Effect of nonsolvent additives on the porosity and morphology of asymmetric TPX membranes

Effect of nonsolvent additives on the porosity and morphology of asymmetric TPX membranes

j o u r n a l of MEMBRANE SCIENCE ELSEVIER Journal of Membrane Science 118 (1996) 49-61 Effect of nonsolvent additives on the porosity and morpholog...

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j o u r n a l of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 118 (1996) 49-61

Effect of nonsolvent additives on the porosity and morphology of asymmetric TPX membranes Juin-Yih Lai *, Fung-Ching Lin, Cheng-Chuan Wang, Da-Ming Wang Department of Chemical Engineering, Chung Yuan Unieersity, Chung Li , Taiwan 32023, ROC

Received 3 November 1995; accepted 7 March 1996

Abstract In this study, nonsolvent was added in casting solution to elevate the porosity of asymmetric TPX (poly(4-methyl-1pentene)) membranes, prepared by the wet phase inversion method. In addition, to prevent the shrinkage of membranes during the formation process, the dope (TPX/cyclohexane/nonsolvent) was evaporated for 30 s before immersing in the coagulation bath (ethanol). Both coagulation value and solubility-parameter difference were proved to be good criteria for choosing suitable nonsolvent additives. Further, it was found that the addition of nonsolvent can have drastic effect on membrane morphology. Three kinds of membrane structure were observed: sponge, finger (macrovoid) and "cellular surface". Our experimental results indicate that macrovoids are easy to grow in case of delayed demixing and that the formation of the "cellular surface" structure is strongly related to the interfacial instability induced by evaporation. Keywords: TPX; Wet phase inversion method; Membrane morphology; Membrane porosity

1. Introduction The application of membrane techniques to separate gaseous and liquid mixtures is continuously increasing nowadays. These techniques include reverse osmosis, pervaporation, gas separation, ultrafiltration and microfiltration. To obtain good separation performance, different membrane processes require different membrane structures [1,2]. The preparation of membranes with a variety of asymmetric or sym-

* Corresponding author.

metric structures can be accomplished by using the wet phase inversion method [3]. The basic procedure of the wet phase inversion method contains two stages. First, the polymer solution is cast over a suitable substrate to form a thin film. Then, the thin polymer film is immersed in a coagulation bath, where the replacement of solvent by coagulant and the precipitation of polymer take place [1,3]. However, this basic procedure is not flexible enough to produce all the desired membrane structures. Modifications of the basic procedure are usually needed, including changing compositions in the casting solution [4,5] or in the coagulation bath [6-8], and introducing additional steps such as evaporation [9] or annealing [10].

0376-7388/96/$15.00 Copyright © 1996 Elsevier Science B.V. All rights reserved Pll S0376-73 88(96)00084-1

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J.-Y. Lai et al. / Journal of Membrane Science 118 (1996) 49-61

Changing the composition in the casting solution or in the coagulation bath is a convenient method to obtain desired membrane structures. Adding solvent in the coagulation bath can delay the occurrence of liquid-liquid demixing in the casting solution and thus results in denser asymmetric membranes [11]. On the contrary, adding nonsolvent in the casting solution can increase the porosity of membranes. For example, Pesek and Koros [5] successfully elevated the porosity of asymmetric polysulfone membranes by adding two different nonsolvents in the polymer solution and Fritzsche et al. [12] found that the free volume in the skin of asymmetric hollow fibers can be increased by adding Lewis-acid nonsolvent in Lewis-base solvent. In addition, the membrane morphology is strongly affected by the amount of nonsolvent additives. Reuvers [13] reported that appropriate amount of nonsolvent additives enhanced the formation of macrovoids (finger-like pores) while too much nonsolvent suppressed their formation. Evaporating the casting solution before immersing it in the coagulation bath is also a common treatment to improve the membrane structure [14]. It has been observed that the thickness of the skin layer in an asymmetric membrane increases with the increasing of evaporation time. In addition, evaporation is known as an efficient method to suppress the formation of macrovoids [9]. In a previous work [15], we reported that the asymmetric poly(4-methyl-l-pentene) (TPX) membranes possessing very high gas flux and suitable selectivity could be prepared by a two-stage wet phase inversion method. However, the toxicity of chloroform (solvent) and methanol (nonsolvent) prohibits the application of this excellent membrane. Viewing this, in the present study, cyclohexane and ethanol were adopted as the solvent and the coagulant, respectively. But, the porosity of the new TPX membrane is too low for practical application. Therefore, nonsolvents were added in the casting solution to elevate the membrane porosity. In our experience, by using the wet phase inversion method alone, smooth TPX membranes can not be obtained because of the serious shrinkage of membranes during the immersion process. This can be prevented by introducing an evaporation step before immersion. The present work focuses on studying the combined effects of the addition of nonsolvent and the evapora-

tion of casting solution on the structure of TPX membranes.

2. Experimental 2.1. Materials The poly(4-methyl-l-pentene) (TPX, MX-002) used in this study was supplied by Mitsui Co. Solvents were purchased from Merck Co. and coagulants were of reagent grade. All the chemicals were used without further purification.

2.2. Membrane preparation 2.2.1. Wet phase inversion process TPX polymer was dissolved at 60°C in a mixture of solvent (cyclohexane) and nonsolvent additives to form a 4.8 vol% polymer solution. The polymer solution was kept at 40°C for 24 h and then cast on a glass plate to a predetermined thickness of 300 ixm with a Gardner knife. The glass plate was immediately immersed into a coagulation bath in which the coagulant was ethanol (99 wt%) and the temperature was maintained at 25°C. After 10 min, the membrane was peeled off, air dried and put in a circulation oven at 50°C for 12 h. 2.2.2. D r y / w e t process The procedures are similar to the wet process, except that the casting solution was first evaporated at 50°C for 30 s prior to the immersion step. 2.3. Determination of coagulation value The coagulation value was determined by the rapid titration method described in the following. TPX polymer and a mixture of solvent (cyclohexane) and nonsolvent additives were stirred in a glass tube to form a homogeneous polymer solution for titration. Coagulant(ethanol) was added in the polymer solution until the solution became milky-white, representing that the cloud point had been reached. The volume of the added coagulant was recorded as the coagulation value. It should be noted that the experiment was performed at 40°C.

J.-Y. Lai et al. / Journal of Membrane Science 118 (1996) 49-61

51

2.5. S E M

The membrane structures were examined by a Hitachi (model $570) scanning electron microscope (SEM). In SEM studies, membrane samples were fractured in liquid nitrogen and coated with gold to 150 angstrom. 2.6. Solubility-parameter difference f~

?) / ¢

In this work, it was assumed that the solubility parameter of a mixture can be represented by the volume average of the solubility parameters of pure components. Hence, the solubility parameter of the mixture of solvent and nonsolvent additive can be calculated by X1V18i, 1 --~ X 2 V 2 ~ i , 2

Fig. 1. Scanning electron micrograph of commercial TPX dense membrane (Mitsui Co.).

2.4. Overall porosity

After the area (A), the mass (Wm) and the thickness (D) of a TPX membrane sample were measured, the overall porosity can be estimated. The volume of the sample (Vm) equals D x A and the volume occupied by the polymer (Vp) can be expressed as Wm/Pp, where pp is the density of the polymer and has a value of 0.838 g / c m 3 for TPX. Then, the porosity can be calculated: Porosity

Vm--V

- -

Vm

D×ADxA

~i,s :

XlVl + X2V 2

i=d,p,h

where X is the molar fraction, V denotes the molar volume, and the subscripts 1 and 2 represent the solvent (cyclohexane) and the nonsolvent additive, respectively. Subscript i stands for the dispersion interaction (i = d), the polar bonding (i = p) and the hydrogen bonding (i = h) components. Then, it follows that the solubility-parameter difference between TPX and the binary mixture can be expressed as ASs_p

[(Sd,s

8

2

2

I.

.21 1/2

where 8i,p denotes the solubility parameter of TPX.

× 100% 2.7. Light transmission experiment

Wm

-Pp

X 100%

A scanning electron micrograph of a sample of the commercialized TPX dense membrane (Mitsui Co.) is shown in Fig. 1. The membrane structure is very dense and has no visual pores, indicating that the porosity is very small. The overall porosity calculated by the above equation is - 0 . 8 % , a value very close to zero. On the basis of this comparison, the reliability of the method presented here is acceptable.

Light transmission experiments have been performed to measure the time of the onset of liquidliquid demixing in the casting solution. For detailed experimental setup and procedures, one can refer to the work of Reuvers [13].

3. Results and discussion

This section is organized in the following manner. First, the effect of different nonsolvent additives on

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J.-E Lai et al./ Journal of Membrane Science 118 (1996) 49-61 100

Table 1 The relationshipbetween the membraneporosity and the coagulation value Nonsolvent additives

Coagulation value a (ml)

Porosity (%)

Control b Ethanol n-Propanol n-Butanol Cyclohexanol Acetic acid NMP ¢ Acetone Ethyl acetate Diethyl ether n-Hexane

8.4 2.4 2.5 2.6 3.0 2.4 2.1 5.0 5.5 8.1 8.1

42.8 71.1 60.1 63.1 57.1 53.0 70.1 44.6 53.5 40.6 37.8

a Polymer solution: 1 g TPX + 20 ml cyclohexane+ 3.75 ml nonsolvent, 40°C. b Polymer solution: 1 g TPX + 23.75 ml cyclohexane, 40°C. c NMP: 1-Methyl-2-pyrrolidone Coagulant: ethanol.

membrane porosity will be presented. Then, the influence of evaporation will be discussed. In the final section, it will be shown that the membrane structure is dramatically affected by the properties of the nonsolvent additives and some interesting membrane structures will be reported as well. 3.1. Effect o f various nonsolvent additives on the porosity o f membranes Table 1 shows the relationship between the membrane porosity and the coagulation value, defined as the volume of coagulant required to bring the polymer solution into the liquid-liquid demixing region [16]. Generally speaking, lower coagulation value indicates that the polymer solution is easier to phase separate, suggesting that the prepared membrane is more porous. It can be seen clearly from Fig. 2 that our experimental results support the above deduction. Therefore, coagulation value is a good criterion for selecting suitable nonsolvent additives to elevate the membrane porosity. A shortcoming of using coagulation value as the criterion for selecting additives is that it needs to be determined experimentally. A criterion that can be evaluated without performing any experiment would be more convenient. The solubility-parameter differ-

80-

o•" ,.#

6O

40

20

0 O0

I

I

I

I

2.0

40

60

80

10.0

Coagulation vah:e (ml) Fig. 2. The relationship between the membrane porosity and the coagulation value.

ence is widely used to estimate the compatibility between two substances [17]. The relationship between the membrane porosity and the solubilityparameter difference between the polymer and the mixture of solvent and nonsolvent additives (A~s_ p) is listed in Table 2 and plotted in Fig. 3. It can be observed that the porosity increases with the increasing of Ag s p. This result is predictable because larger ASs_ p indicates that the polymer is less compatible with the mixture, suggesting that the system is easier to phase separate. Since Ags_ p has such a

Table 2 The relationshipbetween the membraneporosity and the solubility-parameter difference Nonsolvent additives

A~s_ p (J/cm3) 1/2

Porosity (%)

Control Ethanol n-Propanol n-Butanol Cyclohexanol Acetic acid NMP Acetone Ethyl acetate Diethyl ether n-Hexane

1.5 3.6 3.2 2.9 2.7 2.8 2.8 2.5 2.3 1.5 1.2

42.8 71.1 60.1 63.1 57.1 53.0 70.1 44.6 53.5 40.6 37.8

Data from Van Krevelen[18].

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J. - Y. Lai et al. / Journal of Membrane Science 118 (1996) 49-61 100

80

6o "5 2 4o

20

0 0.5

L

I

I

I

L

t

1.0

1.5

2.0

2.5

3,0

3.5

4.0

Solubility parameter difference Fig. 3. The relationship between the membrane porosity and the solubility-parameter difference.

close relationship with the membrane porosity and the data of solubility parameters can easily be found [18], it can be concluded that Ags_ p is a more convenient criterion than the coagulation value.

3.2. Effect of the evaporation step The membranes discussed in the preceding section are of no practical use because there are a lot of wrinkles on the surface resulting from the shrinkage of membrane during the immersion step. To prevent the shrinkage, a 30-s evaporation step was introduced prior to the immersion, a so-called d r y / w e t process.

The evaporation step has drastic effect on the membrane porosity as shown in Table 3. It can be seen that, for those additives with low boiling point, the d r y / w e t process prepared less porous membrane than the wet process. This can be explained by the fact that, when the evaporation rate of the nonsolvent additive is higher than that of the solvent, the evaporation would lead to lower nonsolvent concentration (larger coagulation value) and higher polymer concentration and hence results in a denser structure. For additives such as acetone, diethyl ether and n-hexane, the high evaporation rate can even completely suppress the porosity-elevating effect of nonsolvent additives. When choosing ethanol as the additive, the effect of evaporation can be seen very clearly. The porosity of the membrane prepared by wet process is 0.711 and, after introducing the evaporation step, the porosity reduces to 0.124, which is even smaller than the control (no nonsolvent additive). It appears that the volatility is also an important factor for choosing suitable nonsolvent additives to elevate the membrane porosity in d r y / w e t process. The results show that an additive with larger ASs_ p and lower boiling point can produce a more porous membrane.

3.3. Effect of nonsolvent additives on membrane morphology For the TPX membranes discussed in the present paper, additives such as n-propanol, n-butanol, cyclohexanol, acetic acid and 1-methyl-2-pyrrolidone (NMP) can successfully increase the membrane

Table 3 The effect of the evaporation step on membrane porosity Nonsolvent

Porosity (%)

Boiling point

additives

Wet

Dry/wet

Porosity difference

(°C)

Control Ethanol n-Propanol n-Butanol Cyclohexanol Acetic acid NMP Acetone Ethyl acetate Diethyl ether n-Hexane

32.3 71.1 60.1 63.1 57.1 53.0 70.1 44.6 53.5 40.6 37.8

23.0 12.4 49.5 51.8 60.1 43.1 75.4 21.9 27.5 12.7 16.6

12.3 58.7 10.6 11.3 -3 9.9 - 5.4 22.7 26.0 27.9 21.2

80.7 78.4 80.7 117.5 161.0 118.1 202.0 56.5 77.1 34.5 68.7

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J.-E Lai et al. // Journal of Membrane Science 118 (1996) 49-61

porosity. In fact, not only the porosity but also the membrane morphology is influenced by the addition of nonsolvent. The effect of nonsolvent additives on membrane morphology is shown in Fig. 4. It is obvious that there are two types of membranes: one has smooth surfaces (Fig. 4 (B), (C), (D)) and the other has holes on the surface (Fig. 4 (E), (F)). The alcohol additives (n-propanol, n-butanol, cyclohexanol) belong to the former type, and acetic acid and NMP belong to the latter.

about macrovoid structure (finger-like pores). The formation m e c h a n i s m of m a c r o v o i d s for coagulant/solvent/polymer ternary systems has been studied extensively [9,11,19]. Although the exact mechanism is not known yet, experimental evidences suggest that macrovoids are easy to occur in a system with small coagulation value (easy to phase separate) and high miscibility between solvent and coagulant. According to Table 1, the coagulation value of n-propanol is a little lower than those of n-butanol and cyclohexanol (see Table 1) indicating that the system with n-propanol has a little higher tendency to liquid-liquid demix than the other two systems. In addition, since n-propanol has a higher

3.3.1. Membranes with smooth surfaces As shown in Fig. 4, n-propanol produces spongelike pores and n-butanol and cyclohexanol bring

(I)

(A)

(D)

(B)

(C)

(E)

(F)

Fig. 4. Effect of the addition of nonsolvent in the casting solution on membrane morphology: (I) surface; (II) cross-section; (A) no additive; (B) n-propanol; (C) n-butanol; (D) cyclohexanol; (E) acetic acid; (F) NMP.

J.-Y. Lai et a l . / Journal of Membrane Science 118 (1996) 49-61

55

(II)

(A)

(B)

(c)

(D)

(E)

(F)

Fig. 4 (continued).

miscibility with the coagulant (ethanol) than nbutanol and cyclohexanol, it is reasonable to expect that the casting solution and the coagulant have higher tendency to mix when n-propanol is added. Therefore, the addition of n-propanol should increase the opportunity to form macrovoids. However, this deduction is contrary to the experimental observation. The light transmission experiment has been used to estimate the onset time of liquid-liquid demixing [13]. Generally speaking, macrovoids are formed in case of instantaneous demixing and spongy structure in case of delayed demixing. It has been proved that the addition of nonsolvent in the casting solution can shift the system from delayed to instantaneous

demixing [11]. On basis of the arguments in last paragraph, it can be predicted that the addition of n-propanol would make the casting solution closer to instantaneous demixing than the addition of nbutanol and cyclohexanol. This prediction was verified by the experimental results demonstrated in Fig. 5. Then, we arrived at a conclusion that macrovoids are easy to form in case of delayed demixing, which seems contradictory to the experimental observations reported in literature. This contradiction can be resolved in light of a macrovoid formation mechanism proposed by Smolders et al. [11]. Smolders et al. [11] suggested that the formation mechanism of macrovoids can be split into two stages: initiation and growth. In the initiation stage,

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J.- Y. Lai et al. / Journal of Membrane Science 118 (1996) 49-61

the nucleation of the polymer-poor phase resulting from the liquid-liquid demixing can initiate the formation of macrovoids. In the growth stage, the nuclei expand to macrovoids because the diffusional flow of solvent from the polymer solution to the nuclei is larger than the flow of coagulant from the nuclei to the polymer solution. Therefore, instantaneous demixing is preferred in the initiation stage and delayed demixing is required to provide appropriate environments for the growth of macrovoids. On basis of this mechanism, although the addition of n-propanol promotes the formation of nuclei in the initiation stage, it inhibits the expansion of nuclei to form macrovoids in the growth stage as well. In a coagulant/solvent/polymer ternary system (with no additives), instantaneous demixing or very short delay time is required to initiate the formation of macrovoids [11]. For delayed demixing, a dense skin layer is formed initially which inhibits the permeation of coagulant into the casting solution. Since the coagulant is difficult to diffuse into the polymer solution, the initiation of macrovoids is difficult. However, when nonsolvent is added in the polymer solution, our experimental data suggest that the requirement of instantaneous demixing is not so crucial. For example, in Fig. 5, the addition of cyclohexanol induces the formation of macrovoids in case of delayed demixing. We believe this phenomenon can be explained by the fact that the required amount of coagulant to bring about the initiation of liquidliquid demixing is small (nonsolvent has been added) so that the initiation can happen even after the formation of a dense layer. Since instantaneous demixing is not a requirement in our system, if the macrovoids can occur is controlled by the delayed demixing in the growth stage. Hence, the addition of n-propanol has less opportunity to induce the formation of macrovoids than the addition of n-butanol and cyclohexanol because it promotes instantaneous demixing. The requirement of delayed demixing for inducing macrovoids in our system can be further confirmed by the two experimental observations described below. First, as shown in Fig. 6, the macrovoids can not be induced if there is no evaporation step. Second, adding more nonsolvent in the casting solution suppresses the formation of macrovoids (see Figs. 7 and 8). If instantaneous

lOO

80-~~a) ,~,

60 -

(a)

40-

20-

0 0

I 10

I I 20 30 T i m e (sec)

I 40

50

Fig. 5. Light transmission curves: (a) no additive; (b) n-propanol;

(c) n-butanol; (d) cyclohexanol.

demixing is required to initiate the formation of macrovoids, evaporation should suppress the formation of macrovoids, as being reported in literature [9]. However, we observed the opposite result, indicating that delayed demixing, which can be promoted by evaporation, is the decisive factor for the formation of macrovoids in our system. It has been reported by Smolders et al. [11] that macrovoids can be induced when appropriate amount of nonsolvent is added because the instantaneous demixing is promoted; however, overdose would suppress the formation of macrovoids because the delayed demixing in the growth stage is inhibited. The results presented in Figs. 7 and 8 show similar trend and support the hypothesis that delayed demixing plays an important role in the formation of macrovoids.

3.3.2. Membranes with "cellular surface" surfaces When acetic acid and NMP were added in the casting solution, "cellular surfaces" were observed on the membrane surface as shown in Fig. 4 (E) and (F). There is no obvious pattern for the addition of NMP but a polygonal cell pattern can be found for acetic acid. Some preliminary experiments were carried out to investigate the formation mechanism of the polygonal cells. During the evaporation, it can be observed by using a microscope that, after the addition of acetic

J.-Y. Lai et al. / Journal of Membrane Science 118 (1996) 49-61

acid, the fluidity of the casting solution is much higher near the interface than in the bulk. This might be able to explain why the polygonal holes only occur near the interface because the region with lower viscosity is more unstable when subject to

57

disturbances. The large difference in polarity between acetic acid and the polymer solution (TPX/cyclohexane) might play an important role in the formation of polygonal holes. This deduction is supported by the following observations: the holes

(A)

(B)

(C)

(D)

Fig. 6. Effect of the evaporation step on the formation of macrovoids: (A) without evaporation step, n-butanol; (B) without evaporation step, cyclohexanol; (C) with evaporation step, n-butanol; (D) with evaporation step, cyclohexanol.

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J.- E Lai et al. / Journal of Membrane Science 118 (1996) 49-61

on the membrane surface were completely suppressed after the polarity difference was reduced by the addition of n-butanol (a polar component) in the polymer solution; but the membrane structure remained the same when n-hexane (a nonpolar component) was added.

It was found that the evaporation step is responsible for the formation of polygonal cells. This conclusion was drawn from two experimental observations: the surface holes were completely inhibited when the membranes were prepared without evaporation step, and same polygonal patterns can be produced when

(A)

(B) !! i

(C)

(D)

Fig. 7. Effect of the amount of nonsolvent additive on membrane structure. The volume ratio of n-butanol to cyclohexane: (A) 10/85; (B) 15/80; (C) 20/75; (D) 25/70.

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J.-Y. Lai et a l . / Journal of Membrane Science 118 (1996) 49-61

the membranes were prepared by evaporation only, without immersing the casting solution in the coagulation bath (dry method). Hexagonal patterns have been observed in thin

:~i!ii

film systems such as Bernard-flow [20] and the evaporation of paint [21]. It is known that the formation of the surface pattern is strongly related to the convective flow near the surface induced by interfa-

iiiii~!i!

~ii?!i!i~i!:!ijilji!ill

(A)

(B)

(C)

(D)

Fig. 8. Effect of the amount of nonsolvent additive on membrane structure. The volume ratio of cyclohexanol to cyclohexane: (A) 10/85; (B) 15/80; (C) 20/75; (D) 25/70.

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J.-E Lai et al. / Journal of Membrane Science 118 (1996) 49-61

cial instability. The interfacial instability can be generated by either the density gradient or the interfacial tension gradient. We have proved that the density gradient is not crucial in our system by noticing that the same pattern occurs when the membrane was prepared by turning the casting plate upside down. The interfacial tension gradient could be induced by temperature or concentration gradients because the interfacial tension is strongly dependent on the temperature and the solution composition. The temperature gradient seems not a decisive factor because the surface holes were observed in both cases: the ambient temperature during evaporation is higher (50°C) or lower (25°C) than the film temperature (40°C). Therefore, the concentration gradient stemming from the evaporation might be responsible for the interfacial instability (Marangoni effect [22]). Verification of this hypothesis still remains a challenge.

4. Conclusion The addition of nonsolvent in casting solution was proved to be an efficient method to elevate the porosity of TPX membranes. The nonsolvent additive which results in more porous membrane has smaller coagulation value and larger A~s_ p. Therefore, both the coagulation value and the solubilityparameter difference (ASs_p) are good criteria for selecting suitable nonsolvent additives. When evaporation is part of the membrane formation process, the volatility of the nonsolvent additive should also be taken into account. Less volatile additives are preferred for preparing more porous membranes. The combined effect of the addition of nonsolvent and the evaporation of casting solution can have drastic influence on the membrane morphology. Three types of membrane structure were observed: sponge, macrovoid, and "cellular surface". An interesting phenomenon was observed: macrovoid structure can be found in a "delayed demixing" system. This can be explained in light of the membrane formation mechanism proposed by Smolders et. al. [l 1]: the nucleation of polymer poor phase resulting from instantaneous demixing initiates the formation of macrovoids but it is the delayed demixing which makes these nuclei grow to form macrovoids. In our systems, the instantaneous

demixing (fast exchange between solvent and coagulant) is not required for initiating the nuclei because a certain amount of nonsolvent has already been added in the casting solution. Therefore, if these nuclei can grow is the decisive factor for the formation of macrovoids, indicating that the delayed-demixing is preferred. To our knowledge, the "cellular surface" membrane structure has not been reported in literature. The formation mechanism for this structure is not known yet. Our preliminary experimental results suggest that this structure is strongly related to the interfacial instability induced by evaporation.

Acknowledgements The authors wish to sincerely thank the National Science Council of Taiwan, ROC (NSC 85-2331-B033-001-M08) for the financial support of this project.

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[16] C.W. Yao, R.P. Burford, A.G. Fane and C.J.D. Fell, Effect of coagulation conditions on structure and properties of membranes from aliphatic polyamides, J. Membrane Sci., 38 (1988) 113-125. [17] A. Bottino, G. Camera-Roda, G. Capanneli and S. Munari, The formation of microporous polyvinylidene difluoride membranes by phase separation, J. Membrane Sci., 57 (1991) 1-20.

[18] D.W. Van Krevelen, Properties of Polymers, Elsevier, Amsterdam, The Netherlands, 1990. [19] R.J. Ray, W.B. Krantz and R.L. Sani, Linear stability theory model for finger formation in asymmetric membranes, J. Membrane Sci., 23 (1985) 155-182. [20] C.M. Hansen and P.E. Pierce, Cellular convection in polymer coating. An assessment, Ind. Eng. Chem. Prod. Res. Dev., 12 (1973) 67-70. [21] C.A. Miller and P. Neogi, Interfacial Phenomena: Equilibrium and Dynamic Effects, Marcel Dekker, New York, 1985. [22] C.V. Sternling and L.E. Scriven, Interfacial turbulence: Hydrodynamic instability and the Marangoni effect, AIChE J., 5 (1959) 514-523.