Synergistic Contribution of Spinneret Diameter and Physical Gelation

Article pubs.acs.org/IECR

Synergistic Contribution of Spinneret Diameter and Physical Gelation To Develop Macrovoid-Free Hollow Fiber Membranes Using Single Orifice Spinneret Hossein Fashandi,*,† Kamran Zarrini,† Mostafa Youssefi,† and Mohammad Mahdi Abolhasani‡ †

Department of Textile Engineering, Isfahan University of Technology, Isfahan, 84156-83111, Iran Department of Chemical Engineering, University of Kashan, Kashan, Iran

‡

ABSTRACT: A new, facile, and versatile technique of scientific and industrial interest is introduced for the first time to spin an additive-free polymer solution into the hydrophilic nylon 6 hollow fiber membranes. Specifically, the technique employs a single orifice spinneret immersed directly into the coagulation bath. Hollow fibers are produced when the thickness of gelled surface layer is smaller than the fiber radius, which is in turn determined by the orifice diameter. Otherwise, bore-free fibers are expected. More specifically, the morphology of hollow fibers can be best tailored through choosing spinning dope from different sections in ternary phase diagram of nonsolvent/solvent/polymer. The produced hollow fibers are suitable candidates for water purification purposes, especially forward osmosis and pressure-retarded osmosis.

1. INTRODUCTION Nowadays, polymeric membranes are mainly intended for a wide variety of purification and separation purposes such as reverse osmosis (RO), forward osmosis (FO), gas separation, pervaporation, dialysis, and removal of microorganism and particles by nano-, micro-, and ultrafiltration.1 Polyamide can be undoubtedly considered as one of the best thermoplastic polymers with good chemical, mechanical, and thermal properties to develop polymeric membranes2,3 intended for various scientific and industrial applications including reverse osmosis and water desalination,4−6 steel wastewater reuse,7 preparation of oil-in-water emulsion,8 and efficient membrane solvent back extraction.9 Moreover, aliphatic polyamides have gained a considerable attention in recent years to make membranes for FO or pressure-retarded osmosis (PRO) owing to their hydrophilic nature and less swelling propensity. These features help to reduce internal concentration polarization (ICP) which adversely affects membrane performance.3,10−12 Among various configurations of membranes, there has been a great deal of interest to develop hollow fiber membranes (HFMs). HFM modules are beneficial to separation mainly because of greater surface area provided by them.1,13 The separation performance of HFM modules varies with module characteristics as well as morphology of hollow fibers. The latter can be tuned by controlling operational parameters including phase behavior of nonsolvent/solvent/polymer ternary system, polymer crystallization, rheology of polymer solution,14,15 flow rates of polymer solution and bore fluid,13,16 polymer concentration, take-up speed, and air gap distance.17 Today, hollow fibers in both laboratory and industrial scales are primarily produced based on nonsolvent-induced phase separation (NIPS) through a conventional spinning process. Specifically, a concentrated polymer solution is extruded through an annular gap of a concentric spinneret into the external coagulation bath, so the outer wall of hollow fiber is made. Simultaneously, the so-called bore fluid is ejected from © 2015 American Chemical Society

the internal capillary of the spinneret and the inner wall of the fiber is formed.1,13−17 In practice, producing HFMs following the accepted route poses considerable challenges including the design and manufacture of a spinneret with two concentric orifices as well as supplying the facilities to provide continuous and parallel feeding of both polymer solution and bore fluid with given rates. These factors would limit versatility of the conventional technique to prepare HFMs. Recently, producing HFMs using spinneret with single orifice has been introduced as a new solution to address these problems.18,19 However, it is still controversial whether or not single orifice spinneret can be employed for spinning macrovoid-free HFM without any additive or any post-treatment process. In that respect, to further demonstrate this young method, it is important to investigate how orifice diameter and properties of spinning solution contribute to formation of bore during wet spinning. To this end, solutions of a semicrystalline polymer, nylon 6 (N6), with different concentrations are subjected to wet spinning using two single orifices with different diameters. Viscoelastic analysis is conducted to monitor and rationalize the gelation evolution and bore formation during spinning. The produced hollow fibers are characterized for their mechanical properties as well as pure water and gas permeability.

2. EXPERIMENTAL SECTION 2.1. Materials. The solvent, formic acid (FA), of analytical grade was purchased from Sigma-Aldrich, Inc. and used as received without further purification. Deionized water was used as nonsolvent and wetting solvent. Commercially available Received: Revised: Accepted: Published: 7728

May 1, 2015 July 5, 2015 July 18, 2015 July 18, 2015 DOI: 10.1021/acs.iecr.5b01631 Ind. Eng. Chem. Res. 2015, 54, 7728−7736

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Industrial & Engineering Chemistry Research Table 1. Operating Parameters for Producing Different Fibers fiber ID

spinneret i.d. (mm)

spinneret o.d. (mm)

spinning dope

dope feeding rate (mL/min)

F1 F2 F3 F4 F5

0.3 0.4 0.7 0.80 0.80

0.5 0.6 0.9 1.0 1.0

A1 A1 A1 A1 B5

1.5 1.5 1.5 1.5 1.5

tap tap tap tap tap

water water water water water

temp of coagulation bath (°C) 25 25 25 25 25

take-up speed free free free free free

fall fall fall fall fall

tests, an amplitude scanning was performed to ensure the linearity of the dynamic viscoelasticity. The gelation times of A2−A4 and B4−B6 dopes at 25 °C were measured based on visual evaluation of cessation of the liquid flow inside the tube when it was tilted. This technique was also used by Mal et al.27 to measure gelation rates of PVDF thermoreversible gels. 2.4. Fiber Spinning. To produce hollow fibers, the polymer solution was fed through a single orifice spinneret (inner diameter: 0.3, 0.4, 0.7, and 0.80 mm) directly into the coagulation bath using a syringe pump. The coagulation bath was filled with tap water whose temperature was kept constant at 25 °C throughout the spinning process. Spinning was carried out for two solutions (A1 and B5, as indicated in Figure 1) composed of different concentrations of water (1), FA (2), and N6 (3): (A1) ϕ1 = 0.00, ϕ2 = 0.70, ϕ3 = 0.30; (B5) ϕ1 = 0.17, ϕ2 = 0.58, ϕ3 = 0.25 (ϕi is the volume fraction of component i). The operating parameters to produce fibers have been summarized in Table 1. 2.5. Characterization of Produced Fibers. 2.5.1. SEM Analysis of Fibers. Surface and interior morphologies of produced hollow fibers were examined using scanning electron microscope (SEM) (TESCAN series VEGA 2007 from Czech) performed at 30 kV acceleration voltage. Prior to SEM analysis samples were coated with a 10 nm layer of gold. To monitor the hollow fiber cross section, fiber was fractured in liquid nitrogen and then observed using SEM. 2.5.2. WAXD Patterns of Fibers. Wide angle X-ray diffraction (WAXD) studies of N6 fibers were conducted using 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. 2.5.3. DSC Measurements of Fibers. Differential scanning calorimeter (DSC) from TA Instruments (DSC 2010) was considered to evaluate thermal transitions as well as crystallinity of spun N6 fibers. The samples with approximately equal mass (5.0 mg) were heated from 0 to 250 °C at a scan rate of 10 °C/ min. Indium and deionized water were used to calibrate the temperature and baseline of DSC apparatus before performing experiments. 2.5.4. Gas Permeation of Hollow Fibers. The average pore size of produced hollow fibers was evaluated based on gas permeation test. One end of two hollow fibers was blocked by epoxy resin, and the other was open. Hollow fibers were potted to a stainless steel tubular module with an effective length of 10 cm. N2 was fed to the shell side of the module, and its permeation from the lumen sides of hollow fibers was determined using a flow meter. A pressure regulator before the module was applied to control the N2 feeding pressure in the range of 1−4 bar. The required equations to calculate mean pore size of hollow fibers have been detailed in the literature.16

nylon 6 (N6) was supplied from Tehran Alyaf Co. and dried before use in an oven at 70 °C for 48 h. 2.2. Miscibility Characterization of Water/FA/N6 Ternary System. Tie lines along with binodal boundary included in ternary phase diagram were calculated theoretically for water/FA/N6 system based on equalizing the chemical potentials of a given component in two separated phases. The Gibbs free energy of mixing (ΔGM) (eq 1) was differentiated to obtain chemical potential expressions. Spinodal curve as another part of ternary phase diagram was developed from second derivatives of ΔGM. Further details are found elsewhere.20−23 ΔGM = n1 ln ϕ1 + n2 ln ϕ2 + n3 ln ϕ3 + n1ϕ2g12(u 2) RT + n2ϕ3χ23 (ϕ3) + χ13 n1ϕ3

coagulation bath

(1)

In eq 1, ni and ϕi stand for number of moles and volume fraction of component i (i = 1 (nonsolvent), 2 (solvent), or 3 (polymer)), respectively. R and T, respectively, are the universal gas constant and the absolute temperature. g12(u2) (u2 = ϕ2/(ϕ1 + ϕ2)) and χ23(ϕ3) denote the nonsolvent/solvent and solvent/ polymer concentration-dependent interaction parameters, respectively. χ13 is the nonsolvent/polymer interaction parameter which is considered as a constant. In this work, all interaction parameters were collected from literature.24 Calculated binodal curve was further checked by experimentally measured cloud points. Water (nonsolvent) was added dropwise (while stirring) to N6/FA solutions of various concentrations ranged between 2.5 and 20 wt %, until the initial homogeneous solution turned turbid. Throughout the experiment, the temperature of solution was kept constant at 25 °C. The turbidity concentrations were recorded as cloud points. The crystallization-induced gelation boundary at 25 °C was also determined experimentally. To this end, the N6/FA solution with a given concentration in the range of 1−20 wt % was agitated with a specific amount of water. The mixture temperature was raised to ∼50 °C until the complete dissolution. Then, the homogeneous clear solution was left to age at a constant temperature (25 °C) for 1 week. This process was repeated until the solution turned into a gel. The gelled concentrations were recorded as crystallization-induced gelation points which are connected together by the gelation boundary.25,26 2.3. Evaluation of the Viscoelastic Properties of Polymer Dopes and Measurement of the Gelation Times of Dopes. Oscillatory rheometer, MCR 301 (Anton Paar Co., Germany) equipped with two parallel plates, was employed to evaluate the viscoelastic characteristics of polymer solutions and gels. All experiments were conducted at constant temperature (25 °C). The storage (G′) and loss (G″) moduli were measured at a fixed amplitude of 5% and a swept frequency from 0.01 to 100 Hz. Prior to the frequency sweep 7729

DOI: 10.1021/acs.iecr.5b01631 Ind. Eng. Chem. Res. 2015, 54, 7728−7736

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Industrial & Engineering Chemistry Research 2.5.5. Overall Porosity of Fibers. The overall porosity (ε) of produced hollow fibers was measured according to gravimetric method. This parameter is defined as the ratio of the pore volume to the total volume of the fiber and can be estimated using eq 2.16,28 ε (%) =

(ww − wD)/ρw × 100 ⎡ ww − wD wD ⎤ ⎢⎣ ρ + ρ ⎥⎦ w p

Table 2. Interaction Parameters (IPs) Used To Construct Ternary Phase Diagram of Water/FA/N624,a IP g12(u2) value a

Q (3) ΔP A where Q stands for volumetric permeation rate of pure water (L·h−1). ΔP and A are the transmembrane pressure (bar) and the effective area of the HFMs (m2).10 The salt rejection (Rs) experiment was conducted via circulating a feed aqueous solution (100 ppm of NaCl in water) around the shell side of the fibers at a fixed pressure of 1 bar. Conductivity test was performed on both the feed and permeate solutions using pH and conductivity meter (JENWAY 3540) to measure NaCl concentration. Rs and the salt concentration in the feed (Cf) and permeate solutions (CP) are correlated according to eq 4. The reported data are averages of five measurements.

Cf

× 100

χ13 1.42

g12: water/FA IP. χ23: FA/N6 IP. χ13: water/N6 IP.

Figure 1. Ternary phase diagram of water/FA/N6 at 25 °C. Yellow filled circles in phase diagram refer to compositions of different investigated dopes (A1, B5: spinning dope). Compositions are in volume fractions.

Jw =

Cf − Cp

χ23 −1.03 + 0.20ϕ3

(2)

where ww and wD are weights of wet and dry fibers, respectively. ρw and ρp stand for densities of water and polymer (N6, ρP = 1.13 g/cm3), respectively. To obtain ww, fibers were weighed after immersing in distilled water for 24 h and removing the remaining water on the outer surface of fibers. In the case of hollow fibers, the lumen of fiber was blown by air stream to eliminate the extra water. For each sample, the result was averaged over five measurements. 2.5.6. Water Permeability and Salt Rejection of Hollow Fibers. To measure water permeability in the RO mode, five pieces of hollow fibers with effective length of 18 cm were potted in a pressurized cross-flow filtration module. Distilled water was circulated inside the module around the shell side of fibers under constant pressure of 1 bar. The permeation flux of hollow fibers for pure water (J) (L·m−2·h−1·bar−1) is calculated based on eq 3.

R s (%) =

0.133 − 0.151/(1.0 − 0.803u2), u2 = ϕ2/(ϕ1 + ϕ2)

agree well with calculated binodal curve. Furthermore, three distinct regions are accentuated in this figure: miscibility area, solid−liquid (S−L) immiscibility area, and liquid−liquid (L−L) miscibility gap. Compositions in S−L immiscibility area are unstable with respect to polymer crystallization and eventually separate into two phases: a solid phase (polymer crystals) and a liquid phase whose composition locates on the crystallization-induced gelation boundary. An initially uniform solution inside this region becomes ultimately a gel throughout which polymer crystals act as junction points between polymer chains, and hence, a 3D network can be considered. Inside the L−L immiscibility area the polymer solution precipitates into two liquid phases, i.e., polymer-rich and polymer-lean phases, in thermodynamic equilibrium.20,24,29,30 From another point of view, in ternary system of interest, binodal curve locates far from the solvent−polymer axis and there has been a very wide gap between binodal curve and crystallization-induced gelation boundary. This reflects the high tendency of N6 to mix with water and FA corresponding to low values of water/N6 and FA/N6 IPs as listed in Table 2. The contribution of IPs to locate binodal curve in water/FA/N6 phase diagram is in line with other ternary systems investigated in the literature.20−23 3.2. Transition from Bore-Free Fiber to Hollow Fiber by Adjusting the Spinneret Diameter. Morphologies of fibers spun from the same dopes (point A1 in Figure 1) using spinnerets different in orifice diameters (d1 = 0.3, d2 = 0.4, d3 = 0.7, and d4 = 0.8 mm) exhibit large discrepancies. Spinning by orifice with smaller diameter (d1) leads to a bore-free fiber with some macrovoids (Figure 2a). A bore of small diameter and reduced macrovoids have been adopted because of usage of orifice with increased diameter, i.e., d2, as displayed in Figure 2b. Further increasing the spinneret diameter to d3 and d4 values is followed by formation of macrovoid-free hollow fibers with increased outer and inner diameters as compared in Figure

(4)

2.5.7. Mechanical Properties of Fibers. The mechanical properties of produced fibers were evaluated using Zwick tensile testing machine (model 1446-60). The initial distance between clamps and stretching rate were adjusted to be 4 cm and 2.5 mm/min, respectively. For each sample, five measurements were carried out and the mean value was reported.

3. RESULTS AND DISCUSSION 3.1. Phase Behavior of Water/FA/N6. Miscibility and immiscibility concentrations for water/FA/N6 system were determined by demarcating stable, metastable, and unstable areas in the ternary phase diagram. This was implemented through calculating three interaction parameters (IPs) with high accuracy and drawing the binodal and spinodal curves as well as the gelation boundary; as fully described in section 2.2. These IPs for water/FA/N6 system collected from the literature24 are summarized in Table 2. The phase diagram of water/FA/N6 has been illustrated in Figure 1. As shown, experimentally measured cloud points 7730


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Figure 2. SEM images captured from cross sections of fibers (F1, F2, F3, and F4) spun from dope A1 using four single orifice spinnerets different in diameters (d): (a) d = 0.3, (b) d = 0.4, (c) d = 0.7; (d) 0.8 mm. Images a1, b1, c1, and d1 are magnified parts of images a, b, c, and d, respectively.

Figure 3. SEM images captured from outer parts of cross sections (a, b, c) along with inner (d) and outer surfaces (e) of hollow fibers (F2, F3, and F4) spun from dope A1 using three single orifice spinnerets different in diameters (d): (a) d = 0.4, (b) d = 0.7, (c) d = 0.8 mm.

2c,d and Table 3. However, interestingly, the wall thickness of produced hollow fibers remains approximately unchanged, i.e., ∼200 μm, irrespective of orifice diameter (Figure 2b,c,d and Table 3). Additionally, regardless of spinneret diameter, fibers spun from dope A1 exhibit porous cellular cross sections (Figure 2), porous inner surface (Figure 3d), and nonporous RO-like outer skin (Figure 3e) whose thickness falls in the range of 4−5 μm (Figure 3a−c and Table 3). However, as evidenced by SEM images, the morphology of interior pores varies with orifice diameter. Strictly speaking, produced hollow fibers demonstrate an asymmetric porous structure featuring L−L phase inversion and subsequent evolution of phase separated domain. In solutions composed of semicrystalline polymers like N6, physical gelation may occur as a result of polymer crystallization and/or vitrification of polymer-rich phase. The latter occurs after L−L phase demixing through which a homogeneous polymer solution separates into two coexisting phases in thermodynamic equilibrium, i.e., a polymer-lean phase and a vitrified polymer-rich phase whose glass transition exceeds the operating temperature.25 The way in which these two phenomena, i.e., physical gelation and L−L phase separation, are impressed from each other strongly depends on their sequences. When L−L demixing precedes, further evolution of phase separated domains is limited by gelation of polymer-rich phases. This gelation may be induced as a consequence of either vitrification or crystallization of polymer-rich regions. However, when crystallization-induced gelation surpasses, L−L demixing becomes less probable or even unlikely and crystals will be fixed in their locations.25,29,31 The appearance of hollow bore in N6 wet spun fibers with increasing orifice diameter can be explained based on the interplay of L−L phase inversion as well as physical gelation of fiber surface as a result of water absorption and solvent

extraction from spinning solution below the gelled layer. During spinning, the outer layer of dope comes into contact with a huge volume of water as ejected from the orifice. This volume of water is sufficient to form a stiff gelled layer with a solid-like behavior and specific thickness within a fraction of a second. This layer becomes ultimately a dense nonporous ROlike skin layer as evidenced by Figure 3. The gelation phenomenon, gelation time, and gel properties will be detailed in the following paragraphs. Further, it has been demonstrated that the gelation of fiber surface slows down the diffusion of water through the gelled wall into layers beneath25,31 whose gelation rates sequentially decrease with increasing distance from the surface layer. Simultaneously, solvent molecules leave the core and diffuse through the gelled layers into the coagulation bath. This event causes the polymer chains to deposit on the inner surface of the gelled layers, and hence, a bore is created inside the fiber. The bore becomes larger as time proceeds, and more solvent molecules leave the fiber core. Note that the skin layer on the bath side is dense enough to greatly retard diffusion of water molecules into the fiber during the spinning process. In contrast, the core side of skin layer is porous and poses little resistance against diffusion of solvent molecules. Totally, hollow bore evolution is controlled by surface gelation and subsequent increasing of polymer concentration going away from the fiber center is due to the solvent outflow. This process has been schematically illustrated in Figure 4. Additionally, orifice diameter is another prominent factor to be considered before any attempts to manufacture hollow fibers using single orifice spinnerets. When thickness of gelled wall exceeds the fiber radius, hollow bore disappears and bore-free fiber is expected (Figure 2a). This happens when an orifice with a small diameter is considered for wet spinning. The transition from bore-free fiber to hollow fiber with increment of orifice diameter has been schematically illustrated in Figure 5.

Table 3. Geometrical Characteristics of Produced N6 Hollow Fibers fiber ID

spinneret i.d. (mm)

spinning dope

outer diameter (o.d.) (μm)

inner diameter (i.d.) (μm)

wall thickness (μm)

thickness of RO-like skin layer (μm)

F2 F3 F4

0.4 0.7 0.8

A1 A1 A1

480 720 890

70 320 440

205 200 225

∼5 ∼5 ∼5

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viscoelastic graphs associated with dopes B4 and B5 becomes more pronounced, and eventually a drastic increase is observed for dope B6 which is located in the vicinity of the binodal curve. The values of G″ and G′ for prototypical liquid-like and solid-like materials have been compared in the literature.36,40 G″ > G′ is accordingly the characteristic feature of liquid-like behavior; however, G′ > G″ as well as less frequency dependence of G′ denotes solid-like behavior. In graphs c, d, and e corresponding to dopes B1, B2, and B3, respectively, G″ contains greater values than G′ which leads to tan δ > 1 (plot k) over the whole frequency range and the dopes are in solution state. In graph f, G″ and G′ overlap each other regardless of frequency and tan δ becomes frequency independent as drawn in plot k. This is a sign that shows the gelation phenomenon is happening as demonstrated by Li and Aoki.37 This rheological characterization of dope B4 provides further confirmation for the accuracy of experimentally determined crystallizationinduced gelation boundary in Figure 1. Passing from B4 to B5 and B6 dopes, the gelation proceeds and dopes B5 and B6 become predominantly elastic than viscous corresponding to G′ > G″ and tan δ < 1 as established by plots g, h, and k. The solid-like behavior was also observed for dope A4 which contains the same concentration of water as dope B6 but higher concentration of polymer, N6. These differences in composition are held responsible for enhanced elasticity of dope A4 rather than dope B6 as compared in graph i. Similar comparison can be made between viscoelastic graphs a and c related to dopes A1 and B1. As discussed above, the rheological images superimposed on the ternary phase diagram clearly demonstrate and monitor the variation of viscoelastic properties of spinning dope as well as the occurrence of gelation during immersion of N6/FA solution into the coagulation bath filled with nonsolvent, i.e., water. In other words, as evidenced by Figure 1, from B1 to B6 the water content of polymer dopes gradually increases, and hence, these dopes can be regarded as simulators of either different states that the spinning dope may have in a particular time during spinning or various gelled layers within the fiber structure as schematically investigated in Figure 4. According to the above rheological investigations, when the spinning dope of liquid-like behavior leaves the spinneret and immerses into the water bath, the composition of outer layers due to intimate contact with water quickly enters the gelation region and makes the dope solid-like. This accounts for the formation of a wall whose thickness varies with gelation properties of nonsolvent/ solvent/polymer ternary system. This solid wall is stiff enough (as approved in Figure 6b and Figure 6h) to provide produced hollow fiber with suitable mechanical properties (investigated in Table 5) and prevent it from collapsing. The times required for gelation of dopes A2−A4 and B4−B6 located in S−L immiscibility area in Figure 1 at 25 °C have been tabulated in Table 4. As is obvious, the gelation happens in shorter times as the composition of dope approaches the binodal curve. Additionally, dopes belonging to series A (A2− A4) become gel faster than corresponding dopes of series B because of higher polymer concentration of dopes A. Considering measured gel times and viscoelastic properties, it seems reasonable to postulate that dopes whose compositions fall in the L−L immiscibility area transform to gels within milliseconds, which is fast enough to form the wall of hollow fibers during spinning. In the case of hollow fibers spun from dope A1, no nonsolvent additive was introduced into the dope. However,

Figure 4. Schematic representation of hollow bore formation through wet spinning using a single orifice spinneret.

Figure 5. Schematic representation of transition from bore-free fiber to hollow fiber with increasing orifice diameter of the spinneret.

Overall, it can be said that developing hollow fibers using a single orifice spinneret depends on both orifice diameter and gelation behavior of nonsolvent/solvent/polymer system. The former determines fiber diameter such that its increase favors hollow fiber formation, whereas the latter controls thickness of the gelled wall which may also be influenced by solvent diffusivity. To gain more insight into the gelation process of N6-based solutions, the evaluation of linear viscoelastic properties of solutions composed of N6 in FA/water mixtures with different ratios (points A1, A4, and B1−B6 superimposed on the phase diagram (Figure 1)) is crucial. As shown in Figure 1, points B1−B6 with the same polymer concentration (ϕP = 0.25) occupy the entire area between the polymer/solvent axis and binodal boundary through crossing of the crystallizationinduced gelation curve. These points roughly resemble the state of initially homogeneous spinning solution stepwise after immersion into the coagulation bath. The formation of fibers and membranes via immersion precipitation through which solvent/nonsolvent exchange takes place with a given rate, has been well documented in the literature.32−34 Figure 6 depicts the changes of viscoelastic properties including storage modulus (G′), loss modulus (G″), and loss tangent (tan δ = G″/G′) with frequency for different polymer dopes, i.e., A1, A4, B1−B6. As evident, in all the plots, both G′ and G″ exhibit a growing trend with frequency. Contrarily, tan δ curves show a descending order as frequency rises. Strictly speaking, at high frequencies the polymer dopes behave like an elastic solid; however, with reduced frequency, viscous fluid behavior becomes dominant. Hence, it can be said the response of polymeric liquids including gels and solutions to an imposed stress can resemble the behavior like either a solid or liquid depending on the time sale of a particular deformation.35−39 The variations of G′, G″, and loss tangent from B1 to B6 have been compared in graphs i, j, and k of Figure 6, respectively. Considering the location of dopes B1−B6 in the phase diagram (Figure 1), within the miscibility area the viscoelastic functions, G′ and G″ for dopes B1−B3 follow a regular increment. Crossing the gelation boundary and passing through the S−L immiscibility area, the discrepancy between 7732


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Figure 6. Storage modulus (G′), loss modulus (G″), and loss tangent (tan δ) as functions of radial frequency (ω) for water/FA/N6 solutions of various concentrations (points A1, A4, and B1−B6 in Figure 1): (a) A1, (b) A4, (c) B1, (d) B2, (e) B3, (f) B4, (g) B5, (h) B6, (i) comparison of all G′ values, (j) comparison of all G″ values, and (k) comparison of all tan δ values.

structures of hollow fiber produced from dope B5 using orifice with d3= 0.8 mm have been depicted in Figure 7. Note that the elliptical shape of this hollow fiber (Figure 7a) returns to the sample preparation step for SEM analysis. As evident, hollow fiber produced from dope B5 inside the S−L miscibility gap (Figure 1) gives a symmetric structure with a dense nonporous cross section (Figure 7a1,a2) and surface (Figure 7b). The interconnected crystals primarily contain the interior structure of this hollow fiber where no vestige of L−L phase separation can be found.

Table 4. Gelation Times Measured at 25 °C for Different Polymer Dopes Indicated in Figure 1 dope gelation time

A2

A3

A4

B4

B5

B6

2h

7 min

1 min

3h

22 min

2 min

addition of nonsolvent into the spinning dope accelerates the S−L demixing,29,30 and hence, gelation induced as a result of polymer crystallization would dominate the morphology of produced fiber. SEM images captured from interior and surface 7733


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Figure 8. WAXD patterns of produced hollow fibers (F2, F3, F4, and F5) spun from dopes A1 and B5 using orifices with different diameters.

Figure 7. SEM images captured from cross section (a) and outer surface (b) of fiber (F4) spun from dope B5 using single orifice spinneret with diameter of d = 0.8 mm. Images a1 and a2 are magnified parts of image a.

Comparing the morphology of hollow fibers obtained from dopes A1 and B5, one can easily witness the pivotal contribution of relative rates of L−L and S−L phase separation mechanisms to morphology evolution of N6 hollow fibers. This rate can be tuned by altering the concentration of nonsolvent in the spinning dope. In addition, the discrepancy in fiber structure can be resolved by referring to the phase diagram of water/FA/N6 system (Figure 1). As is evident, dopes A1 and B5 locate in the miscibility area on solvent−polymer axis and S−L miscibility gap, respectively. Compared with dope A1, dope B5 is supersaturated with respect to polymer crystallization. At this condition, S−L demixing precedes L−L phase inversion and nonporous fibers are achieved. However, for the case of dope A1, crystallization takes places within the polymerrich domains attained when L−L phase demixing surpasses polymer crystallization. Therefore, porous morphology dominates the fiber structure. The effect of priority of L−L phase inversion over S−L one and vice versa on morphology of polymeric structures has been also argued in the literature.26,29,30 For the case of fibers spun from dope A1, disappearance of surface pores as illustrated in Figure 3e can be best explained based on spinodal decomposition and subsequent growth of phase-separated domains which is responsible for diminishing surface pores and has been emphasized in the literature.25,41 The influence of orifice diameter on crystalline structure of resultant fibers was also disclosed. In this work, since polymer solutions with the same concentration have been spun with spinnerets of different diameters, an infinitesimal volumetric element of solution inside the orifice with smaller volume is occupied with more population of polymer chains, equivalent to higher polymer concentration. This will lift the storage and loss moduli (see plots a and c of Figure 6) of polymer-rich phases to a higher level, and consequently the crystalline structure of produced hollow fibers varies with both orifice diameter and dope composition as explored in Figures 8 and 9 containing WAXD and DSC graphs, respectively. As shown in Figure 8, the WAXD patterns exhibit two distinct peaks centered around 2θ = 19.9° and 2θ = 23.5° which are characteristics of α crystalline phases.29,42 DSC thermographs record an endothermic peak at temperature about 220 °C (Figure 9) associated with the melting of the crystalline α form.42,43

Figure 9. DSC spectrum of produced hollow fibers (F2, F3, F4, and F5) spun from dopes A1 and B5 using orifices with different diameters.

As is obvious, DSC thermographs confirm corresponding WAXD spectra. On the other hand, the height as well as corresponding areas under the α-related peaks show a descending order with decreasing orifice diameter. It can be explained based on the increased confinement imposed by orifice walls of spinneret with smaller diameter against crystal growth. As well, when the same orifice is used, fibers spun from dope B5 have more crystallinity compared to that produced from dope A1. This can be interpreted by the fact that dope B5 has a high degree of supersaturation with respect to N6 crystallization. The mechanical properties as well as structural features along with pure water permeability and NaCl rejection of obtained hollow fibers have been listed in Table 5. According to data tabulated in Table 5, from a comparison of mechanical properties of fibers F1, F2, F3, F4, and F5, it can be found that creating hollow bore in the fiber structure and increased porosity contribute to weaken the produced fibers. This is due to the fact that the presence of pores would serve as points for stress concentration. The negative influence of porosity on mechanical characteristics of fibers has been also explored in the literature.28,44 Totally, by comparison with the data reported in literature,10,44,45 produced macrovoid-free hollow fibers exhibit a reasonable mechanical strength. However, F5 with higher crystallinity and lower porosity benefits from larger tensile modulus and stress than F2−F4. Contrarily, the measured water permeability for F5 is lower than that of F2−F4 because of lower porosity and higher crystallinity of F5 as is evident from SEM images, WAXD and DSC graphs (Figures 7, 8, and 7734


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Industrial & Engineering Chemistry Research Table 5. Different Properties of Wet-Spun N6 Fibers fiber ID

stress at break (MPa)

F1 F2 F3 F4 F5

Young’s modulus (MPa)

strain at break (%)

burst pressure (bar)

overall porosity (%)

mean pore size (nm)

water permeability L/(m2·h·bar)

Rs (%) (NaCl rejection)

180 160 145 140 170

15 20 26 30 20

8 9 8 9.5

71 70 73 70 20

14 12 15.5 3.1

2.3 3.2 4.5 1.1

83 82 91 95

5.6 5.3 4.9 4.5 5.3

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9). However, F5 represents the highest value for NaCl rejection based on Table 5. From comparison of the mechanical properties, burst pressure as well as water permeability and NaCl rejection of the hollow fibers F2−F4 with those associated with hollow fibers elaborated for FO10 and PRO46 processes, the hollow fibers F2−F4 can be considered as promising candidates for FO and PRO applications. The latter is a composite hollow fiber comprising a thin polyamide RO-like skin layer on a hydrophobic porous substrate, polyethersulfone (PES), which is manufactured during a two-step process. However, hollow fibers F2−F4 have been produced using a simple and single step process as described in preceding sections. From another point of view, the whole structure of F2−F4 including RO-like skin layer (shown in Figure 3) and support have been made from a hydrophilic polymer, N6. This causes all of the pores present in the hollow fiber structure to be wetted when exposed to water and hence imparts more water permeability and reduced ICP to the fiber F3 and F4. The importance of hydrophilicity of support on membrane performance has been explored by McCutcheon et al. and Huang et al.11,47 These features make the hollow fibers produced in this work promising candidates for osmotically driven membrane processes such as FO and PRO.10,47

4. CONCLUSIONS Nylon 6 macrovoid-free hollow fiber membranes for the first time were produced using a single orifice spinneret inside the coagulation bath without any additive or post-treatment process. It was demonstrated that two conditions must be satisfied to guarantee formation of hollow bore when no external additive is used: (1) gelation of fiber surface; (2) thickness of gelled wall must not exceed the fiber radius. Furthermore, morphology of the hollow fibers can be easily elaborated considering the phase behavior of nonsolvent/ solvent/polymer system and it must be emphasized the introduced method is not sensitive to the dope composition. Strictly speaking, selecting spinning dopes from various areas in ternary phase diagram results in desired morphologies suitable for different potential applications like forward osmosis or pressure retarded osmosis.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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REFERENCES

(1) Peng, N.; Widjojo, N.; Sukitpaneenit, P.; Teoh, M. M.; Lipscomb, G. G.; Chung, T. Sh.; Lai, J. Y. Evolution of polymeric hollow fibers as 7735


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Synergistic Contribution of Spinneret Diameter and Physical Gelation

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