Electron Microscopy of Cellulose Acetate Reverse-Osmotic

17 Electron Microscopy of Cellulose Acetate ReverseOsmotic Membranes by Means of the Improved Replication Method

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch017

MASARU KATOH Nissei Sangyo Co., Ltd., S. I. Center, Mori Bldg. 17, Minato-ku, Tokyo, 105 Japan SHO SUZUKI Science University of Tokyo, Department of Chemistry, Faculty of Science, Kagurazaka, Shinjuku-ku, Tokyo, 162 Japan The diameters of the pores of the surface layer of LoebSourirajan-type cellulose acetate membranes have been reported by several authors (1-6). The reported values of the diameters cover the range between 10 Åand 60 Å. For electron microscopic observations, the replication method must be used. In order to obtain the excellently contrasted images the surface of the sample is shadowed with heavy metals in vacuum. In many cases the Pt-Pd alloy has been used as a pre-shadowing metal. But the resolution of the Pt-Pd replica is at the level of about 50 Å, since the size of the evaporated particles is between 20 Åand 40 Å(7, 8, 9). If the pore sizes are in the range of the above -mentioned level, we cannot observe them. By means of the more advanced replication technique using tungsten as a pre-shadowing metal, the electron microscopic observation of both the pores and the morphological changes caused by annealing steps are reported in the present paper. Experimental procedures All experiments were conducted with a Hitachi H-500 type e l e c t r o n microscope, both operated at 100-kV and observed at the m a g n i f i c a t i o n of 100,000 and 200,000. P r e p a r a t i o n of the membranes. The membranes a r e prepared by means'of thë~Manjïkîân s~mëthôd (10). The three kinds of membranes, unannealed, 50°C- and 85°C-annealed ones, a r e ready f o r e l e c t r o n microscopic o b s e r v a t i o n s . T

Samples f o r _ e l e c t r o n microscopy. The membranes a r e t r e a t e d by f r e e z - d r y i n g of the same method as reported by R i l e y , Merten, Gardner (11). The samples immersed i n isopentane are cooled i n l i q u i d n i t r o g e n and then dehydrated i n vacuum. Shadowing_techniques. The p r i n c i p l e s of the present method i s shown schematically i n F i g u r e 1. The evaporation source i s a

0097-6156/81-0153-0247$05.00/0 © 1981 American Chemical Society

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

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N tched-wire flashing method


0.5^0.7+ \

W.Ta.Mo 1.0*

V aperture 0.8*

\ aperture 1.0*

Figure 1. Schematic of evaporating arrangement (in millimeters)

Figure 2.

Use of apertures for shadowing (in millimeters)



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Figure 3. Electron micrograph of evaporated Pt-Pd alloy particles. Carbon particles are shadowed (Xl50,000).

Figure 4. Electron micrograph of evaporated W particles. A carbon particle is shadowed (Xl50,000).



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Figure 5. Electron micrograph of the W preshadowed carbon replica of the surface of an unannealed Loeb-Sourirajantype cellulose acetate membrane (X140,000)

Figure 6. Electron micrograph of the W preshadowed carbon replica of the surface of a membrane annealed for 5 min at 50°C (Xl40,000)

Figure 7. Electron micrograph of the W preshadowed carbon replica of the surface of a membrane annealed for 5 min at 85°C(X140,000)



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a notched-wire, or a tungsten wire of 50 mm i n length and 1.0 mm i n diameter, and i t s middle p o r t i o n i s slimed about 0.7 mm i n diameter. The wire i s s e t i n an ordinary vacuum evaporator of e l e c t r o n microscopy, and an e l e c t r i c power of about 10 V and 60 100 A i s a p p l i e d to i t i n s t a n t l y . As shown i n F i g u r e 2, two more aperture p l a t e s having the hole of 0.8 and 1.5 mm i n diameter are set between the source and s u b s t r a t e to prevent thermal r a d i a t i o n e f f e c t from the evaporation source and to improve shadowing q u a l i t y . With t h i s apparatus, we prepare the one step r e p l i c a , and use tungsten as the pre-shadowing metal. We have compared the r e s o l u t i o n of the tungsten shadowed r e p l i c a with that of the Pt-Pd a l l o y one. The shadowing angle i s 30° and the thickness of carbon f i l m i s about 100 A and that of both W metal and Pt-Pd a l l o y are 30 A. Results_and_discussion Figures 3 and 4 show the h i g h l y magnified e l e c t r o n micrographs of both evaporated tungsten and Pt-Pd a l l o y . The s i z e of Pt-Pd a l l o y evaporated p a r t i c l e s i s at the l e v e l of about 50 100 A and that of tungsten, about 3 - 5 A. So the r e s o l u t i o n of the tungsten pre-shadowed r e p l i c a may be a t the l e v e l of about 15 - 20 A. By means of the one step r e p l i c a with tungsten as pre-shadowing metal, the e l e c t r o n micrograph of the surface of an unannealed membrane i s shown i n F i g u r e 5. In the micrograph we can f i n d the pores whose s i z e s range between 10 and 27 A. The greater part of them i s between 15 - 25 A . The s u r f a c e appears j u s t l i k e c r a t e r and there are a l s o observed some wave-like f e a t u r e s , the s i z e s of which are at the l e v e l of 100 - 500 A. The pores e x i s t uniformly on the s u r f a c e . Figure 6 shows the surface of the 50°C annealed membrane, where we can observe the e x i s t e n c e of c a v i t i e s , the s i z e s of which are between 25 and 50 A. Upon annealing, the f i n e pores as observed i n the unannealed one disappeared and the c a v i t i e s (25 50 A) were dispersed u n i f o r m l y on the s u r f a c e . The features of the surface are very d i f f e r e n t from those of the unannealed one. In the micrograph of the surface of the 85°C annealed membrane (Figure 7), the wave-like features a r e observed, and the s i z e s of the wave range between 25 and 75 A. I t s appearance resembles that of the unannealed one, but the number of pores i s samller than the unannealed one. The pore s i z e s are at the l e v e l about 20 A, and the number of pores i s not so many. As the e f f e c t s of an annealing step on the p r o p e r t i e s of c e l l u l o s e a c e t a t e membranes, the increase of c r y s t a l l i n i t y by means of the X-ray d i f f r a c t i o n method (12, 13) and the changes of pore s i z e s by the BET a d s o r p t i o n method~"(5) have been r e p o r t e d . In our present observations with the unannealed membrane, we can a s c e r t a i n the e x i s t e n c e of a number of pores, which are at the l e v e l of about 10 - 30 A. Then with the 50°C annealing step,


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the features of the s u r f a c e markedly change and such pores as observed i n the unannealed one, can not be observed. The 85°C annealed surface i s subjected once again to the r e o r i e n t a t i o n of molecules, and i t i s considered that the g l a s s t r a n s i t i o n may occur (12, 13). The surface becomes comparatively smooth and the pores of about 15 A, i s very small i n number. The decrease i n the number of pores i s a t t r i b u t e d to the shrinkage of pores. The pores cannot be observed under the present r e s o l u t i o n of the e l e c tron micrograph, and we miss the e x i s t e n c e of such small pores. It should be noted that i n our micrograph, i n which the higher r e s o l u t i o n r e p l i c a method i s used, the semispheres of about 200 A i n diameter can not be observed (6, 14). I t i s concluded that the pore s i z e s of the Loeb-Sourirajantype membrane are under the l e v e l of 10 - 15 A. The remarkable t r a n s i t i o n of the features of the membrane surface by the annealing step i s observed. L i t e r a t u r e Cited

1. Banks, W., Sharples, A. J. appl. Chem., 1966, 16 28. 2. Glueckauf, E . ; Russell, P.J. Desalination. 1970, 8, 351. 3. Meares, P. Eur. Polym. J., 1966, 2, 241. 4. Agrawal, J.P.: Sourirajan, S. J. Appl. Polym. Sci. 1970, 14, 1303. 5. Ohya, H.; Konuma, H.; Negishi, Y. J . Appl. Polym. Sci. 1977, 21, 2515. 6. Schultz, R. D.; Asunmaa, S.K. Recent Progr. Surface Sci. 1970, 3, 291. 7. Bradley, D.E. Nature, 1958, 181, 875. 8. Katoh, M.; Nakazuka, H. J. Electron Microsc., 1977, 26, 219. 9. Katoh, M. Private communication. 10. Manjikian, S.; Loeb, S.; McCutchan, J.W. Proc. 1st Intern. Water Desalination, U.S. Dept. Interior, Office of Saline Water, Wash., D. C. 1965, 2, 159. 11. Riley, R.L.; Merten, U.; Gardner, J.O. Desalination, 1966, 1, 30. 12. Strathmann H.; Scheible, P. Kolloid-Z. u. Z. Polymere, 1971, 246, 669. " 13. Strathmann H.; Scheible, P.; Baker, R.W. J. Appl. Polym. Sci. 1971, 15, 811. 14. Kesting, R.E. J . Appl. Polym. Sci., 1973, 17, 1771. RECEIVED

January 2, 1981.


Electron Microscopy of Cellulose Acetate Reverse-Osmotic

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