Materials Science of Synthetic Membranes - American Chemical

14 Hollow Fiber Membrane Research Morphology, Pervaporation, Gas Separation, and Durability Problems Downloaded by PRINCETON UNIV on October 8, 2013 | http://pubs.acs.org Publication Date: December 12, 1985 | doi: 10.1021/bk-1985-0269.ch014

ISRAEL CABASSO The Polymer Research Institute, State University of New York, CESF, Syracuse, NY 13210

The rapid development of hollow fiber membrane technology frequently has outpaced the advance of s c i e n t i f i c fundamentals. Thus, a revisit to this membrane technology helps to unveil the cause of some outstanding problems and contributes to the improvement of existing processes and products. This paper describes four major topics that are currently being studied in our laboratories: 1) The evolvement of morphology in a nascent wet-spun hollow fiber. 2) Separation of gas mixtures through double-layer composite membranes (1). 3) Stress induced crystallization in cellulose ester, reverse osmosis, hollow fiber membranes (2). 4) The separation of aqueous alcohol mixtures through ion-exchange hollow fibers. Each of these topics is a subject of doctoral thesis and, therefore, only some points of immediate interest w i l l be emphasized here. Method to Trace the Evolvement of Morphology i n a Dry-Jet Wet Spinning Process The dry-jet wet spinning process is commonly used in hollow fiber technology. This spinning method can be employed to obtain almost every known membrane morphology and is described in several publications (3, 4 . In general, a spinning dope composed of a certain polymer dissolved with additives in a water miscible solvent is spun through a tube in an o r i f i c e spinneret into water. The polymer coagulates and the solvents, along with the additives, are eventually extracted through the nascent fiber walls. A method of spinning polysulfone hollow fibers with varied characteristics has been described elsewhere (4). This spinning method is based on the a b i l i t y of polyvinylpyrrolidone (PVP) to form a homogeneous blend with polysulfone and the fact that PVP is a water soluble polymer. Thus, ternary spinning dopes of polysulfone/PVP/dimethylacetamide (DMA) were dry-jet wet-spun to produce asymmetric hollow fiber membranes for applications such as reverse osmosis, ultrafiltration, hemofiltration and gas separations. In the present study, the fact that PVP is a high molecular weight additive with a much lower

0097-6156/85/0269-0305$06.00/0 © 1985 American Chemical Society In Materials Science of Synthetic Membranes; Lloyd, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.

Downloaded by PRINCETON UNIV on October 8, 2013 | http://pubs.acs.org Publication Date: December 12, 1985 | doi: 10.1021/bk-1985-0269.ch014

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mobility than the solvent was exploited. Therefore, segments of the nascent f i b e r can be severed and removed from the spinning l i n e and directed to a subambient chamber without damage, v i r t u a l l y stopping the c o a g u l a t i o n process, u n l i k e the c a s t i n g of f l a t sheet mem branes, the spun f i b e r has to e s t a b l i s h c o n s i d e r a b l e mechanical i n t e g r i t y i n a short p e r i o d of time, < 2 seconds; t h e r e f o r e , dope s o l u t i o n s that c o n t a i n high f r a c t i o n s of polymers, up to 50 wt.%, are often used. In the present study the dope composition contained 15 wt.% PVP ( m o l e c u l a r weight 15,000), 28 wt.% p o l y s u l f o n e ( U d e l l 3500) and DMF. This composition was spun into a water bath at 25°C to produce the f i b e r s t r u c t u r e shown i n F i g u r e 1; that i s , macrovoid-free porous hollow f i b e r s with an external porous skin. The nascent f i b e r was sampled and sectioned at several points in the manufacturing process: the spot at which the spinning solu t i o n enters the c o a g u l a t i o n bath; the bottom of the bath (75 cm depth), where the highly swollen p l a s t i c i z e d f i b e r travels through two guides; and at the c o l l e c t i n g spool (see Reference 3, Figure 7). The sections were quickly removed from the bath and placed i n a cold dry chamber, 2°C or less, where s t i f f n e s s was achieved by bringing the h i g h l y s o l v e n t - p l a s t i c i z e d f i b e r near i t s g l a s s t r a n s i t i o n temperature. Note that the Τ of p o l y s u l f o n e i s ~ 180°C and that the Τ of the PVP i s - 160°C. Considerable depression i n these T s preval.1 when the f i b e r segments c o n t a i n l a r g e amounts of s o l v e n t . Therefore, the morphology of the nascent f i b e r may be altered i f i t i s kept at temperatures above i t ' s depressed Τ . Subtle damage i n the morphology of the s u b s t r u c t u r e can occur a u r i n g the s p i n n i n g procedure due to defective equipment. Analysis of such a case, as w e l l as the formation of porous structures, are shown i n the scan ning electron micrographs (SEM) below. The sample fibers prepared for the SEM were freeze dried to remove the solvent, fractured under l i q u i d n i t r o g e n and subsequently s p u t t e r e d w i t h gold. To a v o i d a l t e r a t i o n of the d e l i c a t e sample, the s p u t t e r i n g time ( u s u a l l y 2 minutes) was cut to f r a c t i o n s of 10 to 20 seconds, w i t h " c o o l i n g " intervals of 40 seconds each. Figure 1 shows an asymmetric macrovoid-free polysulfone hollow f i b e r , spun with the bore off-center, a method that previously (5) proved to be h e l p f u l i n d e t e c t i n g the o r i g i n of i r r e g u l a r i t i e s i n the s u b s t r u c t u r e of such f i b e r s . F i g u r e 2 shows p a r t of a c r o s s s e c t i o n of the f i b e r . Two p a r a l l e l d e f e c t s , " t r a i l s , " s t r e t c h i n g along the f i b e r are denoted with arrows i n the micrograph. Sections of the f i b e r severed i n the spinning l i n e , between the coagulation bath guides, show that s u b s t a n t i a l damage occurred at the f i b e r ' s surface as a result of excessive f r i c t i o n of the nascent f i b e r with the f i r s t guide, F i g u r e 3. The alignment of the guides was sub sequently corrected to y i e l d defect-free hollow fibers. S e c t i o n s of the f i b e r ( F i g u r e 4) severed at the bottom of the coagulation bath, 75 cm below the spinneret, after less than a one second c o a g u l a t i o n p e r i o d , p r o v i d e i n s i g h t to the f i r s t stages of the phase i n v e r s i o n . Non-uniform r a t h e r l a r g e c l o s e d c e l l s are formed as a result of solvent depletion and phase separation. Most of the PVP (75%) i s s t i l l i n the f i b e r i n the f i n a l stage. The result i s the delicate uniform structure shown i n Figures 1 and 2, where tVe PVP and the solvent are completely depleted.

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


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Figure 1:

Asymmetric macrovoid-free polysulfone hollow f i b e r [The f i b e r was spun with bore off-center to detect i r r e g u l a r i t i e s i n the substructure (5)].

Figure 2:

C r o s s - s e c t i o n of asymmetric p o l y s u l f o n e hollow f i b e r . Two p a r a l l e l " t r a i l s " stretched along the f i b e r skin are the r e s u l t of damage i n the s u r f a c e s u b s t r u c t u r e (see F i g u r e 3).



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Figure 3:

Surface view of the f i b e r severed after the run through a guide i n the coagulation bath. The apparent damage i s a r e s u l t of e x c e s s i v e f r i c t i o n of the nascent f i b e r with the guide, (and i s shown i n Figure 2 as two stipes on the f i b e r surface).


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Figure 4:

Cross-section of nascent f i b e r after 1 second coagulation period. (The f i b e r was severed 15 cm under the spinneret i n the coagulation bath).


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Double-Layered Membranes for Gas Separation The introduction of the Prism separator by the Monsanto Company has been a major breakthrough r e g a r d i n g the p r a c t i c a l i t y of membrane technology for gas separation processes. The Prism separator em ployees an asymmetric h o l l o w f i b e r membrane whose s k i n i s the s e p a r a t i n g e n t i t y (same as i n asymmetric membranes f o r r e v e r s e osmosis). To e l i m i n a t e the p o s s i b i l i t y of gas feed mixture flow through the skin's i m p e r f e c t i o n s ( i n t h i s regard gas s e p a r a t i o n processes are much more v u l n e r a b l e than l i q u i d s e p a r a t i o n s ) the Prism membranes are coated with u l t r a t h i n layers of rubbery polymers which plug a l l i m p e r f e c t i o n s . T h i s u l t r a t h i n coating i s prepared from a highly permeable polymer such as s i l i c o n e rubber. The concept of the P r i s m membranes i s s i m i l a r to that of reverse osmosis asymmetric membranes and d i f f e r s s u b s t a n t i a l l y from the p r i n c i p l e s that led to the successful development of thin layer composite membranes f o r reverse osmosis. In the l a t t e r , a t h i n layer consisting of two p r i n c i p a l zones (6) i s deposited on a highly porous s u b s t r a t e . The f i r s t zone i s the s e p a r a t i n g l a y e r which faces the feed l i q u i d . The second zone i s a g u t t e r - l a y e r ( i n t e r mediate layer) which channels the permeate into the substrate pores; thus making use of the entire surface area of the membrane (without the gutter zone the p r a c t i c a l surface area w i l l approach the cumula t i v e value of the substrate's pores s u r f a c e area, which commonly ranges between 1% and 14% of the t o t a l membrane area). The p r i n c i pal advantages of the thin f i l m composite are as follows: 1) There is no need to spin or cast a complex, defect-free asymmetric struc ture from the polymer which displays the desired separation charac t e r i s t i c s . In fact, many polymers cannot be cast and spun to y i e l d this structure. 2) Numerous "exotic" and expensive polymers can be f e a s i b l y employed as an u l t r a t h i n s e p a r a t i n g l a y e r , because the q u a n t i t y of polymer that i s r e q u i r e d i n the d e p o s i t i o n of such a l a y e r (~ l y m ) i s m a r g i n a l when compared to that of the porous s u p p o r t i n g m a t e r i a l . For the l a t t e r , i n most i n s t a n c e s , p o l y s u l f o n e , polypropylene, or g l a s s porous supports (100 to 200 μ m thickness) can be adequately employed. U n l i k e the t h i n f i l m composite membranes that have been s u c c e s s f u l l y employed i n reverse osmosis (for example, NS-100 and PA-300) and which are composed of polysulfone porous supports coated w i t h t h i n f i l m polyamide l a y e r s (6), the d o u b l e - l a y e r composite membranes, that have been c o n s t r u c t e d i n our l a b o r a t o r i e s (see Figure 5), consist of two different polymers deposited on a highly porous glassy polymer (for example, polysulfone). The top layer i s the s e p a r a t i n g e n t i t y , u s u a l l y a g l a s s y polymer, such as poly (2,6 dimethyl-1,4-phenylene oxide), PPO, w h i l e the second l a y e r i s a highly permeable rubbery polymer, such as poly(dimethyl siloxane), which serves as a "sink" and a gutter e n t i t y f o r the permeate which i s channelled to s u r f a c e pores of the support. As shown i n F i g u r e 5, the double l a y e r can be as t h i n as 1 μ m and the s e p a r a t i n g l a y e r can be e a s i l y d e p o s i t e d to d i s p l a y a t h i c k n e s s of 0.2 μ m or l e s s . The membrane performance depends upon the relationship between the thicknesses of the sublayers, the porosity of the substrate and the f r a c t i o n of imperfections i n the s e p a r a t i n g l a y e r s . Q u a n t i t a t i v e correlations among these components w i l l be reported elsewhere (1).



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Cross-section of double-layered composite membrane: the top layer, A, i s a dense poly(l,6-dimethyl-l,4-phenylene oxide), the i n t e r m e d i a t e l a y e r , B, i s made of p o l y ( d i methyl s i l o x a n e ) and the support, C, i s a porous 150 m polysulfone support. The membrane displays a separation f a c t o r of 4.1 toward an O 9 / N 2 a i r mixture and o v e r a l l oxygen p e r m e a b i l i t y of 65 χ 1 0 ~ cm versus cm/cm · seconds-cm Hg. 1 0

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N e v e r t h e l e s s , i t i s apparent that minor damage to the s e p a r a t i n g layers w i l l not cause a t o t a l collapse of the membrane performance. Severe damage to the s e p a r a t i n g l a y e r (> 2% s u r f a c e d e f e c t s ) w i l l reduce the separation a b i l i t y of the membrane to that of the i n t e r mediate layer. Therefore we have further modified this layer (1) to y i e l d better separation and to increase i t ' s inertness. As such, i f the top layer i s damaged, the intermediate layer can be recoated and the membrane's i n i t i a l performance r e s t o r e d , even i n the module. The separation of oxygen and nitrogen i s displayed i n Figure 6. The curves show the enrichment of oxygen plotted versus applied operat ing pressure for three composite membranes made of: poly(dimethyl s i l o x a n e ) , PDMS, m o d i f i e d PDMS (1_) and a d o u b l e - l a y e r t h i n f i l m composite membrane composed of PPO coated on the m o d i f i e d PDMS layer. The membrane's permeability to oxygen i s close to (actually, s l i g h t l y higher than) that of a dense, i s o t r o p i c PPO membrane, thus indicating that the entire surface area of the membrane i s u t i l i z e d , d e s p i t e the f a c t that the e f f e c t i v e s u r f a c e area of the porous polysulfone support i n this instance i s not more than 20%. X-Ray D i f f r a c t i o n Studies on Reverse Osmosis Hollow Fiber Membranes Some of the best polymer materials for use as reverse osmosis mem branes are s e m i - c r y s t a l l i n e polymers; that i s , materials that con s i s t of amorphous and c r y s t a l l i n e regions. The nonpermeable c r y s t a l l i n e domains are thermodynamically the more stable of the two regions. Therefore, the permeability of polymer membranes depends upon the volume f r a c t i o n of the amorphous regions. However, poly mers that e a s i l y c r y s t a l l i z e display an inherent disadvantage; that i s , even i f the membrane processing method minimizes the c r y s t a l l i n e f r a c t i o n , the membrane's amorphous regions w i l l s t i l l have a tendency to c r y s t a l l i z e i n the s o l i d s t a t e over a long p e r i o d of time. This time period w i l l be shortened i f enhancement of polymer chain mobility prevails as a result of operating conditions, such as p l a s t i c i z a t i o n by the permeate combined w i t h exposure to s t r e s s . The p o s s i b i l i t y of stress-induced c r y s t a l l i z a t i o n i n s e m i c r y s t a l l i n e hollow f i b e r membranes should be a major concern. Most melt and wet spun hollow fibers develop l o n g i t u d i n a l orientation associated with the extensive draw of the nascent f i b e r during the spinning process. The fast coagulation of the spun f i b e r does not permit establishment of thermodynamic equilibrium of the "frozen" f i b e r matrix. Relaxa t i o n can perhaps be achieved by annealing the membrane at tempera tures near Τ . A n n e a l i n g above t h i s temperature o f t e n leads to rapid c r y s t a l l i z a t i o n , thus defeating the purpose of obtaining high f l u x membranes v i a reduced c r y s t a l l i z a t i o n . Such i s the case f o r example w i t h c e l l u l o s e t r i a c e t a t e , CTA. Hollow f i b e r s of t h i s polymer are melt spun w i t h c e r t a i n a d d i t i v e s which reduce the spinning temperature far below the melting point range of pure CTA (292 to 314°C) (3). The hot spun thread emerging from the spinneret i s quenched i n a water bath or i s allowed to cool quickly below Τ . Thus, the polymer, which i n slow p r e c i p i t a t i o n from solution y i e l a s ~90% c r y s t a l l i n i t y , i f spun as d e s c r i b e d would y i e l d down to ~10% c r y s t a l l i n e domain only. Of course, this led to the development of e f f e c t i v e CTA hollow f i b e r membrane permeators, such as Dow Chemi cal's DOWEX R0-20K. However, some severe fouling problems (that i s ,


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Oxygen enrichment curves versus pressure. (Feed mixture: a i r at 23°C). Composite membranes: A. PDMS B. Modi f i e d PDMS C. Double-layered PPO and PDMS modified.


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decline i n productivity) of the CTA hollow f i b e r membrane drew our a t t e n t i o n to the d u r a b i l i t y problems of s e m i c r y s t a l l i n e h o l l o w f i b e r s , especially to those made of c e l l u l o s e esters. The f i b e r s i n the permeators are exposed to s t r e s s , p r e s s u r e and permeate p l a s t i c i z a t i o n and they are indeed t a r g e t e d f o r f o u l i n g problems as a result of stress-induced c r y s t a l l i z a t i o n . Some examples of an ex tensive program that deals with the i d e n t i f i c a t i o n , v i a X-ray d i f f r a c t i o n analysis, of progressive c r y s t a l l i z a t i o n of hollow f i b e r s are discussed below. X-Ray D i f f r a c t i o n Analysis. Three materials were examined by X-ray d i f f r a c t i o n techniques for c r y s t a l l i n i t y : a melt-spun CTA hollow f i b e r for sea water desalination by reverse osmosis, a dry-jet wetspun c e l l u l o s e acetate hollow f i b e r , and an ethyl c e l l u l o s e hollow f i b e r (spun i n our laboratory). The f i b e r s were inspected before and after exposure to d i f f e r e n t degrees of longitudinal stress. A l l X-ray data was collected on bundles of f i b e r s . Bundles were placed i n 1.0 mm I.D. X-ray grade thin glass c a p i l l a r i e s or mounted dry on punch card holes f o r i r r a d i a t i o n . For samples to be examined i n l i q u i d media, the c a p i l l a r i e s were sealed with 5-minute cure epoxy. A l l samples were irradiated with Cu Κ radiation (1.542 A wavelength) from a Norelco 1 KV generator. D i f f r a c t i o n patterns were collected on f l a t packs of I l f o r d type G i n d u s t r i a l X-ray f i l m or Kodak DEF-5 X-ray f i l m i n an evacuated pinhole camera. To keep r e l a t i v e inten s i t i e s of d i f f r a c t i o n spots approximately equal, exposures were 24 to 25 hours for unstressed and l i g h t l y stressed samples, and 12 to 14 hours for highly stressed samples. Samples examined were: 1) Cellulose triacetate hollow f i b e r s , CTAHF, as spun, i n d i s t i l l e d water; 2) CTAHF s t r e t c h e d f o r e i g h t days i n water with a 3.57 gram weight (22 p s i stress), i n d i s t i l l e d water; 3) CTAHF a i r d r i e d f o r two weeks, i n an unsealed c a p i l l a r y ; 4) CTAHF stretched for two weeks i n water with a 15.77 gram weight (100 p s i stress) i n d i s t i l l e d water. R e s u l t s and D i s c u s s i o n . Since the melt s p i n n i n g of the f i b e r s approximate ( w i t h some q u a l i f i c a t i o n s ) a s o l u t i o n process, i t i s assumed that any observed c r y s t a l l i n i t y w i l l be i n the r e g i o n s of CTA II l a t t i c e . The CTA II c r y s t a l l i n e l a t t i c e possesses an a n t i p a r a l l e l packing of polymer chains due to the chain fold mechanism of c r y s t a l growth from solution (7)· For comparison purposes Figure 7 shows the d i f f r a c t i o n p a t t e r n from a s m a l l bundle of CTA I I fibers. In this measurement, the camera f i l m to sample distance f o r the CTA I I was roughly the same as f o r the c e l l u l o s e t r i a c e t a t e hollow f i b e r CTAHF. These d-spacings, which were calculated but are not r e p o r t e d here, compare w e l l to those from a c a l i b r a t e d sample (7). F i g u r e 8 shows the d i f f r a c t i o n p a t t e r n f o r the as-spun and w a t e r - s t o r e d CTAHF. The two c o n c e n t r i c rings i n d i c a t e that some c r y s t a l l i n i t y i s present i n the sample, but no o r i e n t a t i o n w i t h respect to the f i b e r macrostructure i s seen. The d-spacings of the r i n g s c o r r e l a t e w e l l w i t h the d-spacing of the s t r o n g e s t CTA I I r e f l e c t i o n s . The only problem i n the comparison occurs at the inner edge of the inner-most ring (highest d-spacing) where a substantial amount of amorphous scattering obscures the exact edge of the ring.



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D i f f r a c t i o n pattern of CTA II f i b e r bundle.

D i f f r a c t o g r a m of as-spun and water-stored f i b e r ( f o r reverse osmosis).

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The d i f f r a c t i o n pattern of the l i g h t l y stretched i n water (22 p s i f o r 8 days) CTAHF i s shown i n F i g u r e 9. From the g e n e r a l l y weaker intensity and s p e c i f i c a l l y from the weaker and narrowed inner r i n g , i t can be deduced that the c r y s t a l l i n i t y i s l e s s i n t h i s sample than i n the o r i g i n a l (Figure 8). The r e l a t i v e l y unaffected outer ring i s probably due to the meridional r e f l e c t i o n ( i n comparison, the CTA II data indicates a long spacing along the chain axis). The observed reduction of c r y s t a l l i n i t y i s contrary to the expected c h a i n and c r y s t a l l i t e o r i e n t a t i o n w i t h respect to the f i b e r a x i s , which normally results from f i b e r stretching. Figure 10 shows the dry d i f f r a c t i o n pattern of the CTAHF. The sample was a i r dried at ambient temperature with no applied stress. Discrete r e f l e c t i o n s appear along the equatorial l i n e underneath the g e n e r a l r i n g p a t t e r n . The appearance of d i s c r e t e r e f l e c t i o n s i s expected, as the removal of water from the m a t r i x would a l l o w the polymer chains to collapse into more ordered regions. Most notably, there appears to be b e t t e r o r i e n t a t i o n of c r y s t a l l i t e s w i t h the f i b e r axis in comparison to the o r i g i n a l sample. The d i f f r a c t i o n pattern of the highly stretched i n water CTAHF i s shown i n Figure 11 (calculated f i b e r stress on this sample i s 100 p s i ) . From the i n c r e a s e d i n t e n s i t y of both r i n g s i t appears that the overall c r y s t a l l i n i t y i s higher, but there s t i l l does not seem to have been much o r i e n t a t i o n of the c r y s t a l l i t e s i n the f i b e r direction. The r e l a t i v e intensity of the two rings i s the same as occurs i n the o r i g i n a l sample, w i t h the inner r i n g much s t r o n g e r than the outer, rather than as i n the l i g h t l y stretched sample where the inner r i n g i s d i f f u s e d . An i n t e r e s t i n g f e a t u r e about t h i s diffractogram i s a broad link-sausage-like intensity pattern around the outer d i f f r a c t e d r i n g . T h i s r e g u l a r p a t t e r n i n d i c a t e s the presence of some order i n the sample. Closer inspection of Figures 9 and 11 r e v e a l the s a u s a g e - l i k e p a t t e r n i s a l s o present i n the outer r i n g s of the p l a i n water s t r e t c h e d sample and the o r i g i n a l sample patterns, but i s less d i s t i n c t . The fibers shown i n Figures 9 and 11 do e x h i b i t , upon d r y i n g , o r i e n t a t i o n which i s s i m i l a r to that shown i n Figure 10, but with much higher intensity. In addition to the CTA hollow f i b e r s , d i f f r a c t i o n patterns from dry-jet wet-spun c e l l u l o s e acetate and ethyl c e l l u l o s e fibers were a l s o c o l l e c t e d . The d i f f r a c t i o n measurement was conducted w i t h bundles of two f i b e r s i n the case of c e l l u l o s e a c e t a t e (CA) and a s i n g l e f i b e r f o r e t h y l c e l l u l o s e (EC), because of the l a r g e f i b e r s i z e and wall thickness. The d i f f r a c t i o n measurement for these two samples was conducted on dry f i b e r s . F i g u r e 12 shows the d i f f r a c t i o n p a t t e r n from the CA sample. Though t h i s p a t t e r n a l s o e x h i b i t s two c o n c e n t r i c r i n g s as w i t h the CTAHF, only the inner rings have the same d-spacing. With this sample the second ring dspacing corresponds to the weaker e q u a t o r i a l and second l a y e r r e f l e c t i o n s of CTA II; the strong t h i r d layer meridional r e f l e c t i o n i s gone. I t appears as i f c r y s t a l l i n i t y i n t h i s sample i s only s l i g h t l y less than i n the CTAHF sample, but most notably c r y s t a l l i t e o r i e n t a t i o n w i t h the f i b e r a x i s i s c o n s i d e r a b l y l e s s . F i g u r e 13 shows the EC d i f f r a c t i o n p a t t e r n . Here we see three d i f f r a c t i o n r i n g s , which are much sharper and accompanied by l e s s amorphous s c a t t e r i n g than i n the other hollow f i b e r s . T h i s suggests that c r y s t a l l i n i t y i s much higher i n this sample, perhaps as much as 40%,



14.

CABASSO

Figure 9:


Diffractogram of CTAHF shown i n Figure 8 after to 22 p s i s t r e s s f o r 8 days.

317

exposure

Figure 10: The d i f f r a c t i o n p a t t e r n of CTAHF, shown i n F i g u r e 8, after drying (without stress). Orientation i n the f i b e r i s c l e a r l y seen.



318


MEMBRANES

Figure 11: D i f f r a c t o g r a m of the f i b e r shown i n F i g u r e 8 a f t e r exposure to 100 p s i stress f o r a period of two weeks.

Figure 12: D i f f r a c t o g r a m of d r i e d c e l l u l o s e a c e t a t e hollow membrane.


fiber

14.

CABASSO

319


but again the continuous rings indicate l i t t l e , orientation along the f i b e r axis.

i f any,

crystallite


Interim Conclusions. The X-ray study on the CTA hollow fibers was undertaken to see i f gross m o r p h o l o g i c a l changes p r e v a i l under working c o n d i t i o n s that i n c l u d e s t r e s s and considerable pressure. The d i f f r a c t i o n patterns of wet CTAHF demonstrate that morphological changes i n the polymer and i t s water c l u s t e r s do e x i s t . The c o l lapse of the m i c r o s t r u c t u r e of the melt-spun hollow f i b e r upon drying follows a "memory pattern" and orientation i s displayed. I t i s most surprising that the dry-jet wet-spun f i b e r do not store any r e c o l l e c t i o n of the s p i n n i n g o r i e n t a t i o n (draw) which i s substant i a l l y high (these f i b e r s were spun with a draw r a t i o n of 7:1). Separation of Aqueous Alcohol Solutions Through Ion-Exchange Hollow Fiber Membranes The separation of organic l i q u i d mixtures through ion-exchange membranes was r e c e n t l y reported from our l a b o r a t o r i e s (8). I t was shown that the nature of the counter-ion i n such membranes a f f e c t s substantially the membrane permeability and s e l e c t i v i t y . Thus, new concepts f o r the s e p a r a t i o n of o r g a n i c l i q u i d mixtures have been conceived, the p r i n c i p a l of which i s that f o r each ion-exchange membrane a myriad of v a r i a t i o n s can be e a s i l y formed, j u s t by exchanging and replacing the counter-ion when needed. The i n t e r a c t i o n of the counter-ion with the permeate determines, to a large extent, the membrane's transport properties. This concept has been demonstrated i n our laboratories as an e f f e c t i v e tool for the separation of alcohols from their aqueous solutions. Some results of this ongoing study are delineated below. Experimental. The hollow f i b e r membranes used for this study were N a f i o n 811, which i s a copolymer of p o l y s u l f o n y l f l u o r i d e v i n y l ether and polytetrafluoroethylene, and sulfonated and/or quaternated derivatives of polyethylene (kindly supplied by Dr. E. Korngold from Ben G u r i o n U n i v e r s i t y i n I s r a e l ) . The aqueous a l c o h o l s o l u t i o n s studied thus far are those of methanol, ethanol and 2-propanol. The s e p a r a t i o n s were accomplished v i a the p e r v a p o r a t i o n process as described i n Reference 9. Counter ions were replaced i n the hollow f i b e r by soaking the permeator f o r twenty four hours i n 1 molar s o l u t i o n s of the p e r t i n e n t ions. For example, experiments were conducted with Na as a counter ion. When this set of experiments was finished, the sodium was exchanged by L i etc. Each data point shown i n F i g u r e 14 c o n s i s t s of 6 to 10 measurements taken over a time p e r i o d of 8 hours. Re-runs w i t h the v a r i o u s counter ions proved that the i n t r i n s i c p r o p e r t i e s of the membrane remain unchanged and the permeability measurements are reproducible. +

+

R e s u l t s and D i s c u s s i o n . F i g u r e 14 demonstrates the p r i n c i p a l of s e p a r a t i o n through ion-exchange membranes, where the counterion/permeate interactions determine the mass transport properties of the system. In t h i s set of experiments, the feed mixture was an azeotropic composition of 2-propanol and water (88.5/11.5 wt.%). A Nafion hollow f i b e r permeator was used i n pervaporation mode. The



320

M A T E R I A L S SCIENCE O F S Y N T H E T I C M E M B R A N E S

Figure 13: Diffractogram of dried ethyl c e l l u l o s e hollow f i b e r membrane .

Counter-Ion

Figure 14: The e f f e c t of counter-ion on flux and separation factor of a Nafion hollow f i b e r . Feed composition: 2-propanol/water 88.5/11.5 Feed temperature: 19°C a


14.

CABASSO

321


permeator was charged w i t h d i f f e r e n t counter i o n s , r e s u l t i n g i n s u b s t a n t i a l d i f f e r e n c e s i n the mass t r a n s p o r t p r o p e r t i e s of the unit. The mono-valent ion series follows the expected trend; that i s , the higher the hydration number of the ion, the higher the flux. The hydrated L i i o n s w e l l s the f i b e r much more than the C s i o n (Figure 15), which exhibits a rather weak association with the water s h e l l around i t . The f a c t that the 2-propanol h y d r a t i o n power i s much lower than the water molecule also contributes to the rather h i g h s e l e c t i v i t y ; that i s , the membrane i s not s w o l l e n enough to allow the 2-propanol "to s l i p by" the ionic conducting channels, and the hydration power of this alcohol i s too small to compete with the water f o r i n t e r a c t i o n s w i t h l a r g e cesium or even potassium ions. Thus, the rates at which the separation factor increases and perme ation decreases with the change i n counters ions are large indeed. As f o r the m u l t i - v a l e n t i o n s , the Ca i s a s t r o n g l y hydrated i o n , but i t also cross-links the conducting channels and even more so i s the A l . Thus, a s u b s t a n t i a l decrease i n the f l u x , without s i g n i f i c a n t chance i n the s e p a r a t i o n f a c t o r , i s shown f o r the s e r i e s L i , Ca , A l ^ " . These p a t t e r n s , shown i n F i g u r e 14 f o r the azeot r o p i c c o m p o s i t i o n , remain the same i f the feed c o m p o s i t i o n i s reversed, as shown i n Table I (for 5.2 wt.% i n the feed mixture).


+

+

+

1

Table I. The E f f e c t of D i f f e r e n t Counter Ions^on Separation Factor and Flux of Nafion 811 Hollow Fiber (Feed Composition: Water/Isopropyl Alcohol: 12/88 wt.%)

Counter No. Ion

+

1

Li

2

Na

3

K

4

Cs

5

Ca^

6

ΑΙ" "**

*

+

+

+

1

Permeate Composition IPA Water

(g

/m hr ) 2

z

(ml /m

hr

57.4

42.6

10.2

742

816.4

76.6

23.4

25.1

364

383.9

86.2

13.8

46.7

124

128.0

87.5

12.5

54.9

107

110.3

56.0

44.0

9.3

531

586.9

50.0

50.0

7.3

232

260.4

Feed temperature: Separation

Flux Separation Factor

29.0°C p

Factor - (X /Xj.) / ( x / x ) w

w

F

I

where X indicates weight f r a c t i o n , subscripts I and W represent isopropanol and water and superscripts Ρ and F represent permeate and feed.




322

Figure 15: The e l o n g a t i o n of a N a f i o n hollow f i b e r with d i f f e r e n t counter ions versus 2-propanol c o n c e n t r a t i o n ( i n i t s aqueous solution).



14.

CABASSO


323

The swelling of the f i b e r i n i t ' s d i f f e r e n t counter-ion v a r i a tions i s i n accord with the mass transport data. The maxima around 20 mole % 2-propanol i n the feed mixture indicates the presence of synergism r e l a t e d to h y d r o p h i l i c / h y d r o p h o b i c contributions. The l a t e r results from the isopropyl s i t e s of the alcohol and suggests s o l u b i l i t y of the 2-propanol i n the amorphous non-ionic regions of the membrane. The v a r i o u s m o d i f i c a t i o n s of the p o l y e t h y l e n e ion-exchange hollow f i b e r membranes a l s o proved that here we have a workable concept. From this family of special interest membranes, results were obtained f o r anion-exchange hollow f i b e r membranes. Separation f a c t o r s of 83, 18 and 3.5 were obtained f o r feed mixtures of 2propanol, ethanol and methanol, respectively (each feed mixture was composed o f 20 wt.% a l c o h o l and the s e p a r a t i o n was conducted a t 23°C). I n t e r i m C o n c l u s i o n . The use of ion-exchange membranes f o r the separation of aqueous organic mixtures i s a novel solution f o r many d i f f i c u l t separations. I t provides a vast number of p o s s i b i l i t i e s based on r a t h e r few membrane m a t r i c e s . Q u a n t i t a t i v e a n a l y s i s of this transport phenomenon i s to be discussed elsewhere (9). Literature Cited 1.

2.

3.

4. 5. 6. 7. 8. 9.

Lundy, K. "Separation of Gas Mixtures via Multi-Layered Composite Membrane" MS Thesis, Chem. Dept., State Univ. of N.Y. ESF Syracuse, N.Y. (1984). Gardiner, E. "Cellulose Ester Blends, Ultrathin Deposits and Membranes" part of Ph.D. Thesis, Chem. Dept., State Univ. of N.Y. ESF Syracuse (1984). Cabasso, I. "Hollow Fiber Membranes" In Kirk-Othmer Encycl. of Chem. Tech.; Grayson, M. and Eckroth, D., Eds.; John Wiley & Sons, Inc.: New York, 1980, 12, 492. Cabasso, I.; K l e i n , E . ; Smith, J.K. J. Appl. Polym. S c i . , 1976, 20, 2377. Ibid 1977, 21, 165. Cabasso, I. In "Synthetic Membranes, Volume I," Turbak, A. F . , Ed.; ACS Symposium Series No. 153, 1981. Roche, E . ; Chanzy, H.; Boudelle, H.; Marchessault, R. M.; Sundararajan, P. Macromolecules, 1978, 11, 86. Cabasso, I. Ind. Eng. Chem. Prod. Res. Dev. 1983, 22, 313. Cabasso, I.; L i u , Z.; Korngold, E., to be published.

RECEIVED September 7, 1984


Materials Science of Synthetic Membranes - American Chemical

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