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Formation and gas flux of asymmetric PMMA membranes
j o u r n a l of MEMBRANE SCIENCE E LS EV l E R
Journal of Membrane Science 109 ( t 996 ) 93-107
Formation and gas flux of asymmetric PMMA membranes Jao-Ming Cheng, Da-Ming Wang, Fung-Ching Lin, Juin-Yih Lai * Department of Chemical Engineering, Chung Yuan UniversiO,. Chung Li, Taiwan 32023 Received 11 October 1994; revised 3 May 1995; accepted 19 July 1995
Abstract In the present work, PMMA membranes were prepared by wet phase immersion methods to improve their gas fluxes. It is found that different membrane structure can be obtained by using different nonsolvent-solvent pairs. To completely describe the membrane formation process, the nonsolvent-solvent miscibility and the interfacial polymer concentration in casting solution should be considered accompanied by the ternary phase diagram. A simplified solution~liffusion model was developed to estimate the interfacial polymer concentration. In addition, the effects of adding solvent into the coagulation bath and adding nonsolvent into the casting solution are discussed. Kevwords." Asymmetric membrane formation; PMMA; Wet phase inversion; Morphology; Gas flux
Coagulation bath
1. Introduction
When membranes are used to separate gas mixtures, an asymmetric membrane is preferred, with a defectfree ultra thin skin layer to retain intrinsic selectivity and a porous sublayer to reduce the resistance of gas permeation. In addition, it possesses proper mechanical strength. Wet phase inversion method is the most widely used technique for preparing asymmetric membranes. In this process, a homogeneous casting solution (polymer solution) is immersed into a coagulation bath. The solvent diffuses out of the casting film and the coagulant diffuses into it (Fig. 1 ), which results in phase transition and polymer precipitation to form a membrane. Based on this method, many researchers [ 1-4] developed modification processes to prepare high performance membranes for gas separation. * Corresponding author. 0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved S S D I 0 3 7 6 - 7 3 8 8 ( 9 5 ) 001 87-5
t Js
Jn~ interface
Js
J ,~'~ Casting Solution
IIIIIII
Support
Fig. l. Fluxes of nonsolvent (J.s, f.,) and solvent (J~, fs) at the interface of the coagulation medium and the cast polymer solution(casting film),
Understanding membrane formation mechanism is essential for developing optimal asymmetric structures. Generally speaking, the asymmetric structure is strongly related to the composition history of the casting film during the immersion process. This behavior can be studied in terms the composition path in a thermodynamic ternary phase diagram of a nonsolvent-
94
J.-M. Cheng et al. / Journal of Membrane Science 109 (1996) 93-107
polymer
solvent
nonsolvent
Fig. 2. Ternary phase diagram of polymer/solvent/nonsolvent. A, B: coagulation path.
solvent-polymer system as shown in Fig. 2. When the composition path directly enters the solidification region, the polymer solidifies (gel type). When the path intersects the liquid-liquid phase separation region, the casting solution separates into polymer-rich and polymer-poor phases and results in porous structure. In addition, it has been shown that the physical properties such as surface tension, viscosity, etc., and the solvent outflow and coagulant inflow rates can also affect the structure of asymmetric membranes. Matz [5] stated that the formation of finger-like pores was due to the hydrodynamic instabilities coupled with surface-tension and viscosity change. Strathmann et al. [6] postulated that low precipitation rates produced membranes of sponge-like structure, whereas high precipitation rates caused large finger-like pores. Koenhen et al. [ 7 ] proposed a mechanism for the formation of sponge structure which is related to liquidliquid phase separation, nucleation and the growth of dispersed phase. Reuvers [ 8 ] pointed out that the interfacial polymer concentration of the casting solution which would form finger-type membranes is generally higher than the one would form sponge-type. Bottino et al. [ 9 ] attempted to find the relationship between the physical properties of casting solution and the membrane characteristics. There were also several investigations [ 10-14] stressing the importance of diffusion kinetics. Poly (methyl methacrylate) (PMMA) has good oxygen-to-nitrogen selectivity but low gas permeability [ 15]. The main purpose of the present work is to show
that the gas flux of a PMMA membrane can be improved by wet phase inversion method. To clearly understand this phenomenon, the effects of solvent and coagulant on membrane morphology and gas flux were studied and will be reported below.
2. Experimental 2.1. Materials
Poly(methyl methacrylate)(PMMA) used in this study was supplied by Aldrich Chemical Co., MW 140 000. All solvents and nonsolvents were of reagent grade and were used without further purification. Compressed dry air was used for gas permeability test. 2.2. Membrane preparation
At room temperature, PMMA polymer was dissolved in proper solvents to form casting solutions. The concentration was represented in vol/vol% (cm 3, polymer/cm 3, solvent). The solution was cast on a glass plate to a predetermined thickness of 300/zm with a Gardner knife, and then immersed into a coagulation bath in which the volume ratio of the coagulation medium to the casting solution was kept about 60. After precipitation, the membranes were peeled off and dried in vacuum for 24 h.
J.-M. Cheng et al. / Journal of Membrane Science 109 (1096) 93-107 2.3. Phase diagram P M M A solutions with different composition were placed in tubes at 25°C and nonsolvents were then slowly added to the solutions until the turbidity values suddenly change (detected by a Hach turbidimeter). This composition represents the transition concentration between the one-phase and the two-phase regions. With this data, a ternary phase diagram can be constructed.
95
~,-Ns = [( 6d.p - 6d,NS)2 + ( 6p.p-- 6p.NS)2 + (&,,,-- &.NS):] '/~
/1)
~P-S = [ ( (~d,P -- ~'~d,S ) 2 _1_ ( •p,p __ (~V,S ) 2
+ ¢ & , p _ &,~)2] ,/2
12)
where 6d stands for the dispersion interaction, 6p represents the polar bonding and 6h denotes the hydrogen bonding component [ 16]. The subscripts refer to the polymer ( P ) , solvent ( S ) and coagulant ( NS ), respectively.
2.4. Light transmission experiments The light transmission apparatus used in this study was similar to Reuvers et al.'s [ 14]. The decrease of light transmittance was caused by the liquid-liquid demixing of casting solution.
2.5. Gas permeation measurement The oxygen and nitrogen pressure-normalized fluxes were determined by Yanaco GTR-10 gas permeation analyzer. The structure of membranes was examined by a Hitachi model $570 scanning electron microscope. For scanning electron microscope studies, membrane samples were fractured in liquid nitrogen and coated with gold to ~ 150 A.
2.6. Precipitation behavior studies A drop of casting solution was placed between two microscope slides and a drop of coagulant was introduced by a syringe. The casting solution and the coagulant would contact because of capillary action. The interface was observed under Olympus BHT-M- 113D optical microscopic. In order to clearly observe the flow pattern in the casting solution, water was dyed with Rhodamine B.
2. Z Difference between solubility parameters The solubility difference between P M M A and nonsolvent is represented by &e-NS and the difference between P M M A and solvent is 6e_s.
Table 1 Structure of asymmetric PMMA membranes prepared by the wet phase inversion methoda Solvent
Coagulation medium Membrane structure
Ethyl acetate
n-hexane
Acetone
n-hexane
Methyl ethyl ketone n-hexane Cyclohexanone
n-hexane
Allyl alcohol~'
n-hexane
Dioxaneb
n-hexane
Tetrahydrofuran
n-hexane
Butyl acetate
n-hexane
NMP
H20
DMAc
H20
DMSO (50°C)
H20
Acetone
H20
Allyl alcohol Dioxane
CHsOH --+H20 C H 3 O H / H 2=0 1
skin exists, spongy sublayer skin exists, spongy sublayer skin exists, spongy sublayer skin exists, spongy sublayer skin exists, spongy subtayer skin exists, spongy sublayer skin exists, spongy sublayer no skin, bamboolike sublayer no skin, finger type sublayer no skin, finger type sublayer no skin, macrovoid sublayer symmetric porous structure no skin, finger type no skin, foamlike structure
~Casting solution: PMMA 14.7 vol%. bNonsolvent is added in the casting solution.
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J.-M. Cheng et al. / Journal of Membrane Science 109 (1996) 93 107
Table 2 The gas separation performance of PMMA membranes which were
3. Results and discussion
3.1. Formation of asymmetric membranes and their gas flux Different asymmetric PMMA membranes can be prepared by wet phase inversion method with different coagulant-solvent pairs. The results are listed in Table 1, in which the PMMA concentration of all casting solutions is 14.7 vol%. This value was chosen because it is the lowest concentration that an integral membrane can be formed by using water and acetone as the coagulant and solvent, a system requires higher polymer concentration to produce integral membrane compared to others. Most of the membranes prepared by using n-hexane as coagulant exhibit an asymmetric structure which consists of a dense skin and a porous
prepared in different coagulant-solvent systems" Coagulant-solvent
02 pressurenormalized flux (GPU)
Selectivity O2/N 2
CH2C12 (dry method) N-Hexane-EA n-Hexane-acetone H20-acetone n-Hexane-MEK n-Hexane-cyclohexane n-Hexane-THF
0.0057 0.0778 0.0768 0.0146 0.0814 0.0657 0.0735
5.7 4.8 5. l 4.6 5.2 4.8 4.8
"Casting solution: 14.7 vol% PMMA/solvent.
7
®
®
Fig. 3. SEM micrographs of PMMA membranes for different coagulant-solvent pairs. (A) n-Hexane-acetone, (B) H20-NMP; (C) (CH3OH ~ H20)-aUyl alcohol; (D) n-hexane-butyl acetate; (E) H20-acetone.
.I.-M. Cheng et al./Journal c (Membrane Science 109 (19u6) 93-107
that P M M A cannot be completely precipitated by u sing methanol as coagulant. In the following, only the single-coagulant system will be discussed. The gas separation performance of the asymmetric P M M A membranes are shown in Table 2, in which only the membranes with good O2/Ne selectivity are listed. Comparing to the membrane prepared by dry method, most of the asymmetric membranes posses much larger gas flux, about 15 times. However, the gas flux of the membrane formed from H,O-acetone system is low - only about twice of the dry method. The reason why H~O-acetone system possesses lower gas flux will be discussed later.
I
I
t00 ~:q====,,~__~
00 0
\
1~
97
20
gO
Time(see) Fig. 4. Light transmission curves: (a) n-hexane-butyl acetate: (b) n-hexane-acetone: (c) H20-acctone: (d) H_,O-NMP: (e) n-hey ane-elhyl acetate. spongy sublayer, as shown in Fig. 3 ( A ) . n - H e x a n e butyl acetate system is an exception. On the other hand, when using water as the coagulant, most of the systems form macrovoid (finger-like) structure as shown in Fig. 3 ( B ) , except for the water-acetone system. Fig. 3( C ) depicts the cross-sectional structure of membranes prepared by using allyl alcohol as solvent, methanol as the I st coagulant and water as the 2nd coagulant. The above system requires two coagulant bath because most alcohol can either swell or dissolve P M M A so
3.2. E1Sect o f coagulant-solvent pairs (m memt~ram' morphology
It has been believed by many that the rate of liquid liquid demixing has a dramatic effect on membrane morphology [6,8,17]. Therefore, a light transmission experiment was conducted to study thc onset of L--L demixing. The results are shown in Fig. 4. Obvious change of transmittance at the very moment of immersion (at time < I s), indicating instantaneous denfixing, was observed for the systems of n-hexane butyl acetate (curve a) and H 2 0 - N M P ( curve b ). The other three systems (n-hexane-acetone, H~O-acetone and nhexane-ethyl acetate) belong to the category of delayed demixing. Our results show that instantaneous demixing forms finger-type membrane and dehwed
PMMA
Fig. 5. Ternary phase diagram of PMMA solution. Nonsolvent solvent: ( + ) H20 NMP: ( O ) H_~Oacetone: t ,k ) n-hexane-butyl acetate:/'_'~) n-hexane-ethyl acetate; ( [] ) n-hexane-acetone.
98
J.-M. Cheng et al./ Journal of Membrane Science 109 (1996) 93-107
Fig. 6. Photo-micrographsof the casting solution/ coagulant interface ( × 200): (A) H20-NMP; (B) HzO-acetone. demixing produces spongy type. The same conclusion was drawn in the work of Reuvers [ 8]. It has been widely accepted that when the binodal curve of a ternary system is closer to the polymersolvent axis, instantaneous demixing is easier to occur [ 18 ], therefore results in a finger-type structure. Hence, water-NMP, n-hexane-butyl acetate are easier to form finger-type membranes (see Fig. 5). However, the experimental results (Fig. 3 and Fig. 4) indicate that water-acetone system forms spongy membranes. This suggests that the thermodynamic phase diagram is not
enough to completely describe the membrane formation mechanism, other factors need to be considered. Reuvers [8] suggested that the miscibility between solvent and coagulant is an important factor in determining if finger-type structure can occur. Generally speaking, if the miscibility is high, the coagulant can easily penetrate into the casting solution and produce macrovoid structure. The miscibility between the solvent and the coagulant can be represented by the interaction parameter between them (g~2). The higher the interaction parameter, the lower the miscibility. The
J.-M. Cheng et al. /Journal (~/'Membrane Science 109 ( I ~96) 93-107 Table 3 Solubility parameter difference between P M M A and coagulant ( solvent ) Nonsolveni
6p NS (J'lelcm31e)
He() n-Hexanc Solvent
36. I 10.6 6e s (J~/-'/cm3/2)
Acelone NMP DMAc Dioxane THF MEK EA Butyl acetate
2.2 4.7 4.9 6.7 3.0 1.8 3.8 4.5
g12 value of H20-acetone is between 2.2 (water concentration is small) and 1.3 (acetone concentration is small) [ 18], and the value of HzO-NMP is between 1.3 (water concentration is small) and 0.65 (NMP concentration is small) [19]. It is obvious that the H20-NMP system has a higher coagulant-solvent miscibility than the H20-acetone system. Therefore, H~O is easier to penetrate into the NMP-PMMA casting solution and produces the finger structure. Whereas, water cannot pass through the acetone-PMMA solution easily. This deduction was verified by observing the inter-diffusion behavior between nonsolvent and casting solution under the microscope (see Fig. 6). It shows that water can break the interface when NMP is the solvent: but water cannot flow into the casting solution when acetone is the solvent. Reuvers [8] also pointed out that a very steep polymer concentration gradient existing at the solutioncoagulant interface would bring about instability of the nascent toplayer and produced the finger structure. The polymer concentration gradient can be estimated roughly by the difference of polymer concentration between the original casting solution and the solutioncoagulant interface. To estimate the polymer concentration at the interface, mathematical models have been developed by Cohen [ 10], Reuvers et al. [ 14], Tsay and McHugh [13]. Their models are quite sophisticated, but difficult to use because of the complicated numerical procedures for solving the simultaneous partial differential equations. In the present work, we developed a simplified solution~liffusion model to
99
estimate the interfacial polymer concentration (see Appendix A). The interfacial polymer concentration can be described by
[ vID,,~/b/D.~/c
where qS~p.4 ~ represent the interfacial polymer and nonsolvent concentration of the casting solution at the initial moment of immersion, k is the partition coefficient of solvent between the polymer rich and poor phases. D,,~/h and D,~/~ stand for the mutual diffusion coefficient of nonsolvent in the coagulation bath and in the casting solution; respectively. &',~and k could also represent the position of the binodal curve in a ternary phase diagram and the slope of tie lines. The binodal curves of H20-NMP, H20-acetone and n-hexanebutyl acetate systems are almost overlapped (see Fig. 5). indicating that the values of d,',,~are the same. Dn~/b is in fact the mutual diffusivity of solvent in nonsolvent (coagulant) at a very dilute concentration bath because the bath is a binary ( solvent and nonsolvent) system. D,~/c can be estimated by D~/J'(&p). D~,~/~ represents the diffusivity of nonsolvent in the casting solution as polymer concentration (~hp) approaches zero and./'(d~p) stands for the effect of pol-
"-a O
5~ &
g--.
•
•
0.10
0.08
N 0.06
0.04
0.02
0.00
~
13.0
i
i
18.0
i
i
i
r
i
i
i
i
1
~
i
i
p
19.0 22.0 2 5 . 0 2 8 . 0
PMMA v o l u m e
fraction
(~)
Fig. 7. Effect of PMMA concentration on gas separation performance. coagulant-solvent: ( O ) n-hexane-acetone: ( i . ) HeO-acetone ~'GPU = 1 × IO ~ ' c m 3 ( S T P ) / c m -" s cmHg.
100
J.-M. Cheng et al. /Journal of Membrane Science 109 (1996) 93-107
(i i!i~i •
-r" 7
'-<,,
p g
ymer. It is assumed that D°~/~ can be substituted by the diffusivity of nonsolvent in solvent at a very dilute concentration. In this study, the polymer concentration was fixed at 14.7 vol% for all systems; therefore, it is reasonable to assume that the effect of polymer are the same for all systems. The binary diffusion coefficients can be estimated by using the average from Tyn-Calus and Hayduk-Minhas methods [20]. The values of ~/Dn~/b/D°/c of H20-NMP, H20-acetone and n-hexane-butyl acetate systems can then be calculated; they are 0.7, 0.44 and 1.44, respectively. The value of H 2 0 acetone system is the smallest. The slope of tie lines (k) can be determined by calculating the binodal curves of PMMA solutions based on the free volume theory [ 18]. Our calculation (will be reported lately) shows that the value of k for the water-acetone system is the largest among the three systems mentioned above. Hence, the interfacial polymer concentration of water-
acetone system is lower than the other two systems. Since the H20-acetone system appears to have the less steep polymer concentration gradient, it prefers to produce sponge-type structure. Although both water-acetone and n-hexane-acetone systems form spongy membrane, the formation mechanisms are different. The membrane formation mechanism of using n-hexane as the coagulant can be described as follows: First, after the casting solution is immersed into a n-hexane bath, the uppermost region can form an integral skin, because of the delayed demixing. The region below the top skin layer remains in fluid state, because the polymer concentration is not high enough to make solidification occur. Then, liquidliquid phase separation takes place because of the diffusion of n-hexane (nonsolvent) from the coagulant bath into the casting solution [7]. Finally, a porous spongy sublayer is developed. The morphology is
J.-M. Cheng et al. / Journal of Membrane Science 109 (1996) 93 107
,
,.~
101
!>
Fig. 8. SEM micrographsof PMMA membranes for systems with differentpolymerconcentrations• (A) n-Hexane-acetone; ( B ) H,O-acetone:
(CI mhexane-butyl acetate; (D) H20-NMP: (D) (CH3OH-~H20)-allyt alcohol. Upper: 21 vol%. lower: 29.4 volq/~. shown in Fig. 3 ( A ) . However, because of the incompatibility between PMMA and water (large 6p,Ns, see Table 3 ), water cannot easily diffuse through the dense skin into the casting solution. Therefore, the completion of membrane formation mainly result from the leaving of acetone. Since the formation mechanism of the sublayer of water-acetone system is mainly due to the leaving of solvent, similar to the mechanism of dry method, the sublayer structure is less porous (see Fig. 3 ( E ) ) and therefore results in low gas flux (Table 2). 3.3. Effect o f polymer concentration on membrane morphology and gas separation performance
As shown in Fig. 7, the 02 flux decreases and 0 2 / N 2 selectivity remains unchanged with increasing
PMMA concentration for both systems of water-acetone and n-hexane-acetone. When the polymer concentration of the casting solution increases, the thickness of toplayer increases and the porosity decreases for n-hexane, as shown in Fig. 8(A). When using acetone as the solvent, the porosity remains unchanged, only the membrane becomes thicker, as shown in Fig. 8(B). As mentioned above, the membrane formation mechanisms for these two systems are different. When n-hexane is used as coagulant, an increase in polymer concentration decreases the amount of casting solution undergoing liquid-liquid phase separation and results in a denser structure. For water, the membrane formation is mainly due to the leaving of acetone from the casting solution, so the increase of polymer concentration does not influence the structure and merely makes total resistance increase
102
J.-M. Cheng et al. / Journal of Membrane Science 109 (1996) 93-107 O
. . . . . .
.
Fig. 9. Effect of adding nonsolvent (formamide) into the casting solution on membrane morphology. Casting solution: 14.7 vol% PMMA + formamide, formamide/acetone: (A) 5 vol%; (B) 6.25 vol%; (C) 7.5 vol%; (D) 15 vol%, solvent: acetone; coagulant: water.
(similar to the dry method). Their O 2 / N 2 selectivity remains the same because nondefective skins have been formed in the beginning of phase inversion. Fig. 8(C), (D) and (E) show the effect of PMMA concentration on membrane morphology for n-hexanebutyl acetate, H20-NMP and (CH3OH ~ HzO)-allyl alcohol, respectively. It clearly indicates that the cavities decreases with an increase of polymer concentration. For the n-hexane-butyl acetate system (Fig. 8 (C) ), an increase of polymer concentration can lead to the disappearance of finger structure (21 vol%) or even make the membrane become homogeneously dense (29.4 vol%). This happens because the polymer concentration is high enough to make the polymer precipitate before the phase separation begins.
3.4. Effect of adding nonsolvent into the casting solution on morphology and gas separation performance It has been noticed [ 1,4] that the adding of nonsolvent into the casting solution leads to a more porous structure or even a finger-type membrane. This can be explained by the fact that the adding of nonsolvent reduces the compatibility between polymer and solvent and makes L - L demixing easy to occur. However, for water-acetone system, adding water into the casting solution results in poor membrane formation, similar to the effect of lowering the polymer concentration in the casting solution. This indicates that a small amount of H20 in acetone does not reduce the solvent power of acetone. To improve the porosity of membrane by
J.-M. Cheng et al. / Journal o f Membrane Science 109 (1996) 9 3 - 1 0 7
surface
cross
103
bottom
O
o
117¸
® !
Fig. 10. Effect of adding various alkanes (nonsolvent) into the casting solution on membrane morphology, casting solution: PMMA : nonsolvent:acetone= 14.7:30:100 (vol/vol%), nonsolvent: (A) n-hexane; (B) n-heptane; (C) n-octane, coagulant: n-hexane.
adding nonsolvent, formamide is added into the casting solution(the nonsolvent in coagulation bath is still water). The results are shown in Fig. 9. When the added formamide is less than 6.25 vol%, both porosity and gas flux slightly increase with the increase of formamide. However, when the added formamide is more than
6.25 vol%, the large cavities exist and the membrane possesses no O2/N2 selectivity. When using n-hexane as the coagulant, the effect of adding alkanes (nonsolvents) in the casting solution on membrane morphology was studied. Fig. 10 shows the porosity of membranes increases in the order of noctane > n-heptane > n-hexane. The O2 pressure-nor-
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J.-M. Cheng et al. /Journal of Membrane Science 109 (1996) 93-107
Table 4 Effect of adding alkanes (nonsolvent) into the casting solution on gas separation performance"
7
-5~ -4
Nonsolvent 02 flux GPU Selectivity Oz/N2 Coagulant toleranceb n-Hexane 0.16 n-Heptane 0.25 n-Octane 0.40
4.6 3.3 2.0
-3 "~
41.5 38.5 36.5
~Casting solution: PMMA/nonsolvent/acetone= 14.7/30/100 (volume ratio) coagulant: n-hexane. bCoagulant tolerance: coagulant/acetone ( v/v%) as casting solution undergoing liquid-liquid phase separation, obtained from turbidity measurements.
-2
o
-0
0.21 0.19
~'~ 0.17 0.15 0.13
malized fluxes increase in this order while 0 2 / N 2 selectivity in the opposite order, as shown in Table 4. This can be explained by the fact that the liquid-liquid phase separation is easier to occur when the coagulant tolerance is small [21 ].
3.5. Membrane modified by adding solvent in coagulation medium When acetone is the solvent of the casting solution, and coagulation bath contains 5 vol% of acetone (or EA) and 95 vol% of n-hexane, poor membrane formation was observed. However, by using EA as the solvent, the good membrane formation can be obtained when the coagulation bath contains proper amount of acetone or EA. The reason might be that the PMMA solubility of EA is smaller(rp_s = 3.8 JJ/2/cm3/2) than a c e t o n e ( r p s = 2 . 2 J1/2/cm 3/2) which makes PMMA easier to precipitate in the EA system. For the n-hexane-EA system, the effect of adding solvent(EA or acetone) in the coagulation bath is depicted in Fig. 11, indicating that the adding of EA in the coagulation bath results in higher gas flux and the slightly decrease of selectivity. When acetone is added, there exists a maximum 02/ N2 selectivity (6.2) at 5 vol%. When the acetone in the coagulation medium is not in a large amount (lower than 13 vol%), acetone can permeate into the casting solution and is absorbed in the membrane. For removing the retained acetone, the membrane have to transfer into another coagulant bath which contains pure nhexane. This step might bring out capillary pressure which makes the polymer particles packing closer and the selectivity of gas higher. On the other hand, as more
_
0.11 0
0.09 0.07
, i , t-i..,
0
5
, i , 1,
10
, , , i , ~ , iI
15
20
, i i ,
25
solvent in coagulation m e d i u m (volg) Fig. 11. Effect of adding solvent into the coagulation medium on gas separation performance. Coagulant: n-hexane, added solvent: (11) acetone; ( 0 ) EA.
acetone was added, the space between polymer particles is too large to make capillary pressure play a role, thereby leading to larger free volume of membrane which possesses low O 2 / N 2 selectivity. Similar results were reported by Rezac et al. [ 3 ]. Fig. 12 shows the morphologies of PMMA membranes which were made by adding 20 vol% of solvent in the coagulation medium. This indicates that adding more acetone would transfer membrane structure to another form.
4. Conclusion In the present work, we have shown that the PMMA asymmetric membranes can be successfully produced by the wet phase inversion method. The gas flux of the membranes prepared by the wet phase inversion method is much higher than the membranes prepared by the dry method. Different membrane structures can be obtained by using different nonsolvent-solvent systems. It is found that the position of binodal curve in a ternary phase diagram can not completely describe the formation mechanism of PMMA membranes. At least two other factors should be considered; the miscibility between solvent and nonsolvent, and the interfacial
105
J.-M. Cheng et al. / Journal of Membrane Science 109 (I 996) 93-107
surface
cross
bottom
0
0
Fig. 12. Effect of solvent content in the coagulation medium on membrane morphology. Casting solution: 14.7 vol% PMMA; solvent: EA, coagulation medium: 80 vol% n-hexane + 20 vol% ( A ) EA; (B) acetone. polymer concentration. A simplified solution-diffusion model was developed which consists of the effects of the position of binodal curves, solvent/nonsolvent diffusivity ratio and the slope of tie lines. in addition, we have also found that the gas selectivity of P M M A membrane can be modified by adding solvent into the coagulation bath, and that the gas flux can be improved by adding nonsolvent into the casting solution.
Acknowledgements The authors sincerely thank the National Science Council of Taiwan, ROC (NSC82-0405-E033-063) for the financial support of this project.
Appendix A. Simplified solution-diffusion model When polymer membrane is prepared by a wet phase inversion process, the ultimate structure strongly depends on the polymer concentration at the interface between the coagulation bath and the casting solution. Here we present a simple way to estimate it. We just consider the system in which the coagulation bath contains only pure nonsolvent, and the casting solution just contains solvent and polymer. J~ and J,,~ ( see Fig. l represent the solvent outflux and the nonsolvent influx in the coagulation bath, and J', and J'n~ are the fluxes in the casting solution. In the very beginning of immersion, the instantaneous local nonsolvent fluxes at the interface can be represented as following [ 22 J.
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J.-M. Cheng et al. / Journal qf Membrane Science 109 (1996) 93-107
J'ns= ~
X (4ins-- 0)
where Dns/b represents the diffusion coefficient of nonsolvent in the coagulation bath and D,s/~ in the casting solution, thr~, thr and ~ns, ~ stand for the volume fractions of nonsolvent and solvent at the interface (casting solution side r and coagulation bath side p), and they are in thermodynamic equilibrium. The superscripts refer to the polymer rich phase (r) and the poor phase ( p ) , respectively. The two fluxes have to be the same, therefore,
x (1
= V
x (,:/,r,s- o)
Since there is no polymer in the coagulation bath,
,/,~s+ 4,s~= 1
by the polymer-rich binodal curve of the ternary phase diagram. It is noticed that the volume fraction of nonsolvent at the polymer-rich binodal (~b~,s) is roughly a constant for most systems [18,23]. For the P M M A systems we studied ( H z O - N M P , H 2 0 - a c e t o n e and nhexane-butyl acetate), the assumption that ~b~ is a constant is quite reasonable (see Fig. 5). It should be noted that the application of this model is limited. Two major assumptions are required to justify the application of this model: first, it is assumed that the binary-diffusion model can be used to describe the ternary diffusion phenomena; second, the effect of the movement of interface is negligible. These two assumptions can be relieved if complicated models are adopted [ 10,13,14 ]. These models are more sophisticated than the present model; however, they are difficult to use. In the present work, we only need to qualitatively predict the interfacial polymer concentration. Therefore, using the simplified model is reasonable. However, if a more accurate value is needed, complicated models are required.
It follows that
4'7-
4,r,.,
References
~/Dns/blDns/c
It is assumed that the partition coefficient (k) of nonsolvent between polymer rich and poor phases is constant.
Then, we obtain
4,r -
k4,~s ~/Dn,
The volume fraction of polymer (~bp) at the interface can be represented by r
r
Hence, the following equation is derived which can be used to estimate the interfacial polymer concentration. k ~/Dnslbl D,slc r
r
thp and ~bns are the volume fractions of polymer and nonsolvent at the interface. Generally speaking, they are not independent and their relationship is described
[ 1] I. Pinnau and J. Koros, Structuresand gas separationproperties of asymmetric polysulfone membranes made by dry, wet and dry/wet phase inversion, J. Appl. Polym. Sci., 43 (1991) 1491-1502. [2] T.S. Chung, E.R. Kafchinski and P. Foley, Development of asymmetric hollow fibers from polyimides for air separation, J. Membrane Sci., 75 (1992) 181-195. [3] M.E. Rezac, J.D.L. Roux, H. Chen, D.R. Paul and W.J. Koros, Effect of mild solvent post-treatments on the gas transport properties of glassy polymer membranes, J. Membrane Sci., 90 (1994) 213-229. [4] J.Y. Lai, M.J. Liu and K.R. Lee, Polycarbonate membrane prepared via a wet phase inversion method for oxygen enrichment from air, J. Membrane Sci. 86 (1994) 103-118. [5] R. Matz, The structure of cellulose acetate membranes I. The development of porous structures in anisotropic membranes, Desalination, 10 (1972) 1-15. [6] H. Strathmann, K. Kock, P. Amar and R.W. Baker, The formation mechanism of asymmetric membranes, Desalination, 16 (1975) 179-203. [7] D.M. Koenhen, M.H.V. Mulder and C.A. Smolders, Phase separation phenomena during the formation of asymmetric membranes, J. Appl. Polym. Sci. 21 (1977) 199-215. [8] B. Reuvers, Membrane Formation-Diffusion Induced Demixing Processes in Ternary Polymeric System, Ph.D. Thesis, University of Twente, 1987, Chap. 7.
J.-M. Cheng et al. / Journal of Membrane Science 109 (1996) 93 107 [9] A. Bottino, G. Camera-Roda, G. Capannelli and S. Munari, The formation of microporous polyvinylidene difluoride membranes by phase separation, J. Membrane Sci. 57 ( 1991 ) 1-20.
[ 10] C. Cohen, Diffusion-controlled formation of porous structures in ternary polymer systems, J. Polym. Sci., Polym. Phys. Ed., 17 (1979) 477-489. [ I 1] Y.S. Kang, H.J. Kim and U.Y. Kim, Asymmetric membrane fnrmation via immersion precipitation method. I. Kinetic effect, J. membrane Sci, 60 ( 1991 ) 219-232. J 12] T.H. Young and L.W. Chen, A diffusion-controlled model for wet-casting membrane fnrmation, J. membrane Sci., 59 ( 1991 ) 169-181. [13[ C.S. Tsay and A.J. McHugh, The combined effects of evaporation and quench steps on asymmetric membrane formation by phase inversion, J. Polym. Sci., Polym. Phys. Ed., 29 ( 1991 ) 1261-1270. [14] A.J. Reuvers, J.W.A. van den Berg and C.A. Smolders, F~wmation of membranes by means of immersion precipitation part 1. a model to describe mass transfer during immersion precipitation, J. Membrane Sci., 34 (1987) 45-65. [ 15] K.E. Min and D.R. Paul, Effect of tacticity on permeation properties of poly(methyl methacrylate), J. Polym. Sci., Polym. Phys. Ed., 26 ( 19881 1021-1033.
1(/7
[161 D.W.V. Krevelen, Properties of Polymers, Elsevier. Amsterdam, 1990. [ 17] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic, Dordrecht, 1991. [18] F. Altena and C.A. Smolders, Calculation of liquid-liquid phase separation in a ternary system of a polymer in a mixture of a solvent and a nonsolvent, Macromolecules, 15 ( 6 ) ( 1982 1491-1497. [191 L. Zeman and G. Tkacik, Thermodynamic analysis of a membrane-fiwming system water/N-methyl-2-pyrrolidnne/ polyethersullone, J. Membrane Sci., 36 11988) 119-140. [20] R.C. Reid, J.M. Prausnitz and B.E. Poling, The Properties of Gases and Liquids, McGraw-Hill, Singapore, 1988. [ 21 I J.G. Wijmans, J.P.B. Baaij and C.A. Smolders, 3"he mechanism of formation of microporous or skinned membranes produced by immersiml precipitation, J. Membrane Sci., 14 ( 1983 ) 263274. [221 E.L. Cussler, Diffusion, Mass Transl2-r in Fluid Syslems, Cambridge University Press, 1984. J 23 J W.Y. Lau, M.D. Guiver and T. Matsuura, Phase separation in polysulfone/solvent/water and polyethersultkme/solvent/ water systems, J. Membrane Sci.. 59 / lt)91 t 2t9-227.
Journal of Membrane Science 109 ( t 996 ) 93-107
Formation and gas flux of asymmetric PMMA membranes Jao-Ming Cheng, Da-Ming Wang, Fung-Ching Lin, Juin-Yih Lai * Department of Chemical Engineering, Chung Yuan UniversiO,. Chung Li, Taiwan 32023 Received 11 October 1994; revised 3 May 1995; accepted 19 July 1995
Abstract In the present work, PMMA membranes were prepared by wet phase immersion methods to improve their gas fluxes. It is found that different membrane structure can be obtained by using different nonsolvent-solvent pairs. To completely describe the membrane formation process, the nonsolvent-solvent miscibility and the interfacial polymer concentration in casting solution should be considered accompanied by the ternary phase diagram. A simplified solution~liffusion model was developed to estimate the interfacial polymer concentration. In addition, the effects of adding solvent into the coagulation bath and adding nonsolvent into the casting solution are discussed. Kevwords." Asymmetric membrane formation; PMMA; Wet phase inversion; Morphology; Gas flux
Coagulation bath
1. Introduction
When membranes are used to separate gas mixtures, an asymmetric membrane is preferred, with a defectfree ultra thin skin layer to retain intrinsic selectivity and a porous sublayer to reduce the resistance of gas permeation. In addition, it possesses proper mechanical strength. Wet phase inversion method is the most widely used technique for preparing asymmetric membranes. In this process, a homogeneous casting solution (polymer solution) is immersed into a coagulation bath. The solvent diffuses out of the casting film and the coagulant diffuses into it (Fig. 1 ), which results in phase transition and polymer precipitation to form a membrane. Based on this method, many researchers [ 1-4] developed modification processes to prepare high performance membranes for gas separation. * Corresponding author. 0376-7388/96/$15.00 © 1996 Elsevier Science B.V. All fights reserved S S D I 0 3 7 6 - 7 3 8 8 ( 9 5 ) 001 87-5
t Js
Jn~ interface
Js
J ,~'~ Casting Solution
IIIIIII
Support
Fig. l. Fluxes of nonsolvent (J.s, f.,) and solvent (J~, fs) at the interface of the coagulation medium and the cast polymer solution(casting film),
Understanding membrane formation mechanism is essential for developing optimal asymmetric structures. Generally speaking, the asymmetric structure is strongly related to the composition history of the casting film during the immersion process. This behavior can be studied in terms the composition path in a thermodynamic ternary phase diagram of a nonsolvent-
94
J.-M. Cheng et al. / Journal of Membrane Science 109 (1996) 93-107
polymer
solvent
nonsolvent
Fig. 2. Ternary phase diagram of polymer/solvent/nonsolvent. A, B: coagulation path.
solvent-polymer system as shown in Fig. 2. When the composition path directly enters the solidification region, the polymer solidifies (gel type). When the path intersects the liquid-liquid phase separation region, the casting solution separates into polymer-rich and polymer-poor phases and results in porous structure. In addition, it has been shown that the physical properties such as surface tension, viscosity, etc., and the solvent outflow and coagulant inflow rates can also affect the structure of asymmetric membranes. Matz [5] stated that the formation of finger-like pores was due to the hydrodynamic instabilities coupled with surface-tension and viscosity change. Strathmann et al. [6] postulated that low precipitation rates produced membranes of sponge-like structure, whereas high precipitation rates caused large finger-like pores. Koenhen et al. [ 7 ] proposed a mechanism for the formation of sponge structure which is related to liquidliquid phase separation, nucleation and the growth of dispersed phase. Reuvers [ 8 ] pointed out that the interfacial polymer concentration of the casting solution which would form finger-type membranes is generally higher than the one would form sponge-type. Bottino et al. [ 9 ] attempted to find the relationship between the physical properties of casting solution and the membrane characteristics. There were also several investigations [ 10-14] stressing the importance of diffusion kinetics. Poly (methyl methacrylate) (PMMA) has good oxygen-to-nitrogen selectivity but low gas permeability [ 15]. The main purpose of the present work is to show
that the gas flux of a PMMA membrane can be improved by wet phase inversion method. To clearly understand this phenomenon, the effects of solvent and coagulant on membrane morphology and gas flux were studied and will be reported below.
2. Experimental 2.1. Materials
Poly(methyl methacrylate)(PMMA) used in this study was supplied by Aldrich Chemical Co., MW 140 000. All solvents and nonsolvents were of reagent grade and were used without further purification. Compressed dry air was used for gas permeability test. 2.2. Membrane preparation
At room temperature, PMMA polymer was dissolved in proper solvents to form casting solutions. The concentration was represented in vol/vol% (cm 3, polymer/cm 3, solvent). The solution was cast on a glass plate to a predetermined thickness of 300/zm with a Gardner knife, and then immersed into a coagulation bath in which the volume ratio of the coagulation medium to the casting solution was kept about 60. After precipitation, the membranes were peeled off and dried in vacuum for 24 h.
J.-M. Cheng et al. / Journal of Membrane Science 109 (1096) 93-107 2.3. Phase diagram P M M A solutions with different composition were placed in tubes at 25°C and nonsolvents were then slowly added to the solutions until the turbidity values suddenly change (detected by a Hach turbidimeter). This composition represents the transition concentration between the one-phase and the two-phase regions. With this data, a ternary phase diagram can be constructed.
95
~,-Ns = [( 6d.p - 6d,NS)2 + ( 6p.p-- 6p.NS)2 + (&,,,-- &.NS):] '/~
/1)
~P-S = [ ( (~d,P -- ~'~d,S ) 2 _1_ ( •p,p __ (~V,S ) 2
+ ¢ & , p _ &,~)2] ,/2
12)
where 6d stands for the dispersion interaction, 6p represents the polar bonding and 6h denotes the hydrogen bonding component [ 16]. The subscripts refer to the polymer ( P ) , solvent ( S ) and coagulant ( NS ), respectively.
2.4. Light transmission experiments The light transmission apparatus used in this study was similar to Reuvers et al.'s [ 14]. The decrease of light transmittance was caused by the liquid-liquid demixing of casting solution.
2.5. Gas permeation measurement The oxygen and nitrogen pressure-normalized fluxes were determined by Yanaco GTR-10 gas permeation analyzer. The structure of membranes was examined by a Hitachi model $570 scanning electron microscope. For scanning electron microscope studies, membrane samples were fractured in liquid nitrogen and coated with gold to ~ 150 A.
2.6. Precipitation behavior studies A drop of casting solution was placed between two microscope slides and a drop of coagulant was introduced by a syringe. The casting solution and the coagulant would contact because of capillary action. The interface was observed under Olympus BHT-M- 113D optical microscopic. In order to clearly observe the flow pattern in the casting solution, water was dyed with Rhodamine B.
2. Z Difference between solubility parameters The solubility difference between P M M A and nonsolvent is represented by &e-NS and the difference between P M M A and solvent is 6e_s.
Table 1 Structure of asymmetric PMMA membranes prepared by the wet phase inversion methoda Solvent
Coagulation medium Membrane structure
Ethyl acetate
n-hexane
Acetone
n-hexane
Methyl ethyl ketone n-hexane Cyclohexanone
n-hexane
Allyl alcohol~'
n-hexane
Dioxaneb
n-hexane
Tetrahydrofuran
n-hexane
Butyl acetate
n-hexane
NMP
H20
DMAc
H20
DMSO (50°C)
H20
Acetone
H20
Allyl alcohol Dioxane
CHsOH --+H20 C H 3 O H / H 2=0 1
skin exists, spongy sublayer skin exists, spongy sublayer skin exists, spongy sublayer skin exists, spongy sublayer skin exists, spongy subtayer skin exists, spongy sublayer skin exists, spongy sublayer no skin, bamboolike sublayer no skin, finger type sublayer no skin, finger type sublayer no skin, macrovoid sublayer symmetric porous structure no skin, finger type no skin, foamlike structure
~Casting solution: PMMA 14.7 vol%. bNonsolvent is added in the casting solution.
96
J.-M. Cheng et al. / Journal of Membrane Science 109 (1996) 93 107
Table 2 The gas separation performance of PMMA membranes which were
3. Results and discussion
3.1. Formation of asymmetric membranes and their gas flux Different asymmetric PMMA membranes can be prepared by wet phase inversion method with different coagulant-solvent pairs. The results are listed in Table 1, in which the PMMA concentration of all casting solutions is 14.7 vol%. This value was chosen because it is the lowest concentration that an integral membrane can be formed by using water and acetone as the coagulant and solvent, a system requires higher polymer concentration to produce integral membrane compared to others. Most of the membranes prepared by using n-hexane as coagulant exhibit an asymmetric structure which consists of a dense skin and a porous
prepared in different coagulant-solvent systems" Coagulant-solvent
02 pressurenormalized flux (GPU)
Selectivity O2/N 2
CH2C12 (dry method) N-Hexane-EA n-Hexane-acetone H20-acetone n-Hexane-MEK n-Hexane-cyclohexane n-Hexane-THF
0.0057 0.0778 0.0768 0.0146 0.0814 0.0657 0.0735
5.7 4.8 5. l 4.6 5.2 4.8 4.8
"Casting solution: 14.7 vol% PMMA/solvent.
7
®
®
Fig. 3. SEM micrographs of PMMA membranes for different coagulant-solvent pairs. (A) n-Hexane-acetone, (B) H20-NMP; (C) (CH3OH ~ H20)-aUyl alcohol; (D) n-hexane-butyl acetate; (E) H20-acetone.
.I.-M. Cheng et al./Journal c (Membrane Science 109 (19u6) 93-107
that P M M A cannot be completely precipitated by u sing methanol as coagulant. In the following, only the single-coagulant system will be discussed. The gas separation performance of the asymmetric P M M A membranes are shown in Table 2, in which only the membranes with good O2/Ne selectivity are listed. Comparing to the membrane prepared by dry method, most of the asymmetric membranes posses much larger gas flux, about 15 times. However, the gas flux of the membrane formed from H,O-acetone system is low - only about twice of the dry method. The reason why H~O-acetone system possesses lower gas flux will be discussed later.
I
I
t00 ~:q====,,~__~
00 0
\
1~
97
20
gO
Time(see) Fig. 4. Light transmission curves: (a) n-hexane-butyl acetate: (b) n-hexane-acetone: (c) H20-acctone: (d) H_,O-NMP: (e) n-hey ane-elhyl acetate. spongy sublayer, as shown in Fig. 3 ( A ) . n - H e x a n e butyl acetate system is an exception. On the other hand, when using water as the coagulant, most of the systems form macrovoid (finger-like) structure as shown in Fig. 3 ( B ) , except for the water-acetone system. Fig. 3( C ) depicts the cross-sectional structure of membranes prepared by using allyl alcohol as solvent, methanol as the I st coagulant and water as the 2nd coagulant. The above system requires two coagulant bath because most alcohol can either swell or dissolve P M M A so
3.2. E1Sect o f coagulant-solvent pairs (m memt~ram' morphology
It has been believed by many that the rate of liquid liquid demixing has a dramatic effect on membrane morphology [6,8,17]. Therefore, a light transmission experiment was conducted to study thc onset of L--L demixing. The results are shown in Fig. 4. Obvious change of transmittance at the very moment of immersion (at time < I s), indicating instantaneous denfixing, was observed for the systems of n-hexane butyl acetate (curve a) and H 2 0 - N M P ( curve b ). The other three systems (n-hexane-acetone, H~O-acetone and nhexane-ethyl acetate) belong to the category of delayed demixing. Our results show that instantaneous demixing forms finger-type membrane and dehwed
PMMA
Fig. 5. Ternary phase diagram of PMMA solution. Nonsolvent solvent: ( + ) H20 NMP: ( O ) H_~Oacetone: t ,k ) n-hexane-butyl acetate:/'_'~) n-hexane-ethyl acetate; ( [] ) n-hexane-acetone.
98
J.-M. Cheng et al./ Journal of Membrane Science 109 (1996) 93-107
Fig. 6. Photo-micrographsof the casting solution/ coagulant interface ( × 200): (A) H20-NMP; (B) HzO-acetone. demixing produces spongy type. The same conclusion was drawn in the work of Reuvers [ 8]. It has been widely accepted that when the binodal curve of a ternary system is closer to the polymersolvent axis, instantaneous demixing is easier to occur [ 18 ], therefore results in a finger-type structure. Hence, water-NMP, n-hexane-butyl acetate are easier to form finger-type membranes (see Fig. 5). However, the experimental results (Fig. 3 and Fig. 4) indicate that water-acetone system forms spongy membranes. This suggests that the thermodynamic phase diagram is not
enough to completely describe the membrane formation mechanism, other factors need to be considered. Reuvers [8] suggested that the miscibility between solvent and coagulant is an important factor in determining if finger-type structure can occur. Generally speaking, if the miscibility is high, the coagulant can easily penetrate into the casting solution and produce macrovoid structure. The miscibility between the solvent and the coagulant can be represented by the interaction parameter between them (g~2). The higher the interaction parameter, the lower the miscibility. The
J.-M. Cheng et al. /Journal (~/'Membrane Science 109 ( I ~96) 93-107 Table 3 Solubility parameter difference between P M M A and coagulant ( solvent ) Nonsolveni
6p NS (J'lelcm31e)
He() n-Hexanc Solvent
36. I 10.6 6e s (J~/-'/cm3/2)
Acelone NMP DMAc Dioxane THF MEK EA Butyl acetate
2.2 4.7 4.9 6.7 3.0 1.8 3.8 4.5
g12 value of H20-acetone is between 2.2 (water concentration is small) and 1.3 (acetone concentration is small) [ 18], and the value of HzO-NMP is between 1.3 (water concentration is small) and 0.65 (NMP concentration is small) [19]. It is obvious that the H20-NMP system has a higher coagulant-solvent miscibility than the H20-acetone system. Therefore, H~O is easier to penetrate into the NMP-PMMA casting solution and produces the finger structure. Whereas, water cannot pass through the acetone-PMMA solution easily. This deduction was verified by observing the inter-diffusion behavior between nonsolvent and casting solution under the microscope (see Fig. 6). It shows that water can break the interface when NMP is the solvent: but water cannot flow into the casting solution when acetone is the solvent. Reuvers [8] also pointed out that a very steep polymer concentration gradient existing at the solutioncoagulant interface would bring about instability of the nascent toplayer and produced the finger structure. The polymer concentration gradient can be estimated roughly by the difference of polymer concentration between the original casting solution and the solutioncoagulant interface. To estimate the polymer concentration at the interface, mathematical models have been developed by Cohen [ 10], Reuvers et al. [ 14], Tsay and McHugh [13]. Their models are quite sophisticated, but difficult to use because of the complicated numerical procedures for solving the simultaneous partial differential equations. In the present work, we developed a simplified solution~liffusion model to
99
estimate the interfacial polymer concentration (see Appendix A). The interfacial polymer concentration can be described by
[ vID,,~/b/D.~/c
where qS~p.4 ~ represent the interfacial polymer and nonsolvent concentration of the casting solution at the initial moment of immersion, k is the partition coefficient of solvent between the polymer rich and poor phases. D,,~/h and D,~/~ stand for the mutual diffusion coefficient of nonsolvent in the coagulation bath and in the casting solution; respectively. &',~and k could also represent the position of the binodal curve in a ternary phase diagram and the slope of tie lines. The binodal curves of H20-NMP, H20-acetone and n-hexanebutyl acetate systems are almost overlapped (see Fig. 5). indicating that the values of d,',,~are the same. Dn~/b is in fact the mutual diffusivity of solvent in nonsolvent (coagulant) at a very dilute concentration bath because the bath is a binary ( solvent and nonsolvent) system. D,~/c can be estimated by D~/J'(&p). D~,~/~ represents the diffusivity of nonsolvent in the casting solution as polymer concentration (~hp) approaches zero and./'(d~p) stands for the effect of pol-
"-a O
5~ &
g--.
•
•
0.10
0.08
N 0.06
0.04
0.02
0.00
~
13.0
i
i
18.0
i
i
i
r
i
i
i
i
1
~
i
i
p
19.0 22.0 2 5 . 0 2 8 . 0
PMMA v o l u m e
fraction
(~)
Fig. 7. Effect of PMMA concentration on gas separation performance. coagulant-solvent: ( O ) n-hexane-acetone: ( i . ) HeO-acetone ~'GPU = 1 × IO ~ ' c m 3 ( S T P ) / c m -" s cmHg.
100
J.-M. Cheng et al. /Journal of Membrane Science 109 (1996) 93-107
(i i!i~i •
-r" 7
'-<,,
p g
ymer. It is assumed that D°~/~ can be substituted by the diffusivity of nonsolvent in solvent at a very dilute concentration. In this study, the polymer concentration was fixed at 14.7 vol% for all systems; therefore, it is reasonable to assume that the effect of polymer are the same for all systems. The binary diffusion coefficients can be estimated by using the average from Tyn-Calus and Hayduk-Minhas methods [20]. The values of ~/Dn~/b/D°/c of H20-NMP, H20-acetone and n-hexane-butyl acetate systems can then be calculated; they are 0.7, 0.44 and 1.44, respectively. The value of H 2 0 acetone system is the smallest. The slope of tie lines (k) can be determined by calculating the binodal curves of PMMA solutions based on the free volume theory [ 18]. Our calculation (will be reported lately) shows that the value of k for the water-acetone system is the largest among the three systems mentioned above. Hence, the interfacial polymer concentration of water-
acetone system is lower than the other two systems. Since the H20-acetone system appears to have the less steep polymer concentration gradient, it prefers to produce sponge-type structure. Although both water-acetone and n-hexane-acetone systems form spongy membrane, the formation mechanisms are different. The membrane formation mechanism of using n-hexane as the coagulant can be described as follows: First, after the casting solution is immersed into a n-hexane bath, the uppermost region can form an integral skin, because of the delayed demixing. The region below the top skin layer remains in fluid state, because the polymer concentration is not high enough to make solidification occur. Then, liquidliquid phase separation takes place because of the diffusion of n-hexane (nonsolvent) from the coagulant bath into the casting solution [7]. Finally, a porous spongy sublayer is developed. The morphology is
J.-M. Cheng et al. / Journal of Membrane Science 109 (1996) 93 107
,
,.~
101
!>
Fig. 8. SEM micrographsof PMMA membranes for systems with differentpolymerconcentrations• (A) n-Hexane-acetone; ( B ) H,O-acetone:
(CI mhexane-butyl acetate; (D) H20-NMP: (D) (CH3OH-~H20)-allyt alcohol. Upper: 21 vol%. lower: 29.4 volq/~. shown in Fig. 3 ( A ) . However, because of the incompatibility between PMMA and water (large 6p,Ns, see Table 3 ), water cannot easily diffuse through the dense skin into the casting solution. Therefore, the completion of membrane formation mainly result from the leaving of acetone. Since the formation mechanism of the sublayer of water-acetone system is mainly due to the leaving of solvent, similar to the mechanism of dry method, the sublayer structure is less porous (see Fig. 3 ( E ) ) and therefore results in low gas flux (Table 2). 3.3. Effect o f polymer concentration on membrane morphology and gas separation performance
As shown in Fig. 7, the 02 flux decreases and 0 2 / N 2 selectivity remains unchanged with increasing
PMMA concentration for both systems of water-acetone and n-hexane-acetone. When the polymer concentration of the casting solution increases, the thickness of toplayer increases and the porosity decreases for n-hexane, as shown in Fig. 8(A). When using acetone as the solvent, the porosity remains unchanged, only the membrane becomes thicker, as shown in Fig. 8(B). As mentioned above, the membrane formation mechanisms for these two systems are different. When n-hexane is used as coagulant, an increase in polymer concentration decreases the amount of casting solution undergoing liquid-liquid phase separation and results in a denser structure. For water, the membrane formation is mainly due to the leaving of acetone from the casting solution, so the increase of polymer concentration does not influence the structure and merely makes total resistance increase
102
J.-M. Cheng et al. / Journal of Membrane Science 109 (1996) 93-107 O
. . . . . .
.
Fig. 9. Effect of adding nonsolvent (formamide) into the casting solution on membrane morphology. Casting solution: 14.7 vol% PMMA + formamide, formamide/acetone: (A) 5 vol%; (B) 6.25 vol%; (C) 7.5 vol%; (D) 15 vol%, solvent: acetone; coagulant: water.
(similar to the dry method). Their O 2 / N 2 selectivity remains the same because nondefective skins have been formed in the beginning of phase inversion. Fig. 8(C), (D) and (E) show the effect of PMMA concentration on membrane morphology for n-hexanebutyl acetate, H20-NMP and (CH3OH ~ HzO)-allyl alcohol, respectively. It clearly indicates that the cavities decreases with an increase of polymer concentration. For the n-hexane-butyl acetate system (Fig. 8 (C) ), an increase of polymer concentration can lead to the disappearance of finger structure (21 vol%) or even make the membrane become homogeneously dense (29.4 vol%). This happens because the polymer concentration is high enough to make the polymer precipitate before the phase separation begins.
3.4. Effect of adding nonsolvent into the casting solution on morphology and gas separation performance It has been noticed [ 1,4] that the adding of nonsolvent into the casting solution leads to a more porous structure or even a finger-type membrane. This can be explained by the fact that the adding of nonsolvent reduces the compatibility between polymer and solvent and makes L - L demixing easy to occur. However, for water-acetone system, adding water into the casting solution results in poor membrane formation, similar to the effect of lowering the polymer concentration in the casting solution. This indicates that a small amount of H20 in acetone does not reduce the solvent power of acetone. To improve the porosity of membrane by
J.-M. Cheng et al. / Journal o f Membrane Science 109 (1996) 9 3 - 1 0 7
surface
cross
103
bottom
O
o
117¸
® !
Fig. 10. Effect of adding various alkanes (nonsolvent) into the casting solution on membrane morphology, casting solution: PMMA : nonsolvent:acetone= 14.7:30:100 (vol/vol%), nonsolvent: (A) n-hexane; (B) n-heptane; (C) n-octane, coagulant: n-hexane.
adding nonsolvent, formamide is added into the casting solution(the nonsolvent in coagulation bath is still water). The results are shown in Fig. 9. When the added formamide is less than 6.25 vol%, both porosity and gas flux slightly increase with the increase of formamide. However, when the added formamide is more than
6.25 vol%, the large cavities exist and the membrane possesses no O2/N2 selectivity. When using n-hexane as the coagulant, the effect of adding alkanes (nonsolvents) in the casting solution on membrane morphology was studied. Fig. 10 shows the porosity of membranes increases in the order of noctane > n-heptane > n-hexane. The O2 pressure-nor-
104
J.-M. Cheng et al. /Journal of Membrane Science 109 (1996) 93-107
Table 4 Effect of adding alkanes (nonsolvent) into the casting solution on gas separation performance"
7
-5~ -4
Nonsolvent 02 flux GPU Selectivity Oz/N2 Coagulant toleranceb n-Hexane 0.16 n-Heptane 0.25 n-Octane 0.40
4.6 3.3 2.0
-3 "~
41.5 38.5 36.5
~Casting solution: PMMA/nonsolvent/acetone= 14.7/30/100 (volume ratio) coagulant: n-hexane. bCoagulant tolerance: coagulant/acetone ( v/v%) as casting solution undergoing liquid-liquid phase separation, obtained from turbidity measurements.
-2
o
-0
0.21 0.19
~'~ 0.17 0.15 0.13
malized fluxes increase in this order while 0 2 / N 2 selectivity in the opposite order, as shown in Table 4. This can be explained by the fact that the liquid-liquid phase separation is easier to occur when the coagulant tolerance is small [21 ].
3.5. Membrane modified by adding solvent in coagulation medium When acetone is the solvent of the casting solution, and coagulation bath contains 5 vol% of acetone (or EA) and 95 vol% of n-hexane, poor membrane formation was observed. However, by using EA as the solvent, the good membrane formation can be obtained when the coagulation bath contains proper amount of acetone or EA. The reason might be that the PMMA solubility of EA is smaller(rp_s = 3.8 JJ/2/cm3/2) than a c e t o n e ( r p s = 2 . 2 J1/2/cm 3/2) which makes PMMA easier to precipitate in the EA system. For the n-hexane-EA system, the effect of adding solvent(EA or acetone) in the coagulation bath is depicted in Fig. 11, indicating that the adding of EA in the coagulation bath results in higher gas flux and the slightly decrease of selectivity. When acetone is added, there exists a maximum 02/ N2 selectivity (6.2) at 5 vol%. When the acetone in the coagulation medium is not in a large amount (lower than 13 vol%), acetone can permeate into the casting solution and is absorbed in the membrane. For removing the retained acetone, the membrane have to transfer into another coagulant bath which contains pure nhexane. This step might bring out capillary pressure which makes the polymer particles packing closer and the selectivity of gas higher. On the other hand, as more
_
0.11 0
0.09 0.07
, i , t-i..,
0
5
, i , 1,
10
, , , i , ~ , iI
15
20
, i i ,
25
solvent in coagulation m e d i u m (volg) Fig. 11. Effect of adding solvent into the coagulation medium on gas separation performance. Coagulant: n-hexane, added solvent: (11) acetone; ( 0 ) EA.
acetone was added, the space between polymer particles is too large to make capillary pressure play a role, thereby leading to larger free volume of membrane which possesses low O 2 / N 2 selectivity. Similar results were reported by Rezac et al. [ 3 ]. Fig. 12 shows the morphologies of PMMA membranes which were made by adding 20 vol% of solvent in the coagulation medium. This indicates that adding more acetone would transfer membrane structure to another form.
4. Conclusion In the present work, we have shown that the PMMA asymmetric membranes can be successfully produced by the wet phase inversion method. The gas flux of the membranes prepared by the wet phase inversion method is much higher than the membranes prepared by the dry method. Different membrane structures can be obtained by using different nonsolvent-solvent systems. It is found that the position of binodal curve in a ternary phase diagram can not completely describe the formation mechanism of PMMA membranes. At least two other factors should be considered; the miscibility between solvent and nonsolvent, and the interfacial
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J.-M. Cheng et al. / Journal of Membrane Science 109 (I 996) 93-107
surface
cross
bottom
0
0
Fig. 12. Effect of solvent content in the coagulation medium on membrane morphology. Casting solution: 14.7 vol% PMMA; solvent: EA, coagulation medium: 80 vol% n-hexane + 20 vol% ( A ) EA; (B) acetone. polymer concentration. A simplified solution-diffusion model was developed which consists of the effects of the position of binodal curves, solvent/nonsolvent diffusivity ratio and the slope of tie lines. in addition, we have also found that the gas selectivity of P M M A membrane can be modified by adding solvent into the coagulation bath, and that the gas flux can be improved by adding nonsolvent into the casting solution.
Acknowledgements The authors sincerely thank the National Science Council of Taiwan, ROC (NSC82-0405-E033-063) for the financial support of this project.
Appendix A. Simplified solution-diffusion model When polymer membrane is prepared by a wet phase inversion process, the ultimate structure strongly depends on the polymer concentration at the interface between the coagulation bath and the casting solution. Here we present a simple way to estimate it. We just consider the system in which the coagulation bath contains only pure nonsolvent, and the casting solution just contains solvent and polymer. J~ and J,,~ ( see Fig. l represent the solvent outflux and the nonsolvent influx in the coagulation bath, and J', and J'n~ are the fluxes in the casting solution. In the very beginning of immersion, the instantaneous local nonsolvent fluxes at the interface can be represented as following [ 22 J.
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J'ns= ~
X (4ins-- 0)
where Dns/b represents the diffusion coefficient of nonsolvent in the coagulation bath and D,s/~ in the casting solution, thr~, thr and ~ns, ~ stand for the volume fractions of nonsolvent and solvent at the interface (casting solution side r and coagulation bath side p), and they are in thermodynamic equilibrium. The superscripts refer to the polymer rich phase (r) and the poor phase ( p ) , respectively. The two fluxes have to be the same, therefore,
x (1
= V
x (,:/,r,s- o)
Since there is no polymer in the coagulation bath,
,/,~s+ 4,s~= 1
by the polymer-rich binodal curve of the ternary phase diagram. It is noticed that the volume fraction of nonsolvent at the polymer-rich binodal (~b~,s) is roughly a constant for most systems [18,23]. For the P M M A systems we studied ( H z O - N M P , H 2 0 - a c e t o n e and nhexane-butyl acetate), the assumption that ~b~ is a constant is quite reasonable (see Fig. 5). It should be noted that the application of this model is limited. Two major assumptions are required to justify the application of this model: first, it is assumed that the binary-diffusion model can be used to describe the ternary diffusion phenomena; second, the effect of the movement of interface is negligible. These two assumptions can be relieved if complicated models are adopted [ 10,13,14 ]. These models are more sophisticated than the present model; however, they are difficult to use. In the present work, we only need to qualitatively predict the interfacial polymer concentration. Therefore, using the simplified model is reasonable. However, if a more accurate value is needed, complicated models are required.
It follows that
4'7-
4,r,.,
References
~/Dns/blDns/c
It is assumed that the partition coefficient (k) of nonsolvent between polymer rich and poor phases is constant.
Then, we obtain
4,r -
k4,~s ~/Dn,
The volume fraction of polymer (~bp) at the interface can be represented by r
r
Hence, the following equation is derived which can be used to estimate the interfacial polymer concentration. k ~/Dnslbl D,slc r
r
thp and ~bns are the volume fractions of polymer and nonsolvent at the interface. Generally speaking, they are not independent and their relationship is described
[ 1] I. Pinnau and J. Koros, Structuresand gas separationproperties of asymmetric polysulfone membranes made by dry, wet and dry/wet phase inversion, J. Appl. Polym. Sci., 43 (1991) 1491-1502. [2] T.S. Chung, E.R. Kafchinski and P. Foley, Development of asymmetric hollow fibers from polyimides for air separation, J. Membrane Sci., 75 (1992) 181-195. [3] M.E. Rezac, J.D.L. Roux, H. Chen, D.R. Paul and W.J. Koros, Effect of mild solvent post-treatments on the gas transport properties of glassy polymer membranes, J. Membrane Sci., 90 (1994) 213-229. [4] J.Y. Lai, M.J. Liu and K.R. Lee, Polycarbonate membrane prepared via a wet phase inversion method for oxygen enrichment from air, J. Membrane Sci. 86 (1994) 103-118. [5] R. Matz, The structure of cellulose acetate membranes I. The development of porous structures in anisotropic membranes, Desalination, 10 (1972) 1-15. [6] H. Strathmann, K. Kock, P. Amar and R.W. Baker, The formation mechanism of asymmetric membranes, Desalination, 16 (1975) 179-203. [7] D.M. Koenhen, M.H.V. Mulder and C.A. Smolders, Phase separation phenomena during the formation of asymmetric membranes, J. Appl. Polym. Sci. 21 (1977) 199-215. [8] B. Reuvers, Membrane Formation-Diffusion Induced Demixing Processes in Ternary Polymeric System, Ph.D. Thesis, University of Twente, 1987, Chap. 7.
J.-M. Cheng et al. / Journal of Membrane Science 109 (1996) 93 107 [9] A. Bottino, G. Camera-Roda, G. Capannelli and S. Munari, The formation of microporous polyvinylidene difluoride membranes by phase separation, J. Membrane Sci. 57 ( 1991 ) 1-20.
[ 10] C. Cohen, Diffusion-controlled formation of porous structures in ternary polymer systems, J. Polym. Sci., Polym. Phys. Ed., 17 (1979) 477-489. [ I 1] Y.S. Kang, H.J. Kim and U.Y. Kim, Asymmetric membrane fnrmation via immersion precipitation method. I. Kinetic effect, J. membrane Sci, 60 ( 1991 ) 219-232. J 12] T.H. Young and L.W. Chen, A diffusion-controlled model for wet-casting membrane fnrmation, J. membrane Sci., 59 ( 1991 ) 169-181. [13[ C.S. Tsay and A.J. McHugh, The combined effects of evaporation and quench steps on asymmetric membrane formation by phase inversion, J. Polym. Sci., Polym. Phys. Ed., 29 ( 1991 ) 1261-1270. [14] A.J. Reuvers, J.W.A. van den Berg and C.A. Smolders, F~wmation of membranes by means of immersion precipitation part 1. a model to describe mass transfer during immersion precipitation, J. Membrane Sci., 34 (1987) 45-65. [ 15] K.E. Min and D.R. Paul, Effect of tacticity on permeation properties of poly(methyl methacrylate), J. Polym. Sci., Polym. Phys. Ed., 26 ( 19881 1021-1033.
1(/7
[161 D.W.V. Krevelen, Properties of Polymers, Elsevier. Amsterdam, 1990. [ 17] M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic, Dordrecht, 1991. [18] F. Altena and C.A. Smolders, Calculation of liquid-liquid phase separation in a ternary system of a polymer in a mixture of a solvent and a nonsolvent, Macromolecules, 15 ( 6 ) ( 1982 1491-1497. [191 L. Zeman and G. Tkacik, Thermodynamic analysis of a membrane-fiwming system water/N-methyl-2-pyrrolidnne/ polyethersullone, J. Membrane Sci., 36 11988) 119-140. [20] R.C. Reid, J.M. Prausnitz and B.E. Poling, The Properties of Gases and Liquids, McGraw-Hill, Singapore, 1988. [ 21 I J.G. Wijmans, J.P.B. Baaij and C.A. Smolders, 3"he mechanism of formation of microporous or skinned membranes produced by immersiml precipitation, J. Membrane Sci., 14 ( 1983 ) 263274. [221 E.L. Cussler, Diffusion, Mass Transl2-r in Fluid Syslems, Cambridge University Press, 1984. J 23 J W.Y. Lau, M.D. Guiver and T. Matsuura, Phase separation in polysulfone/solvent/water and polyethersultkme/solvent/ water systems, J. Membrane Sci.. 59 / lt)91 t 2t9-227.