Ultrastructure of Asymmetric and Composite ... - ACS Publications

19 Ultrastructureof Asymmetric and Composite Membranes 1

ISRAEL CABASSO

Gulf South Research Institute, P.O. Box 26518, New Orleans, LA 70186

Downloaded by UNIV LAVAL on June 22, 2014 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch019

The development of the Loeb-Sourirajan asymmetric cellulose acetate membrane (1) has been followed by numerous attempts to obtain a similar membrane configuration from virtually any available polymer. The presumably simplistic structure of this cellulose acetate membrane - a dense, ultrathin skin resting on a porous structure - has been investigated by transmission and scanning electron microscopy since the 1960s (2,3). The discovery of macrovoids (4), a nodular intermediate layer, and a bottom skin have contributed to the question of the mechanism by which a polymer solution is coagulated to yield an asymmetric membrane. A few empirical and theoretical studies to postulate a general set of rules for the fabrication of asymmetric membranes by phase inversion mechanism (in which the polymer solution is coagulated within a nonsolvent bath) have been attempted. Thus, for example, from the literature which described the formation of asymmetric membranes, Klein and Smith (5) compiled working rules in the early 1970s regarding the requirements of a casting solution: 1.

The c a s t i n g composition s o l u b i l i t y parameters ( 6 ) should be near the s o l u b i l i t y boundary f a c i n g the quench medium. 2. A v o l a t i l e solvent component should be such that i t s l o s s w i l l move the composition out of the s o l u b i l i t y area, rather than into i t . 3. The s o l i d s content at the s o l u t i o n boundary must be high i n order to cause a r a p i d t r a n s i t i o n from s o l u t i o n to g e l . 4. A l l components of the system should be m i s c i b l e with the quench medium (nonsolvent). The above s e t of r u l e s - though a c c u r a t e l y d e s c r i p t i v e of e a r l i e r c a s t i n g procedures - has l e d to s e r i o u s misconceptions p e r t a i n i n g to the formation o f a n i s o t r o p i c membranes, and t h e r e f o r e , misconcept i o n s i n the formulation of new polymeric c a s t i n g s o l u t i o n s . I t i s evident that the polymer s o l u t i o n concentration p r o g r e s s i v e l y increases at the surface l a y e r during the evaporation p e r i o d , and 1

t

Current address: Chemistry Department, State University of New York, Syracuse, NY 13210 0097-6156/81/0153-0267$06.25/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


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formation of the p e l l i c l e i s v i s i b l e to the naked eye. Yet i t was proven that f o r many dope mixtures ( i n c l u d i n g that of c e l l u l o s e a c e t a t e ) , the evaporation p e r i o d i s not necessary and the a n i s o t r o p i c s t r u c t u r e i s formed i n s i t u upon quenching (6,7). (This i s a l s o shown i n the wet-spinning of hollow f i b e r membranes, which are spun d i r e c t l y i n t o the coagulation bath.) Some c a s t i n g compositions i n which one of the solvent components (e.g., chloroform) i s immiscible with the coagulating medium (water) were r e c e n t l y reported to be very e f f e c t i v e (8). In order to b r i n g the c a s t i n g formulation c l o s e r to the s o l u b i l i t y boundary, numerous formulations c i t e d i n the l i t e r a t u r e employ a s o - c a l l e d " s w e l l i n g agent" (9), which simply designates a nonsolvent f o r the polymer, which d i s s o l v e s i n the polymer b e t t e r than i n the coagulant. The s w e l l i n g agent concept i s perhaps the most obscure and misleading item i n the short h i s t o r y of the a n i s o t r o p i c membrane. The f a c t i s that such membranes can be cast from a s i n g l e solvent [e.g., dimethylformamide f o r p o l y s u l f o n e and p o l y ( v i n y l i d e n e f l u o r i d e ) , and dioxane f o r d e r i v a t i v e s of polyphenylene oxide.] In analyzing the myriad formulations conceived f o r c e l l u l o s e acetate membranes, apart from the a d d i t i o n of a " s w e l l i n g agent" (by the general d e f i n i t i o n given above), i t i s hard to recognize a common f e a t u r e among them. On the other hand, c a r e f u l observation of the g e l a t i o n process c l e a r l y i n d i c a t e s that the e n t i r e morphological s t r u c t u r e of a polymeric membrane i s o f t e n a f f e c t e d by the d i f f e r e n c e i n s p e c i f i c g r a v i t y between the cast s o l u t i o n and the coagulation l i q u i d . This aspect has been overlooked i n the past when v a r i a b l e s f o r dope mixture formulation were considered i n the c a s t i n g process. I f t h i s element i s taken i n t o account when e v a l u a t i n g known c a s t i n g formul a t i o n s , one may conclude that the i n t r o d u c t i o n of components ( a d d i t i v e s , s w e l l i n g agents, or solvents) with r e l a t i v e l y high s p e c i f i c g r a v i t i e s (> 1) [e.g., dioxane (7), chloroform (8), phosphoric a c i d (10), and formamide (11)] seems to be very e f f e c t i v e i n the p r e p a r a t i o n of these membranes. T h i s leads us to the conclusion that the s p e c i f i c g r a v i t y of the s o l u t i o n mixture as w e l l as the chemical p r o p e r t i e s of the m a t e r i a l s i n which a polymer candidate i s d i s s o l v e d i s of utmost importance i n shaping membrane morphology. T h i s aspect of membrane f a b r i c a t i o n has been i n v e s t i gated i n the course of the present study and w i l l be discussed i n t h i s manuscript. The s t r u c t u r e of the s o - c a l l e d "composite" membranes used i n reverse osmosis i s a l s o much more complex than the conventional, s i m p l i s t i c d e s c r i p t i o n of the u l t r a t h i n semipermeable f i l m deposited on and supported by a porous s u b s t r a t e . Most of these membranes which e x h i b i t high f l u x and s e p a r a t i o n are composed of an a n i s o t r o p i c , porous substrate topped by an a n i s o t r o p i c , u l t r a t h i n perms e l e c t i v e dense l a y e r which i s e i t h e r h i g h l y c r o s s l i n k e d , or e x h i b i t s a p r o g r e s s i v e l y decreased h y d r o p h i l i c i t y toward the s u r f a c e . The b a s i c d i f f e r e n c e between the conventional a n i s o t r o p i c (asymmetric) membrane and the t h i n f i l m composite i s that the l a t t e r might be


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composed of two or more d i f f e r e n t polymers which e x i s t i n the membrane as separate l a y e r s . The s t r u c t u r a l d i f f e r e n c e s among the membranes described above were analyzed by e l e c t r o n microscopy techniques and are discussed i n the f o l l o w i n g s e c t i o n s .


Experimental The r e l a t i o n s h i p among the c a s t i n g s o l u t i o n compositions, the s p e c i f i c g r a v i t i e s o f the components, and the c o a g u l a t i o n mechanisms were s t u d i e d employing the experimental set-up shown i n F i g ure 1. A polymer s o l u t i o n was cast (thickness of 250-300 um) on a g l a s s s l i d e 8 x 4 cm i n s i z e and then submerged t o a depth o f 7 cm i n the coagulation bath (water), as shown i n Figure 2. The mode of solvent d i s s i p a t i o n from the cast l a y e r was detected by a s c h l i e r e n p a t t e r n . The s c h l i e r e n photographs were taken with a camera set up perpendicular to the long s i d e o f the s l i d e . A b l a c k - r u l e d background was used to detect convection flows i n the coagulation bath. In the experiments described h e r e i n , the polymer s o l u t i o n was cast on the s l i d e and then i n s t a n t a n i o u s l y submerged ( s l i d i n t o the bath a t a 45° angle) so as to e l i m i n a t e turbulance; t h i s part of the experiment ( c a s t i n g and submersion) took 1 to 4 seconds. The l i g h t source and f l a s h l i g h t were adjusted so that the photographs could be taken with a s h u t t e r speed of 1/60 second. Selected polymers and s o l v e n t s used i n t h i s experiment are l i s t e d i n Table 1. In general, a l l observations of coagulations conducted with the m a t e r i a l s s p e c i f i e d i n Table 1 i n d i c a t e a d e f i n i t e linkage between s p e c i f i c g r a v i t i e s of the components (the polymer s o l u t i o n , solvent mixture, and coagulation l i q u i d ) and the phase-inversion mechanism. Only a few t y p i c a l examples w i l l be discussed b r i e f l y here. The r e s u l t s shown i n Figures 2-7 are summarized i n Table I I . TABLE I . SOLVENTS AND POLYMERS Specific Molar S o l u b i l i t y Parameters Gravity Volume (p) (V) (6 ) (6 ) (6 )

Material

D

1,4-dioxane

P

H

3.6

85.7

9.3

0.9

0.89

81.7

8.2

2.8

3.9

0.79

74.0

7.6

5.1

3.4

1.00

18.0

7.6

7.8

20.7

dime thy lformamide (DMF)

0.94

77.0

8.5

6.7

5.5

formamide

1.13

39.8

8.4

12.8

9.3

c e l l u l o s e acetate ( v i s e . 3)

1.31

polysulfone

1.24

1.03

tetrahydrofuran (THF) acetone water

(3500)

M-polyphenylene oxide

^ 1.1


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Experimental set-up for schlieren detection in a coagulant bath

Figure 2. Detection of solvent (acetone) convection flows in a water coagulating bath; (a) surface of cellulose acetate (20 wt % in acetone); (b) acetone convection flow upward; (c) accumulation of acetone on the surface of the coagulation bath. Elapsed time = 45 s.


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TABLE I I . COAGULATION MODES OF CASTING SOLUTIONS (Figures 2-7) Figure Elapsed* Membrane Polymer (wt%) Solvent Time Structure Designation F i g u r e s 2,3

cellulose acetate (20)

acetone

< 4s

dense

Figure 4


tetrahydrofuran (THF)

< 4s

dense

Figure 5

p o l y s u l f o n e (15)

THF

< 4s

dense

Figure 6

polystyrene (15)

THF

< 4s

dense and fragmented

Figure 7

p o l y s u l f o n e (15)

dioxane

none

porous


dioxane

none

porous


acetone/ formamide (i/i)

15 min

porous

time i n t e r v a l between submersion and appearance of s c h l i e r e n pattern. A l l the polymer s o l u t i o n s that were prepared from very low s p e c i f i c g r a v i t y solvents e x h i b i t e d r a p i d d e p l e t i o n o f s o l v e n t , once submerged i n the aqueous bath. Figures 2-5 e x h i b i t a convection flow (upward) from the s u r face of the nascent membrane that l a s t s u n t i l more than 80 wt% of the solvent i s depleted from the cast l a y e r . The heavy accumul a t i o n of the solvents (which are h i g h l y water m i s c i b l e i n nature) on the top of the coagulating bath i s c l e a r l y shown i n Figures 2 and 4. As f o r F i g u r e 6, the whole c a s t i n g l a y e r fragmented i n t o s l i c e s of dense polystyrene (p - 1); the solvent d e p l e t i o n was instantaneous. A t y p i c a l case f o r a l l the c a s t i n g s o l u t i o n s with high s p e c i f i c g r a v i t y solvent mixtures i s shown i n F i g u r e 7. With t h i s type of response, s c h l i e r e n patterns could be detected only by b r i e f s t i r r i n g i n the v i c i n i t y of the nascent membrane s u r f a c e , and even then only a f t e r an elapsed time of more than 500 seconds. Experiments were c a r r i e d out with s o l u t i o n s of c e l l u l o s e acetate i n acetone and acetone/formamide (Table I I I ) (Figures 2 and 3 ) . The gradual i n c r e a s e i n the formamide f r a c t i o n to s o l u t i o n increased the elapsed time before s c h l i e r e n patterns appeared. For the s o l u t i o n composition formamide/acetone (40/60), slow convection flow appeared suddenly a f t e r 650 seconds, but the formation of the p e l l i c l e at the nascent membrane i n t e r f a c e could be c l e a r l y seen 15 seconds a f t e r submersion i n the water bath. Such p e l l i c l e s could not be discerned f o r s o l u t i o n s cast from THF o r pure acetone.


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Figure 3.

Acetone convection flows from the surface of cast cellulose acetate solution. Elapsed time = 10 s (see Table 11).

Figure 4.

THF convection flow pattern in a water coagulating bath from cellulose acetate (20 wt % in THF). Elapsed time = 45 s.



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Figure 5.

THF convection flow from the surface of polysulfone (15 wt % in THF). Elapsed time = 20 s.

Figure 6.

Coagulation of polystyrene (15 wt % in THF) in water. Elapsed time = 10 s.


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TABLE I I I . EFFECT OF FORMAMIDE ON COAGULATION Solvent D e p l e t i o n * Cellulose Solvent Formamide/ Acetate Acetone Specific beginning end Gravity (sec) (sec) (wt%) (ratio) (P )


s

302

20

0/100

.79

< 10

>

126

296

16.6

20/80

.86

40

^

190

A-314

15

30/70

.89

62

*

350

A-307

13.3

40/60

.92

650

A-300

20

33/47

.93

>

734

A-298

25

35/40

.95

>

730

A-294

25

35/40

.95

>

740

As evidenced by s c h l i e r e n p a t t e r n The scanning e l e c t r o n microscopy micrographs shown i n the body of t h i s manuscript were taken by AMR-1000 and J e o l C-35 instruments. A l l specimens were gold-palladium coated. To o b t a i n the c r o s s - s e c t i o n morphologies, the membranes were fragmented i n l i q u i d nitrogen. Discussion Dope Composition and Coagulation Process. The formation of a porous membrane by the c o a g u l a t i o n of a polymer s o l u t i o n i n a nonsolvent bath has been d i s c u s s e d by v a r i o u s i n v e s t i g a t o r s and a c r i t e r i a f o r the d i f f u s i o n - c o n t r o l l e d formation of the porous s t r u c t u r e was attempted r e c e n t l y by Cohen, ejt a l . (12). The model employed by these i n v e s t i g a t o r s assumes that the c o a g u l a t i o n bath i s w e l l s t i r r e d , so that the e x i s t i n g s o l v e n t does not remain i n the v i c i n i t y of the i n t e r f a c e (and the composition of the coagulant at the f i l m boundary i s at a l l times that of the o r i g i n a l b a t h ) . While one can employ such an assumption i n order to make use of c o a g u l a t i o n models ( e s p e c i a l l y i n order to avoid a boundary probl ), the problem i n r e a l i t y i s that convection flows of s o l v e n t and solvent-nonsolvent mixtures through the cast l a y e r i n t e r f a c e (Figure 2) (and w i t h i n the nascent membrane) p r e v a i l c o n c u r r e n t l y with the d i f f u s i v e exchange of s o l v e n t with nonsolvent. In a t y p i c a l case, when a polymer s o l u t i o n i s cast on a g l a s s p l a t e or r e l e a s e paper and then submerged i n a coagulant bath, the cast s u r f a c e ( i t s i n t e r f a c i a l zone f a c i n g the coagulant) undergoes a s e r i e s of t r a n s i t i o n s which, i n g e n e r a l , w i l l be r e s u l t s of one or a combination of the f o l l o w i n g three mechanisms: e

m


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Coagulant Case A. Exchange

Case C. Coagulant In

Case B. Solvent Out

Ttt


Cast Layer Each of the three mechanisms y i e l d d i f f e r e n t micro- and macros t r u c t u r e s . The occurrence depends l a r g e l y on the d i f f e r e n c e s i n the s p e c i f i c g r a v i t i e s of the components. The exchange mechanism can be considered as a pure d i f f u s i o n problem - that c o n t r o l s g e l a t i o n v i a stepwise phase i n v e r s i o n - only when the s p e c i f i c g r a v i t i e s of the polymer s o l u t i o n (p ) , i t s s o l v e n t mixture ( p ) , and the coagulant (p ) are equal (pp = p = p ) and i f the heat of mixing (AH ) and h y d r o s t a t i c pressure of the coagulant above the nascent s o l i d phase are minimal. I f these c o n d i t i o n s are not met, convection flows of the l i q u i d components o f t e n dominate the coagul a t i o n process. F o r example, i f a polymer with high s p e c i f i c grav i t y (such as c e l l u l o s e acetate, pp - 1.3) i s cast from i t s s o l u t i o n i n acetone (p - 0.788) and i s coagulated i n an aqueous bath, the g e l a t i o n process i s represented by a general Case B behavior - the solvent i s m o b i l i z e d from the i n t e r f a c e i n a convection-type flow to the upper l e v e l s of the coagulant bath (Figure 2), while the nascent coagulant a t the i n t e r f a c e i s c o n t r a c t i n g inward toward the bottom ( i . e . , the c a s t i n g p l a t e ) . What i s seen i n Figures 3 and 4 i s q u i t e s i m i l a r to r a p i d evaporation of s o l v e n t , which i s chara c t i z e d by i t s r e l a t i v e l y high vapour pressure. The p r o g r e s s i v e i n c r e a s e i n polymer c o n c e n t r a t i o n and the subsequent s o l i d i f i c a t i o n does not n e c e s s a r i l y y i e l d a porous s t r u c t u r e . p

c

s

s

c

s

Conversely, i n most observed cases where s o l i d i f i c a t i o n occurs as a r e s u l t of continued d e p l e t i o n of s o l v e n t (as d e s c r i b e d i n Case B), the h i g h l y concentrated polymer l a y e r s o l i d i f i e s as a r e l a t i v e l y dense, amorphous, p l a s t i c i z e d f i l m . Water d i f f u s i o n i n t o t h i s h i g h l y p l a s t i c i z e d l a y e r becomes p r e v a l e n t (Case A) a t a stage where the c o n t r a c t i o n has gone "too f a r " to y i e l d even a microporous membrane s t r u c t u r e . The s c h l i e r e n experiments employing s o l u t i o n s with THF and dioxane as s o l v e n t s proved that at l e a s t two d i f f e r e n t c o a g u l a t i o n mechanisms e x i s t i n an u n s t i r r e d bath. I t i s evident that Case B a p p l i e d f o r the THF s o l u t i o n , w h i l e Case A a p p l i e d f o r the dioxane s o l u t i o n . The d e p l e t i o n and removal of the THF from the cast l a y e r i n t e r f a c e v i a convection flow (rather than d i f f u s i v e exchange with water molecules) i s instantaneous, and the r a p i d accumulation of the s o l v e n t on the s u r f a c e of the aqueous c o a g u l a t i o n bath i s n o t a b l e . As f o r the dioxane s o l u t i o n (Figure 7), s c h l i e r e n p a t t e r n s could not be observed even a f t e r 1000 seconds - even when phase



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s e p a r a t i o n was completed. The f a c t that a s c h l i e r e n p a t t e r n appeared upon gentle s t i r r i n g of the r e g i o n c l o s e to the i n t e r f a c e proves that a concentrated l a y e r of dioxane/water s o l u t i o n i s g r a d u a l l y accumulated at the nascent membrane i n t e r f a c e and that d i f f u s i v e exchange (Case A) dominates the coagulation mechanism. Both solvents are c y c l i c ethers with about the same molar volume and s o l u b i l i t y parameters, with THF the more p o l a r of the two (Table I ) . However, no l o g i c a l connection can be e s t a b l i s h e d between the gross morphological d i f f e r e n c e s that are produced upon coagulation of the 15 wt% polymer s o l u t i o n i n water and the chemical p r o p e r t i e s of these two s o l v e n t s . Yet, the d i f f e r e n c e s i n the s p e c i f i c g r a v i t i e s does indeed p r e d i c t the behavior that i s d i s p l a y e d i n Figures 4, 5 and 7), and consequently, s i g n i f i c a n t d i f f e r e n c e s i n the membrane s t r u c t u r e s . Examining the c l a s s i c problem of c e l l u l o s e acetate/acetone, an i d e n t i c a l coagulation response to that shown by THF i s observed (Figures 2 and 3). The i n s t a n t d e p l e t i o n and removal of the acetone-rich l a y e r ( i n the coagulation bath) follows a s o l i d i f i c a t i o n of the c e l l u l o s e acetate i n t o a dense, t i g h t s e m i - i s o t r o p i c membrane. I t was t h e r e f o r e of i n t e r e s t to examine the r o l e of formamide as a necessary component (11) i n the f a b r i c a t i o n of asymmetric and porous c e l l u l o s e acetate membranes. A phase diagram of c e l l u l o s e acetate/formamide and acetone i s shown i n F i g u r e 8. The nine formulations that produce Loeb/Sourirajan-type membranes are shown and were a l l taken from the l i t e r a t u r e (7, 11, 13). The s o l i d l i n e represents the phase boundary. A r e l a t e d s o l u b i l i t y parameter diagram (5) i n d i c a t e s that formamide i s a l s o somewhat of a s w e l l i n g agent f o r the polymer; t h e r e f o r e , coagulation and i t s morphological outcome are commonly viewed to be l a r g e l y a r e s u l t of s p e c i f i c chemical i n t e r a c t i o n which determines the s t a t e of e x i s t e n c e of the macromolecular chains i n the c a s t i n g s o l u t i o n . However, many other solvents which e x h i b i t a range of i n t e r a c t i o n - with c e l l u l o s e acetate - s i m i l a r to that r e f l e c t e d through the s o l u b i l i t y parameters of formamide (Table I) do not y i e l d the d e s i r e d porous s t r u c t u r e . (One must remember that the as-cast c e l l u l o s e acetate reverse osmosis membranes, though a n i s o t r o p i c , are s t i l l porous, and d e n s i f i c a t i o n of the s k i n i s a r e s u l t of a subsequent annealing i n most instances.) Examining the s p e c i f i c g r a v i t i e s of s o l u t i o n formulations presented i n F i g u r e 8, each s p e c i f i c g r a v i t y of the solvent mixture i s w i t h i n the range of 0.9 to 1, i n d i c a t i n g that as concentration of the polymer i n s o l u t i o n increases from 17 wt% to 32 wt%, a s l i g h t decrease i n the s p e c i f i c g r a v i t y of the solvent mixture i s allowed. The s e r i e s of coagulation experiments employing c e l l u l o s e acetate s o l u t i o n s i n various acetone/formamide mixtures (Table I I I ) i n d i c a t e s that the c o n t r i b u t i o n of formamide to the cast s o l u t i o n ' s s p e c i f i c g r a v i t y g r e a t l y i n f l u e n c e s the r a t e and c o a g u l a t i o n mechanism of the cast s o l u t i o n . What seems to be an important point i n the formulation of c e l l u l o s e acetate a n i s o t r o p i c membranes i s that the compositions that b r i n g the c a s t i n g s o l u t i o n s u f f i c i e n t l y c l o s e to the g e l a t i o n point are a l s o those that b r i n g the s p e c i f i c g r a v i t y of the mixture c l o s e to that of the coagulant. Therefore, these two v a r i a b l e s are s y n e r g i s t i c . Skin formation occurs at the



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Figure 7. Coagulation of polysulfone (15 wt % in dioxane). Schlieren patterns were not detected. (This was the case for any coagulation of a polymer solution with a high specific gravity solvent.) Elapsed time = 45 s.

P h a s e diagram of the system c e l l u l o s e acetate-formdmide-acetone. Specific gravity of solvent mixtures: • 1.00, D0.97, A 0.95, *0.94, X 0.92, • 0 . 9 2 , A 0.91, O 0 . 9 0 .

Figure 8.

Phase diagram of the system in cellulose acetate-formamide-acetone


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i n t e r f a c e where the polymer s o l u t i o n i s i n d i r e c t contact w i t h the coagulant. Subsequent l i q u i d - l i q u i d phase s e p a r a t i o n with nucleat i o n and growth of the polymer-rich or polymer-poo. phase i s predominantly c o n t r o l l e d by d i f f u s i v e exchange (Case k). The formation of the i n i t i a l s k i n l a y e r (as observed f o r the r a t i o 40/60 formamide/ acetone i n Table I I I ) acts f u r t h e r as a l i m i t i n g b a r r i e r through which the exchange of solvent f o r nonsolvent i s c o n t r o l l e d . Acetone, however, can be replaced by dioxane to produce a very e f f e c t i v e a n i s o t r o p i c c e l l u l o s e acetate membrane and the whole problem of a d j u s t i n g the s p e c i f i c g r a v i t y i s thus e l i m i n a t e d . Phase s e p a r a t i o n c o n t r o l l e d by d i f f u s i o n exchange o f t e n r e s u l t s i n a s k i n which i s composed of a m i c e l l a r assembly of nodules, as w i l l be discussed below. When extremely hydrophobic polymers (e.g., modified-PPO) are cast from dioxane i n t o water ( p - p - P ) , a dense polymer l a y e r i s formed at the s o l u t i o n ' s i n t e r f a c e that somewhat resembles the type of l a y e r formed by i n t e r f a c i a l polymeri z a t i o n . There i s almost no inward c o n t r a c t i o n of the i n t e r f a c i a l s k i n , and the coagulation process i s c o n t r o l l e d by d i f f u s i o n through the dense, i n t e r f a c i a l t h i n f i l m . These r e s u l t i n an a n i s o t r o p i c membrane with a very f i n e " c o r a l " s t r u c t u r e (Figures 9 and 10). The s k i n , which has a l a t e r a l o r i e n t a t i o n , i s not an i n t e g r a l extension of the porous matrix and tends to crack e a s i l y .


s

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R e l a t i o n s h i p Between Nodular and R e j e c t i n g Layers. Nodular formation was conceived by Maier and Scheuerman (14) and was shown to e x i s t i n the s k i n s t r u c t u r e of a n i s o t r o p i c c e l l u l o s e acetate membranes by Schultz and Asunmaa (3), who i o n etched the s k i n to d i s c o v e r an assembly of close-packed, ^ 188 A i n diameter spheres. Kesting (15) has i d e n t i f i e d t h i s kind of m i c e l l a r s t r u c t u r e i n dry c e l l u l o s e e s t e r reverse osmosis membranes, and Panar, et_ a l . (16) has i d e n t i f i e d t h e i r existance i n the polyamide d e r i v a t i v e s . Our work has shown that nodules e x i s t i n most polymeric membranes cast i n t o a nonsolvent bath, where g e l a t i o n at the i n t e r f a c e i s caused by i n i t i a l d e p l e t i o n of s o l v e n t , as shown i n Case B, which f o l l o w s r e s t r i c t e d inward c o n t r a c t i o n of the i n t e r f a c i a l zone. This leads to a dispersed phase of m i c e l l e s w i t h i n a continuous phase ( d e s i g nated as "polymer-poor phase") composed of a mixture of s o l v e n t s , coagulant, and a d i s s o l v e d f r a c t i o n of the polymer. The formation of such a s k i n i s d e l i n e a t e d i n the scheme shown i n F i g u r e 11. In c a s t i n g processes, the nodular l a y e r may fuse to y i e l d a dense amorphous l a y e r when the solvent moves r a p i d l y (by d i f f u s i o n and convection) from the lower, as-yet uncoagulated zone, to the interface. The development of a n i s o t r o p i c membranes based on a hydrophobic polymer matrix (e.g., p o l y s u l f o n e d e r i v a t i v e s or phosphonylated-PPO) which does not c o l l a p s e upon drying, made p o s s i b l e a more thorough i n v e s t i g a t i o n i n t o the o r i g i n and r o l e of the nodular l a y e r . I t i s now c l e a r that i f the nodular l a y e r extends to the i n t e r f a c e without f u s i o n , the membrane i s open to s o l u t e permeation. Solute separat i o n would then be dependent upon the s e r r i e d n e s s of the nodules



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Figure 9.

Cross-section of a modified-PPO hydrophobic membrane exhibiting a very fine coral structure

Figure 10.

Torn surface of a modified-PPO anisotropic membrane (also shown in Figure 9) after exposure to lateral stress



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bottom surface

Figure 11. Initial mechanisms of phase inversion: (I) polymer solution interface at zero time; (II) initial depletion of solvent, inward contraction, and formation of the nodular layer; (III) end of contraction and establishment of the nodular layer.


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and the s p a t i a l c r o s s - s e c t i o n of the s o l u t e s p e c i e s . T h i s i s a t y p i c a l s i t u a t i o n f o r many u l t r a f i l t r a t i o n membranes. Where s o l u t e dimensions a r e i n the range of sodium and c h l o r i d e i o n s , as f o r a reverse osmosis membrane, the n o d u l e s packing density should be much higher than, f o r example, that shown i n F i g u r e 12. This implies that the s e l e c t i v e l a y e r of reverse osmosis membranes may have a d i f f e r e n t o r i g i n from that of the m i c e l l e s . Such a case i s c l e a r l y i d e n t i f i e d by examination o f the s k i n s t r u c t u r e of c e l l u l o s e acetate/poly(bromophenylene oxide phosphonate) a l l o y membranes (17), which e x h i b i t a high f l u x and high s a l t s e p a r a t i o n (Figure 13). The s k i n r e s t s on an assembly of giant spheres (up to 1 um i n diameter) and i s c e r t a i n l y o r i g i n a t e d by a d i f f e r e n t coagul a t i o n mechanism than that of the spheres. The nodular l a y e r i s one of three compacting zones i n reverse osmosis membranes. Exposure to elevated h y d r a u l i c pressure gradu a l l y increases the s e r r i e d n e s s of t h i s l a y e r . The nodular zones shown i n Figures 12 and 13 are not fused, and the spaces between spheres are f i l l e d with amorphous polymer deposits from the polymer-poor phase i n which the m i c e l l e s were dispersed during coagulation. Upon exposure to elevated h y d r a u l i c pressure, the nodules may fuse i n t o the r e j e c t i n g l a y e r with time ( i n reverse osmosis, t h i s phenomenon i s h i g h l y dependent on the polymer's h y d r o p h i l i c i t y ) . For example, a high s a l t s e p a r a t i o n membrane (phosphonylated-PPO) e x h i b i t e d extensive compaction o f the s k i n zone a f t e r exposure to 13.6 atm pressure (200 p s i ) f o r f i v e months i n a reverse osmosis operation; t h i s compaction of the s k i n zone i s shown i n Figure 14. T h i s seems to be contrary to the conventional b e l i e f that the s k i n thickens as a f u n c t i o n o f pressure and time (through f u s i n g with the matrix immdiately below). The micrographs "show, r a t h e r , d e n s i f i c a t i o n of the t e s t e d s k i n , which r e s u l t s i n an apparent s k i n that i s thinner by h a l f than i t s untested counterpart. Further microscopic a n a l y s i s (18) detects numerous nodules imbedded i n the lower s e c t i o n of the s k i n - only p a r t i a l l y fused to become an i n t e g r a l p a r t of the amorphous l a y e r shown as an apparent s k i n i n F i g u r e 14B. This implies that the r e j e c t i n g l a y e r does indeed t h i c k e n at the expense of the nodular layer.


1

At t h i s pressure range (^ 13.6 atm), the compaction and d e n s i f i c a t i o n of the s k i n ' s nodular zone notably surpasses that of the adjacent macrovoids shown i n the photomicrograph, the reason being that the polymer density around such macrovoids i s very h i g h . T h e i r c o l l a p s e pressure i s s e v e r a l orders of magnitude greater than that of macrovoids which a r e l o c a t e d f a r t h e r below the nodular zone, as w i l l be discussed below. Bottom Skin. The o r i g i n of the bottom s k i n i s d i f f e r e n t from that of the surface s k i n . The bottom surface of the membrane i s u s u a l l y an i n t e g r a l extension of the h i g h l y porous substructure, and as such, should d i s p l a y the same c o n f i g u r a t i o n (Figure 15A). However, many dope formulations y i e l d a h i g h l y s t r u c t u r e d , dense


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Figure 12. Cross-section of the surface zone of a polysulfone hollow fiber (spun from DMF directly into water while conducting nitrogen through the lumen)



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Figure 13. Cross-section of the skin zone of a reverse osmosis membrane consisting of cellulose acetate-polyfbromophenylene oxide phosphonate)

Figure 14. A phosphonylated-PPO reverse osmosis membrane before (A) and after (B) testing for 5 months under a pressure of 13.6 atm



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s k i n that may o r may not be porous (Figure 15B). Bottom s k i n forma t i o n i s l a r g e l y due to the i n t e r a c t i o n between the c a s t i n g surface and the components of the dope mixture. D i f f e r e n t surfaces would y i e l d d i v e r s i f i e d bottom s k i n s t r u c t u r e s . For example, by employing polyphosphonates which e x h i b i t a v a r i e d degree of adherence to the c a s t i n g surface (e.g., g l a s s ) , one may obtain s k i n l a y e r s with a wide range of c h a r a c t e r i s t i c s (19). I t i s important to note that bottom s k i n formation i s not l i m i t e d to the case where the dope mixture i s cast on a s o l i d surface. In the wet-spinning of hollow f i b e r membranes, a gas (e.g., dry nitrogen) i s conducted through the inner o r i f i c e (the f i b e r lumen) while the nascent f i b e r i s coagulated e x t e r n a l l y by an aqueous s o l u t i o n . These f i b e r s o f t e n e x h i b i t an i n t e r n a l s k i n s i m i l a r to the bottom s k i n of f l a t sheet membranes, even though the lumen does not contact any s o l i d s u r f a c e . The gas pressure a p p l i e d i n the lumen (or i n f l a t sheet membranes, the h y d r o s t a t i c pressure a p p l i e d by the coagulant) may a f f e c t the formation of a t i g h t bottom s k i n . This subject i s s t i l l open to f u r t h e r i n v e s t i g a t i o n . The existance of a bottom s k i n contributes to the membrane's r e s i s t a n c e and i f t h i s s k i n i s not porous, membrane p r o d u c t i v i t y i s g r e a t l y impeded. Macrovoids. The formation of macrovoids has been discussed i n d e t a i l i n previous p u b l i c a t i o n s (20). In short, they are almost e x c l u s i v e l y a r e s u l t of coagulant p e n e t r a t i o n through the nascent membrane i n t e r f a c e during the f i r s t stages of coagulation f o l l o w i n g the submersion of the cast l a y e r (or the spun extrudate) i n the coagulating bath. In F i g u r e 16, a reverse osmosis membrane i s shown containing three types ( d i s t i n g u i s h e d by l o c a t i o n i n the membrane morphology) of macrovoids: a., l a r g e macrovoids that extend through most of the membrane's substructure, b. medium-sized macrovoids that extend through o n e - t h i r d of the membrane, and c. small macrovoids s i t u a t e d near the membrane's nodular l a y e r . The membrane shown i n Figure 16A has a s a l t r e j e c t i n g s k i n (^ 95%), and operates s a t i s f a c t o r i l y at low h y d r a u l i c pressures - up to 10 atm (^ 147 p s i ) . However, a gradual increase i n the pressure causes a nonuniform c o l l a p s e of the macrovoids (Figure 16B). The c o l l a p s e of the l a r g e and medium-sized macrovoids causes l a t e r a l tension at the membrane s u r f a c e , which then y i e l d s to the a c t i n g f o r c e s . What was o r i g i n a l l y a f l a t surface now has a "mountainous" topography. Since a h i g h l y permselective s k i n does have minimal c a p a c i t y f o r e l o n g a t i o n , the membrane p e r m s e l e c t i v i t y i s reduced s u b s t a n t i a l l y by macrovoid c o l l a p s e . I t i s important to note once again that the small macrovoids adjacent to the membrane surface n e i t h e r c o l l a p s e nor b u r s t . A membrane's f a i l u r e progresses from bottom to top, and at p o i n t s of high l a t e r a l tension, l o n g i t u d i n a l cracks extending to the r e j e c t i n g l a y e r can be i d e n t i f i e d . The f i n e l y porous morphology of the membrane shown i n Figure 16 i s c h a r a c t e r i s t i c of a dope mixture containing a combination of solvents with i n d i v i d u a l s p e c i f i c g r a v i t i e s i n the same order of



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Figure 15. Bottom surfaces: (A) phosphonylated-PPO reverse osmosis membrane; (B) interior skin at lumen of the polysulfone hollow fiber shown in Figure 12.

Figure 16. Reverse osmosis membrane exhibiting three types of macrovoids: large (a), medium (b), and small (c). (A) Before testing and (B) after exposure to 13.6 atm hydraulic pressure. A longitudinal crack in the skin is designated by the perpendicular arrow.



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magnitude as that of the coagulant (e.g., a mixture of dioxane/ a c e t i c a c i d with water as coagulant). The texture of the porous s t r u c t u r e might d r a s t i c a l l y change i f the polymer i s cast from a solvent mixture c o n t a i n i n g a component with a very low s p e c i f i c g r a v i t y (e.g., acetone). The porous substructure may be almost e n t i r e l y composed of l o n g i t u d i n a l macrovoids (Figure 17). Since the w a l l s of these voids are r a t h e r dense, the s t r u c t u r e maintains a r e l a t i v e l y high r e s i s t a n c e to compaction, and as shown i n Figure 17B, the r e g i o n below the s k i n i s kept u t t e r l y i n t a c t a f t e r s e v e r a l weeks of reverse osmosis t e s t i n g at a working pressure of 13.6 atm (200 p s i ) . This type of substructure stems from the use of low s p e c i f i c g r a v i t y s o l v e n t s i n the dope mixture - i n the present case, acetone/acetic acid/phosphonylated-PPO (37.5/37.5/25). The r a p i d d e p l e t i o n of the acetone from the cast l a y e r i n t o the coagulating medium ( i . e . , water, which i n t r u d e s s u b s t a n t i a l l y i n t o t h i s dope formulation) causes the substructure to coagulate around and over the coagulant i n t r u s i o n s i n a mechanism s i m i l a r to that described i n the preceding s e c t i o n f o r the formation of dense membranes. A notable f e a t u r e of the membrane shown i n F i g u r e 17 i s the absence of a nodular l a y e r i n the v i c i n i t y of the perms e l e c t i v e s k i n , although f u r t h e r microscopic examination detects imbedded spheres i n the macrovoid w a l l s (18). Composite_,Membranes. A general scheme showing the c r o s s - s e c t i o n of a composite membrane [or what i s o f t e n c a l l e d a t h i n f i l m compos i t e membrane (21)] i s shown i n Figure 18. The s k i n zone c o n s i s t s of a r e j e c t i n g l a y e r and a h y d r o p h i l i c , c r o s s l i n k e d g e l l a y e r . The support most commonly used i s an a n i s o t r o p i c porous membrane (e.g., p o l y s u l f o n e ) . The r e j e c t i n g l a y e r can be deposited by dip coating (Figure 12) or i n s i t u p o l y m e r i z a t i o n of monomers which y i e l d an extremely t i g h t , permselective b a r r i e r . The l a r g e v a r i e t y of composite membranes reported i n the l i t e r a t u r e are not n e c e s s a r i l y represented by the s t r u c t u r e shown i n F i g u r e 18. Nevertheless, the membranes which have m a t e r i a l i z e d as commercially f e a s i b l e , operating reverse osmosis u n i t s (e.g., PA-300, NS-100, which are c r o s s l i n k e d polyamide d e r i v a t i v e s u l t r a t h i n f i l m d e p o s i t i o n s on porous p o l y s u l f o n e supports) are a c c u r a t e l y represented by t h i s scheme. In comparing the LoebS o u r i r a j a n type of membrane to the composite membrane, one may be puzzled by the f u n c t i o n of the g e l l a y e r shown i n the scheme. An e a r l y generation of composite membranes, developed by R i l e y , et a l . (21), was based on c e l l u l o s e t r i a c e t a t e (CTA) cast i n an u l t r a t h i n coat from chloroform on the f i n e l y porous s u r f a c e of a c e l l u l o s e n i t r a t e / c e l l u l o s e acetate s u b s t r a t e . These membranes d i d not r e f l e c t a need f o r a h y d r o p h i l i c - g e l intermediate l a y e r . Yet, t h i s membrane s u b s t r a t e i s much more h y d r o p h i l i c than the r e j e c t i n g CTA l a y e r , and high f l u x as w e l l as high separation were conc u r r e n t l y obtained. T h i s i s not the case i f the porous s u b s t r a t e i s h i g h l y hydrophobic. A r e j e c t i n g l a y e r deposited on such a surface would y i e l d an extremely poor p r o d u c t i v i t y due to the l o s s of


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Figure 17.

Cross-section of a reverse osmosis membrane with dense-walled macrovoids



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Cross-section scheme of a composite reverse osmosis membrane



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e f f e c t i v e surface area; a "gutter l a y e r " i s then needed i n order to channel the permeated water i n t o the pores to maintain p r o d u c t i v i t y . This i s the main f u n c t i o n f o r t h i s intermediate l a y e r . A second f u n c t i o n i s to serve as a "cushion" between the r i g i d , uneven support surface and the h i g h l y c r o s s l i n k e d permselective d e p o s i t , which has a minimal c a p a c i t y to absorb l a t e r a l s t r e s s e s . In most cases where c r o s s l i n k e d polyamide d e r i v a t i v e s (22) are deposited on a p o l y s u l f o n e s u b s t r a t e , the f a b r i c a t i o n process c o n s i s t s o f s e r v a l stages that can be summarized as: 1) coating the substrate with a h y d r o p h i l i c polyamine d e r i v a t i v e . 2) chemically c r o s s l i n k i n g the surface zone of the coated l a y e r by employing a c r o s s l i n k i n g agent. 3) inducing thermal ( s e l f ) c r o s s l i n k i n g of the polyamine l a y e r beneath the chemically c r o s s l i n k e d surface ( i . e . , thus producing a g e l l a y e r ) . I t was shown that i f the chemical c r o s s l i n k i n g (23) i s allowed to proceed through the e n t i r e coated l a y e r - thus y i e l d i n g a h i g h l y c r o s s l i n k e d , dense l a y e r on top of the hydrophobic substrate - the performance of the membrane i s reduced to the degree of n o n p r o d u c t i v i t y . In the Loeb-Sourirajan type of membrane, the s k i n and substructure are of the same polymer. Since the substructure extends from the s k i n , the n e c e s s i t y o f producing a l a y e r i s t o t a l l y eliminated; and from a mechanical viewpoint, the nodular l a y e r serves as a supporting element " a s s i s t i n g " the s k i n to absorb minor l a t e r a l stress. Conclusion Thorough a n a l y s i s and e v a l u a t i o n of membrane morphology i s mandatory f o r the understanding of t r a n s p o r t phenomena i n membranes, and e s p e c i a l l y f o r those w i t h rather complex s t r u c t u r e s , as described i n the present manuscript. Each s i n g l e membrane can be viewed perhaps as a "black box" when operating i n a c e r t a i n w e l l defined system. Yet, any deduction on t r a n s p o r t mechanism that i s based s o l e l y on transport data i s h i g h l y s p e c u l a t i v e . F o r example, the presence of a double s k i n , macrovoids, the d e n s i f i c a t i o n of the nodular l a y e r , and other items described h e r e i n cannot be p r e d i c t e d by the a n a l y s i s of transport data. But they can be i d e n t i f i e d , and can be very supportive t o "whoever dares to look i n t o the b l a c k box." Acknowledgements The author would l i k e to acknowledge the support o f the O f f i c e of Water Research and Technology, U.S. Department o f the I n t e r i o r . C r e d i t i s due a l s o f o r experimental work c a r r i e d out by Mr. C S . Eyer, Mr. R. LeBoeuf, and Mrs. B. Zimny. S p e c i a l a p p r e c i a t i o n i s expressed t o Ms. A. Echols f o r her a s s i s t a n c e i n p r e p a r a t i o n o f t h i s manuscript.


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Literature

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Cited

1.

Loeb, S; Sourirajan, S; Adv. Chem. Ser., 1962, 38, 117.

2.

Riley, R.L.; Gardner, J.O.; Merten, U.; Science, 1964, 143, 801.

3.

Schultz, R; Asunmaa, S; Recent Progr. Surface Sci., 1970, 3, 293.

4.

Frommer, M.A.; Lancet, D; "Reverse Osmosis Membrane Research," Lonsdale, H.K. and Podall, H.E., Eds., Plenum Press, New York, 1972; pp. 85-110.

5.

Klein, E; Smith, J . K . ; "Reverse Osmosis Membrane Research," Lonsdale, H.K. and Podall, H.E., Eds., Plenum Press, New York, 1972; pp. 61-84.

6.

Sarboulouki, M.N.; Polym. Letts. Ed., 1973, 11, 753.

7.

Sirkar, K.K.; Agarwal, N.K.; Pandu, G; J. Appl. Polym. Sci., 1978, 22, 1919.

8.

Aptel, P; Cabasso, I; J . Appl. Polym. Sci., 1980, 25, 1969.

9.

Kesting, R.E.; "Permeability of Plastic Films and Coatings," Hopfenberg, H.B., Ed., Plenum Press, New York, 1974; pp. 402.

10.

Kunst, B; Vajnaht, Z.; presentation in Synthetic Membrane Symposium (A.C.S.) Second Chem. Cong. of the North American Cont., Las Vegas, Nevada, August 24, 1980.

11.

Manjikian, S; Ind. Eng. Chem. Prod. Rev., 1967, 6, 23.

12.

Cohen, C; Tanny, G.B.; Prager, S; J . Polym. Sci., (Physics Ed.) 1979, 17, 477.

13.

Fahey, P.M.; Grethlein, H.E.; Desalination, 1979, 9, 297.

14.

Maier, K.H., Scheuermann, E.A.; Kolloid Z . , 1960, 171, 122.

15.

Kesting, R.E.; J. Appl. Polym. Sci., 1973, 17, 1771.

16.

Panar, M; Hoehn, H.; Hebert, R; Macromolecules, 1973, 6, 777.

17.

Cabasso, I; Tran, C.N.; J. Appl. Polym. Sci., 1979, 23, 2967.

18.

Cabasso, I; unpublished results.

19.

Cabasso, I; "Study of Novel Polymers and Alloys in Asymmetric and Composite Membranes," U.S. Dept. of Interior, OWRT Research and Development Report, Feb. 26, 1979.



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20.

Cabasso, I; "Ultrafiltration," Cooper, A, Ed., Plenum Press, New York, 1980; pp. 47.

21.

Riley, R.L.; Hightower, G.R.; Lyons, C.R.; Tagami, M; "Permeability of Plastic Films and Coatings," Hopfenberg, H.B., Ed., Plenum Press, New York, 1974; pp. 375.

22.

Rozelle, L . T . ; Cadotte, J . E . ; Cobian, K.E.; Kopp, C.V. Jr; "Reverse Osmosis and Synthetic Membranes, Sourirajan, S., Ed., National Res. Cons., Canada, 1977; pp. 249.

23.

Cabasso, I; Tamvakis, A.P.; J. Appl. Polym. Sci., 1979, 23, 1509.

RECEIVED

January 5, 1981.


Ultrastructure of Asymmetric and Composite ... - ACS Publications

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