6 Plasma-Polymerized Membranes of 4-Vinylpyridine in Reverse Osmosis Downloaded by PENNSYLVANIA STATE UNIV on July 2, 2012 | http://pubs.acs.org Publication Date: January 1, 1985 | doi: 10.1021/bk-1985-0281.ch006
S. SUZUKI, T. OGAWA, and N. HITOTSUYANAGI Department of Chemistry, Science University of Tokyo, Shinjuku-ku, Tokyo 162, Japan The reverse osmosis membranes, prepared by plasmapolymerization of 4-vinylpyridine using both 50 Hz low-frequency (Lf) and 13.5 MHz (Rf) generator were studied in terms of their membrane characteristics, morphologies, properties of polymerized substances and life. The membrane characteristics were evaluated for both inorganic and organic solutes and compared with those of cellulose acetate membranes. The surface and the cross-section of membranes were observed by scanning electron microscopy and the effects of pHvalues and temperature of feed solution on the membrane life were investigated. The formation of nitril group by glow discharge was observed.
There are two methods of preparing reverse osmosis membranes by plasma polymerization. In the first method the plasma-polymerized substance precipitates on the substrate surface to form a layer, while in the second method the plasma polymerization takes place directly on the top of the substrate which is placed on one of two electrodes (1-5). Not many studies have been reported so far on the choice of the method. In the present study the membrane preparation using 4-vinylpyridine and the characterization of the membrane properties were studied applying the second method for the membrane formation. Further attempt was made to use a luminous tube transformer to generate low frequency (50 Hz) instead of the high frequency generator (13.5 MHz). The properties of both low frequency (50 Hz) and high frequency (13.5 MHz) membranes were compared in terms of their transport properties, morphologies and infrared spectroscopies. Furthermore, the effects of glow discharge on millipore substrate were investigated.
Experimental Reactor. The reactor used is shown in Figure 1. It consists of two 47-mm-diameter stainless steel electrodes enclosed in a 90-mm-
0097-6156/85/0281-0069$06.00/0 © 1985 American Chemical Society
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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Figure 1.
Schematic of experimental apparatus.
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
6. SUZUKI ET AL.
Plasma-Polymerized Membranes of 4- Vinylpyridine
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diameter reactor, the volume of which is 1500 ml. The distance between electrodes was chosen to 15 mm and a substrate was fixed to one of the electrodes with a set of teflon frames. The plasmapolymerized membranes (PPM) were made by using both 50 Hz and 13.5 MHz generators. The 50 Hz low frequency (Lf) generator was a luminous tube transformer (Matsushita Electric Co. Ltd.) and its primary voltage was adjusted by a slide rheostat. The 13.5 MHz radio frequency (Rf) generator was a radio frequency transmitter (Nippon Kohshuha Co. Ltd. Type SKN-05P, power input 20 W ) . These were connected to the electrodes through an impedance matching network. Chemicals. Reagent grade 4-vinylpyridine was purified for plasma polymerization by means of distillation under a reduced pressure. As a substrate Millipore filter (0.025) was used. Operating Conditions. All PPM membranes were pressurized at 80 kg/ cm2 for 2 h prior to reverse osmosis experiment. The operating conditions were as follow: operating pressure; 80, 70, 60 and 40 kg/cm 2 , flow rate; 400 ml/min, feed solution temperature; 25 ± 1°C, effective membrane area; 9.26 cm, feed concentration; 0.01 mol/1. Data Processing. The solute rejection R, the volume flux Jv and the solute flux Js were measured. The hydraulic permeability coefficients Lp was obtained from the pure water permeation rate. The reflexion coefficient was determined by extrapolating the rejection R to the infinite pressure. The solute permeability coefficients P was caluculated by the equation derived from the Spiegler-Kedem equation (6). To evaluate the polymer deposition rate, an aluminum foil was used as substrate, on which the increase of weight was measured. The effect of pH of the feed solution on the membrane performance was studied in the following way: the membrane performances were measured before and after the immersion of membranes in various pH solutions for 100 h. The pH value of the solution was adjusted by Britton-Robinson buffer solution and NaOH solution. Scanning Electron Microscopy. The morphological observation of the surface, the cross-section of membranes and the effects of glow discharge on Millipore filters were carried out by an ALPH-10 scanning electron microscope (Akashi Seisakusho Co. Ltd.). The samples were coated under high vacuum with carbon and gold. In order to observe the cross-section the^membranes were fractured after hardening in liquid nitrogen. Infrared Spectroscopy. The infrared spectra were obtained using a Japan Spectroscopic Model DS-403G, in which the samples were prepared by plasma polymerization on KBr disk. The observation of the surface of membrane by an attenuated reflection (ATR) technique was performed using an IR435-ATR-2A (Shimazu Co.). Results and Discussion Effect of effect of in Figure rejection
the the 2. and
Deposition Pressure on Membrane Performances. The deposition pressure on membrane performances is shown Up to the deposition pressure of 1.0 torr, both water flux increase. Then the former decreases while
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
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the l a t t e r i n c r e a s e s . I t i s n o t e d t h a t r e j e c t i o n e x h i b i t s a maximum value. The membrane c o r r e s p o n d i n g to t h e maximum shows p r o p e r t i e s s u c h a s r e j e c t i o n of 83 - 96% and w a t e r f l u x of 0 . 4 - 0 . 5 m^/m 2 .day (0.1% NaCl 0 . 1 i n f e e d s o l u t i o n and a t 40 k g / c m 2 ) . The e f f e c t o f t h e d e p o s i t i o n p r e s s u r e on t h e d e p o s i t e d q u a n t i t y i s shown i n F i g u r e 3. The d e p o s i t e d q u a n t i t y a l s o p a s s e s through a maximum a t a p r e s s u r e o f about 0 . 4 t o r r . However, t h e p r e s s u r e s , a t which maxima are e x h i b i t e d , are not i d e n t i c a l , therefore i t i s considered that t h e r e i s no c o n n e c t i o n b e t w e e n t h e r e j e c t i o n and t h e d e p o s i t e d quantity. I t i s worth n o t i c i n g t h a t t h e r e j e c t i o n h a s a maximum v a l u e and a f t e r t h e maximum t h e membrane l o s e s i t s f u n c t i o n g r a d u a l l y and a c c o r d i n g l y t h e s e l e c t i v i t y d e c r e a s e s . Comparison of t h e Membrane Performances of CAM and PPM. In T a b l e I t h e s e l e c t i v i t y o f t h e plasma p o l y m e r i z e d membrane by u s i n g low f r e q u e n c y (50 Hz) g e n e r a t o r (Lf-PPM) i s shown i n c o m a r i s o n w i t h t h a t o f t h e M a n j i k i a n t y p e c e l l u l o s e a c e t a t e membrane (CAM). The r e f l e x i o n c o e f f i c i e n t s o f Lf-PPM membrane f o r Na2S0i+, g l u c o s e and NaCl, a r e a l m o s t t h e same v a l u e s a s t h o s e o f CAM, however i n c a s e o f o r g a n i c s o l u t e s t h e s e l e c t i v i t y of Lf-PPM i s b e t t e r t h a n t h a t of CAM, The same c o n c l u s i o n can be d e r i v e d from t h e s o l u t e p e r m e a b i l i t y c o e f f i c i e n t P , w h i c h i s a l s o shown i n T a b l e I . The r e a s o n why t h e PPM membrane shows b e t t e r s e l e c t i v i t y f o r o r g a n i c s o l u t e s than CAM membrane, m i g h t be a t t r i b u t e d t o t h e weak h y d r o p h i l i c i t y o f PPM membrane. Comparison of t h e Membrane Performances of Lf- and Rf-PPM Membranes. As t h e t r a n s p o r t p r o p e r t i e s of b o t h L f - and Rf-PPM membranes w i t h r e s p e c t to i n o r g a n i c and o r g a n i c s o l u t e s a r e shown i n Table I I and III. In c a s e of inorganic s o l u t e s the v a l u e s of r e f l e x i o n c o e f f i c i e n t of Rf-PPM membrane a r e l a r g e r and t h o s e o f t h e s o l u t e p e r m e a b i l i t y c o e f f i c i e n t P , s m a l l e r than Lf-PPM membrane. While i n
Table I .
Comparison of Transport C o e f f i c i e n t s
i n PPM-Lf and CAM
P P M(Lf) a (
)
P (cm/sec)
Na2S0i| Glucose NaCl n-Ci+HgNH n-C3H7CHO n-CH 3 C0C2H 5 n-C3H7COOH n-Ci+HsOH C6H5OH L
P,PPM(Lf): 1
P CAM
(
)
P (cm/sec)
xlO"5
Solute
L
CAM a
5 5x
-
0.99 0.97 0.95 0.81 0.79 0.73 0.71 0.57 0.34
0.12 0.92 2.54 5.14 6.33 8.90 8.07 8.56 29.91
xlO"5 0.99 0.99 0.99 0.66 0.45 0.46 0.47 0.42 -0.27
4.9*10-6day(0.1% NaCl in feed solution and at 40 kg/cm 2 ). The transport property of the Lf-PPM membrane was discussed in terms of reflexion coefficients, the hydraulic permeability coefficients and the solute permeability coefficients obtainable for those membranes and compared with that of the Manjikian type cellulose acetate membrane. The performances of both Lf- and Rf-PPM membranes were compared. The selectivity of the Rf-PPM membrane for inorganic solutes was superior, on the other hand that of the Lf-PPM membrane was superior for organic solutes. The durability of both Lf- and Rf-PPM membrane in the long term operation and at different pH values and temperatures of feed solution was studied. They did not show an excellent durability and therefore it seems necessary to find an appropriate way of membrane reinforcement. The spherical polymers between 0.07 - 0.4 urn were observed on the surface of both plasma-polymerized membranes. The thickness of plasma polymerized layer ranged from 0.A to 0.6 ym. By infrared spectroscopy it was concluded that the pyridine ring decomposed by glow dicharge and the nitrile group formed. The surface layer of millipore substrate was destroyed by glow discharge and the interior of the membranes was exposed. Acknowledgments The authors wish to acknowledge Prof. Drs. Matsuura of NRC, Ohya of Yokohama University and Mrs. E. Tsukuda of the Institute of Physical and Chemical Research, for their valuable advices. Literature Cited 1.
2. 3. 4. 5. 6. 7.
Wydeven, Th; Hollahan R., in "Techniques and Applications of Plasma Chemistry"; Hollahan, J. R. and Bell, A. T., Eds.; John Willey & Sons; New York, 1974; p. 215. Buck, K. R.; Davar, V. K. Br. Polym. J. 1970, 2, 238. Yasuda, H.; Lamaze, C. E. J. Appl. Polym. Sci. 1973, 17, 201. Bell, A. T.; Wydeven, Th.; Johnson, C. C. J. Appl. Polym. Sci. 1975, 19, 1911. Peric, D.; Bell, A. T.; Shen, M. J. Appl. Polym. Sci. 1977, 21, 2661. Spiegler, K. S.; K edem, O. Desalination, 1966, 1, 311. Tazuke, S.; Okamura, S. J. Polym. Sci. A-l. 1967, 5, 1083.
In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
82 8.
REVERSE OSMOSIS AND ULTRAFILTRATION "Atlas of Spectral Data and Physical Constants for Organic Compounds"; Grasselli, J. G.; Ritchey, W. M., Eds.; CRC Press; Ohio, 1975, 2nd Ed.; Vol. I, p. 342 and 346.
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In Reverse Osmosis and Ultrafiltration; Sourirajan, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1985.
