Interplay of Liquid-liquid and Solid-liquid Phase

Fibers and Polymers 2015, Vol.16, No.2, 326-344 DOI 10.1007/s12221-015-0326-0

ISSN 1229-9197 (print version) ISSN 1875-0052 (electronic version)

Interplay of Liquid-liquid and Solid-liquid Phase Separation Mechanisms in Porosity and Polymorphism Evolution Within Poly(vinylidene fluoride) Nanofibers Hossein Fashandi*, Athar Yegane, and Mohammad Mahdi Abolhasani1 Department of Textile Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran 1 Department of Chemical Engineering, University of Kashan, Kashan, Iran (Received August 8, 2014; Revised October 9, 2014; Accepted October 14, 2014) Abstract: Porosity and polymorphism evolution within poly(vinylidene fluoride) (PVDF) nanofibers is discussed in relation to interplay of liquid-liquid (L-L) and solid-liquid (S-L) phase separation mechanisms. To this end, poly(vinylidene fluoride) (PVDF) solutions composed of nonvolatile solvents, dimethylformamide (DMF) and N-methylpyrrolidone (NMP), are subjected to electrospinning under environmental conditions of constant temperature (T=20 C) and different levels of relative humidity (RH) ranging from 20 to 80 %. It is demonstrated that bead appearance, fiber diameter, porosity formation and polymorphism evolution is strongly affected by L-L phase inversion. Increasing RH as well as size of L-L miscibility gap in the ternary phase diagram of nonsolvent (water)/solvent (DMF or NMP)/polymer (PVDF) reduces time required to induce L-L demixing as verified by calculated mass transfer pathways. Therefore, bead-free fibers of larger diameters are expected, meanwhile, growth of β-phase crystals is suppressed. This is why fibers electrospun from DMF-based solution contain less β-phase crystals at high values of RH. In contrast, for solutions composed of NMP, jet stretching due to whipping instability plays pivotal role to form fiber structure as a result of delayed L-L demixing with respect to S-L demixing at high RH, 80 %. Furthermore, retarded L-L demixing in NMP-based systems destabilizes fiber formation at low humid environment which can be enhanced by addition a volatile solvent such as acetone. Additionally, more evidence for increment of β-phase formation with increasing working distance (w.d.) at constant RH is provided. o

Keywords: Poly(vinylidene fluoride), Polymorphism, Electrospinning, Nanofiber morphology, Porous nanofiber, Phase separation

when strength of electric field exceeds the surface tension of the polymer solution. Travelling of jet in the electrical field is accompanied by its continues stretching as a direct result of whipping instability as well as solvent evaporation which makes very fine fibers [17-19]. Depending on process parameters including electrospinning factors [20,21], solution properties [20-25] and environmental conditions [20,25-32], electrospun fibers could exhibit various morphologies among which there has been a great deal of interest in porous morphology, which endues electrospun fibers with high surface area and consequently great potential for many applications. A rich literature has focused on producing nanoporous electrospun fibers through a variety of procedures summarized as follow: • electrospinning at high humid environment [20,25,26,28-33] • using a volatile solvent [20,28,32,34], a mixture of volatile/nonvolatile solvent [13,20,32,35] or volatile/ volatile solvents [36] • electrospinning of a solution composed of two components in a common solvent and selective dissolution [37,38] or pyrolysis of one component [39] • temperature-induced phase separation (TIPS) [40] • electrospinning Lewis acid-based complex solution [41] • controlling ratio of solvent evaporation rate to phase inversion rate [42] • electrospinning a solution located in the miscibility area of ternary phase diagram [43,44] • deposition of fiber on the surface of nonsolvent liquid [45]

Introduction Today polymeric nanofibers have drawn great attention because of their potential applications in a wide range of scientific and commercial areas such as filtration [1], piezoelectric nano-composites [2,3], tissue engineering [4], elaborating surface hydrophobicity and wetting processes [5, 6], membranes in sensing materials [7], catalytic systems [8], protective clothing [9], cleanup the oil contaminations [10,11], three-dimensional nanofibrous macrostructures [12] and sportswear, activewear and workwear [13]. A range of techniques have been introduced to fabricate nanofibers such as drawing, template synthesis, phase separation, self-assembly, electrospinning and recently centrifugal jet spinning and pressurised gyration process [12,14-16]. Two last methods turn out to be suitable for mass production of nanofibers, however, challenges associated to the design of the spinneret and the material features limit their application [15,17]. Overall, electrospinning has been undoubtedly employed as a simple, well-known, efficient and versatile method to produce such type of polymeric fibers with diameters on the order of tens of nanometers up to a few micrometers. This process works based on electrostatic forces acting along an electrically charged jet ejected from a nozzle with specific polarity and accelerated in an electrical field toward a grounded collector. The ejection of jet is created *Corresponding author: [email protected] 326 Downloaded from http://www.elearnica.ir

Porosity and Polymorphism Evolution Within PVDF Nanofiber

Prove has shown that porous morphology is dominantly created and elaborated by L-L phase separation through which a thermodynamically unstable polymer solution is separated into two liquid phases in thermodynamic equilibrium. One of these phases, i.e. solvent-rich phase, eventually transforms into pores distributed in a matrix of polymer-rich phase. Temperature variations and absorption of nonsolvent are two main events which are held responsible for thermodynamic instability. The former is discussed as temperatureinduced phase separation (TIPS) while the latter is known as nonsolvent-induced phase demixing (NIPS) categorized in two groups of VIPS (vapor-induced phase separation) and LIPS (liquid-induced phase separation) depending on physical form of nonsolvent [28,32,40,45]. Among different L-L phase separation mechanisms, VIPS has been recognized as the most relevant and applicable one to account for porous structure within electrospun fibers [28]. Contribution of VIPS event to morphology of fibers electrospun from solutions of amorphous polymers including fiber porosity and diameter as well as bead formation, have been explored in a number of studies [26,28,29,31] in the framework of thermodynamic and kinetic principles of polymer solution through which a variety of observed morphologies can be rationalized. Strictly speaking, time required to induction of VIPS within spinning jet is mainly determined by phase behavior of nonsolvent/solvent/polymer ternary system as well as vapor concentration of nonsolvent in electrospinning atmosphere. Additionally, its priority to other involved phenomenon such as solvent evaporation and buckling instability is necessary to obtain porous morphology. Otherwise a wrinkled or smooth surface along with nonporous cross-section is unavoidable. Pai et al. [26] showed that calculated mass transfer pathways superimposed on ternary phase diagram can be considered as a powerful and promising tool to estimate the occurrence of VIPS event and predict the fiber morphology. The contribution of solvent volatility and size of miscibility area in ternary phase diagram to morphology evolution of fibers obtained from solutions of amorphous polymers was also demonstrated previously [28,29,31]. VIPS has been extensively studied for fabricating polymeric membranes with desired morphologies. Kinetics of phase separation induced by nonsolvent vapor was discussed by Lee et al. [46]. They used Cahn-Hilliard theory to explain the early stage of phase separation. A model for predicting pathway of mass transfer of three components during membrane formation precipitated from vapor phase was developed by Matsuyama et al. [47] and Yip and McHugh [48]. Recently, Bouyer et al. [49] used a model similar to the one presented by Yip and McHugh to investigate morphology of membranes prepared from polyetherimide (PEI)/Nmethyl-2-pyrrolidone (NMP)/water system by VIPS. Role of viscosity and elasticity of polymer-rich phase after VIPS occurrence was discussed by Tsai et al. [50] who showed the

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lacy structure formed after spinodal decomposition could be retained when viscoelasticity of polymer-rich phase is high enough to arrest coarsening of phase-separated domains. Absorption of nonsolvent from vapor phase is responsible for L-L demixing in solutions of amorphous polymers. In the case of semi-crystalline polymers, solid-liquid (S-L) demixing as a direct consequence of polymer crystallization could also occur in which the polymer crystals (S) are in equilibrium with a liquid (L) phase. At this condition, the final morphology of membranes is demonstrated as a result of competition between precipitation based on L-L and S-L phase demixing mechanisms [51]. This has been the subject of many scientific researches focusing on controlling morphology of poly(vinylidene fluoride) (PVDF) membranes and films [5254]. Nowadays, PVDF is the polymer of choice for a variety of scientific and commercial researches because of its favorable properties including flexibility, thermal stability, high mechanical strength and chemical resistance as well as diversity of PVDF potential applications as membrane, piezoelectric and pyroelectric materials in the form of films or electrospun fibers [2,3,55], porous nanofibers [56,57], core-shell nanofibers [58,59] and protection of brittle paper relics from environmental damage [60]. Although numerous studies have focused on exploring and clarifying VIPS process during membrane formation using partially crystalline polymers [51-54], however, more investigations is still needed for a better understanding of different aspects of morphology evolution in fibers electrospun from solutions of semi-crystalline polymers concerning VIPS event. After providing evidence for impression of phase behavior of ternary system and solvent properties particularly its volatility on the final structure of fibers produced from various amorphous polymers such as polystyrene (PS) [28], polyethersulfone (PES) [29] and polyetherimide (PEI) [31], now, the interplay of L-L and S-L demixing events during electrospinning is considered to account for evolution of different morphologies in nanofibers electrospun from a semi-crystalline polymer such as PVDF. In other words, during electrospinning PVDF chains due to solvent evaporation are inevitably directed into diverse crystalline phases. Additionally, electrospinning is mainly performed in atmospheres containing different amounts of humidity which affects L-L demixing and consequently porosity development. The way in which these two processes (L-L and S-L demixing) affect each other, is addresses in present contribution to be able to control morphology of PVDF nanofibers. For this purpose, solutions of PVDF in two non-volatile solvents, dimethylformamide (DMF) and N-methylpyrrolidone (NMP), are subjected to electrospinning under constant temperature of 20 ºC and different values of relative humidity (RH). Observed morphologies for electrospun fibers are discussed in terms of mass transfer pathways added to ternary nonsolvent/solvent/ polymer phase diagram, SEM images captured from surface as well as interior structure of PVDF nanofibers, DSC,

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WAXD and FTIR spectrums. Dry/wet electrospinning is employed as another approach to get more insight into morphology evolution of a semi-crystalline polymer during electrospinning process.

Experimental Materials Commercially available Poly(vinylidene Fluoride) (PVDF) (MFI=6.0-25.0 (450 oF g/10 min ASTM D1238)), was purchased from China. Sample was dried before use in an oven for 24 h at 70 oC. The solvents dimethylformamide (DMF), N-methylpyrrolidone (NMP), acetone and dichloromethane (DCM) of analytical grade used in this study were supplied from Sigma Aldrich, Inc. Deionized water was used as nonsolvent. All solvents were used as received without further purification.

Hossein Fashandi et al.

ΔGM ----------- = n1lnφ 1 + n2lnφ 2 + n3lnφ3 + n1φ2g12(u2 ) RT + n2φ 3χ23(φ 3 ) + χ13n1φ3

In equation (1) subscripts 1, 2, 3 stand for nonsolvent, solvent and polymer, respectively. R is the gas constant; T, the absolute temperature, and ni and Øi denote to number of moles and volume fraction of component i, respectively. g12( u2) in equation (1) is a concentration dependent interaction parameter depending on the volume fraction u2 = φ 2/( φ 1 + φ2 ) of a pseudo binary mixture which can be represented based on either equation (2) (Koningsveld and Kleintjens model) or equation (3) [61,62]. β0 g12( u2) = α0 + ---------------1 – γ0 u2

(2) 2

Electrospinning Fibers were electrospun from solutions of concentration 20 wt.% prepared at room temperature by stirring a given amount of polymer in DMF, NMP or NMP/acetone mixtures of different volume ratios for at least 24 h. The prepared solution was loaded in a 10 ml glass syringe with metal needle. All electrospinning experiments were performed in a chamber under controlled environmental conditions, i.e. temperature (T) and relative humidity (RH), with high accuracy. A potential of 15 kV was applied to the needle of syringe by a high voltage power supply. A syringe pump (TOP-5300) was used to supply a steady flow of 0.5 ml/h of polymer solution to the tip of the needle. Electrospinning experiments were conducted using two different setups: • In setup 1, fibers were produced in a horizontal arrange. Working distance (the distance between nozzle and collector) (w.d.) was set at 20 cm and RH was changed in the range of 20-80 % at constant temperature 20 oC. • In setup 2, a vertical arrange was used to produce PVDF fibers. T and RH were set at 20 oC and 40 %, respectively, and electrospun fibers were collected in a nonsolvent-containing vessel (water or methanol) at w.d. ranging from 2 to 20 cm. At RH: 80 %, since phase separation takes place very fast and prevents fiber stretching and traveling, voltage and w.d. were fixed at 20 kV and 15 cm, respectively, to avoid fiber deposition at the middle of working distance. Ternary Phase Diagram Ternary phase diagram of nonsolvent (water)/solvent (DMF or NMP)/polymer (PVDF) at 20 oC was determined by measuring the cloud points and crystallization-induced gelation data as well as calculating the binodal and spinodal curves along with tie-lines based on Gibbs free energy of mixing (equation (1)).

(1)

3

4

g12( u2) = α0 + β0u2 + γ0u2 + ε0u2 + η0u2

(3)

where α0, β0, γ0, ε0 and η0 are constants. In the present contribution, solvent/polymer interaction parameters (χ23) including DMF/PVDF and NMP/PVDF, were roughly estimated based on solubility parameters (δ) using equation (4) [47]. v 2 χ23 = 0.35 + ------2- ( δ2 – δ3 ) RT

(4)

where v2 is the molar volume of component 2, solvent. Values of δ2:DMF=24.8 (MPa)0.5, δ2:NMP=22.9 (MPa)0.5 and δ3:PVDF=23.2 (MPa)0.5 were obtained from Ref. [63]. χ13 in equation (1) is the water/polymer interaction parameter, often assumed as a constant and measured using water uptake (at T=20 oC) of a film cast from PVDF/DCM solution. The procedure has been completely explained in Ref. [28,29]. 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. [64]. To further verify the calculated binodal curves, cloud points of different ternary systems were measured at constant temperature, i.e. 20 oC according to instruction detailed in our previous works [28,29]. The crystallization-induced gelation boundary at 20 oC was measured for water/DMF/PVDF as well as water/NMP/ PVDF ternary systems. For this purpose a given quantity of PVDF was mixed with a specific solvent (DMF or NMP) in a sealed glass bottle. Temperature of the mixture was then raised to completely dissolve polymer. A specific amount of water was added to this solution. The mixture was agitated at ca. 85 oC to obtain a clear homogeneous solution. After that the solution was maintained at a chamber of constant temperature of 20 oC for at least 10 days. The composition at which clear solution started to precipitate was considered as crystallization-induced gelation point. The crystallization-



induced gelation boundary is depicted by drawing a line through the gelation points.

differences approach in one dimension (radial direction, r) using central differences in space and forward difference in time was performed to calculate φ1 and φ2 within fiber at various times. Volume fraction of polymer (φ3) at a given time was calculated based on conserved relation, viz., φ 3 = 1 – φ1 – φ2 . Since fiber radius varies with time as a result of solvent outflow and nonsolvent inflow, to immobilize the air-fiber interface and simplify the problem, the following coordinate transformation was considered [26,47]:

Mass Transfer Pathway Model Description Mass transfer of components, i.e. nonsolvent, solvent and polymer, during electrospinning was calculated based on the diffusion model developed by Tsay and McHugh [65] and Matsuyama et al. [47] and superimposed on the ternary phase diagram to estimate the times on which these paths meet the binodal curve as well as crystallization-induced gelation line. This model was employed by Pai et al. [26] to rationalize resultant morphology of fibers electrospun from 30 wt.% PS/DMF solution at different RHs. The time evolution of concentration of nonsolvent and solvent is given by equations (5) and (6), respectively [26]. dφ 1 V -------- = D11∇2φ 1 + D12 -----1 ∇2φ2 ∂t V2

(5)

dφ 2 -------- = D2 ∇2φ 2 ∂t

(6)

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r η = --------- for 0 ≤ r ≤ R( t ) R( t )

(7)

where R(t) denotes to fiber radius at time t and can be calculated based on equation (8) [26]. ⎛ πR20φ 30 ⎞ -⎟ R( t ) = ⎜ --------------------------1 ⎝ ∫0 2πηφ 3dη⎠

1/2

(8)

The initial and boundary conditions after coordinate transformation as well as details of equations have been reviewed in Refs. [26,47]. Evaluation of Model Parameters At the current contribution, math transfer paths were calculated for water/DMF/PVDF and water/NMP/PVDF ternary systems. Required parameters have been listed in Tables 1 and 2. Some parameters in these Tables are calculated as discussed in appendix A.

where Dij is the appropriate phenomenological diffusion coefficient for the ternary system and D2 stands for the mutual diffusion coefficient of the solvent in the binary system. Vi (cm3/g) is the partial specific volume of component i in the fiber. Here, subscripts 1, 2, 3 indicate nonsolvent, solvent and polymer, respectively. Determination of mass transfer path in fibers during electrospinning requires time-dependent partial differential equations (5) and (6) to be solved numerically in cylindrical coordinates. Finite

Characterization of Electrospun Nanofibers SEM Analysis Surface morphology along with cross-section of electrospun o

Table 1. Model parameters used to calculate mss transfer paths for water/DMF/PVDF and water/NMP/PVDF ternary systems at 20 C and different values of RH Parameter Value Parameter Value M (g/mole) 18 w 0.2 * M (g/mole) 254000 V3 (cm /g) [48] 0.565 ρ (g/cm ) 1 V (cm /g) Table A.1 ρ (g/cm ) 1.78 V (cm /g) [26,47] 335 d 10 τ1 ( − ) 0.413 w d w 0.8 τ2 ( − ) -1.65 V for NMP was assumed to be the same as DMF. 1

30

3

3

3

3

1

1g

3

3

3

a

2g

-30

10

20

Parameter ∞ ρ1g (g/cm ) 0 P1 (atm) 0 P2 (atm) ρg (g/cm ) Pt (atm) μ (pa·s) (20 C) [67]

Value Table A.3 Table A.4 Table A.4 Table A.5 1 1.81×10

3

3

o

-4

g

Parameter ∞ ρ2g (g/cm ) T (K) 12 D ( φ 1 = 1 ) (cm /s) (×10 ) D (cm /s) [26,47] K /γ (cm /(g·K)) [47,48] K23 – Tg3( K ) [47,48] 3

g3

2

2

1g

3

13

-5

Value 10 235 1.12 [47,66] 0.267 0.000273 -127 -30

a

2g

o

Table 2. Model parameters unique to the two ternary systems, water/DMF/PVDF and water/NMP/PVDF, at 20 C Parameter M (g/mole) 2

ρ (g/cm ) 3

2

*

Water/DMF/PVDF 73.09

Water/NMP/PVDF 99.13

0.94

1.03

3

V2 (cm /g) 0.926 [47,48] 0.841 [48] 5×10 2×10 D (cm) ξ(−) 0.74 0.91 Estimated based on equations available at Ref. [68]. -4

c

a

a

-4

Parameter K /γ (cm /(g·K)) 3

12

K22 – Tg2( K ) 2

-4

D (cm /s) (×10 ) D (cm /s) 20

2

2g

Water/DMF/PVDF 0.000976 [26,47,48]

Water/NMP/PVDF 0.000963 [48]

-43.8 [26,47,48]

-48.496 [48]

8.48 [21,39,40] 0.023 [26,47,48]

3.137 [48] 0.0075 [48]

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fibers were observed using scanning electron microscope (SEM) (TESCAN series VEGA 2007 from Czech) performed at 30 kV acceleration voltage. To observe the interior structure (cross-section) of fibers, the integrated composite of fibers was 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 value and standard deviation were obtained by readying the diameters of 50 randomly selected fibers. X-ray Diffraction Wide angle X-ray diffraction (WAXD) studies of PVDF electrospun mats were performed using PANalytical XRD diffractometer operated at 40 kV and 20 mA in the range of 2θ =5-40 o. FTIR Spectroscopy The variation of crystal polymorphism in the structure of PVDF nanofibers was also verified by recording attenuated total reflectance (ATR) spectrum using BOMEM FTIR MBseries, MB-100 (Hartmann & Braun, Canada). The electrospun web was put directly on the surface of a flat crystal (ZnSe, a 45 o ATR prism). The spectrum was measured at a wave number resolution of 4 cm-1 for a spectral range from 400 to 4000 cm-1. DSC Measurements Thermal analysis was performed using a differential scanning calorimeter from TA Instruments (DSC 2010). The PVDF electrospun webs with approximately similar mass (5.0 mg) were heated from 0 to 220 oC at scan rate of 10 oC/ min. The temperature and baseline of DSC apparatus were calibrated before performing thermal analysis. Indium and deionized water were used as calibration substances.

followed by formation of polymer-rich phase and polymer crystals as characteristic features of L-L and S-L phase separation mechanisms, respectively. These two phases benefit from different properties and are thought to affect morphology of resultant fibers in a different manner. Therefore, having knowledge about time of L-L and S-L phase demixing and their sequence would be of significant importance to rationalize different morphologies achieved during electrospinning of semi-crystalline polymers. Ternary Phase Diagrams Construction of ternary phase diagrams requires three interaction parameters to be precisely determined. These parameters used for various systems have been summarized in Table 3. Coefficients required to calculate gwater/DMF based on equation (3) were collected from Ref. [62]. These values for calculating gwater/NMP according to equation (2) in ternary system composed of PVDF were obtained from Ref. [69]. Calculated phase diagrams for ternary systems of water/ PVDF/DMF and water/PVDF/NMP have been included in Figure 1. These phase diagrams contain binodal and spinodal curves, tie lines and crystallization-induced gelation lines. Solutions of semi-crystalline polymers would become thermodynamically unstable with respect to L-L demixing and/or polymer crystallization. Hence, associated ternary phase diagram can be divided into three distinct regions using binodal and crystallization-induced gelation boundaries: S-L miscibility gap, L-L miscibility gap and miscibility area. When a homogeneous solution in miscibility area is brought across binodal into L-L miscibility gap, the solution has the potential for instability with respect to L-L demixing and separates into two liquid phases in equilibrium, i.e. solventrich and polymer-rich phases. Gelation of polymer solution happens, if the S-L miscibility gap is entered through crystallization-induced gelation line above which an uniform solution becomes ultimately a gel. An initially uniform solution composed of semi-crystalline polymer and solvent is located on the solvent-polymer axis in ternary phase diagram. Upon the addition of nonsolvent, the location of mixture varies with time and leads it to cross the phase boundaries, i.e. binodal or gelation lines, followed by L-L and S-L demixing, respectively. As evident from Figure 1, S-L miscibility gap locates immediately after solvent-polymer axis and therefore it can be considered that crystallization takes place in advance of L-L demixing as a result of nonsolvent addition. However,

Results and Discussion Exploring morphology development in fibers electrospun from solutions of partially crystalline polymers under different environmental conditions needs thermodynamic as well as kinetic aspects of ternary phase behavior to be clarified. For this purpose construction of ternary phase diagram is the first step. Mass transfer pathways superimposed on phase diagram could provide insight into relative time scale of occurrence of phase inversion according to L-L and S-L mechanisms within the electrospun fibers. Phase demixing into two phases in thermodynamic equilibrium is o

Table 3. interaction parameters for different ternary systems at 20 C Ternary system Water/DMF/PVDF

T (ºC) 20

g (u )= 2 3 4 0.50 + 0.04u2 + 0.80u2 – 1.20u2 + 0.8u2

χ 3.0

g 0.43

Water/NMP/PVDF

20

0.468 0.316 + -------------------------1 – 0.499u2

3.0

0.35

12

2

13

23



331

o

Figure 1. Ternary phase diagram of (a) water/NMP/PVDF and (b) water/DMF/PVDF at 20 C. Figure 1(b) also contains cloud points of methanol/DMF/PVDF system (filled square symbols) at 20 C. o

Figure 3. Comparison of concentration-dependency of water/ DMF and water/NMP interaction parameters at 20 C. o

Figure 2. Comparison of phase diagram of water/DMF/PVDF and water/NMP/PVDF at 20 C. o

other factors such as slow kinetic of polymer crystallization as well as higher rate of solvent/nonsolvent exchange may precede L-L phase inversion over crystallization. For the sake of easier comparison, ternary phase diagrams of systems based on PVDF have been enlarged in Figure 2. As evidenced by both Figures 1 and 2, when NMP is used as solvent, binodal is located farther from solvent-polymer axis compared to DMF. This means more nonsolvent, water, is required to induce L-L phase separation when NMP is considered as solvent. Further evidence is provided by good agreement established between theoretically calculated binodal curves and experimental cloud point data. This difference in location of binodal curve can be investigated by considering water/solvent and solvent/polymer interaction parameters as tabulated in Table 3 and compared in Figure 3.

According to Figure 3, water/NMP displays higher interaction parameter over whole concentration range indicating lower tendency of these components to mix. This corresponds higher concentration of water for PVDF/NMP system to meet requirements of L-L demixing. Further, based on data listed in Table 3, χPVDF/DMF > χPVDF/NMP. This means NMP is better solvent for PVDF compared to DMF. Hence, L-L demixing takes place in higher water concentration when NMP is solvent. Our investigations on the qualitative in addition to quantitative influence of solvent/polymer as well as nonsolvent/solvent interaction parameters on phase diagram are in good accordance with theoretical calculations argued extensively in literature [61,62]. Another discrepancy between phase diagrams of ternary systems water/DMF/PVDF and water/NMP/PVDF lies in the size of miscibility area which is again larger when NMP is used as solvent (Figures 1, 2). On the other hand, S-L

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Table 4. Physical properties of DMF [71] and NMP [72] Physical property Molecular weight (g/mol) DMF 73.10 NMP 99.13

Boiling point (ºC) 153.0 202.0

3

Density (g/cm ) 0.940 1.028

Dipole moment (Debye) 3.82 4.10

Dielectric constant 37.06 33.00

demixing happens in higher polymer and nonsolvent concentration which originates from more favorable interaction of NMP/PVDF rather than DMF/PVDF and decreased g23 for NMP compared to DMF. This is consistent with theoretical calculations of Witt et al. [70]. Regarding physical properties of DMF and NMP listed in Table 4, no significant differences can be distinguished. Therefore, it is a reasonable conjecture to discuss phase behavior of nonsolvent/solvent/polymer ternary system as main factor responsible for differences in PVDF fibers electrospun from DMF- and NMP-based solutions under various environmental conditions. Calculated Pathways of Composition Change As discussed in preceding section, solutions of PVDF, a semi-crystalline polymer, in organic solvents such as DMF and NMP precipitate either into a gel or two liquid phases in thermodynamic equilibrium as a result of occurrence of S-L or L-L phase separation, respectively. Ternary phase diagram only determines the compositions at which mixtures undergo phase demixing but it is of no use to predict the time at which phase separation with specific mechanism begins. Such data are of significant importance to predict and analyze the final morphology of a special skeleton [26,28,29]. Pathways of composition changes during electrospinning of PVDF solutions under different humid environments, RH: 5-80 %, have been superimposed on ternary phase diagram and displayed on Figure 4 for two nonvolatile solvents, DMF and NMP. As depicted in Figure 4(a), when DMF is regarded as solvent, calculated mass transfer pathways are a strong function of relative humidity of the environment under which electrospinning is carried out. When RH is less than or equal to 20 %, composition paths settle on the S-L immiscibility area of phase diagram and do not cross the binodal. Under these conditions, phase demixing is likely to occur based on crystallization, however, L-L demixing cannot be expected. By increment of RH to 30 %, in systems composed of DMF, mass transfer paths intersect binodal curve. Further increasing of RH to 40 % and higher values is accompanied by crossing the binodal curve through shorter trajectories which consequently leads to shorter times for induction of L-L demixing. Increasing RH increases the driving force for penetration of water molecules into electrospinning jet from vapor phase which helps to induction of phase inversion at a given time depending on the phase behavior of ternary system. Totally, according to composition pathways, regardless of operating RH, composition paths enter the S-L demixing

Figure 4. Theoretical mass transfer paths calculated for (a) water/ DMF/PVDF and (b) water/NMP/PVDF ternary systems at 20 C and different values of RH. Each data point in figures a and b displays a time interval of 0.05 and 0.25 s, respectively. o

area immediately after coming into contact with humid atmosphere. While crossing the binodal curve and coming to L-L demixing area needs more times depending on operating RH. Comparison of images included in Figure 4 proves that composition variations go through different paths when DMF is replaced by NMP. In other words, once DMF used as solvent, the concentration of polymer was found to be an increasing function with the elapsed time indicating higher rates of solvent outflow than nonsolvent (water) inflow (Figure 4(a)). Contrarily, NMP solution behaves oppositely and polymer concentration reduces as time proceeds (Figure 4(b)). Thus, it can be concluded that by using NMP, solvent



333

Table 5. Calculated times (s) for beginning liquid-liquid (L-L) phase separation (crossing binodal curve) and solid-liquid (S-L) phase separation (crossing crystallization-induced gelation boundary) at different RHs during electrospinning of PVDF/DMF and PVDF/NMP solutions Ternary system Water/DMF/PVDF Water/NMP/PVDF

Phase separation type L-L L-L S-L

5 >1 >1 >1

10 >1 >1 >1

outflow to the surrounding becomes negligible in comparison with nonsolvent inflow. Similar behavior was observed by Yip and McHugh [48] and Caquineau [73] during study VIPS process for evaluation membrane morphology cast from NMP solutions. Additionally, in spite of DMF, paths depicted for NMP solutions do not change significantly by operating RH. The reason of observed discrepancies between performance of DMF and NMP can become clear by evaluating their saturated vapor pressures and comparing with that of water (Table A.4). As listed in Table A.4, NMP suffers from very low value of saturated vapor pressure which is approximately 107 times lower than that of water while this value for DMF levels off to about 6. When RH1 >1 >1

Relative humidity (RH) (%) 30 40 0.8 0.55 >1 >1 >1 0.37

60 0.30 0.85 0.25

80 0.20 0.65 0.15

Figure 5. SEM micrographs of webs electrospun from 20 wt.% (a) PVDF/DMF and (b) PVDF/NMP solutions at 20 C and RH: 20 %. o

beads observed during electrospinning process [28]. According to literature [21] bead formation during electrospinning is likely once capillary instability is permitted to act. This originates from surface tension of polymer solution and is prohibited as viscoelastic stresses overcome it. Viscoelastic stresses can be controlled by adjusting electrospinning parameters such as solution concentration, solution viscosity, polymer molecular weight as well as solvent volatility [74,75]. L-L Phase demixing can also contribute to raise the viscoelasticity of jet due to formation of polymerrich domains. In our previous studies [28,29,31] it was demonstrated that a delayed solidification leads capillary

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Figure 6. SEM micrographs of webs electrospun from 20 wt.% PVDF/DMF and PVDF/NMP solutions at 20 C and different RHs (RH: 40, 60 and 80 %). The inset in the SEM image of web electrospun from PVDF/DMF solution at RH: 80 % represents water contact angle (WCA) of PVDF electrospun mat.

instability to overcome viscoelastic stresses, resulting bead with high density to develop. Similar discussion can be presented here for formation of beaded fibers at low values of RH. In the case of PVDF/DMF solution, when spinning is carried out at RH: 20 %, S-L demixing can take place in solution jet immediately after ejecting from needle tip, but L-L demixing needs more time to occur (>1 s), as demonstrated by composition pathway and corresponding calculated time (Figure 4(a) and Table 5). Hence, PVDF crystals are likely to form as shown in Figure 5(a). By contrast, for NMP solution at RH: 20 %, neither L-L nor S-L demixing are not probable within 1 sec as illustrated in Figure 4 and Table 5 which leads to PVDF beads of lower crystallinity (Figure 5(b)). The crystallographic data of PVDF electrospun webs investigated in the framework of FTIR, DSC and WAXD spectrums in Figures 9 through 11 confirm the lower crystallinity of beads obtained from PVDF/NMP solution at RH: 20 %. This will be discussed in detail in following sections. Presented in Figure 6 are SEM micrographs of webs electrospun from PVDF/DMF and PVDF/NMP solutions under conditions of temperature 20 oC and different RHs, 40, 60 and 80 %. Obviously, increasing RH to values of 40 and

60 % leads to fibers with bead-on-string morphology for DMF solutions, while webs obtained from NMP solutions at these RHs are still composed of particles which are different in shape from those produced from this solution under RH: 20 % (Figure 5(b)) and are too similar to PVDF crystallites as observed for web produced from DMF solution at RH: 20 % (Figure 5(a)). This is not surprising by considering shorter times of S-L demixing estimated for RH: 40 % and 60 %, relative to RH: 20 %, as reported in Table 5. Further increasing of RH to 80 % leads to bead-free fibers regardless of solvent type (Figure 6). However, as evident, DMF leads to more uniform and densely packed fibers compared to NMP. Suppression of bead formation and producing uniform fibers by increasing RH can be attributed to promoting L-L demixing and increased viscoelasticity of polymer-rich phase to overcome capillary instability. This is also accompanied by increasing fiber diameter due to formation of polymer-rich domains which prevent further stretching of electrospinning jet as a result of whipping instability [28]. As inserted in Figure 6, the hydrophobicity feature of PVDF electrospun web was also confirmed via measuring contact angle of water droplet on the mat composed of uniform and bead-free PVDF fibers electrospun



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Figure 7. SEM images captured from surface as well as cross-section of fibers electrospun from 20 wt.% PVDF/DMF and PVDF/NMP solutions at 20 C and different RHs. o

from DMF-based solution at RH: 80 %. Surface along with corresponding interior morphologies of fibers obtained from PVDF/DMF and PVDF/NMP solutions at different RHs have been displayed and compared in Figure 7. As shown in this figure, fibers obtained from PVDF/DMF solutions at RH: 40 % and 60 % benefit from smooth surface while those produced at RH: 80 %, without paying attention to solvent type, exhibit a porous and rough surface populated with small isolated pores. In addition, electrospinning at RHs: 60 and 80 % also leads to porous cross-section composed of inter-connected networks of pores. Both surface and interior porosities can be described by formation of polymer-lean domains as a result of spinodal decomposition (SD) mechanism following L-L demixing.

Differences in surface and interior pores is argued by considering coarsening of phase-separated domains on the surface of fibers driven by interfacial tension in the solutionair interface which is absent in the fiber cross-section [28]. The contribution of this factor to porosity evolution becomes more pronounced in the case of fibers produced from PVDF/ DMF solution at RH: 60 % which have solid non-porous surface and porous cross-section (Figure 7). As discussed above, porosity within nanofibers confirms the occurrence of L-L phase demixing resulting to polymerrich and solvent-rich domains. Meanwhile, morphology of PVDF electrospun webs and fibers is also influenced by PVDF crystallization after which S-L demixing and crystallizationinduced gelation becomes probable. Here, PVDF crystallinity

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is regarded to study the S-L demixing and its contribution to morphology of PVDF electrospun nanofibers. WAXD, FTIR and DSC graphs are used to follow crystallinity evolution in fibers and webs produced from PVDF/DMF and PVDF/ NMP solutions under different environmental conditions. These spectrums have been included in Figures 8 through 10. As investigated in literature, PVDF chains of different conformations can pack into five distinct crystalline phases known as non-polar α & δ phases, polar β-phase and γ- & εphases. The conformations of chains in these phases are TGTG', TGTG', TTT, T3GT3G' and T3GT3G', respectively, which T and G stands for Trans and Gauche conformations [3,76]. Among these phases, α, β and γ are the most studied and discussed in scientific and applied researches [3,76,77]. WAXD patterns of all fibers and webs (Figure 8) except one produced from PVDF/NMP solution at RH: 20 % show three distinct peaks at 2θ around 18.6, 20.3 and 27.1 o which correspond to, respectively, α (020), β ((200) (110)) and α (111) reflections of PVDF crystallites [3,76,77]. As obvious,


when DMF is employed as solvent, the intensity of β phase increases with RH up to 40 % above which β-phase intensity exhibits an descending order with RH. Using NMP, intensities increase with RH ranging from 40 to 80 %. For RH: 20 % no distinct peak is available in diffraction pattern. In both DMFand NMP- based solutions the variation of height of peaks related to α phase with RH, goes in opposite direction with respect to the peaks of β crystallites. Totally, from diffraction patterns it can be concluded that for NMP-based solutions, higher RH favors more β crystalline phase. FTIR absorbance in the range of 400-1500 cm-1 measured for webs and fibers produced from DMF- and NMP-based solutions under various operating conditions have been depicted in Figure 9. The peaks appeared at 614, 765 and 975 cm-1 are attributed to chain conformations with short trans (T) sequence namely α-phase, while bands at 840 and 1275 cm-1 are related to polymer chain conformations characterized with long trans sequence which can be found in β- and γ-phases of PVDF [3,76,77]. In fact, these two crystalline phases, β and γ, have absorbance bands at similar

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Figure 8. WAXD patterns of webs and fibers electrospun from 20 wt.% (a) PVDF/DMF and (b) PVDF/NMP solutions at 20 C and different RHs (working distance: 20 cm).

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Figure 9. FTIR spectrum of webs and fibers electrospun from 20 wt.% (a) PVDF/DMF and (b) PVDF/NMP solutions at 20 C and different RHs (working distance: 20 cm).


wavenumbers making it very hard to distinguish between them [76]. In 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 quantitative evaluation of absorbance bands, all spectrums were normalized using the peak at 877 cm-1 as reference band [3]. Regarding FTIR spectrums of mats obtained from NMP solutions (Figure 9(b)), one can witness the continuous increment and decline of absorbances corresponding to α and β-phases, respectively, with RH, indicating the growth of β-phase. However, as can be observed in Figure 9(a), the absorbances associated to β-phases of mats produced from DMF solutions is an increasing function of RH when electrospinning is performed under low humid environment (RH