1997 - formation of ultrathin high-performance polyethersulfone hollow-fiber membranes

ELSEVIER Journal of Membrane Science 133 (1997) 161-175

journal of MEMBRANE

SCIENCE

Formation of ultrathin high-performance polyethersulfone hollow-fiber membranes

Tai Shung C h u n g a'b'*, S o o Khean T e o h b, Xudong H u b

aInstitute of Materials Research and Engineering, 10 Kent Ridge Cresent, Singapore 119260, Singapore bDepartment of Chemical Engineering National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore

Received 22 November 1996; received in revised form 1 April 1997; accepted 1 April 1997

Abstract

We have demonstrated, for the first time, that ultrathin skin-layer hollow-fiber membranes with a skin layer of 474 ,~ can be prepared using mainly a one-polymer and one-solvent system. This is one of the thinnest skin-layer asymmetric hollow-fiber membranes that have ever been reported in the literature for air and gas separation. This work implies that, in order to yield a high-permeance polyethersulfone (PES) membrane with a skin layer of approximately 500 A, the addition of non-solvents into spinning dopes may not be the pre-condition to form ultrathin skin-layer hollow-fiber membranes for gas separation. The keys to fabricate ultrathin skin-layer hollow-fiber membranes are (1) to control the chemistry of the internal coagulant and the bore- fluid flow rate and (2) to have a dope exhibiting significant chain entanglement. The newly developed polyethersulfone (PES) hollow fibers have an O2/N2 selectivity of 5.80 with a permeance of 9,3 x 10 -6 cc(STP)/cm2 s cmHg for O2 at room temperature. The skin layer thickness was calculated to be 474 ,~. These hollow fibers were wet-spun from a 35/75 (weight ratio) PES/N-methyl-pyrrolidone (NMP) dope using water as the external coagulant and 80/20 NMP/H20 as the bore fluid. The hollow fiber must be coated with a silicone elastomer. This work also suggests that in order to yield a high-permeance PES membrane with a skin layer of approximately 500 A, there might not exist a critical solvent molar volume when preparing the dope solvent mixture, as previously suggested by the Permea research group. SEM observation of skin nodules suggests that the skin layer thickness is less than 700 ,~.

Keywords: Air-separation membranes; Ultrathin skin layer; Hollow fiber spinning; Asymmetric membranes; Polyethersulfone membranes

1. Introduction

Hollow-fiber membranes used for gas separation have received world-wide attention during the last two decades. This is due to the fact that membrane separation processes may offer more capital and energy efficiency when compared to the conventional separa-

*Corresponding author. Fax: +65 779 1936.

0376-7388/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PI I S 0 3 7 6 - 7 3 8 8 ( 9 7 ) 0 0 1 0 1-4

tion processes in some applications. Presently, membrane based separation technologies have been developed and commercia l ized for the recovery of Ha from chemical industrial gases containing CH4, N2,

or CO; the separation of acidic gases (CO2, H2S) from natural gases; and the enrichment of 02 and N2 from air for industry and instrumentation uses. Both asymmetric membranes and microporous composite membranes have been used in these commercial

162 T.S. Chung et al./Journal of Membrane Science 133 (1997) 161-175

membrane-separation technologies. In general, a microporous composite membrane for gas separation consists of a microporous support layer and a thin skin selective layer. Several review articles on the formation of asymmetric membranes and microporous composite membranes have been published recently [ 1-4].

Phase inversion process is one of the most important means to prepare asymmetric membranes for air separation. The resultant membranes have a skin layer which integrally bonded in series with a thick porous substructure. The skin and the substructure are com- posed of the same material. The skin layer, which contains the effective separating layer, is one of the key elements in determining the membrane permeability and selectivity. To have a high-performance air- separation membrane, this skin layer has to be as thin as possible and must contain minimum defects. Henis and Tripodi [5,6] had invented a caulking technology to eliminate membrane surface defects by depositing a thin silicone-rubber coating on asymmetric membranes. Although their technology has been exten- sively practiced in both academia and industry, there is a limitation of this technology as previously men- tioned in their patent [6]. They noticed in their patent that the silicone coating technology could not work for some membranes if the substructure had a high resistance. In other words, membrane's substructure (support layer) also plays an important role on selectivity. Experiments, conducted by Pinnau and Koros [7], had shown that low selectivities could be resulted from substructure resistance to gas transport even when the skin layer was defect-free.

From the marketing viewpoint, a high-selectivity membrane must have a high permeance (productivity) in order to lower production costs. Thin asymmetric hollow-fiber membranes with a skin layer less than 1000 ,~ used to be the most desirable in the 70s and 80s. In order to meet the target skin thickness of less than 1000 ,~, researchers at Permea led technology breakthrough in the late 80s using Lewis acid : base complex solutions to fabricate asymmetric hollow fibers with a graded-density skin [2,8-10], while research group at the University of Texas (UT) led the development of ultrathin asymmetric fiat membranes. The former may have a skin layer thickness probably less than 1000 ,~, while the latter less than 600,~ [11-13]. Interestingly, they both employed solvent/non-solvent mixtures when preparing the

casting or spinnin~ dopes in order to meet the target of less than 1000 A. Since their inventions, it appears to most membrane scientists that there is a hypothesis that one must employ solvent/non-solvent or multi- solvent approach in dope formulation in order to fabricate membranes with an ultrathin selective layer. This hypothesis has become questionable because, recently, Hachisuka et al. [14] demonstrated that ultrathin skin layer membranes could be fabricated from a binary system. However, only fiat membranes were developed in Hachisuka et al.'s work.

It is important to point out that Kesting et al. [2,8- 10] of Permea and Koros et al. [ 1,7,11-13] of UT have different explanations on the importance of non-solvents in the membrane formulation. Permea believed that the incorporation of a high level of non-solvents that formed a Lewis acid : base complex solution with NMP could induce a high level of frozen free volume among nodules in the selective layer, and therefore created a high-permeance membrane. They also observed a relationship between solvent molecular volume and the free volume and permeance of the resultant membranes, as illustrated in Fig. 1 [8,10]. They believed that a critical solvent molar volume existed (~147 cc/mole for polysulfone) for the spinning solutions of ultrathin membranes. When one used a solvent having a molar volume greater than this critical volume to prepare spinning solutions, the fabricated membranes would have a substantial increase in free volume and permeance. Since pro- pionic acid and NMP formed a complex solvent pair that had a molar volume greater than 147 cc/min and

5 0 A

~ 4 0

J ~ ~°

2 O

• I ~ :NMp

• pA :O~C

D ~ C •

Solvent Molar Volume ( c c /mo le )

Fig. 1. Kesting et al. data on the relationship between solvent molar volume and oxygen permeance for polysulfone membranes [8,10].

T.S. Chung et al./Journal of Membrane Science 133 (1997) 161-175 163

because this complex fluid dissociated rapidly in water, this solvent mixture worked. Since they revealed their technology in 1989, their hypothesis on the existence of a critical solvent molar volume has not been fully investigated in membrane community. Koros' group explained the importance of non-solvent in their system based on evaporation-induced phase separation mechanism. The evaporation of high-vapor pressure solvents and non-solvents can induce phase instability at outermost skinlayer and form an almost defect-free selective layer. The subsequent immersion in a coagulation bath not only creates a porous substructure, but also (1) controls the thickness propaga- tion (increase) of the evaporation-induced phase separation occurred at the skin layer and (2) prevents the newly formed ultrathin skin layer from redissolu- tion.

Since Kesting et al. and Koros et al.'s inventions, many research groups have tried to improve, modify and extend these newly developed technologies to yield high-performance asymmetric hollow fibers. There are a few successes. For example, Chung et al. at Hoechst Celanese (HC) developed various 6FDA-polyimide membranes for air separation [15,16] by modifying Permea's technology in their solvent system. Teo et al. at the National U. of Singapore (NUS) produced polyethersulfone membranes using NMP and alcohol solvent systems [17,18]. Pesek and Koros extended the UT original approach on fiat membranes to hollow fibers and produced asymmetric polysulfone hollow fiber with a skin layer thickness of about 1200 ,~ [11,13]. Their work indicates that the development of an ultrathin skin-layer hollow-fiber membranes is more difficult than that of fiat membranes.

Since all these high-performance air-separation hollow-fiber membranes utilize solvent/non-solvent or Lewis acid and base mixtures in their spinning dopes and since no one has demonstrated that a one- polymer/one-solvent system can yield an ultrathin selective-layer hollow-fiber membrane, there is a challenge for membrane scientists as to whether one must use solvent/non-solvent or multi-solvents in dope formulations in order to fabricate hollow-fiber membranes with an ultrathin skin layer. Is it true that there exists a critical solvent molar volume, of below which there is no way to form a high-permeance hollow-fiber membrane?

The objectives of this work are trying to answer these challenging questions. Polyethersulfone (PES) was chosen in this study because Henis and Tripodi [6], Kesting et al. [9] and Fabre [19] have used this material in their patents and some works have been done to produce polyethersulfone membranes using NMP and alcohol solvent systems [ 17,18]. In addition, various PES and alkyl-substituted PES membranes prepared from other systems have been published elsewhere [20-24]. The selectivities of PES for O2/N2 was reported to be 6.1 with a permeability of 0.51 Barrer at 30°C and to be 5.1 with a permeability of 0.81 Barrer at 50°C [18]. One may calculate permeability of O2 at 25°C to be about 0.44 Barrer by using an Arrhenius relationship between permeability and temperature.

2. Experiments

2.1. Materials

Polyethersulfone (Radel A-300) was kindly provided by Prof. W.K. Teo at NUS who purchased from Amoco Performance Products Inc. Ohio, USA. It has a weight-average molecular weight of about 15000 Dalton. N-methyl-2-pyrrolidone (NMP, 99+%), was the solvent supplied by MERCK and used as-received.

2.2. Dope preparation and viscosity measurements

Various concentrations of PES/NMP polymer solutions have been prepared. This is the dissolving pro- cedure: PES polymeric powders was first dispersed in a cold NMP solvent (0-3°C) with a high speed stirrer. The chilled solvent reduced the dissolving rate of PES powders and thus prevented powders from agglomeration. The dope container was then agitated in an ice bath for 1.5 h and at room temperature until it was fully dissolved.

Solution viscosity was measured using a Brook- field @ cone-and-plate viscometer (Model: HB DV- III). Basically, the measurement was accomplished by rotating a polymer solution between a cone and a plate. The viscosity of a fluid is determined from the measured torque necessary to overcome the viscous resistance when a cone-shape disc rotates in a fluid. The viscosities corresponding to different polymer


A 60000

40000

* Shear Rate = ~ 0/sec

/

. / / //I

0 2O 22 24 26 28 30 32 34 36 38

C o n c e n t r a t i o n (w t%)

Fig. 2. Viscosity vs. concentration for PES/NMP system.

N2 pressure

H P L C p u m p

spinneret ~

coagulation bath

spinning dope water wash , , , , ,

h o l l o w f i b e r , ' ,

. . . . . .

concentration at the spinning temperature (25°C) were plotted as shown in Fig. 2.

30 and 35 wt% of PES/NMP solution were chosen in this study in order to confirm if chain entanglement in a spinning dope plays an important role in fiber performance. Based on the 6FDA-polyimide hollow- fiber results, Chung et al. [ 14] have hypothesized that a dope exhibiting significant chain entanglement may be one of the requirements to yield an air-separation hollow fiber with minimum defects. Since the viscosity data, as illustrated in Fig. 2, exhibits a dramatical slope change from a dope with polymer concentration below 30% to that above 37%. This change implies that a significant increase in the degree of chain entanglement occurs at a concentration of about 35 wt% (defined as the critical concentration in this work). This value is determined by the intersection of the two slopes in these two regions. Fibers spun from 30 wt% dopes (below the critical concentration) and 35 wt% dopes (at about the critical concentration) were therefore prepared to verify this hypothesis.

2.3. Spinning process

Fig. 3 illustrates the schematic diagram of the hollow fiber spinning apparatus. The formulated dope was fed under nitrogen pressure and bore fluid was fed by 500D Syringe Pumps, made by ISCO. The accu- racy of this ISCO precision pump was -t-0.5% of flow rate. Once the spinning dope and bore fluid met at the tip of the spinneret, they passed through an air-gap ranging from 0-14.4 cm before entering the coagulation bath. All nascent fibers were not extended by drawing which means that the take-up velocity of the

Fig. 3. The schematic diagram of the hollow fiber spinning process.

hollow fiber was nearly the same as the free falling velocity in the coagulation bath. After formation of the hollow fiber, the fibers were stored in the water bath for at least one day and then transfer to a tank containing fresh methanol for at least 1 h to remove the residual NMP completely. Hollow fibers thus treated were used for further test and study.

2.4. Scanning electron microscope (SEM)

Membrane samples for SEM study were first immersed in liquid nitrogen and fractured, and then sputtered with gold using Jeol JFC-1100E Ion Sput- tering Device. We employed a Jeol ® JSM U3 electron microscope and a field emission scanning electron microscope Hitachi ® S-4100 to investigate fiber morphology.

2.5. Module fabrication and tests

Fig. 4 depicts the schematic diagram of a testing apparatus to measure the gas permeation of hollow fibers. Several fibers with a length of 10 cm were assembled into bundles. One end of the bundles was sealed with a 5 min rapid solidified epoxy resin (Araldite ®, Switzerland), while the shell side of the other end was glued onto an aluminum holder using a regular epoxy resin (Eposet®). It took 8 h to fully cure the Eposet ® resin. The prepared module was fitted into a stainless steel pressure cell for the gas permeation measurement at 200 psi (13.6 bar).

T.S. Chung et al./Journal o f Membrane Science 133 (1997) 161-175 165

3

I I G ~ Cylinder 2. P r e ~ Regulator 3 P r e ~ Gauge 4 M e m b ~ e Module 5 Purge Gas

• e ~ Gas

6

Fig. 4. The schematic diagram of the testing apparatus for gas permeation measurements.

The permeances, P/L, of gases through the hollow- fiber module were determined using a bubble-flow meter and calculated using the following equation:

P - Q - Q (1) L ~PA n ~ D l A P

where P=permeability of the separation layer (Barrer); /=effective length of the fibers (cm); AP=transmembrane pressure drop (cmHg); A=membrane effective surface area (cm2); Q=the gas flux reading (cm3/s); n=number of tested fibers; D=outer diameter of the fibers (cm). We use GPU as the gas permeation units, and one GPU is equal to 1 × 10 -6 cm 3 (STP)/cm 2 s cmHg.

The ideal separation factor of an asymmetric membrane can be determined from:

- (P/L)------rA ¢2) (P/L)B

In this study, single gases were used to determine membrane permeance and selectivity. The effective skin layer thickness is estimated from the intrinsic oxygen permeability coefficient and the pressure-nor- malized flux of oxygen from the following equation:

L -- Po2 (3) ( P / L ) o2

2.6. Silicone rubber coating

In order to seal the membrane defects, an assembled hollow-fiber module was immersed in a coating solution containing 3 wt% polydimethysiloxane (Sylgard- 184) in n-hexane for 5 min. We waited 48 h to cure the silicone rubber coating at room temperature before conducting permeation tests. The oxygen and nitrogen permeabilities of this polydimethylsiloxane at 30°C were reported to be 649 and 354 Barrers, respectively [18].

3. Results and discussion

3.1. Fibers spun from 30% PES solutions

Table 1 summarizes the spinning conditions for 30 wt% PES/NMP. By using a 40/60 NMP/H20 mixture as the bore fluid, PES hollow fibers were wet-spun as well as dry-jet wet-spun into a water bath. The air-gap distance for the dry-jet wet-spinning process was 14.4 cm. Table 2(A and B) compare their gas separation performance before and after silicone rubber coating. Before silicone coating, a dry-jet wet-

Table 1 Process parameters and spinning conditions for 30 and 35 wt% PES/NMP dopes

ID A B C

Polymer concentration PES/NMP (by weight) Viscosity (poise) at 10 S - 1

Dope pressure (psi) Dope fluid rate (g/min) Spinning speed (cm/min) Spinning temperature (°C) Bore fluid composition (NMP/H20 by weight) Bore fluid flow rate (cc/min) External coagulant Coagulation bath temperature (°C)

30% 35% 35% 8493 34 444 34 444

10 25 25 0.32 0.234 0.216

81 41.4 67.8 25 25 25

40/60 60/40 80/20 0.083 0.05 O. 1

Water Water Water 25 25 25


Table 2 Permeance of wet-spun and dry-jet wet-spun hollow fibers (spun from 30 wt% PES/NMP dopes using 40/60 NMP/H20 as the bore fluid)

Air-gap distance (cm) 02 permeance (GPU) N2 permeance (GPU) Selectivity 02/N2

A: Before silicone coating 14.4 22.9 25.2 0.91 0 75.8 74.9 0.99

B: After silicone coating 14.0 12.8 13.5 0.95 0 15 6.1 2.38

spinning process produces fibers with 0 2 / N 2 selectivities of approximately 0.91-0.94, while a wet-spinning process produces fibers with selectivities close to 1. These results suggest that dry-jet wet-spun fibers have smaller pore sizes than that of wet-spun fibers. Fig. 5 shows the SEM pictures of external surface of these fibers at 50 000 magnification. The wet-spun fibers have a looser structure than dry-jet wet-spun fibers. However, both fibers have visible defects at external skins. These morphologies are consistent with the measured O2/N2 selectivities.

Since both selectivities of wet-spun and dry-jet wet- spun fibers were not impressive, a silicone coating was deposited on these fibers. Table 2(B) summarizes the results. Interestingly, the selectivities of silicone- coated dry-jet wet-spun fibers do not increase as much as that of silicone-coated wet-spun fibers. This phenomenon is mainly due to the effect of substructure resistance on membrane selectivity, as predicted by Pinnau and Koros [7]. Figs. 6 and 7 show the cross- section morphologies of these two fibers. One can easily see that fibers spun from a long air-gap distance tends to have a thicker and tighter cross-section morphology near the outer skin than that of wet-spun fibers. Since the substructure in a dry-jet wet-spun fiber is relatively thick and has pore sizes suitable for Knudsen diffusion, the determining step of the overall selectivity is the Knudsen diffusion occurred in the substructure. Additional silicone coating only seals defects of the outermost skin, thus the overall fiber selectivity is not improved.

Wet-spun and dry-jet wet-spun fibers have different finger-like void structures. This suggests that air-gap distance significantly affects precipitation paths and changes hollow fiber's substructure. In a wet-spinning process, a nascent hollow-fiber experiences 'convec-

!i

Fig. 5. The external surface morphology of wet-spun and dry-jet wet-spun hollow fibers (spun from 30 wt% PES/NMP dopes using 40/60 NMP/H20 as the bore fluid) (air-gap distance, top=14.4 cm, bottom=0 cm) (magnification: x50000).

tive type' coagulation at its internal and external surfaces simultaneously; while in a dry-jet wet-spinning process, a nascent fiber undergoes different coagulation paths. A dry-jet wet-spun fiber first


Fig. 6. The cross-section morphology of wet-spun hollow fibers (spun from 30 wt% PES/NMP dopes using 40/60 NMP/H20 as the bore fluid) (magnification: top × 150, bottom x750).

Fig. 7. The cross-section morphology of dry-jet wet-spun hollow fibers (30 wt% PES/NMP dopes, 40/60 NMP/H20 as the bore fluid, 14.4 cm air-gap distance) (magnification: top × 150, bottom x 750).

experiences a 'convective type' internal coagulation and a 'non-convective type' external coagulation in the air-gap region simultaneously, then rapid solvent exchange takes place at its external surface when the fiber is immersed in the coagulation bath. As a result, both fibers have quite different morphologies; the wet- spun fibers have a relatively loose skin structure with two arrays of finger-like voids in the fiber cross- section, while the dry-jet wet-spun fibers have a relatively tight skin structure with one array of finger-like voids in the fiber cross-section. The real causes of multiple layer finger-like voids are still in debate [25-33]. In addition, very rare attention has been given to the effect of air-gap distance on macrovoid formation and membrane separation performance [25,26]. Macrovoids play an important role on membrane long-term performance. Cracks usually start at the tips of macrovoids [27,33]; therefore, an under-

standing of the mechanisms of macrovoid formation as a function of spinning conditions is urgently needed.

The word 'convective-type' used here is a relative term to describe the vigorousness of coagulation process. A significant amount of data have suggested that volume change and convective flows occurring during the precipitation may be the cause [26-29]. For example, SEM micrographs shown by Strathmann et al. [27] indicated that finger-like voids were directly connected to the precipitation solution, even though the rate of the moving front of the precipitation was found to be a function of the square root of time (i.e. diffusion-controlled process). Cabasso [28] observed extensive convective flow at the interface between the coagulant and the as-cast flat membrane, and saw schlieren pattern developed across the membrane thickness during the precipitation. In fact, the inter-

168 T.S. Chung et aL /Journal of Membrane Science 133 (1997) 161-175

facial turbulence (extensive convective flow) has been observed in many liquid-liquid interfaces and has been reviewed in elsewhere [30,31]. One of the inter- facial instabilities, known as Maragoni effect, may develop ripples, erratic pulsations, regular and irre- gular surface motion due to the surface tension varia- tion from point to point at the interface and density gradient between two liquids. This wavy motion may break the thin polymeric solution layer, intrude into the bulk of the solution and form macrovoids. Perez et al. [32] and McKevey and Koros [33] have different explanations on the formation of macrovoids, they suggested that the osmotic pressure was the dominant force to generate them in gas-separation membranes. In both explanations, it is reasonable to say that finger- like voids are probably created by spinodal decom- position with the aid of unbalance localized stresses from surface tension, nucleation, osmotic pressure, solvent/coagulant agglomeration, volumetric change, radially convective flows of the inner and outer coa- gulants. Since a wet-spun fiber experiences two convective-type coagulations, it has two arrays of finger- like voids. Since a dry-jet wet-spun fiber experiences one convective-type internal precipitation and one external moisture-induced precipitation in the air- gap region, it has only one array of finger-like voids.

3.2. Fibers spun from 35 wt% of PES solutions

3.2.1. Bore fluid composition: 60/40 NMP/H20 Since fibers spun from 30 wt% of PES have outer

skins full of defects, we increase the dope concentration to 35 wt% which exhibits a sign of significant entanglement during viscosity measurements. In order to reduce substructure resistance, we replace the 40/60 NMP/H20 mixture by a 60/40 NMP/H20

Fig. 8. The external surface morphology of wet-spun fibers (spun from 35 wt% PES/NMP dopes using 60/40 NMP/H20 as the bore fluid) (magnification: ×50000).

mixture as the bore fluid. The spinning conditions for 35 wt% PES/NMP have been summarized in Table 1. Table 3 compares gas separation performance of as-spun fibers before and after the silicone rubber coating. The membranes show poor separation performances. Before silicone coating, wet-spun and dry-jet wet-spun fibers have O2/N2 selectivities in the range of 0.93 to 0.96. Fig. 8 demonstrates the SEM pictures of the external surface of a wet-spun fiber at 50 000 magnification. Compared to Fig. 5, fibers wet spun from 35 wt% of PES have a tighter and less defective outer skin than that spun from 30 wt% dopes. This skin morphology is consistent with our hypothesis that a dope exhibiting significant chain entanglement is one of key elements to yield useful gas-separation membranes.

Similar to the previous case (Table 2B), Table 3(B) suggests that wet-spun fibers have a better performance in O2/N2 selectivity than that of dry-jet wet-

Table 3 Permeance of wet-spun and dry-jet wet-spun hollow fibers (spun from 35 wt% PES/NMP dopes using 60/40 NMP/H20 as the bore fluid)

Air-gap distance (cm) 02 permeance (GPU) N 2 permeance (GPU) Selectivity O2/N 2

A: Before silicone coating 6.0 68 71 0.96 0 28 30 0.93

B: After silicone coating 6.0 36 37 0.97 0 15.9 6.33 2.51



spun fibers. This is also due to substructure effect. In other words, the substructure (inner structure) in dry- jet wet-spun fibers is relatively thick and has pores suitable for Knudsen diffusion. Additional silicone coating only seals defects of the outermost skin, thus the overall fiber selectivity is not improved. The cross- section morphologies, shown in Figs. 9 and 10, support our analysis. Although the wet-spun fibers prepared from the new conditions have a slightly better O2/N2 selectivity than that of the previous case (Table 2), the improvement is very trivial. In addition, the former has a selectivity well below 90% of the inherent selectivity of PES. The cause of this low selectivity may also arise from the same substructure effect. Fig. 11 exhibits the inner skin structure of a wet-spun fiber at 10000 magnification and indicates that its skin is quite dense with a few tiny pores. This

Fig. 10. The cross-section morphology of dry-jet wet-spun hollow fibers (35 wt% PES/NMP dopes, 60/40 NMP/H20 as the bore fluid, 6.0 cm air-gap distance) (magnification: top × 150, bottom × 750).

dense structure was created due to the fact that there was a significant difference in solubility parameter between the bore fluid and the bulk of nascent fibers. A bore fluid of 60/40 N M P / H 2 0 mixtures has a solubility parameter of 16.08 (cal/cm3) °5, while a 65/35 PES /NMP dope has a solubility parameter of 11.45 (cal/cm3) °'5. As a result, it is very likely that the low O2/N2 selectivity may be attributed to the effect of substructure resistance on fiber performance.

3.2.2. Bore f lu id composition: 80 /20 N M P / H e O In order to reduce the substructure resistance on

hollow-fiber performance, we have to change the substructure morphology. An 80/20 N M P / H 2 0 mixture was employed to replace 60/40 N M P / H 2 0 as the bore fluid. The objective is to create a relatively loose inner skin structure. What happened when an 80/20

170 T.S. Chung et al./Journal o f Membrane Science 133 (1997) 161-175

Fig. 11. The inner surface morphology of wet-spun hollow fibers (fibers spun from 35 wt% PES/NMP dopes using 60/40 NMP/H20 as the bore fluid) (magnification: x 10 000).

NMP/H20 mixture was employed as the bore fluid? We observed that one could not conduct dry-jet wet spinning even with an air-gap distance of 1 cm. Clearly, this was caused by the fact the solubility parameter difference between the 80/20 NMP/H20 and 65/35 PES/NMP is much smaller than that between 60/40 NMP/H20 and 65/35 PES/NMP (2.19 versus 4.63 (cal/cm3)°'5). From a thermody- namic viewpoint, a smaller solubility-parameter difference between the nascent fiber and the bore fluid results in a smaller heat of mixing. Thus, the Gibbs free energy may become negative which favors a stable solution mixing. In this case, liquid-liquid diffusion and mixing between solvents of as-spun fiber and bore fluid occur before demixing or phase separation. The resultant fiber tends to have a porous inner skin and its porosity is uniformly distributed.

Fig. 12 give SEM pictures of the fiber cross section and shows that the region near the inner surface consists of no finger-like voids and no skin layer. The finger-like voids near the inner skin, which previously observed when using a 60/40 NMP/H20 mixture as a bore fluid, were eliminated by this new bore fluid formulation. Fig. 13(A) depicts an SEM picture of the fiber external skin and shows no visible defects. The skin quality appears to be better than that of Fig. 8. This improvement in fiber external skin may be due to the effect of bore fluid chemistry and flow rate on external fiber surface morphology. Fig. 13(B) illustrates the detailed structure near the outer layer at a magnification of 80 000 and indicates that this layer consists of nodules. The outermost skin


has a thickness of approximately 600-800 ,~ and the substructure beneath the outermost skin is quite porous and has a tiny finger-like structure. Since the gold coating thickness on SEM samples is around 100- 150 ,~, this SEM picture suggests that the selective layer should be less than 600 A. Fig. 14 reveals the inner surface morphology. The inner surface is more porous and has less resistance for air transport than the previous case (Fig. 11).

Table 4 summarizes separation performance of newly developed hollow fiber for air separation at 25°C. The uncoated fiber has an O2/N2 selectivity of about 0.96 and this value is significantly improved to 5.80 after a silicone coating. Similar data have been reproduced in our laboratory. Table 5 compares our data to the previous work done by Kesting et al. [5] and Wang [12,13]. Although most of their data were


(a)

(b)

Fig. 13. The external surface and cross-section skin morphology of wet-spun fibers (spun from 35 wt% PES/NMP dopes using 80/20 NMP/H20 as the bore fluid) (A (top): external surface, x20000, B (bottom): cross-section morphology near the outer-skin, x80000).

measured at 50°C, we almost have the thinnest appar- ent skin layer thickness. Since they all have similar skin layer thicknesses, the present work suggests that, in order to yield a high-permeance as-spun PES membrane with a skin layer of approximately 500 .~, there might not exist a critical solvent molar volume when preparing the dope solvent mixture. The keys to fabricate ultrathin PES skin-layer hollow-fiber membranes with a skin layer of approximately 500 ~, are to control both the chemistry and bore-fluid flow rate of the internal coagulant. However, it is very important to point out that this work does not mean that there is no critical solvent molar volume when preparing high-permeance PES membranes with a skin layer thickness of much less than 400 ,~. One can only make that conclusion when more experimental data are available.

Fig. 14. The inner surface morphology of wet-spun fibers (spun from 35 wt% PES/NMP dopes using 80/20 NMP/H20 as the bore fluid) (magnification: top: x2000 and bottom x 10 000).

Table 4 Permeance of PES wet-spun fibers (spun from 35 wt% PES/NMP dopes using 80/20 NMP/H20 as the bore fluid)

02 permeance N2 permeance Selectivity (GPU) (GPU) O2/N2

Before silicone 258 269 0.96 coating After silicone 9.3 1.60 5.80 coating

3.2.3. Data verification using Henis and Tripodi's resistance model approach

Using the resistance model approach proposed by Henis and Tripodi [5], we employed the following equation to calculate the permeance of a gas: )l

P ll -~ P2,i d- PI,i(A3/A2) (4)


o

e .

8

m

e~

I .

e~

< ~.

0

o

~ = = = =

~ . . . . o

where Pill=gas i permeance;/1=coating thickness (it is 2.8 ~m in this case); lz=effective skin thickness (it is 474 ]k in this case); Pl,i--intrinsic permeability of the coating material to gas i; P2,i----intrinsic permeability of the dense polymeric membrane to gas i; and (A3/A2)=surface porosity ratio (it is assumed to be 0.00001).

Selectivity of gas A to gas B is defined in Eq. (2). The calculated selectivity of O2/N2 and permeance of 02 from the resistance model are 5.9 and 9.1 GPU, respectively, whereas the selectivity and permeance that we got from experiments are 5.8 and 9.3 GPU. The agreement between the theoretical calculations and experimental results is excellent, thus verifies that our data are correct. For readers' information, we used the permeability data at 30°C published by Wang [17] for the silicone rubber coating material in the calcula- tion.

3.2.4. Mechanisms to form hollow fibers with an ultrathin skin layer

Since we have demonstrated that ultrathin skin- layer hollow-fiber membranes with a skin layer of 474 .~ can be prepared using mainly one-polymer and one-solvent system with a proper bore fluid chemistry, this proves that the addition of non-solvents into spinning dopes is not the prerequisite to form ultrathin skin-layer hollow fiber for gas separation. Although Kesting and many others have explained the roles of non-solvent in ultrathin skin-layer hollow-fiber formation, this work suggests that their explanations are not fully completed. One may still ask the real func- tions of a non-solvent in a ternary dope formulation for hollow fiber spinning and formation. Perhaps Fig. 15 can illustrate the role of non-solvents in practical hollow-fiber formation. Let's use Permea's hollow fiber spinning process as an example. According to their work, water was employed as the bore fluid as well as the external coagulant. Since water is a powerful coagulant for PES/NMP binary systems, the precipitation of an as-spun PES/NMP nascent fiber in water would immediately result in thick and skin layers. Similar phenomenon has been reported elsewhere [15,16] and Figs. 6 and 7 also show similar results. A tight inner skin delays subsequent demixing between the bore fluid and the solvents in a nascent fiber. As a consequence, it limits or slows solvent exchange within a nascent fiber and with the surround-


~emarj sp~anlng ~ l u a o a ana ~ e a t t l l ~ r

Water a s a

bore fluid

Qmck p,ec~p~maon with a ~ /an~ly po ,o~

mnerskin

8 0 / 2 0 NMP/water a s a b o r e fluid

80/20 binary s p ~ g b, l]~p/H20 mlut mn and

~ e n l fiber

Fig. 15. Solvent exchange mechanisms in wet-spun fibers using different bore fluid chemistries.

Polymer/Solvent Polymer/Solvent/Non-Solvent Binary System Ternary System

a t igh a n d ~ m p ~ t skin structure a loose skin structure

Fig. 16. Inner skin morphologies of wet-spun fibers precipitated from binary and ternary spinning solutions when contacting a strong coagulant.

ings. Due to the slow release of NMP, the inner and outer skins of the nascent fiber gradually become thick and dense. The presence of non-solvents in a spinning dope or a nascent fiber can prevent the tightness or slow the rate of tightness of the inner skin formation and provide the channel for continue demixing. In other words, since the presence of non-solvents reduces the solubility parameter difference between the bore fluid and the bulk of a nascent fiber, it results in a slow precipitation process occurred in the inner skin, and the tightness (contraction of molecular chains) of the inner skin and the rate of tightness are no more vigorous and rapid.

Fig. 16 qualitatively illustrates the resulting morphologies of extended random coil chains precipitated from binary and ternary (containing non-solvent) systems when contacting a strong coagulant. It is known that random flexible polymer chains tend to be fully swollen or extended in a good solvent. When a non-solvent is suddenly added into a polymer solution, the extended random coil chains contract in order to reduce the Gibbs' free energy. The speed of chain contraction depends on the dope viscosity (friction of chain movement during contraction), the rate of solvent exchange, and the amount and the flow rate of non-solvents adding to the system. When a binary spinning solution contacts a powerful coagulant, such as water, the extended random polymer chains contract suddenly and almost instantaneously. The sudden contraction results in some of the molecular chains trapped in neighboring nodules because it takes time for chains to shrink and retreat in a viscous solution. Therefore, the precipitated skins may have significant chain-to-chain interactions (entanglement) and a

dense structure is formed. On the other hand, random coil polymeric chains are not fully extended in a ternary spinning solution containing a significant amount of non-solvents. There exists some degrees of chain shrinkage or contraction even before the spinning. When they contact with water, the precipitated skins may have much less chain-to-chain interactions (entanglement) and a loose structure is therefore formed.

If the above hypotheses are correct, one may be able to partially imitate the desired precipitation process of a ternary solution system for a binary solution system by changing the solubility parameter of the bore fluid. In other words, a bore fluid with a relatively close solubility parameter to the bulk of nascent fibers will not only facilitate solvent exchange (demixing) within the nascent fiber but also prevent the tightness of the inner skin. The facilitation of demixing is mainly due to the fact that this bore fluid may almost eliminate the formation of a dense inner skin structure.

3.2.5. Future work on high-permeance

PES hollow fibers Although we have made an advancement on mem-

brane technology to fabricate high-permeance hollow fibers for air separation, there are many unknowns in this technology. For example, we need to optimize the overall process and search for the optimal conditions on bore fluid rates and chemistry. The aging phenomenon of this new high-permeance hollow fiber is one of the subjects worthwhile studying. Fibers spun from binary and ternary systems may have different aging paths. We will discuss it in future publications.


4. Conclusion

We have demonstrated that ultrathin skin-layer PES hollow fiber can be prepared using mainly a binary (one polymer and one solvent) system. The addition of non-solvents into spinning dopes is not the pre-condition to form ultrathin skin-layer hollow-fiber membranes for gas separation. The keys to fabricate ultrathin skin-layer hollow-fiber membranes are to control both the chemistry and bore-fluid flow rate of the internal coagulant. The newly developed polyethersulfone (PES) hollow fibers have an O2/N2 selectivity of 5.8 with a permeance of 9.3 × 10 -6 cc(STP)/cm2s cmHg for 02 at room temperature. The skin layer thickness was calculated to be in the range of 474 ,~. This work also suggests that, in order to yield a high-permeance PES membrane with a skin layer thickness of approximately 500 ,~, there might not exist a critical solvent molar volume when preparing the dope solvent mixture, as suggested by the Permea research group. It is likely that there exists an optimal solubility parameter difference between the dope solvent mixture and the internal coagulant with an appropriate bore-fluid flow rate during the fiber formation.

Acknowledgements

The authors gratefully acknowledge the financial support provided by the National University of Sin- gapore (NUS) and its research fund (No: 960609A). Thanks are also due to Prof. W.K. Teo and Dr. Wang for kindly providing us with PES materials and the design of module tests, Mrs. C.M. Ho and Mr. K.S. Ng of the Departments of Electrical Engineering and Mr. S.K. Tung of the Departments of Mechanical Engineering in NUS for the use of their SEM microscopes.

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1997 - formation of ultrathin high-performance polyethersulfone hollow-fiber membranes

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