Effect of liquid-liquid demixing on the membrane morphology, gas permeation, thermal and mechanical properties of cellulose acetate hollow fibers

journal of MEMBRANE SCIENCE Journal of Membrane Science 140 (1998) 67-79

ELSEVIER

Effect of liquid-liquid demixing on the membrane morphology, gas permeation, thermal and mechanical properties of cellulose acetate hollow fibers Jyh-Jeng

S h i e h a, T a i S h u n g C h u n g a'b'*

a Department of Chemical Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore b Institute of Materials Research and Engineering, 10 Kent Ridge Crescent, Singapore 119260, Singapore

Received 3 July 1997; received in revised form 2 October 1997; accepted 2 October 1997

Abstract The polymer/solvent/nonsolvent systems with different L-L demixing rates were prepared by employing a binary solvent mixture consisting of two solvents - one exhibits an instantaneous liquid-liquid (L-L) demixing process, while the other exhibits a delayed L-L demixing process. It was found that an increase in the delay time of L-L demixing results in a denser membrane structure, an increase in fiber mechanical strength, a delay desorption of moisture in membrane, and a decrease in gas permeance, for a hollow fiber fabrication system consisting of cellulose acetate (CA) (polymer), N-methyl-pyrrolidone (NMP) (solvent having an instantaneous L-L demixing property), tetrahydrofuran (THF) (solvent having a delayed L-L demixing property) and water (nonsolvent). Hollow fibers prepared under an instantaneous L-L demixing process tends to have more mechanically weak points (flaws) than those prepared under a delayed L-L demixing process. Surprisingly, SEM observation suggests that membranes wet-spun from solutions containing both THF and NMP tend to have a rough outer skin morphology. Inconsistent demixing and the collapse of the outer nascent skin may be the main causes. In addition, the effect of bore fluid chemistry on fiber performance is much more pronounced for systems having a delayed L-L demixing mechanism than that having an instantaneous L-L demixing. © t998 Elsevier Science B.V. Keywords: Hollow fiber; Binary solvent system; Liquid-liquid demixing process; Fiber property; Bore fluid

I. Introduction The asymmetric membrane prepared via phaseinversion technique is the most widely employed membrane structure for a variety of separation processes. The study of membrane formation in the phase-inversion process has become a main topic in the membrane research field [1]. Interest is focused on *Corresponding author. 0376-7388/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0376-7388(97)00267-6

the formation mechanism of skin layer (nodular structure) [1,2], macrovoids [3,4], and sublayer (closed cell or open cell) [5]. It has been pointed out that l i q u i d liquid ( I ~ L ) demixing plays a central role in this process [6]. A general mechanism for the membrane formation has been suggested by Smolders et al. [7]: the top skin layer, where most likely gelation takes place, is caused by direct desolvation and the L - L phase separation is responsible for substructure morphology. Qualitatively, membranes with a thin top

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J.-J. Shieh, T.S. Chung/Journal of Membrane Science 140 (1998) 67-79

layer of porous nodular structure and macrovoid-filled open cell substructure are often obtained under instantaneous L - L demixing process, which generally possess ultrafiltration and hyperfiltration properties, while membranes having a dense, thick top layer, and a closed cell substructure are always formed from the delayed L - L demixing process, which are in principal suitable for gas and pervaporation applications if the substructure resistance can be kept low [5]. From the mass transport modeling, Yilmaz and McHugh [8] concluded that any change made in the conditions of the polymer/solvent/nonsolvent system will affect membrane formation mainly through changing the flux ratio value (nonsolvent in-diffusion to solvent out-diffusion). Therefore, the onset of L - L phase separation is considered to directly relate to the flux ratio value. Membranes, when in service, are subjected to forces or sinusoidal stress. In such situations it is necessary to know the characteristics of the membranes and to design the membrane module properly so that any resulting deformation will not be excessive and fracture will not occur. Shilton et al. [9] demonstrated that the tensile properties are insensitive to dope extrusion rate and jet stretch ratio, while the tenacity and modulus increase with increasing the dope concentration. Yang and Chou [10] studied the effect of the drawing conditions on the mechanical properties of polyacrylonitrile hollow fibers and found that the breaking strength and breaking elongation increased with the draw ratio for the samples in dry state. While the thermal and mechanical properties are rarely studied with respect to phase-inversion hollow fiber membranes, the thermal properties of a membrane can give an insight on the membrane structure. Kesting et al. [11] observed an elevated glass transition temperature for the polysulfone hollow fiber membranes spun from Lewis acid:base complexes and attributed this to an increased free volume of membrane due to the rapid sol-to-gel transition accelerated by the water disruption into the complex. Chung and Hu [12] reported that an increase in airgap distance results in a hollow fiber with a layer of less finger-like voids and a significant lower permeance. Their data suggested that the outer skin of wet-spun fibers has a long-range random, un-oriented chain entanglement, but loose structure, while the

outer skin dry-jet-wet-spun fibers has a short-range random, compact, and slightly oriented or stretched structure. As a result, the Tg of a dry-jet-wet-spun fiber prepared from one-polymer/one-solvent systems is lower than that of a wet-spun fiber. It is known that the cellulose acetate (CA)/Nmethyl-pyrrolidone (NMP)/water system exhibited an instantaneous L - L demixing process, while the CA/tetrahydrofuran (THF)/water system exhibited a delayed L - L demixing process [4]. Therefore, it is possible to prepare binary solvent mixtures that exhibit different L - L demixing properties by adjusting the solvent ratio of NMP and THE This kind of information provides us with an opportunity to study the effect of L - L demixing process on the hollow fiber membrane morphology, gas permeation performance, thermal and mechanical properties. This work emphasizes on the hollow fiber formation because the controlling factors for hollow fiber morphology are quite different from that for fiat membranes. It is a known fact that it is very difficult to simulate hollow-fiber spinning process by adopting the process conditions developed for asymmetric fiat membranes. There are two coagulation processes taking place in hollow fiber spinning (internal and external surfaces), while there is only one coagulation surface for an asymmetric fiat sheet membrane. If liquids are used as bore fluids, the internal coagulation process for a hollow fibers starts immediately after extrusion from a spinneret and then the fiber goes through the external coagulation, while there is usually a waiting period for an asymmetric fiat membrane before immersing it into a coagulant. As a result, the L - L demixing mechanisms may be more complicated in hollow fiber formation than that for flat asymmetric membranes. In addition, the spinning dope suitable for fabricating hollow fibers generally has a much higher viscosity and elasticity than that for fiat membranes, this may complicate the L - L demixing mechanism thermodynamically and kinetically. Since different L - L demixing mechanisms yield different membrane structures, membranes will have different properties. In order to study the effect of L - L demixing on hollow fiber membrane structure, mechanical properties, and separation performance, we purposely design our experiments by choosing spinning solutions consisting of cellulose acetate (CA) (polymer), N-methyl-pyrrolidone (NMP) (sol-

J.-J. Shieh, T.S. Chung/Journal of Membrane Science 140 (1998) 67-79 vent having instantaneous L - L demixing property), tetrahydrofuran (THF) (solvent having delayed L - L demixing property)and water.

2. Experimental 2.1. Materials Cellulose acetate (type HB-105) was kindly supplied by Dr. John Chen at Hoechst Celanese, Rock Hill, SC, USA. N-Methyl-pyrrolidone (NMP), tetrahydrofuran (THF), N-propanol and heptane were purchased from Merck, and used as received. 2.2. Spinning Process A 23 wt% CA dope was prepared in a binary solvent system consisting of THF and NMP with different ratios. The spinning CA dope was extruded under a nitrogen pressure of 60 psi through a spinneret with the dimensions of 0.8 mm OD and 0.5 mm ID. The bore fluid rate was precisely controlled at 0.1 cc/min by an ISCO 500 D syringe pump. External coagulant was water of 25°C. No air gap was employed in the spinning process. The spinning process was operated at 25°C. All the nascent fibers were drawn by carefully controlling the take-up speed of hollow fibers in order to eliminate any extra stress, except that due to gravity. These spun fibers were stored in a water bath of 25°C for at least one day, then transferred to a tank containing n-propanol for 2 h, followed by exchanging npropanol with heptane for another 2 h, and finally dried at room temperature for one day. The purpose of multi-step solvent exchange process was to prevent the pore collapse resulting from the capillary force during drying [13,141. A detailed description of hollow fiber spinning apparatus can be found elsewhere [121. 2.3. Scanning electron microscope (SEM) Fiber samples for SEM study were cryogenically fractured in liquid nitrogen and then sputtered with gold of 200-300 ,~ of thickness using Jeol JFC-1100 E Ion Sputtering Device. A field emission scanning electron microscope Hitachi ® S-4100 was employed to investigate the membrane morphology.

69

2.4. Tensile properties An Instron 5542 Material Testing Instrument was used to carry out the tensile property measurements in an environment of 25°C and 80% relative humidity. Each data was obtained from the average value of five tests for each fiber. A fiber gauge length of 50 mm and a drawn speed of 10 mm/min were used throughout the tests. The main parameters of interest are (1) fiber strength (MPa) (calculated based on the solid cross section area) and elongation (%) at break; (2) Young's modulus (MPa), which is the slope of stress-strain curve before yield point; (3) yield stress and strain which are obtained at the yield point determined by the threshold slope of 90% - a function provided by the Instron 5542. 2.5. Thermal analysis The thermal analysis was performed using PerkinElmer Pyris 1 differential scanning calorimeter with a heating rate of 5°C/min from 20°C to 100°C. The hollow fibers were vacuum-dried at room temperature for 24 h to remove any possible solvent residue. After the first DSC run, the testing sample was cooled down to 20°C at a cooling rate of 200°C/min and subsequently started with the second DSC run. The weight loss of each testing sample was calculated by comparing the weight after the second DSC run to its original weight. 2.6. Module fabrication and gas permeation tests Each membrane module was comprised of a bundle of 4 fibers with length of 15 cm. One end of the bundle was sealed with a 5 min rapid solidified epoxy resin (Araldite ®, Switzerland), while the shell side of the other end was glued into an aluminum holder using a regular epoxy resin (Eposet®). The prepared module was fitted into a stainless steel pressure cell for gas permeation measurement. The set-up of the gas permeation apparatus is described elsewhere [12]. The fluxes of gases through a hollow fiber module were measured by employing a soap bubble meter and thepermeance (cm3(STP)/cm2 s cmHg),P/L, is givenby

P L

a AAp

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where P : p e r m e a b i l i t y of the separation layer (cm3(STP) cm/cm 2 s cmHg); A p = t r a n s m e m b r a n e pressure difference (cmHg); A=effective membrane surface area (cm2); Q--gas flux (cm3(STP)/s). Gas Permeation Unit (GPU) is used in this study and is equal to 1 × 10 -6 cm3(STP)/cm 2 s cmHg. The ideal separation factor of gas A over gas B for a hollow fiber is given by (P/L) A O{

- -

(P/L)B

-

-

The coating was applied by dipping the fiber modules into a solution of 3 wt% polydimethylsiloxane (Sylgard-184, Dow Coming) in hexane for 5 min. After coating, the modules were stored in a clean environment for 48 h to allow curing before conducting permeation tests.

3. Results and discussion

3.1. Spinning characteristics Table 1 lists the spinning conditions of C A dopes as a function of dope formulations. Since no drawing was applied to the as-spun fibers except the gravity, it is very interesting to find that the take-up speed of C A hollow fibers increases with increasing T H F content. It is due to the fact that the viscosity of a polymer

solution is higher in a thermodynamically good solvent than a thermodynamic poor one [15]. Based on the calculated total Hansen solubility parameter of a binary solvent system, ~t (Table 1), N M P is a thermodynamically better solvent than T H E Thus, the viscosity of C A / N M P / T H F solutions should decrease with an increase of T H F content. Therefore, the take-up speed increases as increasing the T H F content. Note that the viscosity data of spinning dopes are not listed in the table since they are too viscous to be measured by our viscometer (Brookfield ® cone-and-plate viscometer, HB DV-III). With respect to L - L demixing, the CA/NMP/water system exhibits an instantaneous one, while C A / T H F / water system a delayed one with a delay time of about 68 s [4]. A binary solvent system consisting of N M P and T H F should show that the onset time of L - L phase separation increases with an increase in the T H F content. The last column in Table 1 gives a qualitative comparison of the L - L demixing processes for different C A / N M P / T H F spinning dopes coagulated with water. The dimensions of the resulting hollow fibers are also listed in Table 1. 3.2. Fiber morphology Fig. 1 shows the cross-sectional view of C A hollows prepared from the binary solvent system, using H 2 0 / N M P (20/80 by weight) mixture as bore fluid.

Table 1 The spinning conditions of CA hollow fibers Dope comp. (wt%)

Bore fluid

Take-upspeed (cm/min)

Fiber OD/ID (ktm/~tm)

6t b (MPa)t/2

L-L demixing

CA

NMP

THF

23 23 23 23 23

77 58 38.5 19 0

0 19 38.5 58 77

H20/NMP a H20/NMP H20/NMP HzO/NMP H20/NMP

40.3 60.8 68.6 84.7 90.5

775/500 750/475 825/475 800/450 825/475

22.9 21.9 21.0 20.2 19.4

instantaneous I I .L delayed

23 23 23 23

77 58 38.5 19

0 19 38.5 58

H20 H20 H20 H20

43.1 60.4 74.6 75.8

775/525 775/475 750/450 750/550

22.9 21.9 21.0 20.2

instantaneous I 1 delayed

a 20/80 by weight. b The total Hansen solubility parameter of the binary solvent system, calculated by 6t=6yt,~,Vr,a~+t~a'nvVa'r~ where V is the volume fraction. 6t of CA is about 27.5 (MPa)1/2 [16].

J.-J. Shieh, T.S. Chung/Journal of Membrane Science 140 (1998) 67-79

(a)

(b)

(c)

(d)

71

(e) Fig. 1. SEM pictures of the cross section of CA hollow fibers: the THF content (a) 0 wt%; (b) 19 wt%; (c) 38.5 wt%; (d) 58 wt%; (e) 77 wt% (bore fluid=H20/NMP 20/80 by weight).

The THF content increases from 0 wt% to 77 wt% as shown in Fig. l(a)-(e). The teardrop type macrovoids can be found for all the fibers and their sizes decrease with an increase in THF content. Whereas, the pore size in the microporous substructure increases with an

increase in the THF content. Empirically, it has been pointed out that the formation of macrovoid structure in a flat membrane can be avoided if the phase inversion process undergoes a slow precipitation rate of polymer or delayed L--L demixing process [4]. In

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J.-J. Shieh, T.S. Chung/Journal of Membrane Science 140 (1998) 67-79

this study, the CA/THF/water system shows a very delayed L - L demixing process. Therefore, more the THF in the spinning dope the more delayed L - L demixing it will become, and the macrovoid structure diminishes gradually. On the other hand, in a delayed L - L demixing system the available time for nucleation and growth mechanism [5] increases; consequently, the pore size of microporous substructure increases as observed in Fig. 1. It is also interesting to notice that the macroviods always exist in the outer region of CA hollow fiber even the solvent used is THF (the macrovoids structure disappears when using THF as solvent in preparing the flat membrane [4]). It may be due to the more vigorous coagulation step in preparing the hollow fiber, which induces a convective solvent exchange process. The word "convective" used here is a relative term to describe the vigorousness of coagulation process in a wet-spinning process. A significant amount of data has suggested that volume change and convective flows occurred during the precipitation of polymer solutions [12,17-19]. Since in a wet-spinning process, the extruded nascent fiber immediately contacts with a wavy coagulation medium, the convective flow phenomenon may be much enhanced in a wet-spinning hollow fiber process than that in an as-cast flat membrane process. In other words, the outer skin of an as-cast fiat membrane may be immediately and slightly modified (coagulated) after casting due to the contact of moisture in air. As a result, its skin viscosity may increase and retard the degree of interfacially convective flow during the subsequent coagulation. Fig. 2 shows the outer surface of CA hollow fibers having the same preparation conditions as in Fig. 1. Hollow fibers spun from one-polymer/one-solvent systems, such as CA/NMP or CA/THF dopes, have the smoothest outer surfaces. This phenomenon may be due to a consistent demixing process occurred in the precipitation of an one-polymer/one-solvent system. In the case of CA/NMP/THF(one-polymer/twosolvent) system, Fig. 2 suggests that the surface roughness increases first, from (a) to (c), then decreases, from (c) to (e), with an increase of THF content. This interesting phenomenon is due to the fact that the solvent exchange rate between H20 and NMP is different from that between H20 and THF. Therefore, the inconsistent solvent exchange step occurs during the entire precipitation process of CA/NMP/

THF systems. When an extruded hollow fiber membrane is in contact with the coagulant bath (H20), NMP exchanges very fast with water due to its instantaneous L - L demixing characteristics. A nascent and mechanically weak polymeric outerskin is immediately formed. At this moment, THF still resides in the membrane and will diffuse out of the membrane later because of its delayed L - L demixing characteristics. Once the out-diffusion of THF processes, the mechanically weak nascent skin starts to collapse because of the partial re-dissolution of membrane skin. Therefore, very rough surfaces are created as shown in Fig. 2 (b) and (c). While at a higher THF content, the rough surface may mostly re-dissolve by the presence of a large amount of THF and the final fiber surface becomes smoother. The formation of macrovoids underneath the outer skin surface may be contributed to the intrusion of nonsolvent into the weak spot between different membrane surface structures, which is resulted from the different exchange rates between nonsolvent and solvents [17,20]. Fig. 3 shows the inner surface of the same series of CA hollow fibers using a 20/80 H20/NMP mixture as bore fluid. The surface porosity is observed to decrease with increasing THF content. This is consistent with the trend observed in flat asymmetric membranes that a more delayed L - L demixing process results in a denser, more compact structure [4]. 3.3. Fiber mechanical property The effect of dope composition on hollow fiber's strength, break elongation, and Young's modulus is shown in Table 2. The Young's modulus, or modulus of elasticity, may be thought of as stiffness, or a material's resistance to elastic deformation. For the CA hollow fibers using H20/NMP (20/80 by weight) as bore fluid, their Young's moduli show no difference in terms of THF content, while for those using H20 as bore fluid, their Young's moduli increase with increasing the THF content. In the latter case, the increase in Young's moduli may be due to increased cohesion between polymer molecules as they become more densely packed on fiber solidification, resulting from the more delayed L - L demixing when the THF content increases. On the other hand, a bore fluid having higher solvency ability, like H20/NMP (20/80 by

J.-J. Shieh, T.S. Chung/Journal of Membrane Science 140 (1998) 67-79

73

I,. f~:

(a)

(b)

(c)

(d)

(e)

Fig. 2. SEM pictures of the outer surface of CA hollow fibers: the THF content (a) 0 wt%; (b) 19 wt%; (c) 38.5 wt%; (d) 58 wt%; (e) 77 wt% (bore fluid=H20/NMP 20/80 by weight).

weight) in the former case, may destroy the dense membrane structure from the lumen side during the later vitrification step. As a consequence, their Young's moduli show no apparent difference as shown in Table 2.

The stress-strain behavior of a material changing from elastic to plastic deformation is called yielding. From a molecular perspective, plastic deformation corresponds to the breaking of bonds with original molecule neighbors and then reforming bonds with

74

J.-J. Shieh, T.S. Chung/Journal of Membrane Science 140 (1998) 67-79

(a)

(b)

(c)

(d)

(e)

Fig. 3. SEM pictures of the inner surface of CA hollowfibers: the THF content(a) 0 wt%; (b) 19 wt%; (c) 38.5 wt%; (d) 58 wt%; (e) 77 wt% (bore fluid=H20/NMP 20/80 by weight).

new neighbors as large numbers of molecules move relative to one another; upon removal of the stress they do not return to their original positions. Therefore, stronger the yield stress, stronger is the original molecular bonding force per unit cross-sectional area. The

yield stress of CA hollow fibers has a similar trend as the Young's modulus and suggests that a dense membrane structure possesses a higher intermolecular interaction, as discussed above. While, the yield strain seems to be independent of dope composition.

J.-J. Shieh, T.S. Chung/Journal of Membrane Science 140 (1998) 67-79

75

Table 2 The tensile property of CA hollow fibers Dope composition(wt%)

Bore fluid

Break Stress (MPa)

Strain (%)

H20/NMP HEO/NMP H20/NMP H20/NMP

6.04-0.3 8.04-0.8 7.94-0.4 6.34-0.3 7.94-0.7

H20 H20 H20 H:O

6.24-0.2 8.94-0.3 9.94-0.7 >14.85:0.9

CA

NMP

THF

23 23 23 23 23

77 58 38.5 19 0

0 19 38.5 58 77

H 2 0 / N Mp a

23 23 23 23

77 58 38.5 19

0 19 38.5 58

Young's modulus

Yield

(MPa)

Stress (MPa)

Strain (%)

9.74-1.0 20.74-0.6 27.84-2.7 30.64-1.5 28.44-1.8

217.84-27.1 214.84-32.3 207.74-26.2 159.24-22.7 190.6+27.0

2.54-0.2 3.14-0.2 2.8-4-0.3 2.04-0.1 3.14-0.2

1.64-0.4 1.34-0.1 1.54-0.3 1.74-0.5 1.64-0.1

8.04-1.0 22.74-1.2 24.84-4.7 >21.85:3.8b

212.34-08.8 233.04-12.3 291.94-09.4 487.34-12.5

3.14-0.1 3.35:0.2 4.14-0.5 6.6+0.2

1.45:0.1 1.74-0.5 1.75:0.2 1.95:0.5

20/80 by weight. b The tensile properties exceed the maximumload. a

The strength at break is a measure of the strength of polymer within a fiber and essentially depends on the conditions of fiber failure or flaw. It can be observed that the CA fibers using pure NMP as solvent system have the lowest break stress and strain. This indicates that more fiber failure and flaws may be generated in an instantaneous L - L phase separation. Note that the tensile strength of the fibers prepared from the dope with THF content of 58 wt% using H20 as bore fluid exceed the m a x i m u m load of Instron. This indicates that their breaking strength and elongation are higher than the data listed in Table 2. 3.4. Thermal property

The glass transition temperature (Tg) is always used to interpret membrane structure when employing a thermal analysis on a membrane [11,12]. A higher Tg indicates that membrane possesses more free volume fraction, therefore, a looser structure and vice versa. In this study, we find that the membrane structure can be illuminated in terms of other thermal property rather than Tg. Fig. 4 shows typical first-run DSC curves for CA hollow fibers. A broad endothermic peak is present in the first run, while it disappears in the second run. Since the endothermic peak appears again when we place the sample after the second run in atmosphere (temperature=25°C, relative humidity=80%) for 2 h, it is identified to be caused by the loss of moisture

THF content, wt*/, =

58 385 19

> "--"- " - . / ~,-" ~ . / "

\, N',,,

,.. \,,

x.

"c-'~z. " )

i

i

i

i

~

i

i

i

20

30

40

50

60

70

g0

90

100

Temperature. °C

Fig. 4. The typical DSC traces of CA hollow fibers: the THF content (a) 0 wt%; (b) 19 wt%; (c) 38.5 wt%; (d) 58 wt%; (e) 77 wt% (bore fluid=H20/NMP20/80 by weight).

absorbed in the membrane. The moisture content in CA hollow fibers is approximately 4 - 6 wt% as shown in Table 3. It is interesting to find that endothermic peak shifts to a higher temperature region if a higher THF content is used in the spinning dope. A summary of the DSC results is listed in Table 3. The m a x i m u m temperature, Tm~x, which is the peak temperature of the endothermic region, increases with increasing THF content. This may be explained from the perspective of membrane structure. As discussed in the previous sections, membrane structure becomes denser with an increase in THF content (a more delayed L - L demixing process), therefore, the trapping

76

J.-J. Shieh, T.S. Chung/Journal of Membrane Science 140 (1998) 67-79

Table 3 The effect of THF on the maximum temperature (Tmax)of CA hollow fibers. Dope composition (wt%)

Bore Fluid

Weight Loss (%)

Tm~x

CA

NMP

THF

(°C)

23 23 23 23 23

77 58 38.5 19 0

0 19 38.5 58 77

H20/NMP a H20/NMP H2OfNMP H20/NMP H20/NMP

56.7 56.7 58.8 59.0 62.2

5.9 3.7 4.3 4.3 4.0

23 23 23 23

77 58 38.5 19

0 19 38.5 58

H20 H20 H20 H20

55.3 56.7 62.0 64.3

5.4 6.7 5.0 5.0

a 20/80 by weight.

moisture needs more energy to escape from a denser membrane and Tmax increase. In addition, the Tm~x observed for fibers using H20 as bore fluid is higher than that for fibers using H 2 0 / N M P (20/80 by weight) as bore fluid, indicating that a much denser membrane structure is obtained for the former than the latter, which is consistent with the data of tensile property in Table 2. 3.5. Gas permeation

The use of two solvents with different L - L demixing processes allows a finer adjustment of polymer coagulation rates in wet-spun fiber formation. Peinemann and Pinnau [21], Pinnau et al. [22], Pinnau and Koros [23] and Pesek [24] used solvent mixtures consisting of a less volatile solvent and a volatile solvent in the casting solution to prepare defect-free asymmetric membranes via the dry/wet phase inversion technique. Their efforts were focused on the formation of defect-free skin layer. The selective skin thickness was determined by the solvent evaporation step that induced a concentrated outermost skin region in a nascent membrane. There is, however, a host of other formation parameters that are affected by a change in the solvent ratio including the relative volatility, the re-dissolution rate and the kinetics of wet phase separation process in a dry/wet phase inversion technique. Lai and co-workers [25,26] prepared poly(methyl methacrylate) (PMMA) and polycarbonate gas separation membranes from a variety of solvent/nonsolvent systems via a wet phase inversion

method and obtained different membrane structures. They concluded that in order to prepare a suitable gas separation membrane, except the position of binodal curve in a ternary phase diagram, at least two other factor should be considered; the miscibility between solvent and nonsolvent, and the interfacial polymer concentration. These two factors apparently relate to the L - L demixing property of a casting system. This section mainly focuses on the influence of L - L demixing on the gas permeation characteristics of C A hollow fiber when applying a wet phase inversion technique. Figs. 5 and 6 show the 02 permeance and O2/N2 selectivity as a function of THF content for the uncoated C A hollow fibers. A m a x i m u m permeance is observed for both bore fluids at 20 wt% THF, then

104

io3

I02

I01

io o

10-1

10-2 10

20

30

40

50

60

70

80

THF content, wt*/o

Fig. 5. 02 permeance vs. THF content for the uncoated CA hollow fibers (bore fluid: H/O/NMP 20180 by weight (0); H20 (O)).

J.-J. Shieh, T.S. Chung/Journal of Membrane Science 140 (1998) 67-79

Intrinsic O211'42selectivity of CA

L-

i3

O2/N2 selectivity for Knusen flow 0 10

20

30

40

50

60

70

80

THF content, wt%

77

proceeds gradually from the shell side to the lumen side of the hollow fiber and this results in a loose membrane structure. Chung et al. [27] addressed the same argument when preparing the PES hollow fiber membrane using different bore fluids consisting of water and NMP in different ratios. However, the current work clearly demonstrates that the effects of bore fluid chemistry on membrane performance are much more pronounced for systems having a delayed L - L demixing mechanism than that having an instantaneous L - L demixing mechanism. Figs. 7 and 8 show the gas permeation characteristics for the silicone-coated CA hollow fibers. Appar-

Fig. 6. O2/N2 selectivity vs. THF content for the uncoated CA hollow fibers (bore fluid: HzO/NMP 20/80 by weight (O); H:O (©)). 104

decreases with increasing THF content. The trend is especially remarkable for the fibers using H20 as a bore fluid. As shown in the SEM photograph, the most defective (or rough) surface skin is observed for the hollow fiber prepared from dopes containing about 20--40 wt% of THF, and this may account for the observed maximum permeance. As for the effect of bore fluid on separation performance, no significant differences in permeance and selectivity are observed for the fibers spun with different bore fluids (H20 or 20/80 H20/NMP) when the THF content is less than 20 wt%. However, the situation changes when the THF content is becoming greater. Fibers prepared from dopes having a THF content higher than 20 wt% exhibit 3-4 orders of magnitude lower when using H20 rather than 20/80 HaO/NMP as bore fluid. In addition, the selectivity increases with an increase in the THF content in the spinning dope. Since H20 is a powerful coagulant, it produces a dense structure at the lumen side of a hollow fiber if there is a delayed demixing at the interface, thus the total resistance increases and the O2 permeance reduces. In the case of using HeO/NMP (20/80 by weight) as bore fluid, Fig. 6 suggests that the resultant hollow fiber membranes exhibit the Knudsen flow mechanism (O2/N 2 selectivity ~ 0.94) irrespective of the THF content in the spinning solutions (except for the one prepared from the CA/THF solution). This interesting phenomenon is due to that fact 20/80 HaO/NMP is still a solvent for CA, no precipitation will occur in the lumen side of a hollow fiber at first. Precipitation

103

102

i

8.

101

l00 10-I

10-2 10

20

30

40

50

60

70

80

THF content, wt%

Fig. 7. 02 permeance vs. TI-IF content for the silicone-coated CA hollow fibers (bore fluid: H20/NMP 20/80 by weight (O); H20

(0)).

Intrinsic O2/N2 selectivity of CA

'~ 3

g 2

O2/N 2 selectivity for Knuscn flow

10

20

30

40

50

60

70

80

THF content, wt%

Fig. 8. O 2 / N 2 selectivity vs. THF content for the silicone-coated CA hollow fibers (bore fluid: H20/NMP 20/80 by weight ( 0 ) ; H20 (O)).

78

J.-J. Shieh, T.S. Chung/Journal of Membrane Science 140 (1998) 67-79

ently, the 02 permeance reduces significantly after silicone coating, but the selectivity remains almost unchanged. Coating seals the surface pore and reduces the permeance. According to the resistance model proposed by Henis and Tripodi [28] and Pinnau and Koros [29], the same selectivity for membranes before and after coating suggests that there is significant substructure resistance in these membranes.

4. Conclusions We have investigated the effect of L - L demixing process on the properties of hollow fiber preparing from the CA/NMP-THFPrI20 system. Different L - L demixing systems are prepared by adjusting the ratio of NMP and THF in a binary solvent. The fiber with large tear-drop type macrovoids and a fine porous substructure is observed from an instantaneous L - L demixing system, while the fiber with small tear-drop type macrovoids and a substructure of large pore size is found from a delayed L - L demixing system. Furthermore, the hollow fibers prepared from a delayed L - L demixing system, i.e. more THF content in the dope solution, possess stronger mechanical strength, higher Tm~x in the endothermic peak and lower gas permeation flux. Experimental data also show that the effect of L - L demixing on the membrane morphology can not only be observed microscopically, i.e. macrovoids or porous substructure, but also from a molecular scale by employing the tensile test, thermal analysis and gas permeation test. In addition, the choice of bore fluid plays an important role on determining the final membrane structure. A dense membrane structure resulting from a delayed L - L demixing system may be destroyed by a bore fluid with high solvency power. This work also suggests that the effects of bore fluid chemistry on membrane performance are much more pronounced for systems having a delayed L - L demixing mechanism than that having an instantaneous L - L demixing mechanism.

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 Ms. Teoh S.K. and Dr. S. Mullick for their useful help and comments, and the Department of Electrical Engineering and Mechanical Engineering in NUS for the use of their SEM microscopes. Special thanks are due to Dr. John Chen and Hoechst Celanese for the provision of CA flakes.

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