P84 copolyimide dual-layer hollow fiber membranes with delamination-free morphology

Journal of Membrane Science 294 (2007) 132–146

A morphological and structural study of Ultem/P84 copolyimide dual-layer hollow fiber membranes with delamination-free morphology Natalia Widjojo, Tai Shung Chung ∗ , William B. Krantz Department of Chemical and Biomolecular Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 117602, Singapore Received 17 July 2006; received in revised form 15 February 2007; accepted 17 February 2007 Available online 21 February 2007

Abstract We have studied dual-layer Ultem/P84 hollow fiber membranes with various morphologies by using dual coagulation baths and different spinneret designs in this work. The effects of first external coagulant and bore-fluid chemistry as well as air-gap distance on the outer and inner layer morphology of the dual-layer hollow fibers have been investigated systematically. It is found that dual-layer hollow fiber membranes spun with a longer air gap show a larger size closed-cell structure compared to those spun at a shorter air gap possibly due to the partial phase inversion induced by water vapor at 65% relative humidity. In addition, the outer layer of hollow fibers spun using water or methanol in the first coagulation bath shows mostly an open-cell structure, whereas those spun using ethanol or 2-propanol exhibit mostly a closed-cell structure. To fulfill the delamination-free requirement for an ideal dual-layer hollow fiber for pressure-driven separation processes, two novel methods have been proposed in this work: (1) the addition of aluminium oxide (Al2 O3 ) nanoparticles in the inner layer followed by heat treatment; and (2) the introduction of early convective premixing with the aid of an indented and heated dual-layer spinneret. The first method has reduced the degree of shrinkage of the inner layer during heat treatment and thus lowers the heat-treatment temperature to avoid any delamination, e.g., from 175 ◦ C for 1 h to 150 ◦ C for 2 h. The second method facilitates interlayer molecular diffusion and thus eliminates delamination during the spinning process. No post-heat treatment is needed. © 2007 Elsevier B.V. All rights reserved. Keywords: Ultem/P84 copolyimide membranes; Dual-layer hollow fiber; Dual coagulation bath; Delamination; Nanoparticles; Dual-layer spinneret

1. Introduction The superiority of dual-layer over single-layer hollow fiber membranes has been demonstrated in recent years since the invention of the simultaneous co-extrusion process for duallayer hollow fibers about 25 years ago [1–14]. Not only does the dual-layer hollow fiber membrane have the advantages of traditional single-layer hollow fiber membranes, such as large surface area and reduced transport resistance by the asymmetric skin structure, but it also has the flexibility and potential to maximize separation performance as well as lower material costs by allowing the choice of two different materials for the outer and inner layers based on application requirements. The development of fluoropolyimide/polyethersulfone (PES) dual-layer hollow fibers for natural gas purification and

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separation [2], polysulfone (PSf)-zeolite/polyimide [3] and PES-zeolite/P84 [4] dual-layer hollow fibers for various gas separations are recent examples. The fluoropolyimide used in the work of Liu et al. [2] has an impressive separation performance, but it is extremely expensive. When co-extruding this material with polyethersulfone, the resultant dual-layer membrane consists of a 10–15 ␮m thick fluoropolyimide functional layer and a 100–150 ␮m thick PES supporting layer. The fluoropolyimide layers were further chemically modified to achieve a higher separation performance with suitable anti-plasticization properties. Mixed matrix materials made of Matrimid, PSf or PES and nano-size zeolite particles are new-generation membrane materials with enhanced separation performance [3–5]. However, spinning mixed matrix materials into a hollow fiber configuration with desirable particle distribution and morphology is a challenging task. Jiang et al. and Li et al.’s pioneering works demonstrate that the dual-layer co-extrusion process is far superior to the single-layer spinning process for the development of hollow fibers comprising an ultra-thin mixed matrix

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material layer. The former gives membrane scientists more freedom than the latter to manipulate the nanoparticle distribution across the membrane. The development of dual-layer hollow fiber membranes with desirable morphology and separation performance by simultaneously co-extruding two different dope solutions via a triple-orifice spinneret is still not a trivial task. An ideal dual-layer hollow fiber membrane for medium pressure-driven applications should have the following morphological characteristics: (1) the outer layer should possess a thin dense selective layer while the interface between the two layers and the surface of the lumen side must be porous to reduce transport resistance [6]; (2) no gap can be tolerated between the outer and inner layers of the dual-layer hollow fiber membrane—that is, the membrane must be delamination-free for consistent performance and longterm use; (3) the D/2h (dual-layer fiber diameter divided by twice the wall thickness) ratio should be approximately 2 in order to withstand high pressures [15,16]. To meet the above requirements, one must control both the inner and outer dope formulations, coagulant chemistry and precipitation conditions, spinning parameters, and spinneret design according to the polymer chemistry and structure, and physicochemical properties of the polymer dopes. It is widely known that a proper selection of the internal and external coagulants plays an important role in obtaining the desired membrane structure and morphology. Reuvers and Smolders [17] defined two types of liquid–liquid demixing processes in the polymer solutions during membrane formation via phase inversion method, denoted as delayed and instantaneous demixing. Not only does the type of demixing affect the formation of the skin layer but it also influences the formation of macrovoids in the membrane cross-section. Basically, the type of demixing depends on the interaction strength between the solvent and nonsolvent. A poor interaction between a solvent and a nonsolvent can result in delayed demixing, whereas a strong interaction between them can lead to instantaneous demixing. By utilizing dual coagulation baths, one should be able to tailor the desirable membrane structure so that it incorporates the desired characteristics of both types of demixing, e.g., a thin dense selective layer with porous support structures and macrovoid-free. The effect of dual coagulation baths on the gas-separation performance of both flat and single-layer hollow fiber membranes has been reported by Van’t Hof et al. [18]. They found that by using a series of alcohols with different molecular volumes and diffusion coefficients as the external coagulant they were able to affect the membrane morphology and separation performance. The choice of the first coagulation bath is critical since it determines the type of membrane demixing process. Based on Reuvers et al. and Van’t Hof et al.’s works, we believe that the dual-bath coagulation technique combined with a suitable choice of internal coagulant are worthy of investigation for fabricating dual-layer hollow fiber membranes in order to meet the first requirement, that is, to produce a thin dense selective layer with a highly porous substructure. Another important factor in fabricating dual-layer hollow fiber membranes is the choice of the outer and inner layer polymeric materials. The compatibility between the outer and inner

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layer materials could prevent delamination. An extensive study to produce delamination-free dual-layer hollow fiber membranes has been done by Li et al. [6]. They found that a good attachment between the outer and the inner layer of the dual-layer hollow fibers can be obtained by adjusting the air gap prior to immersion in the coagulation bath, bore-fluid composition, and polymer concentration of the inner layer dope solution. Thermal treatment has also been identified by Li et al. [4] as an effective way to close the gap between the outer and inner layers of a mixed matrix dual-layer hollow fiber. However, the major drawback of this method is that it significantly decreases the gas permeance of the dual-layer hollow fiber due to the densification of the membrane selective layer. Furthermore, the third requirement can be achieved by using a spinneret with a favorable dimension of diameter to wall thickness and adjusting the dope flow rate. The purpose of this study is to investigate the feasibility of fabricating dual-layer hollow fiber membranes consisting of Ultem polyetherimide as the outer layer. Ultem has been chosen for the following reasons: (1) it possesses a reasonably high separation factor for important gas separations (αO2 /N2 = 7.3, αCO2 /CH4 = 38.8) [19]; (2) since its glass-transition temperature (Tg ) is approximately 209 ◦ C [19], it has a greater potential to be the polymeric matrix for the mixed matrix material development [19,20]; and (3) its moderate cost in comparison to other polyimide materials makes it very attractive for membrane development. To date, no publications have appeared that discuss the production of dual-layer hollow fiber membranes using Ultem polyetherimide as an outer layer that fulfill the aforementioned three requirements. Pereira et al. [11] have produced dual-layer hollow fiber membranes using Ultem as their outer layer and PES as the inner layer; however, their dual-layer hollow fiber membranes displayed severe delamination. It appears harder to fabricate delamination-free dual-layer hollow fiber membranes using Ultem polyetherimide because its physicochemical properties are quite different from those of the fluoropolyimides and Matrimid. Therefore, the purpose of this research was to conduct a fundamental study on the science and engineering of developing Ultem/P84 dual-layer hollow fiber membranes with a delamination-free interface. Because there are many important factors affecting the morphology during the fabrication of dual-layer hollow fiber membranes, we first examine how the outer and inner layer structures are affected by altering the external and internal coagulants in combination with changing the air-gap distance. This research lays the foundation for choosing the internal and external coagulant chemistries in order to achieve desired structures for both the outer and inner layers. Subsequently, two novel methods to eliminate delamination are proposed and tested in this work: (1) the addition of aluminium oxide (Al2 O3 ) nanoparticles in the inner layer followed by heat treatment; and (2) the introduction of early convective premixing with the aid of an indented and heated dual-layer spinneret. To our best knowledge, no such approaches have ever been proposed in the literature for the fabrication of dual-layer hollow fiber membranes.

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Fig. 2. Critical concentration of outer layer dope solution (Ultem/NMP). Fig. 1. Chemical structures of (a) Ultem and (b) P84 copolyimide.

2. Experimental 2.1. Materials Ultem® 1010 (Tg = 209 ◦ C), supplied from General Electric, was used as the outer layer polymer material, whereas P84 copolyimide (Tg = 315 ◦ C) from HP Polymer Gmbh, Austria was selected as the inner supporting layer. Fig. 1 shows their chemical structures. N-Methyl pyrrolidone (NMP) (>99%) from Merck was used as the solvent for both polymeric materials. An alcohol series (methanol, ethanol, or 2-propanol from Merck) was used as the bore-fluid and for the first external coagulation bath. In addition, ethanol was used as an additive in the outer and inner layer dope solutions. Aluminium oxide (Al2 O3 ) with an average particle size of approximately 0.2 ␮m from Nanoscale Materials Inc. was employed as an additive in the inner dope solution for an exploratory study that will be discussed later. Methanol and hexane from Merck were used for the solvent exchange. All chemicals were used as received.

Fig. 3. Binodals for the ternary NMP/Ultem/water system (a) and NMP/Ultem/ ethanol system (b).

the dense selective layer, as suggested in the literature [21,22]. Fig. 3 shows a portion of the ternary phase diagrams for the Ultem/NMP/water and the Ultem/NMP/ethanol systems. The latter was selected because it has a slower precipitation rate that provides a wider operational window than the latter. Table 1 lists the sample identification ID code and compositions of the outer and inner layer dope solutions. Both the outer and inner dope solutions were prepared according to the following procedure. The mixture of NMP (solvent) and ethanol (nonsolvent) was stirred at 0–5 ◦ C inside an ice bath for 30 min. Then, the polymer was added slowly to the stirred solution. The

2.2. Spinning dope composition and preparation Fig. 2 shows the relationship between dope viscosity and Ultem concentration in NMP measured by an ARES Rheometer at a shear rate of 10 s−1 at 25 ◦ C. A polymer concentration of 32 wt% therefore was chosen in order to reduce defects in

Table 1 The dope compositions of Ultem/P84 copolyimide dual-layer hollow fiber membranes Solution ID

OL-1 IL-1 IL-2 IL-3 a b c

Layer

Outer layer Inner layer Inner layer Inner layer

Polymer Components

Concentration in solvent (wt%)c

Ultem 1010 P84 P84 P84

32 26 26 26

Dope viscosity was measured at 25 ◦ C with shear rate 10 s−1 . Tg was measured at second run with heating rate 10 ◦ C/min. Weight ratio of solvent to nonsolvent (NMP/EtOH) = 6.4/1.

Al2 O3 loading in polymer (wt%)

Dope viscosity (cP)a

Tg of dried membrane (◦ C)b

0 0 15 25

79,823 14,402 23,740 25,172

209 315 322.5 324.8

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Fig. 4. Dual-layer hollow fiber spinning setup with two coagulation baths: (A) bore-fluid tank, (B) dope solution tank, (C) syringe pump, (D) filter, (E) spinneret, (F) coagulation bath 1, (G) coagulation bath 2, (H) water sprayer and (I) take-up drum.

temperature of mixture was then increased gradually to room temperature. The dope solution was kept at high speed agitation until the polymer dissolved completely. All the solutions were degassed in the three-neck flask for 24 h and then placed into two ISCO syringe pumps. Spinning took place after another 24 h of degassing. The dope preparation for the inner layer with the addition of aluminium oxide nanoparticles was slightly different from that without nanoparticles. The desired amount of aluminium oxide was dispersed in the mixture of NMP and ethanol at a high speed mixing for 3 h. One fifth of the total polymer amount first was added slowly into the solution. Once it fully dissolved, the remainder of the polymer was added. The subsequent steps were the same as those outlined above for the dope preparation in the absence of nanoparticles.

and then transported over rollers to the second coagulation bath (i.e., water). A take-up drum was used to collect the fibers. The as-spun dual-layer hollow fiber membranes were cut into pieces of approximately 30 cm and immersed in a clean water bath for 2 days. The fibers were solvent-exchanged by three consecutive 30-min immersions in a methanol circulation. The same procedure then was repeated using n-hexane. Finally, these fibers were dried in air at ambient temperature. 2.4. Heat treatment protocol Heat treatment processes were performed in a precision high-temperature programmable furnace (CenturionTM Neytech

2.3. Dual-layer hollow fiber spinning process Fig. 4 illustrates the setup for spinning dual-layer hollow fiber membranes with two coagulation baths. The dope solutions and bore-fluid were extruded at specified flow rates through a triple-orifice spinneret using three ISCO syringe pumps. A dryjet wet-spinning process was used. The spinning procedure has been described in our previous work [2,5,7–9]. The same nonsolvent such as water or one of the alcohols (i.e., methanol, ethanol, and 2-propanol) was used simultaneously for both the bore-fluid and first external coagulant. Two dual-layer spinnerets with a flat exit feature (referred as “the original design”) and indent premixing feature (referred as “the modified design”) were fabricated. Their dimensions are indicated in Fig. 5. Table 2 tabulates the sample ID and spinning conditions using the original design spun at room temperature, whereas Table 3 summarizes the sample ID and spinning conditions for the original and modified designs spun at various temperatures and flow rates. The resultant fibers were immersed in the first coagulation bath for less than 30 s depending on the take-up speed and air-gap distance

Fig. 5. Original and modified dual-layer spinneret designs.

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Table 2 Spinning conditions of Ultem/P84 copolyimide dual-layer hollow fibers using the original dual-layer spinneret at 25 ◦ C* Spinning ID

Outer dope (OL-1) flow rate (cm3 /min)

DL-1 DL-2 DL-3 DL-4 DL-5 DL-6 DL-7 DL-8 DL-9 DL-10 DL-11 DL-12 DL-13 DL-14 DL-15 DL-16 *

0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06 0.06

Inner dope Solution ID

Flow rate

IL-1 IL-1 IL-1 IL-1 IL-1 IL-1 IL-1 IL-1 IL-1 IL-1 IL-1 IL-1 IL-1 IL-1 IL-1 IL-1

0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6 0.6

(cm3 /min)

Bore-fluid and first external coagulant

Air gap (cm)

Water Methanol Ethanol 2-Propanol Water Methanol Ethanol 2-Propanol Water Methanol Ethanol 2-Propanol Water Methanol Ethanol 2-Propanol

0 0 0 0 1 1 1 1 5 5 5 5 10 10 10 10

Bore-fluid flow rate: 0.3 cm3 /min; second external coagulant: water; coagulant bath and spinning temperature: 25 ◦ C; relative humidity: 65%.

Qex). The heat treatment was used to eliminate delamination in the dual-layer hollow fiber membranes. Its procedure was to anneal the as-spun fibers from room temperature to a high temperature (125,150, or 175 ◦ C) at a heating rate of 1 ◦ C/min under vacuum; then hold them for a period of time (1 or 2 h) before cooling down naturally. 2.5. Membrane characterization The morphology of the dual-layer hollow fiber membrane was observed using field emission scanning electron microscopy (FESEM JEOL JSM-6700LV) and scanning electron microscopy (SEM JEOL JSM-5600LV). The sample was fractured in liquid nitrogen and coated with platinum before FESEM or SEM analysis. The glass-transition temperature (Tg ) of the inner layer of the dual-layer hollow fiber membranes was measured using a Mettler-Toledo Model 822E Differential Scanning Calorimeter. Pure gas permeation tests were carried out using the constant volume method described elsewhere [23]. Each module

consists of 10 fibers with an effective length of approximately 10 cm. The gas permeation tests were run at 100 [psi] for all experimental conditions. The permeance, P/L, was determined using the following equation: Q Q P = = L AP nπDlp

(1)

where P is the permeability of separating layer (Barrer), L the thickness of the apparent dense-selective layer (cm), Q the pure gas flux (cm3 /s), n the number of fibers in one testing module, D the outer diameter of the testing fibers (cm), l the effective length of the modules (cm), and p is the gas pressure difference cross the membrane (cm Hg). The permeance unit is GPU (1 GPU = 1 × 106 cm3 (STP)/cm2 s cm Hg). The ideal separation factor, αA/B , can be determined from the following equation: αA/B =

(P/L)A (P/L)B

(2)

Table 3 Spinning conditions of Ultem/P84 copolyimide dual-layer hollow fibers Spinning ID

DL-17 DL-18 DL-19 DL-20 DL-21 DL-22 DL-23 DL-24

Outer dope (OL-1) flow rate (cm3 /min)

Inner dope Solution ID

Flow rate (cm3 /min)

0.09 0.09 0.09 0.06 0.06 0.06 0.06 0.06

IL-1 IL-2 IL-3 IL-1 IL-1 IL-1 IL-1 IL-1

0.9 0.9 0.9 0.6 0.6 0.6 0.6 0.6

Bore-fluid and first external coagulant

Spinneret temperature (◦ C)

Type of dual-layer spinneret

Ethanol Ethanol Ethanol Water Water Water Water Water

25 25 25 60 70 25 60 70

Original Original Original Original Original Modified Modified Modified

Air-gap distance, 1 cm; bore-fluid flow rate, 0.3 cm3 /min; second external coagulant, water; coagulant and spinning temperature, 25 ◦ C; relative humidity, 65%.

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3. Results and discussion 3.1. Overall morphology of the as-spun dual-layer hollow fiber membranes Fig. 6 illustrates a typical morphology of the Ultem/P84 copolyimide dual-layer hollow fiber membranes for samples: DL-1 to DL-16. At a magnification of 50,000, the dual-layer fiber has a dense skin in the outer layer. The interfacial layers, i.e., the inner surface of the outer layer and the outer surface of the inner layer, are porous but de-laminated. It appears that different first coagulation bath media and bore-fluids do not result in significant morphological differences in the outer selective skin; however, the bore-fluid chemistry does affect the inner surface morphology of the inner layer. Generally, as shown in the upper right-hand side of Fig. 6, all inner surfaces of the inner layers spun for the conditions in Table 2 are porous but with different degrees of porosity. Those fibers spun using water or methanol as the bore-fluid result in a tighter inner surface morphology compared to those spun using ethanol or 2-propanol. 3.2. Outer layer morphology The outer layer of the dual-layer hollow fiber membranes usually determines the separation performance, while the inner layer provides the mechanical strength to withstand high pressures.

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To obtain a high separation performance, a thin dense selective layer with an asymmetric structure is desirable for the outer layer. Dual coagulation baths provide the advantage of using only a small amount of organic solvent in the first coagulation bath to tailor the desired outer layer structures, while still using a large amount of low-cost water to precipitate the entire membrane. Fig. 7 illustrates the effect of different media in the first coagulation bath on the outer layer morphology for fibers spun with a 1 cm air gap (i.e., samples ID: DL-5 to DL-8). Note that the right-hand side of each SEM image is close to the outer skin of the outer layer, while the left-hand side is close to the inner skin of the outer layer. Note also that the outer layer of the hollow fibers spun using water or methanol in the first coagulation bath shows mostly an open-cell structure, whereas those spun using ethanol or 2-propanol exhibit mostly a closed-cell structure. The explanation for this structure difference is complex. One of the possible reasons arises from different NMP diffusion coefficients in these nonsolvents, which are summarized in Table 4. NMP has a higher diffusion coefficient in methanol and water than in ethanol and 2-propanol. In addition, the contributions to the solubility parameters for the solvent and nonsolvents summarized in Table 5 indicate that water and methanol have much stronger hydrogen bond strength than ethanol and 2-propanol. Therefore, the former two should have a greater interaction with NMP than the latter two. Table 5 also permits determining the difference in solubility parameters between Ultem and these nonsolvents, the order for which is as

Fig. 6. SEM images of different skins of Ultem/P84 copolyimide dual-layer hollow fiber membranes.

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Fig. 7. SEM images of the outer layer cross-section morphology spun at 1 cm air gap. The bore-fluid and the first external coagulant are (A) water, (B) methanol, (C) ethanol and (D) 2-propanol. (The right-hand side of each picture is the outer skin, whereas the left-hand side is the inner skin of the outer layer.) Table 4 Properties of solvent and nonsolvents (adapted from Refs. [18,24]) Nonsolvent

˚ 3) Molecular volumea (A

Boiling point at 1 atm (◦ C)

Vapor pressure (mmHg at 20 ◦ C)

Specific gravity

Viscosity (cP)

o b DNMP/NS −6 2 (10 cm /s)

Water Methanol Ethanol 2-Propanol NMP

30 67 97 127 160

100 64.5 78 82 202

17.5 96 43.89 33 –

1.00 0.792 0.789 0.790 1.026

1.00 0.60 1.22 2.40 1.65

8.70 16.20 8.50 4.70 –

a b

The volume per molecule is calculated by the molecular weight divided by the density and Avogadro number. Diffusion coefficients of NMP in almost pure nonsolvent (infinite dilution of NMP in nonsolvent).

follows: (Ultem–water) > (Ultem–methanol) > (Ultem–ethanol) > (Ultem–2-propanol). A greater solubility parameter difference usually implies a faster precipitation rate. Furthermore, these nonsolvents have different molecular sizes following the order of water < methanol < ethanol < 2-propanol. Consequently, the outer layer of the dual-layer hollow fibers precipitated in water or methanol are more likely to undergo instantaneous demixing and form an open-cell structure, whereas those precipitated in ethanol or 2-propanol should lead to delayed demixing and form mostly a closed-cell structure. Table 5 Solubility parameter of polymers, solvent, and nonsolvents (adapted from Refs. [25–27]) Chemicals

δd

δp

δh

δt

Water Methanol Ethanol 2-Propanol NMP Ultem P84

15.60 15.10 15.80 15.80 18.0 – –

16.0 12.30 8.80 6.10 12.30 – –

42.30 22.30 19.40 16.40 7.20 – –

47.80 29.60 26.50 23.50 22.90 23.70 36.90

δd , dispersive parameter (J/cm3 )1/2 ; δp , polar parameter (J/cm3 )1/2 ; δh , hydrogen bonding parameter (J/cm3 )1/2 ; δt , total solubility parameter (J/cm3 )1/2 .

3.3. The effect of air-gap distance on the outer layer morphology Fig. 8 shows the effect of air-gap distance to the outer layer morphology using water as the bore-fluid and first external coagulant (samples ID: DL-1, 5, 9, and 13). Interestingly, the wet-spun (i.e., no air gap) dual-layer hollow fiber has an outer layer filled with a sponge-like microporous structure, while those spun at longer air gaps bring about cellular macrovoids near the outer skin. As shown in Fig. 8B–D, the size and the number of cellular macrovoids seem to increase with an increase in the air-gap length. In addition, these cellular macrovoids gradually transform from open-cell to more closed-cell structures. Fig. 9 shows the outer layer cross-section morphology as a function of precipitation medium (samples ID: DL-13, 14, 15, and 16). Those spun using pure water or methanol as the first external coagulant were composed of both closed-cell cellular macrovoids and a microporous sponge-like structure. Approximately two thirds of the cross-sectional area near the outer skin contains closed-cell cellular macrovoids, however the remaining area near the interface between the two layers has a microporous structure. Not surprisingly, those fibers spun using ethanol or 2-propanol as the first external coagulant consist of mainly a closed-cell structure in the outer layer.

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Fig. 8. SEM images of the outer layer cross-section of the dual-layer hollow fiber membranes spun with water as the first external coagulant and bore-fluid at (A) 0 cm, (B) 1 cm, (C) 5 cm and (D) 10 cm air gaps. (The right-hand side of each picture is the outer skin, whereas the left-hand side is the inner skin of the outer layer.)

The most likely causes for this morphological change in the outer layer of dual-layer hollow fibers are due to moisture-induced phase separation [28–32], possibly enhanced by gravitational forces [33,34], and the solubility parameter difference. When a fiber is extruded through a spinneret, there are two coagulations taking place in a dry-jet wet-spinning process. If a strong coagulant is chosen as the bore-fluid, phase separation begins immediately on the lumen side after extrusion from the spinneret, whereas a small amount of moisture-induced phase separation can occur near the external surface of the hollow fiber

in the air-gap region [28,29]. Complete phase separation can take place near the external surface of the hollow fiber only when the fiber is fully immersed in the coagulant bath. Since the solvent used for the spinning dope is nonvolatile (i.e., the NMP boiling point is 202 ◦ C), the evaporation of the solvent during the air-gap exposure is negligible [30]. However, NMP likes water and adsorbs water. In addition, the evaporation of the nonsolvent additive (i.e., ethanol) in the spinning dope solution helps the intake of water vapor. Several researchers have studied water adsorption on flat membranes made of polysulfone

Fig. 9. SEM images of the outer layer cross-section morphology spun at 10 cm air gap. The bore-fluid and the first external coagulant are (A) water, (B) methanol, (C) ethanol and (D) 2-propanol. (The right-hand side of each picture is the outer skin, whereas the left-hand side is the inner skin of the outer layer.)

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Table 6 Permeance tested at 100 psi and 25 ◦ C Sample ID

DL-1 DL-2 DL-3 DL-4 DL-5 DL-6 DL-7 DL-8 DL-13 DL-14 DL-15 DL-16

First coagulant and bore-fluid

Air gap (cm)

Water Methanol Ethanol 2-Propanol Water Methanol Ethanol 2-Propanol Water Methanol Ethanol 2-Propanol

0 0 0 0 1 1 1 1 10 10 10 10

Before coating

After silicone rubber coating

PO2 (GPU)

PN2 (GPU)

αO2 /N2

PO2 (GPU)

PN2 (GPU)

αO2 /N2

17.15 58.38 13.67 6.02 9.44 11.53 9.24 2.91 3.37 6.32 3.02 0.87

17.37 61.01 14.35 2.92 9.59 12.18 7.43 1.24 3.48 6.55 1.29 0.26

0.99 0.96 0.95 2.06 0.98 0.95 1.24 2.35 0.97 0.97 2.34 3.39

5.74 47.96 4.76 2.01 3.75 4.38 3.46 1.09 0.9 2.3 2.31 0.32

5.12 42.83 2.37 0.53 1.58 3.74 0.93 0.25 0.14 1.88 0.5 0.06

1.12 1.12 2.01 3.82 2.37 1.17 3.74 4.42 6.34 1.22 4.64 5.24

P = permeance (GPU) and α = selectivity.

[28] and polyetherimide [31,32]. They reported the amount of water intake increased with an increase in humidity. Morphological results very similar to those reported here were obtained by Tsai et al. [28] who observed their polysulfone (PSF) fibers changing from a bicontinuous structure to a cellular structure with an increase in the air-gap length, whereas Caquineau et al. [31] found the uppermost layer (which is adjacent to the air) of their flat Ultem membranes becoming a cellular structure with an increase in humidity. Both studies reported a greater degree of closed-cell structure with higher humidity. Clearly, the adsorbed water initiates a phase separation in the as-prepared membrane and induces a structure change in the cross-section near the outer surface. Khare et al. [33] also reported a distinct layer having large-pores at the top surface of dry-wet cast flat films. Their mathematical analysis indicates that the water sorption from air to the cast film leads to the development of large NMP gradients in the top layer (which is facing the air) and thus results in localized polymer dilution, which in turn leads to the formation of the large-pore region at the top surface. A comparison of our SEM images with those obtained from flat membranes [31] indicates that both have a similar thickness (around 5–12 ␮m) of closed-cell structure from the outer surface. Since the duration of the nascent hollow fiber exposure to air (in the air-gap region) is much shorter than that for the drywet cast flat film, the development of a closed-cell structure in hollow fibers must take place much faster than those in flat membranes. It is interesting to speculate how the cellular structure can become fully developed in the outer fiber layer in this short time. The formation of a cellular structure might possibly be facilitated by the gravitational forces and elongational stresses [34] as well as slow precipitation rates associated with a small solubility parameter difference. Prior studies [35] indicate that the external spin-line stresses created by gravity, the weight of the fiber and the take-up force could enhance the phase instability and facilitate phase separation during the spinning process. The small solubility parameter difference between Ultem and ethanol, and Ultem and isopropanol suggest delayed demixing that could facilitate morphological rearrangement inside the nascent fiber.

Generally, the effects of air gap, bore-fluid, and the first coagulant chemistry on membrane morphology can be characterized by their air separation performance. Table 6 shows the permeance of these dual-layer hollow fiber membranes prepared using various air-gap lengths, bore-fluid chemistry and first external coagulants. Because of the development of a closed-cell structure, the permeance decreases with an increase in air-gap length. This phenomenon is true for all samples studied as long as they were spun from the same bore-fluids and first external coagulants. Knudsen diffusion prevails in those fibers spun using water or methanol as the bore-fluid and the first external coagulant because their O2 /N2 selectivity is less than 1. In contrast, some manifestation of a solution-diffusion separation mechanism can be observed for hollow fibers spun using ethanol or 2-propanol as the bore-fluid and the first external coagulant because their O2 /N2 selectivity is greater than 1. In addition, the selectivity of the latter increases with an increase in air-gap length because of an increase in the thickness of the closed-cell cellular structure. 3.4. Inner layer morphology and macrovoid formation The presence of macrovoids in the membrane is usually detrimental and unfavorable for high pressure-driven separation processes because it tends to reduce membrane mechanical strength [36,37]. Fig. 10 (sample ID: DL-1, 5, 9, and 13) represents the effect of air-gap length on macrovoid formation in the inner layer of the dual-layer hollow fiber membranes spun using water as the internal and external coagulants. The number of macrovoids increases with an increase in air-gap length. A similar phenomenon has also been found during the spinning of single-layer P84 copolyimide hollow fiber membranes [38]. Some researchers have suggested that solutocapillary convection in the dope solution can enhance the nonsolvent intrusion that causes macrovoid formation [38–40]. This possibly can occur for the following reason. The lumen-side structure is frozen almost instantaneously in the air-gap region due to the use of water as a strong internal coagulant, while the outer layer

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Fig. 10. The SEM images of the cross-section of the dual-layer hollow fiber membranes spun with water as the first external coagulant and bore-fluid at (a) 0 cm, (b) 1 cm, (c) 5 cm and (d) 10 cm air gaps.

skin structure has not been fixed yet. Although NMP and water are highly miscible, the rapid precipitation densifies the inner skin that hinders the further transport of NMP to the lumen side. The early loss of NMP into the lumen side creates a steep concentration gradient across the membrane thickness, while the early diffusion of water through the lumen side results in a localized homogenous supersaturated solution. Both effects are destabilizing and could promote demixing. On the other hand, the moisture-induced surface-tension gradient at the interface of the dispersed phase microdroplets and the die-swell-induced chain relaxation that occurs at the outer moving surface help to drive the solutocapillary convection within the dope solution. Thus, an instantaneous liquid–liquid demixing could occur in the homogenous supersaturated solution. The nucleation of droplets and their subsequent growth enhanced by solutocapillary convection in the polymer-lean phase results in macrovoids in the cross-section of the inner layer. In addition to the air-gap length, the type of the first coagulation bath and bore-fluid also have a significant impact to the formation of macrovoids in the inner layer. Fig. 11 indicates that the number of macrovoids in the inner layer of hollow fibers spun at an air gap of 5 cm increases as a function of bore-fluid and first external coagulant chemistry according to the following order: water > methanol  ethanol = 2-propanol. No macrovoids can be found in the inner layer of the dual-layer hollow fiber membranes spun using either ethanol or 2-propanol as the first coagulation bath and internal coagulant. Clearly, a change in precipitation type from instantaneous, i.e., using water or methanol as internal and external coagulants, to delayed demixing, i.e., using ethanol or 2-propanol as internal and external coagulants, can prevent the formation of macrovoids in the inner layer. As previously discussed and shown in Tables 4 and 5,

ethanol and 2-propanol have larger molecular volumes, lower diffusion coefficients, and lower solubility differences with P84 than do either water or methanol. These physicochemical characteristics are thought to be the main causes for the delayed demixing and the absence of macrovoids. It seems that there is a minor difference on the order with respect to the number of macrovoids in the inner layer as a function of bore-fluid and first external coagulant chemistry between the wet-spun and dry-jet wet-spun fibers. The former (Fig. 12) followed the order of methanol > water = ethanol  2propanol, while the latter (Fig. 11) followed the order of water > methanol  ethanol = 2-propanol. This phenomenon may arise from the competitive effects of diffusivity and solubility parameter differences among the nonsolvents and the nascent hollow fiber on membrane formation. From Tables 4 and 5, it can be seen that the diffusivity coefficient (D) of NMP is greater in methanol than in water, whereas the solubility difference (δ) between methanol and NMP was smaller than that between water and NMP. The solubility parameter difference determines the precipitation rate of the nascent membrane, while the diffusivity affects the speed of the moving front and the rate of solvent exchange. For the wet-spun fiber, both the inner and outer skins precipitate at almost the same time. The higher diffusivity between NMP and methanol may be the cause of the slightly higher degree of nonsolvent intrusion and the subsequent formation of macrovoids. For the dry-jet wet-spun fiber, the inner skin of the inner layer would precipitate much faster than its outer skin. The greater solubility parameter difference between NMP/P84 and water may result in a more rapid outer skin precipitation and subsequently supersaturated environments favorable to macrovoid formation.

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Fig. 11. The SEM images of the cross-section of the dual-layer hollow fiber membranes spun at 5 cm air gap with the first external coagulant and bore-fluid (A) water, (B) methanol, (C) ethanol and (D) 2-propanol.

3.5. The delamination phenomenon in the dual-layer hollow fiber membranes Fig. 12 shows the SEM images of the cross-sectional morphology of wet-spun dual-layer hollow fibers (samples ID: DL-1 to DL-4). It can be observed that delamination has occurred in all dual-layer fibers spun with various first coagulation baths

and bore-fluids. This phenomenon might arise from the fact that there is a large difference in shrinkage between the inner and outer layers [6,7]. Delamination is usually observed after solvent exchange processes because the inner layer has a higher shrinkage rate than the outer layer. Previous studies by our research group on the PESzeolite/P84 dual-layer hollow fiber indicate that the delami-

Fig. 12. SEM images of the cross-section of the dual-layer hollow fiber membranes spun at 0 cm air gap with the first external coagulant and bore-fluid (A) water, (B) methanol, (C) ethanol and (D) 2-propanol.

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Fig. 13. The effect of (A) 0 wt%, (B) 15 wt%, and (C) 25 wt% Al2 O3 loading in the inner layer, heat-treatment temperature, and time on delamination.

nation can be eliminated after heat treatment over 200 ◦ C for 2 h [4]. In the studies reported in this paper, delamination can be eliminated by heat treatment at 175 ◦ C for 1 h, as shown in Fig. 13A (sample ID: DL-17). Clearly, the outer layer shrinks more than the inner layer because a low Tg polymer (i.e., Ultem) and a high Tg polymer (i.e., P84) were chosen as the outer and inner layer materials, respectively; there is a huge difference in Tg between these two materials. As a result, the outer layer can adhere nicely to the inner layer after heat treatment. However, the drawback of this method is the densification of the outer layer that causes a significant flux reduction. In order to reduce this drawback, we explored the following two approaches: (1) adding aluminium oxide particles into the inner layer dope solution to reduce the shrinkage of the inner layer as well as lower the heat-treatment temperature; (2) introducing early convective premixing with the aid of an indented and heated dual-layer spinneret. The first approach aims to reduce the inner layer shrinkage, while the second approach attempts to create interlayer-diffusion and prevent delamination. 3.5.1. The effect of Al2 O3 particle loading on delamination To study the feasibility of incorporating Al2 O3 particles into the inner layer on preventing delamination, we spun dual-layer hollow fibers using different particle loadings (i.e., 0; 15; and 25 wt%). Ethanol was chosen as the first coagulation bath and internal coagulant during the spinning (sample ID: DL-17 to DL19) and a short air gap of 1 cm was used because these conditions produced hollow fiber membranes spun from dopes without Al2 O3 with a desirable morphology, i.e., no macrovoid formation and porous structures. Al2 O3 particles were used because they were inexpensive and readily available.

Fig. 13 validates our hypothesis that the addition of Al2 O3 particles can decrease the inner layer shrinkage relative to that observed in the absence of any particle loading. For example, the delamination could be eliminated with heat treatment at 175 ◦ C for 1 h for the dual-layer sample without Al2 O3 loading, whereas it could be eliminated at a lower heat-treatment temperature (150 ◦ C for 2 h) for the dual-layer fibers with 15 and 25 wt% particle loadings. Therefore, we concluded that although these studies indicate that the addition of Al2 O3 particles cannot eliminate delamination, their addition can lower the heat-treatment temperature required to eliminate the delamination. The degree of shrinkage of the inner layer of the dual-layer hollow fiber membranes is defined as follows: DS (%) =

OD before heat treatment − OD after heat treatment OD before heat treatment ×100 (3)

Fig. 14. % Shrinkage of the outer diameter of the inner layer as a function of heat-treatment temperature (1 h). (For interpretation of the references to colour in this artwork, the reader is referred to the web version of the article.)

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Fig. 15. SEM images of the cross-section of the dual-layer hollow fiber membranes spun using (a) original and (b) modified dual-layer spinneret at different spinneret temperatures.

where DS is the degree of shrinkage, and OD is the outer diameter of the inner layer of the dual-layer hollow fiber membranes. Fig. 14 shows the degree of shrinkage of the outer diameter of the inner layer as a function of heat-treatment temperature for 1 h. At the heat-treatment temperature of 125 ◦ C, the inner

layer without Al2 O3 particle addition shows 1.11% shrinkage, whereas there is no shrinkage in those inner layers that contain Al2 O3 particles. A possible hypothesis to explain the reduction in shrinkage and heat-treatment temperature is that the presence of the Al2 O3

Fig. 16. SEM images of the cross-section of the dual-layer hollow fiber membranes spun using (a) original and (b) modified dual-layer spinneret at different spinneret temperatures.

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particles could rigidify the polymer chains [41,42]. Table 1 indicates that the Tg of the inner layer increases with an increase in Al2 O3 particle loading, which provides strong evidence for chain rigidification. Hence, we speculate that the overall shrinkage of the inner layer during fiber drying and heat treatment is due to the rigidification of the polymer chains near the Al2 O3 particles. 3.5.2. The effect of an indented and heated dual-layer spinneret on delamination Another method for avoiding delamination as well as enhancing the interlayer adhesion is to facilitate the interlayer-diffusion between the outer and inner polymer dope solutions. There are three possible approaches for achieving this objective: (1) by heating the spinneret at 60–70 ◦ C; (2) by utilizing a modified dual-layer spinneret as shown in Fig. 15; and (3) by combining methods (1) and (2). The first approach aims to lower the dope viscosity and thereby promote an enhanced interlayer-diffusion rate, while the second could initiate the premixing of the outer and inner dope solutions before extrusion from the dual-layer spinneret. The high temperature initiated interlayer-diffusion has been reported by Jiang using an unmodified dual-layer spinneret [9]. Fig. 15 illustrates that neither the first (sample ID: DL-20 and DL-21) or second approach (sample ID: DL-22) alone can prevent delamination. For the first approach, it is due to the fact that there is not enough contact time between the two polymer dope solutions before immersion in the coagulation bath. For the second approach, although a longer contact time is provided by the indented dual-layer spinneret, a low interlayer-diffusion rate at room temperature possibly hinders the molecular diffusion induced by this modified spinneret. Therefore, a combination of the indented spinneret with a high spinning temperature was explored to offset their respective drawbacks and thereby to avoid delamination. Figs. 15b and 16b (sample ID: DL-23 and DL-24) validate this approach and show that the outer layers are closely bound to the inner layer relative to those in Figs. 15a and 16a (sample ID: DL-20 and DL-21). 4. Conclusions Delamination-free Ultem/P84 dual-layer hollow fiber membranes have been successfully fabricated by a dry-jet wetspinning phase-inversion technique using premixing facilitated by an indented triple-orifice spinneret and high temperatures. The following conclusions can be drawn from this work: (1) The outer layer of the dual-layer hollow fibers spun using water or methanol as the first coagulant shows less closed cell but a more porous structure than those spun using ethanol or 2-propanol due to the different types of demixing (i.e., instantaneous versus delayed demixing). (2) Water vapor during the air-gap exposure could induce phase separation; therefore, the dual-layer hollow fibers spun at longer air-gap lengths (i.e., 5 or 10 cm) normally form more closed-cell structures in the outer layer than those spun at shorter air gaps (i.e., 0 or 1 cm).

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(3) To fabricate delamination-free dual-layer hollow fiber membranes, two approaches have been proposed: (1) incorporating aluminium oxide particles into the inner layer followed by heat treatment; (2) introducing an early convective premixing with the aid of an indented and heated dual-layer spinneret. The first method reduced the degree of shrinkage of the inner layer during the heat treatment and thus lowered the heat-treatment temperature to mitigate the delamination, while the second method successfully eliminated delamination during the spinning process without any post treatment. Acknowledgements The authors are grateful to NUS for funding this research via grant R-279-000-184-112. Ms. Widjojo would like to thank UOP for additional support via grant N-279-000-010-001. Special thanks are extended to Ms. Jiang Lanying and Mr. Li Yi for their useful suggestions. The authors would like to thank the reviewers for their valuable comments on macrovoid formation. References [1] W. Henne, G. Dunweg, W. Schmitz, R. Poble, F. Lawitzki, Method of producing dialyzing membrane, US Patent 4,164,437 (1979). [2] Y. Liu, T.S. Chung, D.F. Li, R. Wang, High-selectivity anti-plasticization cross-linked polyimide/polyethersulfone dual-layer hollow fiber membranes, Ind. Eng. Chem. Res. 42 (2003) 1190. [3] L.Y. Jiang, T.S. Chung, S. Kulprathipanja, An investigation to revitalize the separation performance of hollow fibers with a thin mixed matrix composite skin for gas separation, J. Membr. Sci. 276 (2006) 113. [4] Y. Li, T.S. Chung, Z. Huang, S. Kulprathipanja, Dual-layer polyethersulfone (PES)/BTDA-TDI/MDI co-polyimide (P84) hollow fiber membranes with a submicron PES-zeolite beta mixed matrix dense-selective layer for gas separation, J. Membr. Sci. 277 (2006) 28. [5] L.Y. Jiang, T.S. Chung, C. Cao, Z. Huang, S. Kulprathipanja, Fundamental understanding of nano-sized zeolite distribution in the formation of the mixed matrix single- and dual layer-asymmetric hollow fiber membranes, J. Membr. Sci. 252 (2005) 89. [6] D.F. Li, T.S. Chung, R. Wang, Morphological aspects and structure control of dual-layer asymmetric hollow fiber membranes formed by a simultaneous co-extrusion approach, J. Membr. Sci. 243 (2004) 53. [7] Y. Li, C. Cao, T.S. Chung, K.P. Pramoda, Fabrication of dual-layer polyethersulfone (PES) hollow fiber membranes with an ultrathin dense selective layer for gas separation, J. Membr. Sci. 245 (2004) 53. [8] D.F. Li, T.S. Chung, R. Wang, Y. Liu, Fabrication of fluoropolyimide/polyethersulfone dual layer asymmetric hollow fiber membranes for gas separation, J. Membr. Sci. 198 (2002) 211. [9] L.Y. Jiang, T.S. Chung, D.F. Li, C. Cao, S. Kulprathipanja, Fabrication of Matrimid/polyethersulfone dual-layer hollow fiber membranes for gas separation, J. Membr. Sci. 240 (2004) 91. [10] D. Wang, K. Li, W.K. Teo, Preparation of annular hollow fiber membranes, J. Membr. Sci. 166 (2000) 31. [11] C.C. Pereira, R. Nobrega, K.V. Peinemann, C.P. Borges, Hollow fiber membranes obtained by simultaneous spinning of two polymer solutions: a morphological study, J. Membr. Sci. 226 (2003) 35. [12] T. He, M.H.V. Mulder, H. Strathmann, M. Wessling, Preparation of composite hollow fiber membranes: co-extrusion of hydrophilic coatings onto porous hydrophobic support structures, J. Membr. Sci. 207 (2002) 143. [13] O.M. Ekiner, R.A. Hayes, P. Manos, Novel multicomponent fluid separation membranes, US Patent 5,085,676 (1992). [14] Y. Kusuki, T. Yoshinaga, H. Shimazaki, Aromatic polyimide double layered hollow filamentary membrane and process for producing the same, US Patent 5,141,642 (1992).

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