C) and high humidity environment (RH 80%). First, solutions are characterized for associated ternary phase behaviors and rheological properties of polymer-rich domains through which images captured from surfaces and interior structures of fibers are rationalized. Theoretical investigations are conducted in the framework of determining the ternary phase diagram including the physical gelation boundary.

2. EXPERIMENTAL SECTION 2.1. Materials. The solvents dimethylformamide (DMF), 2pyrrolidone (2P), and dichloromethane (DCM) of analytical grade were purchased from Sigma-Aldrich, Inc. Deionized water was used as nonsolvent. All solvents were used as received without further purification. Two kinds of polymers were investigated in the present research work: 1. Amorphous polymers investigated included polystyrene (PS) from Sigma-Aldrich, Inc. (Mw = 280 000 g/mol); poly(ether imide) (PEI), type Ultem 1000 (General Electric, USA) (Mw = 32 800 g/mol); poly(ether sulfone) (PES), type Ultrason E6020P (BASF, Germany) (Mw = 49 000 g/mol); and polysulfone (PSf), type Udel P-3500 (Ridgefield, CT, USA) (Mw = 50 800 g/mol). 2. A semicrystalline polymer, poly(vinylidene fluoride) (PVDF) (Kynar 710, MFI = 19.0−35.0 (450 °F, g/10 min, ASTM D1238)) (Arkema, U.K.), was also investigated. All polymer samples were dried before use in an oven for 24 h at 70 °C. Used polymers exhibited low water absorption as obvious from high values of the water/polymer interaction parameters listed in Table 2. Therefore, all polymers in this study can be categorized as hydrophobic polymers. 2.2. Ternary Phase Diagram. The Gibbs free energy of mixing (ΔGM) based on Tompa’s expansion40 for ternary systems consisting of nonsolvent/solvent/polymer (eq 1) was considered to construct the thermodynamic phase diagrams of different ternary systems at 20 °C. ΔGM = n1 ln ϕ1 + n2 ln ϕ2 + n3 ln ϕ3 + n1ϕ2g12(u 2) RT + n2ϕ3χ23 (ϕ3) + χ13 n1ϕ3

(1)

In eq 1, subscripts 1, 2, and 3 stand for nonsolvent, solvent, and polymer, respectively. R and T indicate to the gas constant and the absolute temperature, respectively. ni and ϕi denote the number of moles and the volume fraction of component i, respectively. g12(u2) shows the concentration-dependent nonsolvent/solvent interaction parameter depending on the volume fraction u2 = ϕ2/(ϕ1 + ϕ2) of a pseudobinary mixture. g12(u2) can be represented based on either eq 2 (Koningsveld and Kleintjens model) or eq 3.41,42 241

DOI: 10.1021/ie503848v Ind. Eng. Chem. Res. 2015, 54, 240−253

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Industrial & Engineering Chemistry Research g12(u 2) = α0 +

β0 1 − γ0u 2

g12(u 2) = α0 + β0u 2 + γ0u 2 2 + ε0u 2 3 + η0u 2 4

γ* = (2)

Table 1. Glass Transition Temperature (Tg) of Different Used Materials material Tg (K)

PS 378

PES 498

PSf 460

PEI 488

PVDF 238

DMF 129

2P 163

According to the literature,37,38 Tg depression of a polymer is insensitive to nonsolvent content in the polymer solution. Thus, a vitrification line parallel to the solvent−nonsolvent axis was depicted in all ternary phase diagrams. The vitrificationrelated gelation boundary was considered a special tie line, defined as the solidus tie line, that crosses the intersection point of the binodal and the vitrification line. This point is described as Berghmans point.37,38,48 The structure formation region (SFR) superimposed on ternary phase diagrams was regarded as an area limited by the vitrification-related gelation boundary and the binodal curve.48 The crystallization-induced gelation boundary at 20 °C was measured for the water/DMF/PVDF system. For this purpose a given quantity of PVDF was mixed with a specific solvent (DMF) in a sealed glass bottle. The temperature of the mixture was then raised to completely dissolve the polymer. A specific amount of water was added to this solution. The mixture was agitated at ca. 85 °C to obtain a clear homogeneous solution. After that the solution was maintained in a chamber of constant temperature of 20 °C for at least 10 days. The composition at which clear solution started to precipitate was considered the crystallization-induced gelation point. 2.3. Rheological Measurements. A Physica MCR 300 rheometer (Anton Paar) with parallel plate geometry of diameter 50 mm was used to evaluate viscoelastic properties of polymer-rich phases. The measurements were taken under conditions of fixed angular frequency (ω) of 100 rad/s and strains scanned from 0.1 to 10% within the linear viscoelastic regime of the sample. The temperature was kept constant at 20 °C over all experiment time. To obtain polymer-rich phases, a polymer mixture containing nonsolvent/solvent/polymer of volume fractions 0.05/0.70/ 0.25 was prepared. The mixture was allowed to separated into two phases for 2 weeks. Then the upper phase was removed and the lower one was characterized for its rheological characteristics. Note that the selected composition is located in the SFR for all systems. 2.4. Electrospinning. Fibers were electrospun from solutions of concentration 20 wt % prepared at room temperature by stirring a given amount of polymer in solvent (DMF or 2P) for at least 24 h. The polymer solution was stored at room temperature for at least 1 day to degas. The degassed solution was loaded in a 1 mL syringe with positively charged needle. All electrospinning experiments were performed in a chamber under controlled environmental conditions, i.e., T and RH, with high accuracy. To produce fibers, the T and RH of the electrospinning chamber were kept constant at 20 ± 1 °C and 80 ± 5%, respectively. Fibers were electrospun in a

3

where v2 (cm /mol) is the molar volume of component 2, DMF. Values of δ2:DMF = 24.8 (MPa)0.5 and δ3:PVDF = 23.2 (MPa)0.5 were obtained from ref 44. χ13 in eq 1 is the nonsolvent (water)/polymer interaction parameter, often assumed as a constant and measured based on water uptake (at T = 20 °C) of a film cast from polymer/DCM solutions. The procedure has been thoroughly explained in refs 16, 20, and 22. More information about the mathematical/ numerical treatment to obtain binodal and spinodal curves as well as tie lines may be found, e.g., in ref 42. To further examine the validity of the calculated binodal curves, cloud points of different ternary systems were measured at constant temperature, i.e., 20 °C. For this purpose, differently concentrated polymer solutions (1, 2, 5, 10, 15, and 20 wt %) were prepared by dissolving required amounts of polymer granules in solvent (DMF or 2P). The polymer solutions were stored at room temperature for at least 1 day to degas. The titration method was used to calculate cloud points of degassed solutions and was performed at 20 °C inside a chamber with controlled temperature. Before titration, polymer solutions as well as titration solution were kept in the chamber under gentle stirring at the temperature of interest (20 °C) for at least 2 h to reach equilibrium conditions. Titration was begun by introducing one droplet of titration solution to the polymer solution under stirring. Before the addition of more titration solution, the cloudy solution was agitated to become homogeneous again. Otherwise the obtained turbid point was regarded as the onset of the cloud point and its composition was determined by calculating the amount of polymer, solvent (DMF or 2P), and nonsolvent (water) in the turbid mixture. In the case of amorphous polymers, the vitrification line at T = 20 °C was obtained theoretically by calculating the composition of solvent and polymer whose glass transition temperature (Tg) equals the operating temperature, T = 20 °C. The Tg depression of fiber-forming systems (polymer and DMF or 2P) was evaluated using eq 5.38,45 αT ϕ2Tg2 + ϕ3Tg3 αT ϕ2 + ϕ3

(6)

In eq 6, Tm, Tb, and Tg stand for the melting, boiling, and glass transition temperatures of a solvent, respectively. γ* has been estimated to be 1.36.47 Tg’s of the employed polymers and solvents have been tabulated in Table 1.

(3)

where α0, β0, γ0, ε0, and η0 are constants. In the present contribution, as listed in Table 2, solvent (DMF)/amorphous polymer (PS, PEI, PES, or PSf) interaction parameters (χ23) at 20 °C were collected from our previous papers16,20,22,43 based on vapor pressure osmometry (VPO) data. χDMF/PVDF was roughly estimated based on solubility parameters (δ) using eq 4.24 v χ23 = 0.35 + 2 (δ2 − δ3)2 (4) RT

Tg =

Tm + Tb T − Tb + 0.6 m Tg + Tb Tm + Tb

(5)

where ϕi and Tgi denote the volume fraction and glass transition temperature of component i (i = 2, solvent; i = 3, polymer), respectively. αT is related to the thermal expansion coefficients of different components and is generally taken to be 2.1.45 Tg’s of amorphous polymers were measured using differential scanning calorimetry (DSC). The Tg of DMF was obtained from ref 46, while this parameter for 2P was estimated using an empirical equation (eq 6) derived by Li.47 242


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Industrial & Engineering Chemistry Research Table 2. Interaction Parameters for Different Ternary Systems at 20 °Ca ternary system

water/2P/PSf

T (°C)

20

χ13b

g12(u2)

0.32 +

0.47 1 + 0.50u 2

χ23

2.5

0.527

3.0

0.43c

0.276 1 + 0.622u 2

3.1

0.4916

0.276 1 + 0.622u 2

2.3

0.366 − 0.061ϕ3 [refs 20, 43]

0.276 1 + 0.622u 2

2.4

0.443 − 0.017ϕ3 [ref 43]

0.276 1 + 0.622u 2

2.5

0.498 + 0.008ϕ3 [refs 22, 43]

[ref 27] water/DMF/PVDF

20

0.50 + 0.04u 2 + 0.80u 2 2 − 1.20u 2 3 + 0.8u 2 4 [ref 42]

water/DMF/PS

20

0.218 + [ref 49]

water/DMF/PES

20

0.218 + [ref 49]

water/DMF/PSf

20

0.218 + [ref 49]

water/DMF/PEI

20

0.218 + [ref 49]

g12, nonsolvent/solvent interaction parameter; χ13, nonsolvent/polymer interaction parameter; χ23, solvent/polymer interaction parameter. b Measured based on water uptake experiment (at T = 20 °C) as explained in section 2.2. cCalculated based on solubility parameters (δ) using eq 4. a

electrospun fibers, and the density of the matrix, i.e., paraffin, allows one to calculate the density of produced fibers. It is worth noting that before density measurement the volume of the vessel should be calibrated by a liquid with a given density. In this work, distilled water with specific density at the working temperature was used as standard liquid to calibrate the volume of the vessel. All density measurements were carried out at a constant temperature of 25 °C. The results have been averaged over five measurements for each sample. Details of mathematical calculations have been described elsewhere.16 2.5.3. X-ray Diffraction. Wide angle X-ray diffraction (WAXD) studies of a PVDF electrospun mat was performed using a Philips X-ray diffraction model X’Pert-MPD. The start angle, the end angle, and the step size were 5°, 50°, and 0.04°, respectively. The WAXD scan was recorded with Cu Kα radiation (λ = 1.54 Å) generated at 40 kV and 30 mA. 2.5.4. FTIR Spectroscopy. The variation of crystal polymorphism in the structure of PVDF nanofibers was also verified by recording the attenuated total reflectance (ATR) spectrum using a BOMEM FTIR MB-series, MB-100 (Hartmann & Braun, Canada). The electrospun web was put directly on the surface of a flat crystal (ZnSe, a 45° ATR prism). The spectrum was measured at a wavenumber resolution of 4 cm−1 for a spectral range from 4000 to 400 cm−1.

horizontal setup in which a negatively charged aluminum foil was used as collector. The working distance (nozzle-tocollector distance) (wd) was set at 15 cm, and a potential of 18 kV was applied to the needle of a syringe by a high voltage power supply. A syringe pump (TOP-5300) was used to supply a steady flow of 0.5 mL/h polymer solution to the tip of the needle. 2.5. Characterization of Electrospun Nanofibers. 2.5.1. SEM Analysis. Surface morphology and the cross section of electrospun fibers were observed using a scanning electron microscope (SEM; TESCAN series VEGA 2007 from Czech) performed at 30 kV acceleration voltage. To monitor the interior structure (cross section) of fibers using SEM imaging, fibers were integrated in a composite such that a surrounding layer of polymer matrix tightly encompasses the fibers. The composite was then broken in liquid nitrogen and observed using SEM. Before SEM analysis samples were coated with a 10 nm layer of gold. Based on SEM images of produced webs, diameters of fibers were also determined using an image analysis software. The mean and standard deviation values were obtained by readying the diameters of 100 randomly selected fibers. 2.5.2. Measurement of Fractional Interior Pore Volume (FPV) of Electrospun Fibers. The density of electrospun fibers was considered as a robust tool to evaluate the total interior pore volume of produced fibers. The method was first successfully applied in our previous work to measure the interior pore volume of electrospun PS fibers.16 For this purpose, fibers were mixed with liquid paraffin in a glassy vessel of 1.7932 cm3 volume to form an integrated composite. Liquid paraffin was chosen as the matrix due to its nonpolar and hydrophobic nature, two essential factors for complete wetting of hydrophobic electrospun fibers. In addition, large paraffin molecules cannot diffuse into the interior porous structure of the fibers. Knowing the total volume of the composite corresponding to the volume of the vessel, the mass of

3. RESULTS AND DISCUSSION Electrospinning of polymer solutions under high humidity environment (RH 80%) is accompanied by solvent outflow and nonsolvent inflow through the air−solution jet interface. During this process the composition of the electrospinning jet is brought into the phase demixing region and VIPS becomes likely.16,24,25,28 After VIPS, one-phase polymer solution is transformed into two coexisting liquid phases, i.e., polymer-rich and polymer-lean phases, which are different in properties from the initial polymer solution. Therefore, after 243


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induced gelation line above which a uniform solution becomes ultimately a gel. Based on data presented in Table 3 and calculated binodal curves and solidus tie lines, SFRs for ternary systems made of different amorphous polymers are highlighted and compared in Figure 1. Compositions located inside this area are demixed into two liquid phases whose polymer-rich phase is not vitrified. Outside the gelation boundary the polymer-rich phase reaches a composition whose Tg exceeds the operating temperature. After drying of the solvent-rich phase, the initial polymer solution finally turns into a porous glass. The mobility of the vitrified polymer-rich phase is low enough to arrest the solvent-rich phase, hinder its coalescence, and lock in the fiber morphology. At this condition, a liquid-containing polymer gel is produced.37,38 As evidenced by phase diagrams included in Figure 1 and data listed in Table 1, as high as the polymer Tg, as small as the SFR. Obviously, the largest and smallest SFRs belong to ternary systems based on PS and PEI, respectively. It should be noted that, although Tg,PES >Tg,PEI, SFRPEI is smaller than SFRPES due to very steep tie lines, particularly the solidus tie line of PEI. The size of SFR can influence the gelation time defined by Li et al.38 as the time interval between the occurrence of L−L demixing and the beginning of vitrification. Strictly speaking, systems with relatively longer associated gelation times have more chance for evolution of morphology after phase inversion. As is obvious, since Tg,PVDF is lower than 20 °C (Table 1), the vitrification boundary cannot be defined for PVDF and hence vitrification-related gelation is a meaningless concept for PVDF. The time of L−L phase demixing (tdem) is another feature deduced by comparison of the locations of the binodals in ternary phase diagrams. tdem is defined as the time taken by composition of the electrospinning jet to meet the binodal. As is obvious, when solvent, nonsolvent, and ambient conditions are the same for all systems, tdem will be predominantly determined by the size of the miscibility area (for amorphous polymers) or the S−L miscibility gap (for the semicrystalline polymer, PVDF). Binodal curves for different polymers have been compared in Figure 2, where DMF and water are used as solvent and nonsolvent, respectively. PEI and PS exhibit phase diagrams which are approximately alike but remarkably different from that of PES. From Figure 2, it can be clearly concluded that solutions of PEI and PS in DMF require almost the same amount of water to meet the requirement for L−L phase demixing. However, PES solution in DMF precipitates when higher amounts of water are available, corresponding to a larger L−L miscibility area. Additionally, in the case of PVDF, the binodal has the greatest distance from the polymer−solvent axis, compared with those for other polymers. The change in binodal location can be investigated by considering differences in the solvent/polymer as well as water/polymer interaction parameters as depicted in Figure 3 and Table 2. As illustrated in Figure 3, χDMF/PS and χDMF/PEI are approximately equal to 0.5 over the whole concentration range. χDMF/Polymer’s for PSf, PES, and PVDF polymers contain values less than 0.5, regardless of polymer concentration. Therefore, DMF can be considered as a good solvent for PSf, PES, and PVDF and a theta solvent for PS and PEI.51 Similarly, 2P acts as a theta solvent for PSf. Furthermore, χDMF/PES contains the smallest values over the entire concentration range denoting a more favorable interaction between DMF and PES compared to PVDF, PEI, PS, and PSf. This corresponds to more nonsolvent (water) for

phase separation and prior to solidification of the jet, the properties of these two phases, particularly the polymer-rich phase, will dictate the final morphology of electrospun fibers. Since the polymer concentration in the polymer-rich phase is too high, it would be crucial to study whether or not polymer type can affect the contribution of the polymer-rich phase to the morphology of electrospun fibers. To this end, exploring the phase behavior of various systems is the first step. 3.1. Phase Diagram of Different Polymer/DMF/Water Ternary Systems. Construction of a ternary phase diagram needs three interaction parameters to be precisely determined as explained in section 2.2. These parameters for ternary systems of interest have been included in Table 2. Furthermore, Table 3 contains vitrification compositions of systems under investigation which are required to draw the vitrification boundary in the phase diagram. Table 3. Vitrification Composition (Tg,composition = 293 K) for Different Polymer/Solvent Systems polymer

solvent

Tg (K)

ϕ2

ϕ3

PS PES PSf PEI PSf

DMF DMF DMF DMF 2P

292.88 292.90 293.12 292.90 292.86

0.22 0.41 0.36 0.39 0.38

0.78 0.59 0.64 0.61 0.62

Ternary phase diagrams of various water/DMF/amorphous polymer (PEI, PS, PES, or PSf) systems have been illustrated in Figure 1 in which miscibility areas are separated from twophase regions by binodal curves. As shown, regardless of polymer type, the experimentally measured cloud points are in quite good agreement with theoretically calculated binodal curves. This implies the accurate determination of binary interaction parameters (Table 2). However, some deviations can be observed for PVDF which may be assigned to PVDF crystallization. When a homogeneous solution in the miscibility area is brought across the binodal into the liquid−liquid (L−L) miscibility gap, the solution has the potential for instability with respect to L−L demixing and separates into two liquid phases in thermodynamic equilibrium, i.e., solvent-rich and polymerrich phases. These two phases are connected through a tie line. The spinodal boundary is located inside the L−L miscibility gap and separates the metastable region from the unstable one. These areas are, respectively, characterized with different phase separation mechanisms, i.e., nucleation and growth (NG) and spinodal decomposition (SD).50 The former leads to isolated pores embedded in a polymer matrix if the polymer-lean phase is nuclei, while SD is responsible for bicontinuous structure. No phase separation is taken for solution if the composition is changing in the miscibility area. Figure 1 also contains the ternary phase diagram of a semicrystalline polymer, PVDF, which includes a crystallizationinduced gelation boundary in addition to binodal and spinodal curves. Solutions of semicrystalline polymers would become thermodynamically unstable with respect to both L−L demixing and polymer crystallization. Hence, the associated ternary phase diagram can be divided into three distinct sections using binodal and crystallization-induced gelation boundaries: solid−liquid (S−L) miscibility gap, L−L miscibility gap, and miscibility area. Gelation of polymer solution happens, if the S−L miscibility gap is entered through the crystallization244


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Figure 1. Phase diagrams of different water/DMF/polymer (PS, PES, PSf, PEI, and PVDF) ternary systems at T = 20 °C. For amorphous polymers (PEI, PS, PES, and PSf) diagrams contain theoretically calculated binodal and spinodal curves, tie lines, vitrification boundaries, and structure formation region (SFR) along with experimentally measured cloud points. For PVDF crystallization-induced gelation data is also added to the ternary diagram.

• path through which the composition of components changes along with the time of phase separation • type of phase separation mechanism, i.e., NG or SD • coarsening and domain growth of phase-separated structures which may be restricted by high viscosity and elasticity of the polymer-rich phase • vitrification boundary, angle of tie lines with solvent− nonsolvent axis, and, totally, size of SFR • crystallization-induced gelation boundary in the case of semicrystalline polymers In the following, the contribution of above-mentioned parameters to the morphology development of electrospun fibers is discussed. 3.2. Morphology of Electrospun Webs and Fibers. Included in Figure 4 are SEM images obtained from mats electrospun from different 20 wt % polymer/DMF solutions at T = 20 °C and RH 80%. As is evident, amorphous polymers, PS, PEI, PSf, and PES, exhibit bead-free fibers (Figure 4a−d),

precipitation. Moreover, though the interaction of water with PVDF and PS contains approximately the same values, this parameter for water/PES, water/PEI, and water/PSf pairs includes smaller values indicative of a higher tendency of PES, PEI, and PSf to mix with water rather than PVDF and PS. This is equivalent to more water needed to bring about phase separation. However, it seems the DMF/polymer interaction parameter predominantly contributes to changing the size of the L−L miscibility gap in ternary systems of PVDF, PS, PEI, or PES/DMF/water, compared to the water/polymer interaction parameter. After rationalizing the location of the binodal in different systems, tdem’s can be compared for various systems; obviously, as large as the miscibility area, the longer the tdem. On the basis of above discussion, it seems reasonable to suspect that the morphology of porous fibers electrospun under high humidity environment might be influenced by the following events: • size of the miscibility area in the ternary phase diagram 245


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electrospinning at RH 80% and prevent bead formation (Figure 4a−d). In the case of PVDF, based on Figure 1, the composition of components in the electrospinning jet changes in a relatively large S−L miscibility gap, where the polymer solution becomes unstable with respect to polymer crystallization. Furthermore, this large S−L immiscibility area makes L−L phase separation less probable to happen in a suitable time interval during electrospinning to stop bead formation. The positive influence of polymer crystallization to prevent bead development becomes clear by comparison of webs electrospun from PVDF/DMF solution (displayed in Figure 4e) with that produced in our previous work (included in ref 20 and redrawn in Figure 4f) from PES/NMP solution under conditions of T = 40 °C and RH 60%. Strictly speaking, although miscibility areas in both water/DMF/PVDF and water/NMP/PES systems are almost the same in size, the web produced from PES/NMP solution contains circular separated beads, resembling structures obtained from the electrospraying process. In contrast, the PVDF web exhibits beads as a part of fibers known as bead-on-string morphology. This may be attributed to formation of PVDF crystals which stabilize fiber formation. However, the induction time required for polymer crystallization is a possible cause of why some beads can be observed in the web electrospun from PVDF/DMF solution. From another point of view, accelerated phase demixing prevents further stretching of the solution jet in the working distance from nozzle to collector and is responsible for making fibers of larger diameter and broader distribution as depicted in Figure 5. As compared in Figure 2, although water/DMF/PS and water/DMF/PEI represent homogeneous regions of approximately equal size, properties of corresponding webs electrospun from PS/DMF and PEI/DMF solutions are clearly different (Figures 4a,b and 5). PS fibers are more uniform in diameter in comparison to PEI fibers. Additionally, it is hard to find a PEI fiber with the same diameter throughout the fiber length. Some beads can be observed along the PEI fibers. These beads are created due to instantaneous formation of PEI-rich phase in the electrospinning jet rather than domination of capillary instability. The former makes fiber stretching impossible and nonuniform fibers are created. Discrepancy between PS and PEI webs becomes more pronounced when surface and cross-sectional morphologies of fibers electrospun from PS/DMF and PEI/DMF solutions at high humidity atmosphere are compared in Figures 6 and 7, respectively. As illustrated in Figure 6, PS fibers benefit from a smooth surface with some isolated pores stretched along the fiber axis, whereas PEI fibers have high asperities on their surfaces. The surface roughness reduces in PSf fibers and becomes more shallow in PES and PVDF ones. PS, PSf, PES, and PVDF fibers are porous in their cross sections, while PEI fibers show a solid nonporous interior morphology (Figure 7). Note that all of these fibers with a broad range of microstructures have been electrospun from one nonvolatile solvent, DMF, under the same environmental conditions, T = 20 °C and RH 80%. Hence, differences in the phase diagram especially SFR are held responsible for the structural discrepancies of produced fibers. In the light of SEM images and phase behaviors of the systems under investigation, for the surface morphology of fibers electrospun at a humid atmosphere, four mechanisms along with corresponding pathways of composition variation in

Figure 2. Comparative illustration of binodal location for different water/DMF/polymer (PS, PES, PSf, PEI, or PVDF) ternary systems at T = 20 °C.

Figure 3. Solvent (DMF or 2P)/polymer (PS, PEI, PSf, PES, or PVDF) interaction parameters as a function of polymer concentration (χ23(ϕ3)) at T = 20 °C.

while the semicrystalline PVDF (Figure 4e) contains beaded fibers. Bead formation during electrospinning is probable once capillary instability originated from the surface tension of polymer solution is permitted to act. This type of instability involved in electrospinning can be overcome by adjusting the viscoelastic properties of the solution jet.52 These properties may be enhanced as a direct consequence of (1) solvent drying, (2) L−L phase separation, or (3) S−L phase separation. Case 1 is only true for solutions composed of volatile solvents and yields to a high concentration of polymer throughout the jet structure. During the second mechanism formation of polymerrich domains is regarded to explain bead disappearance. Totally, systems with larger miscibility areas are more susceptible to bead production due to delayed L−L demixing. The third mechanism triggers nucleation of polymer crystals. These crystals act as junction points between polymer chains which raise the solution elasticity and cease bead production. However, polymer crystals require time to nucleate and growth53 which may disparage the contribution of S−L demixing to developing bead-free fibers. Comparing the associated ternary phase diagrams in Figure 1, one can witness for amorphous polymers, PS, PEI, PSf, and PES, the sizes of associated miscibility areas are small enough to trigger L−L phase demixing in an appropriate time during 246


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Figure 4. SEM images captured from webs electrospun from different 20 wt % polymer solutions: (a) PS/DMF, (b) PEI/DMF, (c) PSf/DMF, (d) PES/DMF, (e) PVDF/DMF, and (f) PES/NMP. Environmental conditions of electrospinning: (a−e) T = 20 °C and RH 80%; (f) T = 40 °C and RH 60%. Image (f) has been selected from our previous work, ref 20. (Adapted with permission from ref 20. Copyright 2014 Springer.)

the minimization of the interfacial tension between polymerlean and polymer-rich domains. This process is further driven by surface tension between the polymer solution and humid air.27 The first mechanism is declined due to the small size of the metastable region as clearly shown in Figure 1. NG needs the polymer solution to stay in the metastable region to initiate nuclei.15 The electrospinning process cannot afford enough time for polymer solution to occur NG, and hence, SD becomes the dominant mechanism. In addition, the surface of the fiber is in intimate contact with humid air. Therefore, sufficient concentration of nonsolvent molecules would be available to move the solution into the unstable region of the phase diagram. The smooth skin cannot be generated after crossing the gelation boundary due to formation of a porous glass. Thus, the fourth mechanism is refused owing to a large SFR. For PEI fibers, the rough porous surface can be described based on the fourth mechanism due to a too-small SFR region in the ternary phase diagram included in Figure 1. The slight size of the SFR reduces the gelation time at which the polymerrich domain vitrifies, and hence, the resultant bicontinuous structure is preserved. The vitrified phase can also account for the large diameter of electrospun PEI fibers. Compared with PEI, PES and PSf fibers exhibit surfaces with slight roughness in spite of SFRs with almost equal sizes (Figure 1). Considering tdem and the visual assessment of surface asperities of PEI, PSf, and PES fibers, an inverse relationship can be established between tdem and surface roughness. On the other hand, a larger miscibility area and longer tdem increase the amount of nonsolvent to enter the gelation area. At this condition, gelation due to vitrification of polymer-rich phase is not likely. Hence, the ability of polymerrich phases to coalesce and grow after the occurrence of SD is believed to be of the most relevance for surface morphology

Figure 5. Averages and standard deviations of diameters of fibers electrospun from different polymer (PS, PEI, PSf, PES, or PVDF)/ DMF solutions at T = 20 °C and RH 80%.

the ternary phase diagram (illustrated in Figure 8) can be suggested: (1) L−L phase demixing based on NG (path I) which causes isolated pores (2) L−L phase demixing based on SD (path II) which may cause interconnected or separated pores depending on whether or not subsequent coarsening of the polymer-rich phase happens (3) crossing the vitrification boundary (path III) which leads to a solid nonporous surface (4) crossing the vitrification-related gelation boundary and formation of a porous glass (path IV) which eventually leads to a porous surface For PS fibers, the smooth skin populated with not joined pores can be interpreted by the second mechanism, according to which the interconnected pores are converted to isolated pores. This is caused by growth of demixed phases, driven by 247


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Figure 6. Surface morphologies of fibers electrospun from different 20 wt % polymer/DMF solutions at T = 20 °C and RH 80%: (a) PS/DMF, (b) PEI/DMF, (c) PSf/DMF, (d) PES/DMF, and (e) PVDF/DMF.

As also displayed in Figure 6e, PVDF fiber has a smooth skin populated with imprints left by water droplets, whereas the cross section of this fiber is constructed from an interconnected network of pores (Figure 7e). On the fiber surface crystallization-induced gelation precedes L−L phase demixing due to high concentration of water molecules near the air−jet interface as well as evaporation of solvent from the surface of the jet which makes it cool. Condensation of water droplets on the cooled skin leaves some holes on it. Furthermore, the gelled skin slows down diffusion of solvent molecules through the skin layer and, consequently, inside the fiber L−L demixing surpasses gelation owing to PVDF crystallization and fiber with a porous cross section is expected (Figure 7e). Formation of PVDF crystals was also confirmed by WAXD and FTIR spectra obtained from PVDF electrospun mat and is shown in Figure 12. The WAXD pattern of PVDF fibers represents three distinct peaks at 2θ around 18.6, 20.3, and 27.1° which correspond to, respectively, α (020), β ((200) (110)) and α (111) reflections of PVDF crystallites.55 In the FTIR absorbance the peaks appearing at 614, 765, and 975 cm−1 are attributed to chain conformations with a short trans (T) sequence, namely αphase, while bands at 840 and 1275 cm−1 are related to polymer chain conformations characterized with a long trans sequence found in β- and γ-phases of PVDF.55 In fact, these two crystalline phases, β and γ, have absorbance bands at similar wavenumbers making it very hard to distinguish between them.56 In the present contribution, the bands at 840 and 1275 cm−1 are considered to follow β-phase evolution, while γ-phase can be best described using bands at 809 and 1233 cm−1. To quantitatively evaluate absorbance bands, all spectra were normalized using the peak at 877 cm−1 as reference band.55 As is clearly shown, PVDF fibers undergo crystallization-induced gelation during electrospinning, but in contrast to vitrificationrelated gelation, this type of gelation does not lock in the PVDF fiber morphology. Stretching of the jet is essential to achieving

evolution of PES and PSf fibers (second mechanism) as observed in the case of PS fibers. Viscoelastic properties of the polymer-rich phase provide insight into the growth rate of the polymer-rich domain. Table 4 contains experimental results of the measurement of the storage modulus and the loss modulus as well as the shear viscosity of polymer-rich phases of different systems. Considering data in Table 4 together with SEM images displayed in Figure 6, one can clearly relate the surface morphology of fibers to viscoelastic properties of corresponding polymer-rich phases. The PS-rich phase with the lowest values of elastic and loss moduli exhibits less resistance to coarsening. This favors electrospun fibers with smooth skins (Figure 6a). In the case of PES and PSf polymers, the rheological properties of PES- and PSf-rich domains are high enough to oppose coarsening and consequently some asperities are created on surfaces of fibers. The lower values of the rheological properties of the PEI-rich phase compared to PSf- and PES-rich phases highlights the greater importance of the SFR to the morphology development of electrospun fibers. SEM images obtained from fibers electrospun from PSf/2P solution at T = 20 °C and RH 80% (Figure 9) provide further evidence to clarify the relationship between the resistance of the polymer-rich phase against evolution and the surface morphology of fibers. As shown in Figures 10 and 11, water/ 2P/PSf is demixed into two phases at the same compositions as water/DMF/PSf; in addition, the corresponding SFRs are almost the same in size (Figure 1). But surface of fiber produced from 2P-based solution (Figure 9b) is more rough than that produced from DMF-based solution (Figure 6c). This can be assigned to the larger values of elastic and loss moduli of the PSf-rich phase when 2P is solvent, as compared in Table 4. However, the mean diameter of fibers produced from 2P-based solution (2.04 μm, standard deviation 0.98) is nearly equal with those electrospun from DMF-based solution as a direct consequence of the same location of the binodal curve in associated ternary phase diagrams (Figure 11). 248


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Figure 7. Interior morphologies of fibers electrospun from different 20 wt % polymer/DMF solutions at T = 20 °C and RH 80%: (a) PS/DMF, (b) PEI/DMF, (c) PSf/DMF, (d) PES/DMF, and (e) PVDF/DMF.

Table 4. Viscoelastic Properties of Polymer-Rich Phases of Different Systems Measured at Fixed Angular Frequency of ω = 100 rad/s and Strain of 10% composition of initial polymer mixture ternary system water/DMF/PS water/DMF/PEI water/DMF/PES water/DMF/PSf water/2P/PSf a

ϕ1

0.05

ϕ2

0.70

ϕ3

G′ (Pa)

G″ (Pa)

ηa (cP)

0.25

25.2 153.5 364.8 298.0 606.2

165.0 682.1 852.7 735.9 1169.1

1657.0 6991.6 9274.6 7939.5 13169.2

Calculated using the equation η = [(G′/ω)2 + (G″/ω)2]0.5.54

the large intensity of β-phase related peaks in WAXD and FTIR spectra, whereas freezing the morphology prevents the electrospinning jet from further stretching. Interior morphologies of fibers electrospun from both amorphous and semicrystalline polymers (Figure 7) can also be rationalized based on phase separation and gelation concepts. The cross sections of PS, PSf, PES, and PVDF fibers all contain interconnected networks of pores which can be best explained based on the SD mechanism. However, the measured fractional interior pore volumes (FPVs) for these fibers are

Figure 8. Schematic pathways (I, II, III, and IV) to rationalize surface morphologies of fibers electrospun at constant temperature and high humidity atmosphere.

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Figure 9. SEM images captured from (a) web and (b) surface of fibers electrospun from 20 wt % PSf/2P solution at T = 20 °C and RH 80%.

Figure 10. Ternary phase diagram of water/2P/PSf ternary system at T = 20 °C (SFR, structure formation region).

Figure 12. WAXD and FTIR spectra of PVDF mat electrospun from 20 wt % PVDF/DMF solution at T = 20 °C and RH 80%.

with a solid cross section, FPVPEI = 0.02. Here, it can be said that gelation of the surface retards the diffusion of nonsolvent through the skin surface to the layers beneath and so SD becomes unlikely. The role of vitrification to slow down nonsolvent penetration was also demonstrated by Lin et al.37 They introduced physical gelation as a new approach to obtaining macrovoid-free membranes.

4. CONCLUSION In conclusion, surface and interior porosities of fibers electrospun at high humidity atmosphere (T = 20 °C and RH 80%) from solutions of amorphous polymers, polystyrene, poly(ether imide), polysulfone, and poly(ether sulfone), dissolved in nonvolatile solvents such as dimethylformamide can be elaborated by considering three main factors: • liquid−liquid phase demixing time (tdem) • size of structure formation region • viscoelastic properties of polymer-rich phase Accelerated liquid−liquid demixing as a result of a small miscibility area as well as a small structure formation region expedites gelation of the fiber surface. The gelled surface slows down the diffusion rate of nonsolvent molecules to layers beneath. Finally, fibers with a rough porous surface and solid cross section are electrospun, as observed for poly(ether imide) fibers. Polystyrene with a phase behavior similar to that of

Figure 11. Comparative illustration of binodal location for water/ DMF/PSf and water/2P/PSf ternary systems at T = 20 °C.

different (FPVPS = 0.22, FPVPSf = 0.12, FPVPES = 0.13, and FPVPVDF =0.15) as is obvious from SEM images. This may be interpreted by coarsening of phase-separated domains which is intensely influenced by viscoelastic properties of polymer-rich domains. The PS-rich phase with the lowest values of elastic and loss moduli (Table 4) results in fibers with the highest porosity, whereas fibers produced from solutions of PSf and PES contain relatively lesser interior porosities. This originates from the high force employed by PSf- and PES-rich phases to oppose coarsening. Similarly, for PVDF electrospun fibers physical gelation due to polymer crystallization can be considered as the main factor accounting for the low value of FPV. Contrarily, vitrification-related gelation endows PEI fibers 250


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Industrial & Engineering Chemistry Research poly(ether imide) results in fibers with a smooth surface and porous cross section due to delayed gelation (large structure formation region) along with subsequent growth of phaseseparated domains. In the case of polysulfone and poly(ether sulfone), delayed liquid−liquid demixing is followed by retarded gelation. Additionally, the viscoelastic properties of polymer-rich domains are high enough to oppose coarsening of polysulfone- and poly(ether sulfone)-rich domains and fibers with slight roughness on their surfaces and porous cross sections are produced. Totally, it can be said phase separation and vitrification-related gelation are two complementary events and together form a promising tool to design fibers with desirable morphology for a specific application when electrospinning is performed under a high humidity atmosphere. In the case of semicrystalline polymers like poly(vinylidene fluoride), observed physical gelation as a result of polymer crystallization can well account for increased elasticity of polymer solution and consequently fibers with a smooth surface and less interior porosity are expected. However, it was observed that crystallization-induced gelation cannot lock in the fiber morphology similar to that observed for vitrificationrelated gelation.

■

Variables

T = absolute temperature (K) Tg = glass transition temperature (K) Tm = melting temperature (K) Tb = boiling temperature (K) RH = relative humidity (%) ΔGM = Gibbs free energy of mixing n = number of moles R = gas constant (8.314 J mol−1 K−1) ϕ = volume fraction χ23 = solvent/polymer interaction parameter χ13 = nonsolvent/polymer interaction parameter g12 = nonsolvent/solvent interaction parameter u2 = volume fraction of a pseudobinary mixture α0, β0, γ0, ε0, η0 = constants required to calculate g12 v = molar volume (cm3 mol−1) δ = solubility parameter (MPa)0.5 G′ = storage modulus (Pa) G″ = loss modulus (Pa) η = shear viscosity (cP) Mw = weight-average molecular weight (g/mol) αT ∼ 2.1 = related to the thermal expansion coefficient of different components ω = angular frequency (rad/s) wt = weight concentration (%) α, β, γ = PVDF crystalline phases tdem = time of L−L phase demixing

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +98-31-3391-1091. Fax: +98-31-33912444.

Indices

Notes

The authors declare no competing financial interest.

■

NOMENCLATURE

■

Acronyms

2P = 2-pyrrolidone ATR = attenuated total reflectance DCM = dichloromethane DMF = N,N-dimethylformamide DSC = differential scanning calorimetry FTIR = Fourier transform infrared spectroscopy FPV = fractional interior pore volume LIPS = liquid-induced phase separation L−L = liquid−liquid MFI = melt flow index (g/10 min) NG = nucleation and growth NIPS = nonsolvent-induced phase separation NMP = N-methylpyrrolidone PEI = poly(ether imide) PES = poly(ether sulfone) PS = polystyrene PSf = polysulfone PVDF = poly(vinylidene fluoride) SD = spinodal decomposition SEM = scanning electron microscope SFR = structure formation region S−L = solid−liquid T, G = trans, gauche conformation of bonds in polymer chain TIPS = temperature-induced phase separation VIPS = vapor-induced phase separation VPO = vapor pressure osmometry WAXD = wide-angle X-ray diffraction wd = working distance (nozzle-to-collector distance) (cm)

1 = nonsolvent (ns) 2 = solvent (s) 3 = polymer (p) M = mixture

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