Materials Science of Synthetic Membranes - American Chemical Society

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Phase Inversion Membranes R. E. KESTING SPMK, Inc., 4625 Green Tree Lane, Irvine, CA 92715

The science of membrane formation via phase inversion and the technology of producing phase inversion membranes are related in this paper. The discussion begins by presenting the phase inversion mechanism and continues by distinguishing and describing in detail four membrane formation processes used to achieve phase inversion. The structure of the resulting membranes are discussed in terms of asymmetry or skinning and anisotropy. Skinned membranes are classified as integrally- and nonintegrally-skinned microgels and ultragels. Finally, structural irregularities such as wavemarks, macrovoids and blushing are discussed. Piiase inversion refers to the process by which a polymer solution (in which the solvent system is the continuous phase) inverts into a swollen three-dimensional macromolecular network or gel (where the polymer is the continuous phase). As a thin film designed for use as a barrier, such a gel constitutes a phase inversion membrane. Mechanism of Phase Inversion Phase inversion typically begins with a molecularly homogeneous, single-phase solution (Sol 1) which undergoes a transition into a heterogeneous, metastable solution of two interdispersed, liquid phases (Sol 2) which subsequently form a gel (Sol 1 ·* Sol 2 G e l ) . A l t e r n a t i v e l y , S o l 2 may serve as the starting point f o r the formation of the g e l (Sol 2 G e l ) . The dispersed phase of S o l 2 consists of spherical droplets or micelles which are coated with polymer molecules. The composition i n the i n t e r i o r of the micelles and i n the continuous phase d i f f e r from case to case depending upon the p a r t i c u l a r v a r i a t i o n of the phase inversion process. The m i c e l l a r structure which exists i n the primary g e l immediately following gel formation d i f f e r s only i n f i n i t e s i m a l l y from that of Sol 2 just p r i o r to gelation. Therefore, S o l 2 i s conceded s t r u c t u r a l as w e l l as temporal primacy over the g e l (1). In fact

0097-6156/ 85/ 0269-0131 $09.50/ 0 © 1985 American Chemical Society In Materials Science of Synthetic Membranes; Lloyd, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

M A T E R I A L S SCIENCE OF SYNTHETIC M E M B R A N E S

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as well as i n theory, the structure and function of the f i n a l phase inversion membrane i s primarily controlled by adjustments to Sol 2. By comparison, post formation changes to the gel have only a secondary impact on membrane structure. The reader may find i t h e l p f u l at t h i s junction to consider the phenomenological model o r i g i n a l l y developed by Cahn (2) to describe two phase metal alloys and more recently used i n conjunction with polymer a l l o y s . This model explains the appearance of i s o t r o p i c , interdispersed domains i n terms of spinodal decomposition. This may y i e l d some insight into the reasons why " u p h i l l " d i f f u s i o n (that i s , d i f f u s i o n against the concentration gradient) occurs i n phase inversion. The reader i s also referred to the contribution of Strathmann i n the present volume (3). Sol 2 i s present either when one phase separates into two phases or when two phases are prevented from recombining into a single phase. I t i s expedient to e n t i t l e t h i s factor incompatibility, and to discuss the various phase inversion processes i n terms of the reasons for incompatibility. In the sections to follow four phase inversion processes are discussed: a dry process, a wet process, a thermal process and a polymer assisted phase inversion process. The Dry Process The dry or complete evaporation phase inversion process i s the oldest and easiest to understand. I t can be i l l u s t r a t e d by a t y p i c a l cellulose n i t r a t e (CN) casting solution (see Table I ) . This system can be used to exemplify the various macroscopically observable stages involved i n the formation of membranes by the dry process: (1) Loss of v o l a t i l e solvents and the inversion of a clear one-phase solution into a turbid, two-phase solution. (Alternatively, the solution may be turbid and two-phase to begin with.) Ease of processing and r e p r o d u c i b i l i t y are enhanced i f the solution begins with a Sol 1 or at least a Sol 2 which i s somewhat removed from the point of i n c i p i e n t gelation. In most cases, i t i s desirable to formulate a clear single phase solution which does not invert u n t i l some time after i t has been cast. (2) Gelation, which i s marked by a decrease i n the r e f l e c t i v i t y of the cast solution (3) Contraction of the gel accompanied by syneresis. In the case of skinless membranes, syneresis causes expelled l i q u i d to appear at the a i r / s o l u t i o n interface. I f the membrane i s cast on a porous support, l i q u i d may appear at both surfaces. In the case of membranes which are skinned at the a i r / s o l u t i o n interface only, syneresis occurs downward into the porous support. Where no such support e x i s t s , syneresis does not occur at a l l . In such a case, drying can be a slow process requiring the d i f f u s i o n of vapor rather than l i q u i d through what may be a r e l a t i v e l y impervious skin layer (4) Capillary depletion. Here the largely nonsolvent l i q u i d encompassed by the gel departs, leaving behind empty c a p i l l a r i e s . Frequently t h i s results i n the formation of snowflake patterns that gradually f i l l i n u n t i l the entire membrane becomes opaque. The reason f o r this i s l i g h t scattering by the micrometre-sized empty

In Materials Science of Synthetic Membranes; Lloyd, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

7. KESTING

133

Phase Inversion Membranes


Table I. Decrease i n Casting-solution Weight and Thickness with Drying Time Evaporation time, min 0 0.40 0.83 1.58 2.08 2.8 4.0 5.16 6.67 8.25 10.50 13.16 20.33 24.16 31.0 35.5 43.0 47 54 74 130 900

Solution weight, g 10.5 9.5 9.0 8.5 8.0 7.5 7.0 6.5 6.0 5.5 5.0 4.0 3.5 3.0 2.5 2.0 1.75 1.50 1.25 1.25 0.99 0.82

Thickness of nascent membrane, ym 650

450 500

350* 300 280 250 220 200 170 155 150 135

115

125

115

-

100

*From t h i s point on values r e f e r to the thickness of the g e l exclusive of the layer of expelled l i q u i d . Evaporation at 21±1°C i n a 62±2% r e l a t i v e humidity environment. O r i g i n a l casting solution: 5% c e l l u l o s e n i t r a t e , 54.2% methyl acetate, 23.7% ethyl alcohol, 12,3% butyl alcohol, 3.3% water and 1.5% g l y c e r o l . Reproduced with permission from Ref. 4.

Copyright 1960.



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voids. On the other hand, those membranes which contain voids that are less than 0.5 μΜ i n diameter can be opalescent or clear. Subtle differences i n void size can sometimes be discerned from the t u r b i d i t i e s of the dry gels once they have been wet by water. (5) Loss of residual nonsolvent ( f i n a l drying). Depending on such factors as the v o l a t i l i t y and concentration of residual l i q u i d s i n the membrane, the amount of membrane on the take-up r o l l and storage temperature, f i n a l drying can require between two weeks and six months. I t i s also possible to take up the membrane i n an e s s e n t i a l l y dry condition by passing i t over heated r o l l e r s . In either case, no membrane should be handled unless i t i s completely dry. Otherwise i t i s subject to shrinkage and warpage while s t i l l i n a p l a s t i c i z e d condition. In t h i s process, f i n a l membrane thickness i s only a f r a c t i o n of the as-cast thickness owing to solvent loss and the resultant increase i n the concentration of polymer per unit volume. However, because of the i n c l u s i o n of voids, the f i n a l membrane thickness i s substantially greater than the thickness of a dense membrane containing an equivalent amount of polymer. The weight and thickness as functions of evaporation time for a t y p i c a l CN membrane casting are shown i n Table I. The sequence of events on the c o l l o i d a l l e v e l corresponding to the f i v e macroscopically observable stages outlined above has been deduced from the nature of the gel network i n the finished membrane (4,5) and from the ghosts of the nascent membrane; that i s , the frozen and l y o p h i l i z e d nonvolatile remnants of the membrane in its various formative phases (6). The polyhedral c e l l structure of the f i n a l membrane g e l i s considered to be an immobilized and flattened version of the s o l precursors which exists i n the solution immediately p r i o r to the s o l - g e l t r a n s i t i o n . As the loss of v o l a t i l e solvent progresses, the solvent power of the solution decreases; that i s , i t s c a p a b i l i t y f o r retaining the polymer i n a homogeneous single phase Sol 1 solution i s diminished. I f only polymer and solvent are present, then at least three situations are possible: (1) Separation into two l i q u i d phases may not occur p r i o r to gelation. This would be the case i f polymer and solvent are i n f i n i t e l y miscible. Even a f t e r gelation the solvent w i l l continue to act as a p l a s t i c i z e r . When combined with the e f f e c t of gravity, t h i s can lead to collapse and d e n s i f i c a t i o n of the g e l , ultimately r e s u l t i n g i n a dense f i l m . (2) Phase separation may occur p r i o r to gelation i f there i s only limited polymer s o l u b i l i t y i n the solvent. However, even i n t h i s case residual solvent can act as a p l a s t i c i z e r and a dense or nearly dense (low porosity) f i l m may r e s u l t . (3) For those cases i n which polymer-polymer interactions are unusually strong; f o r example, i n the evaporation of solutions of Nylon 6,6 i n 90% formic acid (7), gelation w i l l occur with the formation of strong v i r t u a l (perhaps c r y s t a l l i n e ) cross-links. Such a g e l can overcome the combined e f f e c t s of p l a s t i c i z a t i o n and gravity so that porosity i s maintained throughout complete evaporation. After phase inversion has occurred and p r i o r to gelation, the s o l structure exhibits long range order. V i r t u a l l y any disruption of t h i s order or nucleation i n the s o l by rapid



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135

agitation or f i n e f i l t r a t i o n w i l l produce larger pores than would have otherwise resulted. Both the i n t e r i o r of the micelles and the continuous phase of a two component system consist of polymer-poor regions. The micellar wall consists of polymer-rich regions. In the c e l l w a l l , polymerpolymer interactions predominate over polymer-solvent interactions. Most dry process casting solutions, however, consist of three or more components: polymer, v o l a t i l e solvent and one or more poreformers from the nonsolvent side of the polymer-solvent interaction spectrum. The nonsolvent should be substantially less v o l a t i l e than the solvent. A p r a c t i c a l rule of thumb i s a 30 to 40°C minimum difference i n b o i l i n g points between the two. Although Sol 1 i s homogeneous on the c o l l o i d a l l e v e l , compatibility decreases as evaporation proceeds. Eventually the solvent power of the remaining solvent system i s i n s u f f i c i e n t to maintain Sol 1, and inversion into Sol 2 occurs. Most of the polymer molecules d i s t r i b u t e themselves around the micelles which have formed. Relatively few polymer molecules (perhaps 0.5 percent) remain dispersed i n the l i q u i d matrix. In t h i s case, the i n t e r i o r of the micelle consists of a l i q u i d with a large concentration of the nonsolvent components of the casting solution. The presence of nonsolvent i n the casting solution and/or strong polymer-polymer interaction forces leads to phase inversion, gelation and the maintenance of gel porosity i n spite of forces which act to collapse the g e l . In other words, i n the dry process incompatibility i s an internal c h a r a c t e r i s t i c of the system. Because solvent loss continues a f t e r phase inversion, the spherical micelles approach one another and eventual make contact i n the i n i t i a l phase of gelation. As the gel network contracts, the micelles deform into polyhedra and the polymer molecules d i f f u s e into the walls of neighboring micelles causing an intermingling of polymer molecules at the i n t e r f a c e . If the walls are s u f f i c i e n t l y t h i n , contraction causes a tearing of the walls which then r e t r a c t and form the hoselike skeleton which constitutes the gel network. This occurs when a high i n i t i a l concentration of components other than polymer and solvent causes the formation of numerous micelles with a large t o t a l surface area. A s i m i l a r phenomenon occurs during the bursting of soap bubbles (8) and the formation of open-celled polyurethane foams. However, the micelles may be covered with such a thick polymer coating that rupturing of c e l l walls i s hindered or e n t i r e l y inhibited. In t h i s case, either mixed open- and c l o s e d - c e l l or e n t i r e l y c l o s e d - c e l l structures r e s u l t . Consider the p r i n c i p a l factors which determine the porosity and pore s i z e c h a r a c t e r i s t i c s of dry process membranes: (1) Polymer volume concentration i n Sol 2 i s inversely proportional to gel porosity. (2) The r a t i o of nonsolvent volume/polymer volume i n Sol 2 i s d i r e c t l y proportional to gel porosity. (3) The difference i n b o i l i n g points between solvent(s) and nonsolvent(s) i s proportional to porosity and pore s i z e . (4) The r e l a t i v e humidity of the evaporation environment i s proportional to porosity and pore s i z e . (5) The presence of more than one polymer with less than perfect compatibility increases porosity.



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MEMBRANES

(6) The presence of high molecular weight polymer tends to increase porosity because these polymers tend to be less compatible. The e f f e c t s of polymer and nonsolvent concentration upon c e l l structure, porosity and permeability are discussed next. Because dry process solutions use nonsolvent poreformers, the polymer d i s s o l v i n g capacity of the solvent system i s severely limited. In spite of t h i s , the casting solution must be s u f f i c i e n t l y viscous to permit handling during f l a t sheet and tubular casting or hollow f i b e r spinning. This dilemma i s resolved by u t i l i z i n g high molecular weight polymers which, although s l i g h t l y less soluble than t h e i r low molecular weight counterparts, do contribute s i g n i f i c a n t l y to solution v i s c o s i t y . However, most engineering p l a s t i c s are available only i n the low to intermediate molecular weight range because they are designed for melt processing applications such as i n j e c t i o n moulding and extrusion. This can, and often does, l i m i t the application of the dry process. Methods for circumventing t h i s obstacle include the preparation of special grades of high molecular weight polymers (9), the u t i l i z a t i o n of v i s c o s i t y enhancers such as a second polymer (10,11) or f i n e l y divided c o l l o i d a l s i l i c a , and casting at low temperatures. In the absence of any nonsolvent poreformer or strong polymer-polymer interactions, phase inversion (Sol 1 ·* gel) does not take place and a dense, high-resistance membrane or f i l m i s formed. As a f i r s t approximation, such a structure consists of a single dense skin layer. With low concentrations of nonsolvent, membranes possessing closed c e l l s , low porosity and substantial resistance to material transport are encountered. However, the thickness of the dense skin layer i s s u b s t a n t i a l l y diminished. At intermediate concentrations of nonsolvent, a mixture of open and closed c e l l s i s formed. The dense skin layer has thinned considerably and a t h i n t r a n s i t i o n layer consisting of closed c e l l s i s d i s c e r n i b l e . Polymer density i n t h i s t r a n s i t i o n layer i s intermediate between that of the dense skin layer and that of the porous, open-celled substructure which i s found i n the bulk of the membrane. Permeability i s small but measurable at t h i s point. At high concentrations of nonsolvent, a bilayered structure comprised of a thin skin and a porous, open-celled substructure i s found. There i s a break i n the curve of permeability versus porosity at that concentration at which mixed open and closed c e l l s give way to open c e l l s . As the concentration of nonsolvent i s increased beyond t h i s point, skin thickness decreases and permeability increases. Eventually the skin becomes s u f f i c i e n t l y thin that i t s i n t e g r i t y i s breached i n places and the porous substructure becomes v i s i b l e . At extremely high nonsolvent concentrations, the dense skin layer i s absent altogether and both surface and i n t e r i o r regions consist of open c e l l s with tears i n the walls. In t h i s case, the porous surfaces c h a r a c t e r i s t i c of m i c r o f i l t r a t i o n membranes are encountered. As nonsolvent concentration i s increased beyond t h i s point, c e l l s i z e , pore size and permeability continue to increase. Eventually the i n t e g r i t y of the porous substructure cannot be maintained. If structures with pore sizes greater than about 5 μΜ are desired, processes other than phase inversion are employed.



7.

KESTING


137

The relationship between Δ b.p. (nonsolvent b o i l i n g point solvent b o i l i n g point) of the two solvents acetone (b.p. 56°C) and dioxolane (b.p. 75°C) and the l e v e l of a single nonsolvent isobutanol (b.p. 110°C) required to produce equivalent skinned membranes (as deduced from t h e i r equivalency i n permeability and permselectivity) i s shown i n Table I I . Both solvent and nonsolvent evaporate simultaneously. Therefore, i f a c r i t i c a l solvent/nonsolvent r a t i o must be reached before gelation occurs, a less v o l a t i l e solvent w i l l require a higher i n i t i a l concentration of a given nonsolvent to reach t h i s r a t i o at a given porosity than w i l l a more v o l a t i l e solvent. S i m i l a r l y , the concentration of nonsolvent i n the casting solution required to achieve a given porosity i s inversely related to i t s volatility. Bjerrum and Manegold (12) were among the f i r s t to observe the influence of the atmosphere above the desolvating solution upon membrane structure and function. A high concentration of solvent vapor retards gelation, whereas high temperatures and high a i r v e l o c i t i e s w i l l hasten i t . Skinning i s enhanced by high a i r flow rates and high polymer concentration. Atmospheric moisture hastens gelation and thus increases average pore size and permeability (Table I I I ) . The inclusion of water i n the casting solution has a pronounced e f f e c t i n those cases i n which water plays a nonsolvent role (Table IV). In hydrophobic s o l s , water acts both to hasten gelation and to increase the size of voids i n the g e l structure. This i s attributable to two factors: f i r s t , a substantial degree of incompatibility with the solvated polymer component of the casting solution, and second, high surface tension. Both factors act to cause water to separate from the remainder of the solution i n the form of comparatively large micelles which then result i n coarse microgels. The presence of a microgel structure i n membranes from moderately hydrophilic polymers such as c e l l u l o s i c s and polyamides confers the important property of wet-dry r e v e r s i b i l i t y on these membranes (6). This occurs because the magnitude of the c a p i l l a r y forces coming into play upon drying depends on the i n t e r n a l surface area of the membrane which i n turn depends on c e l l s i z e . Microgel membranes possess large (1 to 10 uM diameter) c e l l s . Therefore, such membranes have a r e l a t i v e l y small i n t e r n a l surface area and as a result w i l l not lose porosity during drying. U l t r a g e l membranes, on the other hand, have small (1.5 to 0.5 μΜ diameter) c e l l s and consequently possess a larger i n t e r n a l surface area. Ultragels are therefore more l i k e l y to lose porosity during drying and less l i k e l y to be wet-dry r e v e r s i b l e . In comparison to the wet process, the dry process tends to use more d i l u t e solutions and less compatible poreformers (both of which promote the formation of microgels). Therefore, the dry process i s more l i k e l y to produce microgels than i s the wet process. However, there are many exceptions to t h i s rule and i t i s possible both to produce microgels by a wet process and u l t r a g e l s by a dry process. Because the p r i n c i p l e s which govern the dry process are now so w e l l understood and because the process i t s e l f i s so amenable to control, the dry process i s considered by the present author as the technique of choice whenever i t can be applied.


In Materials Science of Synthetic Membranes; Lloyd, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985. Nonsolvent concentration (glBA/formulation) 38 54 Permea bility* (gfd) 5.6 5.5

Salt** Rejection (%) 97.9 97.8

**0.5% NaCl feed @400 p s i and 25±1°C

* Total Polymer Cone. 10% W/V; Polymer Ratio 6/1 JLF-68CA/TMA s a l t of CA 11-bromoundecanoate (made from E-383-40 CA with 0.3D.S. quat); methanol, 10g/formulation

Acetone Dioxolane

Solvent

Δ b.p. (b.p. nonsolventb.p . solvent) (°C) 54 35

Equivalent Nonsolvent Concentrations i n Acetoneand Dioxolane-Solutions* f o r Dry-RO Blend Membranes of CA and the TMA Salt of CA 11-bromoundecanoate

boiling point (°C) 56 75

Table I I .



7. KESTING

Table I I I .

Relative humidity at 20°C, % 80 60 40

139


E f f e c t of Relative Humidity upon Permeability and Pore Size F i l t r a t i o n time f o r 500 ml H 0/12.5 cm at 70 cm Hg, sec 2

Average pore diameter, nm ^600 ^500 ^400

25-40 40-60 60-80

Reproduced with permission from Ref. 4.

Copyright 1960.


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M A T E R I A L S SCIENCE OF SYNTHETIC M E M B R A N E S


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7. KESTING The Wet


Process

The wet or combined evaporation-diffusion> technique i s that v a r i a t i o n of the phase inversion process i n which a viscous polymer solution i s either allowed to p a r t i a l l y evaporate p r i o r to immersion i n a nonsolvent gelation bath, or immersed d i r e c t l y into a nonsolvent gelation bath. Upon immersion, any residual solvent or poreformer i s exchanged f o r nonsolvent. A wet process solution must be r e l a t i v e l y viscous (>^10,000 centipoise) at the moment of immersion i n the nonsolvent so that i t w i l l r e t a i n i n t e g r i t y throughout gelation. When the solution i s too f l u i d , the primary gel w i l l be subject to disruption by both the weight of the nonsolvent and the currents coming into play during immersion. The requirement f o r high v i s c o s i t y and hence high polymer concentration i s , i n most cases, inconsistent with the attainment of high porosity v i a the i n c l u s i o n of nonsolvent poreformers. Therefore, when they are required, poreformers are frequently chosen from the swelling agent—weak solvent side of the polymer-solvent interaction spectrum. Moreover, the presence of poreformers within the casting solution i s not a requirement of every wet process solution. In many instances, p a r t i c u l a r l y when nonvolatile solvents with a strong a f f i n i t y for the nonsolvent gelation medium are u t i l i z e d , the phase inversion sequence Sol 1 Sol 2 g e l i s evoked by the simple act of immersion into nonsolvent without p r i o r evaporation. In such a case, the nonsolvent bath represents an external source of incompatibility. A two-component solution (polymer + solvent) becomes a threecomponent solution (polymer + solvent + nonsolvent poreformer) as a result of the d i f f u s i o n of nonsolvent i n t o , and solvent out of, the nascent membrane g e l . The effect of the nonsolvent gelation medium may be influenced by the presence of certain components of the casting solution. For example, lyotropic s a l t swelling agents from the Hofmeister series cause aggregation of water molecules around the e l e c t r o p h i l i c cations, thereby modifying the properties of a water gelation medium (13). This interaction causes a change i n the role of water from that of a nonsolvent to that of a swelling agent (Table V). Other polar nonsolvents such as a l i p h a t i c alcohols function i n much the same manner as water, except that t h e i r nonsolvent tendencies are less pronounced. Increasing the concentration of the weak nonsolvent poreformer, ethanol, i n a casting solution containing CA and acetone increases the porosity of the resultant membranes (Table VI). Because the poreformer i s a nonsolvent, solution compatibility decreases with increasing ethanol concentration. As the concentration of ethanol i s increased, the solution approaches the point of incipient gelation; that i s , the perimeter of the s o l u b i l i t y envelope. Because a solution containing a high concentration of nonsolvent can be presumed to be of the Sol 2 type and close to gelation, immersion into a nonsolvent bath and subsequent gelation w i l l be accompanied by less gel concentration than would occur i f the solution were further removed from the perimeter of the s o l u b i l i t y envelope. The result i s that the micelle diameter i n Sol 2, as w e l l as the porosity and permeability of the f i n a l membrane, s


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Materials Science of Synthetic Membranes - American Chemical Society

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