Effect of Solvent Exchange on the Morphology of Asymmetric

Chapter 8

Effect of Solvent Exchange on the Morphology of Asymmetric Membranes 1

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H . C. Park , Y . S. Moon , H . W. Rhee , J . Won , Y . S. Kang , and U . Y . Kim 1

Division of Polymer Science and Engineering, Korea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul, Korea Department of Chemical Engineering, Sogang University, Seoul, Korea

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 4, 2015 | http://pubs.acs.org Publication Date: December 29, 1999 | doi: 10.1021/bk-2000-0744.ch008

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The effect of solvent exchange and the subsequent drying process on membrane morphology was investigated for microporous and integrally-skinned asymmetric membranes by using scanning electron microscopy (SEM) and atomic force microscopy (AFM). For the microporous membranes, significant changes in morphology and transport properties were observed due to the collapse of micropores in the top skin layer. In the case of integrally-skinned asymmetric membranes, only negligible differences in membrane structure were observed upon solvent exchange. The effects of solvent exchange depend strongly on the capillary forces imposed on the membrane matrix by a liquid present in the membrane pores. A membrane dried using supercritical CO experienced no capillary forces upon drying, and, hence, the membrane morphology observed by S E M showed the nascent, original membrane structure formed by the phase inversion process. 2

When a membrane is prepared by the nonsolvent-induced phase inversion process by precipitating a polymer solution in a coagulation medium, its pores are filled with the coagulation medium (2-5). Thereafter, the wet membranes are typically dried. The nascent membrane morphology can change due to the capillary forces exerted by the coagulation medium on the membrane matrix during the drying process. The capillary pressure arises from the curvature of liquid in the interstitial capillaries. It provides a force that can deform the membrane matrix whereas the mechanical modulus of the material resists the deformation. This concept was previously studied in the formation of homogeneous films from disperse latex particles (4) and adopted to qualitatively explain the morphological changes of

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© 2000 American Chemical Society

In Membrane Formation and Modification; Pinnau, I., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1999.

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Ill phase inversion membranes on drying by Pinnau (5). A liquid in the interstitial capillary system develops a different capillary pressure depending on the pore radius ((5). The capillary pressure exerts in a direction normal to the water-polymer interface and tends to deform the polymer particles in that direction (4-6). Brown developed a simple mathematical model to calculate the capillary pressure of a close-packed sphere structure using the Young-Laplace equation (4). Here, the capillary pressure was expressed by 12.9 σ/r, where σ and r are the surface tension of the liquid and the radius of particles, respectively. For a closely packed system of polymer particles with a diameter of 0.01 μιη in a liquid with a surface tension of 70 dyne/cm, the capillary pressure is calculated to be 1.8xl0 kg/cm . Because of this extremely high capillary pressure, the nascent membrane can be deformed during the drying process.


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A typical morphological change in the membrane is the collapse of micropores in the top skin layer. In an asymmetric membrane, the pores are usually increasing in size from the top (skin layer) to bottom structure and the capillary force will, then, be the highest in the top skin layer. Therefore, in most cases, the skin morphology of a membrane can be changed significantly on drying, and its transport properties will be altered markedly. For controlling membrane morphology during drying, the solvent exchange method has been commonly employed. The purpose of the solvent exchange is to lower the surface tension of the liquid present in the membrane pores and, consequently, preventing the collapse of the micropores in the top skin layer (7-10). In this study, the effect of solvent exchange was investigated on the morphological changes of microporous and integrally-skinned asymmetric membranes during drying. In addition, the supercritical fluid drying method was utilized to minimize the morphological changes. The supercritical fluid does not undergo any phase transition and, thereby, imposes negligible capillary pressure on the membrane matrix during drying. Using this procedure, the original nascent membrane morphology can be observed even after drying. Experimental Membrane Preparation. Two types of asymmetric phase inversion membranes were prepared from polysulfone (PSf, Amoco Co., P3500,Mw 35,000 g/mol). The casting solutions were prepared by dissolving PSf (25 wt.%) in N-methyl-2pyrrolidone (NMP, Aldrich Chemical, Inc.) or a mixture (6:4 by weight) of N M P and tetrahydrofuran (THF, Merck Co.). The solution was cast onto a glass plate using a doctor blade of 250 μιη clearance. The PSfTNMP solution was immersed into a water/NMP (1:1 by weight) coagulation bath, whereas the PSffNMP/THF solution was quenched into a water coagulation bath. The former is a microporous membrane and will be designated as T l in this paper. The latter is an integrallyskinned asymmetric membrane and will be designated as T2. After the phase


112 inversion process was completed, membranes were kept in a water bath for 2 days at room temperature to replace the residual solvent, N M P and/or THF, with H 0 and then were subject to the solvent exchange.For the solvent exchange,water in a membrane was first replaced with ethanol by soaking the membrane in an ethanol bath for 2 days, and then ethanol was replaced with hexane for 2 days. Membrane samples were taken at each step and dried in air for 2 days and then in a vacuum oven for another 2 days at room temperature. The samples dried after being kept in the water bath are denoted as T1W and T2W, the samples dried after ethanolexchange as T I E and T2E, and the samples dried after hexane-exchange as T1H and T2H, respectively. Membrane samples which were dried using supercritical C 0 after ethanol-exchange are denoted as T1SC. The surface tension of water, ethanol, and hexane are 72.5,22.7, and 17.9 dyne/cm, respectively. 2


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Supercritical C 0 Drying. To investigate the effect of solvent exchange on the membrane morphology, the supercritical C 0 drying method was used. After the replacement of the casting solvent N M P with water, the membrane sample was soaked in an ethanol bath for 5 days to ensure the complete replacement of water and the residual N M P with ethanol. The ethanol-soaked membrane was placed in the reactor cell filled with ethanol. The pressure and temperature in the cell were increased up to 100 bar and 40°C, and then C 0 was introduced and maintained for 12 hours under the same conditions (the critical pressure and temperature of C 0 are 100 bar and 30°C, respectively). After complete replacement of ethanol with C 0 , the pressure and temperature inside the reactor cell were lowered to atmospheric conditions. The drying path of the membrane is shown schematically in Figure 1. 2

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Investigation of Membrane Morphology and Performance. The cross-sectional morphology of a membrane fractured in liquid nitrogen was investigated by field emission electron microscopy (FE-SEM, Hitachi S-4200). The S E M images were taken for membrane samples coated with Pt-Pd at an accelerating voltage of 15 k V . The surface morphology of a membrane was investigated with an Atomic Force Microscope ( A F M , CP, Park Sci. Inst.). The permeance of pure 0 and N was measured by using a constant-pressure gas permeation apparatus with a bubble flowmeter at a feed gas pressure of 70 psig at room temperature (~25°C). 2

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Results and Discussion Membrane with a Porous Skin Layer (Tl). T l type membranes which were produced by coagulation of PSf (25 wt.%)/NMP solution in aNMP/water (1/1 by weight) mixture had a rather thick porous skin layer on top of a sponge-like substrate. These membranes showed very interesting features after the solvent



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Supercritical fluid region Pressure

Drying path

Temperature Figure 1.

Supercritical C 0 drying path in a P-T diagram. 2



114 exchange. The top skin layer contained many micropores and, therefore, it was affected strongly by the capillary forces during the drying step. Figure 2(a) shows the F E - S E M image of the cross-section of a T l W membrane that was dried in air directly after removing the residual solvent from the membrane for 2 days in the water bath. Because the membrane pores were filled with water whose surface tension was the highest among the liquids used in this study, the largest capillary pressure was exerted during the drying process for a given pore size. The nodule-like particles in the top skin layer seem to be fused together and the skin layer is compact and thick. Figure 2 (b) shows the FE-SEM image of the membrane cross-section dried after ethanol exchange. It also shows a compact and thick skin structure, which is, however, thinner than that of T1W. The cross-section of a membrane dried after ethanol-hexane exchange is shown in Figure 2(c). The hexane-exchanged membrane shows a much more open structure than the former two membranes. The less compact structure resulted from the decreased capillary force imposed on the pore walls due to the lower surface tension of hexane present in the membrane pores during drying. Figure 2(d) shows the cross-section of a T1SC membrane dried by using supercritical C 0 . The cross-sectional morphology of the top skin layer was quite different from those of the membranes dried in air with or without solvent exchange. The FE-SEM image of the T1SC membrane showed a very uniform structure composed of nodule-like particles in the skin layer down to a depth of 500 nm from the top surface. No densified skin layer was observed in the cross-section investigated by FE-SEM. When the liquid in the membrane pores was replaced by a supercritical fluid, the membrane can be dried essentially without a capillary force because the supercritical fluid did not undergo a phase transition. Therefore, the membrane retains its nascent morphology formed initially by the immersion precipitation process. The region under the top skin of the Τ ISC membrane showed a seemingly uncollapsed and loose structure. From the cross-section structure of the T1SC membrane, it can be speculated that the top skin layer of an asymmetric membrane initially composed of nodule-like particles was compacted to a dense layer during drying due to the capillary action of the liquid. 2

A F M is a very useful method for studying the surface structure of polymeric membranes (11-16). The changes in the surface, structure of the skin layers investigated by the A F M images (Figures 3 and 4) coincide with the FE-SEM images shown in Figure 2. The collapse of micropores during drying results in the change of the cross-sectional morphology as well as the surface of the skin layer. The skin surface of the T1W membrane appears much rougher than that of the T1SC membrane, which can be confirmed and quantified by the surface roughness parameters: (i) average roughness, (ii) root-mean-square roughness, and (iii) lateral mean diameter. The average roughness, R , is given by the average deviation from the average of the height, avg



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Figure 2. FE-SEM images of microporous polysulfone membranes (Tl type) dried under different conditions: (a) T1W (water), (b) T I E (ethanol), (c)TlH (hexane), and (d) T1SC (supercritical C 0 ) . 2



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Height

Height Profile

(A)

Distance: 2905 A

Trace Distance ( μ )

Height: 6.7 À

Figure 3. A F M image of the surface and height profile of a microporous polysulfone membrane (Tl W) dried in air without solvent exchange.



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H e i

8

h t

Height Profile

(A)

Distance: 836 A

T r a c e D i s t a n c e (

V

1

Height: 1.5 À

Figure 4. A F M image of the surface and height profile of a microporous polysulfone membrane (T1SC) dried using supercritical C 0 . 2


118 _.

Ν

R

= ^\z -Z\/N

avg

(1)

t

where Z is a current Ζ (height) value, Ζ is a mean height, and Ν is the number of points within a given area. The root-mean-square roughness, R ^ , is given by the standard deviation of the data, n

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Rims — Σ(Ζη-ζ)

/(N-l)

(2)

Because contains square terms, the large deviation from the average Ζ height is weighted more heavily than that in R . The lateral mean diameter, P , is a convenient parameter to compare the various sizes together with both R ^ and R . It can be obtained by fitting the ellipse to the height profile of A F M images as shown in Figure 5. The calculated R , R ^ , and P values are given in Table I. avg

L

avg

avg

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Table I. Surface roughness of microporous polysulfone membranes (Tl) dried under different conditions. R (A) R™ (A) Drying conditions 68 T1W 55 23 T1SC 18 Ra : average roughness; R ^ : root-mean-square roughness; P : lateral mean diameter of hemispherical structure. a v s

PL(Â)

S

1,420 592

Vg

L

As expected, all values of R , R ^ , and P are smaller in Τ ISC than in T l W. This result demonstrates clearly that the surface roughness decreases with a decrease in the surface tension of liquid present in the membrane pores during drying. This might suggest that the capillary action caused the aggregation of smaller particles to bigger ones and eventually to dense film formation leading to a thick skin layer. The morphological changes of the top skin layer during drying affected the gas permeances of the membranes, as shown in Table II. avg

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Table II. The effect of solvent exchange on th,e gas transport properties of microporous polysulfone ( T l ) membranes. Membrane

T1W TIE T1H

Surface tension (dyne/cm) 72.5 22.7 17.9

Permeance (P/l) (10- cm (STP)/cm •S'cmHg) N 6

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Selectivity (0 /N ) 2

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2

2

11.6 24.7 25.8

11.9 25.2 26.2


0.98 0.98 0.99


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Height

Distance: 1154 Λ

Height Profile

T

r

a

c

e

Distance ( μ )

H

e

i

g

h

t

:

3

6

5

A

Figure 5. Lateral mean diameter P calculated from the height profile of the A F M image. L


120 The gas permeance decreased with an increase in the liquid surface tension. The decrease in permeance can be explained by an increase in the skin layer thickness. In addition, although the T1W membrane appeared to have a gas-tight top skin layer by FE-SEM, it did not show any 0 / N selectivity.


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Membrane with a Thin Gas-Tight Skin Layer (T2). T2 type membranes which were produced from a 25 wt.% solution of PSf in a N M P / T H F (6/4 by weight) solvent mixture by coagulation in pure water had a gas-tight skin layer and a finger like substructure. This membrane type was not much affected by the solvent exchange treatment. As can be seen in Figure 6, morphological differences are hardly observed in the cross-sections of the top skin layers from S E M images. The permeance of 0 and N as well as the 0 / N selectivity were also hardly affected by the solvent exchange conditions, as shown in Table III. 2

Table III.

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The effect of solvent exchange on the gas transport properties of integrally-skinned asymmetric polysulfone (T2) membranes.

Membrane

Surface tension (dyne/cm)

Permeance (P/l) (10- cm (STP)/cm •S'cmHg) N 2 0.79 2.41 2.52 0.83 0.83 2.53 6

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T2W T2E T2H

72.5 22.7 17.9

2

Selectivity (0 /N ) 2

2

2

3.1 3.0 3.0

However, a noticeable difference in the surface morphology was observed. Figures 7 and 8 show the A F M surface images and the height profile of the T2 type membranes. As given in Table IV, the surface morphology and height profile from A F M images show that the surface becomes smoother by replacing water with ethanol and then with hexane. Table IV.

Liquid

water ethanol hexane

The effect of liquid surface tension on the membrane surface roughness of integrally-skinned asymmetric polysulfone (T2) membranes. Surface tension (dyne/cm) 72.5 22.5 17.9

Ravg(A)

Rrms(A)

PL(A)

81 37 37

103 47 47

1,150 924 880

Conclusions For asymmetric membranes made by the phase inversion process, the drying step can cause additional changes in the membrane morphology. The top skin layer of a



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Figure 6. F E - S E M images of integrally-skinned asymmetric polysulfone membranes dried under different conditions: (a) T2W (water), (b) T2E (ethanol), and (c) T2H (hexane).



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Distance: 1154 A

0.4 0.6 Trace Distance { μ )

0.8 Height: 365 Â

Figure 7. A F M image of the surface and height profile of integrally-skinned asymmetric polysulfone membrane (T2W) dried in air without solvent exchange.



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Height Height Profile {A)

240 200 160 120 0.2 Distance: 517 À

0.4 T r a c e

0.6 Distance ( μ )

0.8 HcIj

1 , . hl

9 g Λ

Figure 8. A F M image of the surface and height profile of integrally-skinned asymmetric polysulfone membrane (T2H) dried in air after ethanol-hexane solvent exchange.



124 microporous asymmetric membrane densified on drying due to the collapse of micropores. Such a trend became less severe as the surface tension of the solvent decreased in the order of water, ethanol, and hexane. It was also observed from A F M analysis that the surface became smoother with a decrease in the liquid surface tension. Such a smooth and loose skin layer resulted in an increase of the gas permeance of the membrane. However, integrally-skinned asymmetric membranes with a very thin gas-tight skin layer on top of a finger-like substrate showed negligible change in the cross-sections and only a small change in the surface roughness. The solvent exchange did not show any effect on the gas transport properties of integrally-skinned asymmetric membranes. The F E - S E M image of the cross-section of the membrane dried using supercritical C 0 after exchangeof water with ethanol showed that the top skin layer of the membrane consisted of nodule like particles, which might represent the nascent morphology of the membrane before drying. 2

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

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Kesting, R.E. Synthetic Polymeric Membranes - A Structural Perspective; 2 Ed.; John Wiley and Sons, New York, N Y , 1985; pp. 237-286. Mulder, M . Basic Principles of Membrane Technology; 2nd Ed.; Kluwer Academic Publishers, Dordrecht, The Netherlands, 1996; pp. 75-140. Wienk, I.M.; Boom, R.M.; Beerlage, M . A . M . ; Bulte, A.M.W.; Smolders, C.A.; Strathmann, H. J. Membrane Sci. 1996, 113, 361. Brown, G.L. J. Polym. Sci. 1956, 22, 423. Pinnau, I.; Koros, W.J. J. Polym. Sci: Part B: Polym. Phys. 1993, 31, 419. Brinker, C.J. Sol-Gel Science; Academic Press, New York, N Y , 1990. Fritzsche, A . K . ; Arevalo, A.R.; Connolly, A.F.; Moore, M.D.; Elings, V.; Wu, C . M . J. Appl. Polym. Sci. 1992, 45, 1945. Merten, U . ; Gantzel, P.K. U.S. Patent 3,415,038, 1968. MacDonald, W.; Pan, C.-Y. U.S. Patent 3,842,515, 1974. Manos, P. U.S. Patent 4,080,743, 1978; U.S. Patent 4,080,744, 1978; and U.S. Patent 4,120,098, 1978. Ohya. H.; Konuma, H. J. Polym. Sci. 1977, 21, 2515. Dietz, P.; Hansuma, P.K.; Herrmann, K . H . ; Inacker, O.; Lehmann, H.D. Ultramicroscopy 1991, 35, 155. Kim, J.; Fane, A . G . ; Fell, C.J.D.; Suzuki, T.; Dickson, M.R. J. Membrane Sci. 1990, 54, 89. Bowen, W.R.; Robert, N.H.; Lovitt, W.; Williams, P . M . J. Membrane Sci. 1996, 110, 229. Krausch, G.; Patterson, D. Macromolecules 1994, 27, 6768. Bowen, W.R.; Robert, N.H.; Lovitt, W.; Williams, P.M. J. Membrane Sci. 1996, 110, 233.


Effect of Solvent Exchange on the Morphology of Asymmetric

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