Morphology of aromatic polyamide type asymmetric reverse osmosis membranes

Morphology of aromatic polyamide type asymmetric reverse osmosis membranes

Desalination, 34 (1980) 97-112 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands MORPHOLOGY OF AROMATIC POLYAMIDE TYPE...

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Desalination, 34 (1980) 97-112 © Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

MORPHOLOGY OF AROMATIC POLYAMIDE TYPE ASYMMETRIC REVERSE OSMOSIS MEMBRANES* CHEN MAYAN', BI SHUCHUN 2 , ZHANG XINGDA 2 AND ZHENG LINGYING I 1 Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian (China) 2 Institute of Forestry and Pedology, Chinese Academy of Sciences, Shenyang (China) (Received September 18, 1979) SUMMARY

The morphology of aromatic polyamide type reverse osmosis membranes has been investigated with the aid of the scanning and transmission electron microscopes (SEM and TEM)_ The majority of the samples are treated by the method of critical point drying (CPD) and then observed with a SEM . It has been found that in the dense layer of these asymmetric membranes prepared with significant solvent evaporation, there exists a micellar structure, but various types of defects have been observed on the surface of some parts of such membranes . The variation in structure in the cross-section of these membranes due to different membrane materials and preparation techniques was observed . In the membranes we studied, we found the RO performance to be correlated not only with the degree of densification and integrity of the surface layer but also with the structure of the cross-sections . The crosssection structure characteristics of such membranes with higher RO performance are rather different from those reported in the literature on cellulose acetate and Nomex membranes without notable evaporation . The influence of the casting conditions of the membranes on the morphology is discussed preliminarily . INTRODUCTION

RO is more and more widely accepted as one of the important processes for industrial separations and purifications . Besides attempting to enhance solute rejection, solvent permeability, chemical and mechanical stability of the membranes as well as economical availability, intensive attention was * Submitted to the Second Symposium on Synthetic Membranes in Science and Industry, Tdbingen, September 17-19, 1979 .



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also paid to basic research on the relationship between morphological structure and RO performance of membranes as well as to the correlation of structure with the polymeric materials and conditions of membrane preparation . The electron microscope has been used by many authors and has proven to be effective for the study of the structure of membranes . It has been used (i) to investigate the layer structures of membranes and subsequently the influence of technological parameters on membrane structures and properties [1-41, (ii) to investigate the structural unit of membrane and to explore their nature and cause of formation [5-81, (iii) to discover the change in structure of membrane cross-sections in the course of membrane formation, particularly in the gelation process [9-121, (iv) to elucidate the correlation between the formation of membrane skin and surface tension of the polymer solution [13] as well as to compare the influence of solvents on membrane structures [14] etc . We have also reported some results of observations of the morphological features of asymmetric RO membranes of the aromatic polyamide type by TEM where the samples were treated by the ultra-microtome and replication method [15, 16] . In the last 15 years or so a certain quantity of photomicrographs of membrane structures have been accumulated . But there still remain many controversial questions on the membrane structure and the mechanism of RO, and much work must still be done in order to guide the practice of membrane preparation by morphological investigation with the electron microscope . Although a number of types of natural and synthetic macromolecules have been studied for use as RO membrane materials, till now only two types of asymmetric RO membrane, i.e ., CA and aromatic polyamide, are broadly received in industry and are commercially available [17, 131 . Therefore, it is important to investigate the structure of these two types of membranes . In the early days, the morphological investigation of RO membranes focused on CA membranes . Recently also some electron microscopic investigations of aromatic polyamide membranes have been made. Some systematic work has been done, but most of it was carried out under simplified conditions of membrane preparation, i.e . without thermal evaporation of solvent from the casting solution [101 . This work on evaporated membranes has not yet been published systematically [2, 12, 18, 191 . As is known, most commercial aromatic polyamide membranes were made by thermal evaporation-gelation technology . Therefore it is interesting to compare the structure of evaporated membranes with that of membranes without evaporation . The structure should obviously be influenced by some new factors during membrane formation and the situation should be more complicated . The present paper reports the results of the combined electron microscopic and optical microscopic observation of aromatic polyamide



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membranes prepared by thermal evaporation-gelation techniques . The sample is treated mainly by the CPD method. The structure of top and bottom surfaces and the cross-section of these membranes have been examined. Discussion is limited to the relation between geometric factors in the structure and performance of the membranes . For a complete elucidation of the relation between membrane nature and RO performance, many other chemical and physical factors [18, 201 should be considered also . They will be investigated in the future . Some results of preliminary investigations about the influence of membrane casting conditions on membrane structure are also considered here . EXPERIMENTAL

1.

Preparation and evaluation of aromatic polyamide type RO membranes The aromatic polyamides and aromatic polyamide-hydrazides used in this work were all synthesized by low temperature solution polycondensation in N,N-dimethyl acetamide (DMAC) . The polycondensate solution was poured into water and the polymer was precipitated . After washing and drying, the polymer and a fixed amount of lithium nitrate were added into a polar solvent (DMAC) heated with agitation to complete dissolution . The solution was filtered and evacuated to remove the impurities and dissolved gas and then poured onto a dried clean glass plate and shaped into a film with expected thickness by means of a glass knife . The glass plate together with the liquid film was immediately placed on an electro-thermoplate or in an oven . When the solvent was partially evaporated, the glass plate with the membrane was immersed in water or some other gelating agent . The RO performance determinations were carried out in membrane test equipment. An operating pressure of 50 kg/cm 2 was employed for the 0 .5% NaCl solution . (Except polym . II 1-1 with 70 Kg/cm 2 and 3 .4% NaCl Solution .) 2. Treatment of samples for EM and observation method A biological treatment technique was used for a membrane sample without substantial modification . In order to preserve the original structure of the membrane, the samples are generally fixed by 2% Os0 4 aqueous solution, then washed in water with the help of ultrasonic waves. The samples are dehydrated gradually by the addition of a dehydrating solvent, e .g. ethyl alcohol or acetone . The various types of subsequent treatment are then used according to different technical requirements of electron microscopy . (a) Sample treatment for TEM The samples can be obtained either by ultra-microtome or by replication . Ultra-microtome . For observation of cross-sections of the membrane, the solvent in the samples described above was gradually exchanged by epoxy-



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solvent, and then the samples were embedded in epoxy and they were cured subsequently . Then the sample embedded in epoxy was sliced with the ultramicrotome . Replication . Negative replicas were obtained by covering with CA papers on both sides of the membrane and by pressing . These negative replicas were shadowed in vacuum with Cr at about tan - ' 0 .7 angle, and then a thin film of carbon was obtained by a vertical sputtering process . The CA paper was then dissolved in acetone and the replicas picked up on copper grids for EM examination . This technique gives a positive replica of the surface . (b) Sample treatment for SEM Method of paraffin wax embedding and slicing . This method was used with an optical microscope . The sample was embedded in paraffin wax and sliced with a razor blade, the wax was washed out by xylene and the sample was sputtered in vacuum with C/Au . Critical point drying (CPD) method [211 . The alcohol in the above samples was displaced with amyl acetate, the latter then thoroughly displaced with liquid CO,, in a CPD chamber under ambient temperature . Let the gaseous CO ,) escape gradually to dryness at a temperature somewhat higher than the critical point . The sample is then glued with an electric conducting paint (if the cross-section is to be observed, a freshly sliced surface is necessary) and shadowed C/au in vacuum . The above samples were examined under JEM-100B electron microscope either by transmission or by scanning as necessary . The photomicrograph of the membrane fine structure may be directly obtained by the electron microscope, but its field of view is restricted and it is difficult to get an impression of the bulk structure of the membrane . It is even difficult to judge whether the photomicrograph represents an actual situation . And furthermore the sample was observed under a high vacuum and procedures of sample treatment were very tedious . Therefore we adopted an optical microscope as a means of preliminary observation . Although the resolution of the latter is low, it is possible to observe the whole structure of the cross-section even of water-containing membranes . There are two advantages of observation with the optical microscope . At first the procedure of sample treatment is simple, and then it is sure that the original structure is kept . Fig . 1 shows a set of optical microscope and SEM photographs . (B) and (C) are the magnified pictures of parts of membrane which are of particular interest in (A). Here the representative characters of the micrograph can be assured . Further, resolution of TEM is high . However, procedure of sample treatment is most elaborate and to judge the location observed is even more difficult than in the SEM . Therefore it is not convenient for adoption as a common method of examination . Micrographs obtained by SEM can give stereoscopic information while CPD may prevent shrinkage of the sample or lighten the degree of contrac-



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Fig 1 . Cross-section view of needle-like pores of a RO membrane prepared from aromatic polyamide-hydrazide_ Comparison of three methods of observation on the same membrane . A . Optical micrograph of wrapped free membrane ; B . SEM photomicrograph of a part of membrane, embedded in paraffin wax and sliced ; C . SEM photomicrograph of a part of membrane, treated by the CPD method . tion caused by surface tension in the phase change process . So we generally use the SEM method of examination, and TEM is utilized if it is necessary . It is found that the pictures obtained with CPD samples better conserve the fine structure . In view of these advantages, the CPD method for sample treatment in combination with the SEM technique was mainly adopted in this paper .

RESULTS AND DISCUSSION

1 . Micellar structure in membranes Shultz et al . [5] and Kesting [6] have observed the surface of CA membranes with the aid of the EM . Panar et al . [7 .8] have observed surface skin of polyamide-hydrazide with freeze-cleave techniques . Micrographs of spherelike nodules were illustrated in these papers . These aggregates of macromolecules were called micelles which some authors supposed might be the fundamental units of the membrane . The existence of micelles has been reflected in our SEM and TEM micrographs of membranes with clear features . It is not restricted to the method of sample treatment (CPD or ultramicrotome) . In Fig . 2, (A) (B) and (C) are photomicrographs observed with SEM on the top surface of aromatic polyamide-hydrazide membrane, aromatic polyamidehydrazide membrane modified with aromatic amine and chemically crosslinked aromatic polyamide-hydrazide respectively (CPD treatment) . Fig. 2D is obtained by means of ultramicrotome-TEM examination_ The figure shows the cross-section of the part near the top surface of aromatic polyamide

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A

Fig_ 2_ Photomicrographs of micelles in RO membranes of distinct aromatic polyamides . A, B and C are views of top surface obtained with CPD-SEM . D . is a view near the top edge of the cross-section of a membrane obtained with ultra-microtome-TEM .

asymmetric membrane . The sphere-like nodules with a diameter of several hundred Angstroms have been observed in all of these four RO membranes, although salt rejection of membranes whose structure is shown in Figs . 2A, B and C was as high as >98^->99% ; the salt rejection of Fig . 2D was only 10%. These photomicrographs are similar to those reported in the reference, but the method used is different . Entity of micelles exists in many various types of aromatic polyamide asymmetric RO membranes formed with evaporation also . Thus the existence of micelles in the various types of asymmetric RO membranes appears to be a general phenomenon . Possibly the difference of performance between various membranes is mainly due to the discriminating nature of linkages between such micelles . 2. Morphological characteristics on membrane surface Sufficient results of EM observations show that the state of the membrane surface was in fact not all very ideal and homogeneous . There can be various kinds of defects, but the influence of dissimilar defects on the membrane performance is very different . The data and results shown in Table I reveal the above conclusion . Fig . 3A, B and C are photographs of two typical surface defects respectively . Fig. 3A shows that there are some large

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TABLE I THE RELATIONSHIP BETWEEN SURFACE STRUCTURE FEATURES AND REVERSE OSMOSIS PERFORMANCE FOR AROMATIC POLYAMIDE TYPE MEMBRANE

Sample No.

Fig. No.

Surface structure feature

Rejection

Flux 1 /m2/day

Polymer I 1-1 1-2 1-3 2-1 2-2 2-3 3-1

3A 3B 3C 4A(a) 4A(b) 4A(c) 4B(a)

Pores are large and penetrating Pores are small and penetrating Penetrating pores absent Rough Dense Denser Larger aggregates arrange in rows

1 28 97 .2 54 94 .6 96 .1 98 .7

7 .3 x 10 3 3 .1 x 10 3 680 400 614 360 664

Polymer II 1-1

4B(b)

Larger aggregates arrange in rows

99 .2

371

penetrating pores with a width of 20-30,u on the surface of the membrane . Fig . 3B shows that there are many small penetrating pores nearly 1g wide . These pores penetrate into the support layer to a certain extent . Consequently this membrane shows a very low rejection power, but its flux is very large . Fig . 3C shows that although there are some defects on the membrane surface, they do not get into the support 'Layer . Therefore the presence of this kind of defect does not reduce solute rejection performance even if their number is high . Besides, permutation of micelles on the surface layer shows a strong effect on membrane performance . Fig . 4A is a TEM photomicrograph of an aromatic polyamide-hydrazide membrane obtained by the method of replication . The RO performance of these pieces of membrane is given in Table I . The top surface of polymer I No . 2-1 appears to be rather rough, linkages among micellae are not close, its rejection reaches only 54% . Polymer I No . 2-2 shows the increasing evaporation quantity of solvent ; polymer I No . 2-3 shows that after the same time of evaporation as No . 2-2, the sample undergoes a few minutes of standing at ambient temperature . Then they were put into a gelating agent . As a result of such treatment, the degree of densification of the membrane surface should be raised, and actually a higher rejection is obtained . In addition, we have found by both the replica-TEM and CPD-SEM methods that on the surface of membranes with good performance larger aggregates than general micelles occurred frequently . Why such aggregates arranged themselves in rows is still a question to be more fully explained (Fig . 4B (a) and Fig. 4B(b) ) .

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Fig . 3 . Defects in the top surface of aromatic polyamide-hydrazide RO membrane and their cross-section structure . A . Pores are large and penetrating ; B . Pores are small and penetrating . C . Penetrating pores absent ; (a) is a view of the top surface, (b) is a view of the cross-section .



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Strathmann et al . made systematic SEM observations on aromatic polyamide Nomex RO membranes without substantial evaporation of solvent and explained their results from the point of view of phase separation . They concluded that there are two typical structures for the cross-section of membrane - finger-like and sponge-like. Moreover, they indicated that the flux of the former type membranes is large but the rejection is low, while the flux of the latter type membranes is low but the rejection is high [101 . As the result of the work on various asymmetric membranes prepared from evaporation at a relatively high temperature from distinct types of aromatic polyamide materials and with different preparation conditions, we have found that besides finger-like (Fig . 5A) and sponge-like (Fig . 5B) structures, there can be other typical structures of the cross-section, i .e . needle-like (Fig . 1 and Fig . 5C) and pseudo-poreless (Fig . 5D i .e . no pore was observable with given resolution by the EM) . These results of SEM observation are consistent with that of the optical microscope . The reason we put particular emphasis on these latter two structure types is that although the appearance of the needle-like structure appeared to be nearly the same as that of narrow finger-like, their rejection performances were substantially different . The pseudo-poreless structure was found more frequently than the sponge-like one and its performance varied widely . By examination of more varieties of membranes prepared from different polymers and formed under various conditons, a series of intermediate morphological structures was found . For example a membrane may contain pores whose form is intermediate between the finger-like and the needle-like . These pores were often found near the bottom of a membrane (Fig. 5E)- There were also structures with only a few short needle-like pores . Such a structure was close to the pseudo-poreless (Fig . 5F) . From the results of our investigation of the asymmetric RO membrane of the aromatic polyamide type with evaporation at moderate temperature it should be pointed out that if the membrane has the structural feature characterized by the presence of needle-like pores, its flux would be large and the rejection would be high (up to 98-99%) . Those membranes having short needle-like pores near the bottom sometimes have an even higher rejection . The basic difference between the finger-like and the needlelike structure could be that the end of the pore of the finger-like structure is near the surface of the membrane or even penetrates the surface . Therefore the flux of such a membrane is large but the rejection power is low or even down to zero . But the sublayer between the end of the needle pore in cross-section and the membrane surface seems to play the role of a barrier for the property of solute rejection . Consequently, in membranes of needlelike structures, excellent rejection performance was obtained . Why certain relationships were found between the morphological characteristics in cross-section and the RO performance may be explained by

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Fig. 5 . SEM photomicrographs of several typical structures in cross-section of aromatic polyamide-hydrazide RO membrane prepared with thermal evaporation .



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the correlation of structure between the cross-section and the surface layer of asymmetric RO membranes cast under certain technical conditions . That is, certain morphology of cross-section should coexist with the corresponding surface layer morphology . So the cross-section morphology may be related to the membrane performance. 4. The influence of some technological factors on membrane structure There are many different factors in the membrane manufacturing process . Here we discuss primarily the influence of the extent of solvent evaporation, existence of salt and the nature of gelating agent . Blais and Sourirajan [18] have studied asymmetric membranes with an aromatic polyamide-CaCl 2 (4%)-DMAc casting solution . Tye varied evaporation time at 95 ° C accompanied with a change of amount of residual solvent in casting solution from 70% to 20%, examined the characteristics of membrane cross-section with frozen drying-SEM and°obtained a series of various degrees of matrix porosity structures . The size of pore decreased with the duration of evaporation time and this change in turn was related to the performance of the membrane . We prepared membranes with aromatic polyamide-hydrazide (20 .6%)-LiNO 3(6 .2%)-DMAc casting solution . At 70° C we varied evaporation time from 3-100 minutes and studied the structure of membranes thus obtained with CPD-SEM . We obtained a series of structures with various degrees of matrix porosity ; their size varied from relatively large to small (Fig . 6) . Owing to the difference in the composition of casting solution and the operating conditions between Blais' work and ours, there appeared to be a difference between these two sources in the structure shown in the photomicrographs, but trends were similar . Experiments have indicated that there is a marked difference in performance between membranes made from salt-containing and salt-deficient casting solutions . Consequently we studied the change of membrane structure in their cross-section prepared under different evaporation times . Figs . 6 and 7 show the different structures of membranes which were prepared with salt-containing and salt-deficient casting solutions respectively . After comparing the structure of these two series of membranes as well as other results, we found that the presence of salt and its quantity have a noticeable influence on the structure of the cross-section . Consequently in the casting process with evaporation, we must not only pay attention to the influence of evaporation time on membrane structure, but also to the influence of the addition of salt . Frommer [9] and Strathmann [10] systematically studied the influence of different gelating agents on the structure of CA and NOMEX membranes without thermal evaporation . We have compared in this paper the influence of four gelating agents which have different rates of precipitation on the cross-section structures of thermal evaporation aromatic polyamide-



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D

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Fig. 6 . SEM photomicrographs of cross-section of aromatic polyamide-hydrazide RO membranes from LiNO 3-containing casting solution at 70° C and for various duration of evaporation. (A) 3, (B) 8, (C) 20 and (D) 100 minutes .

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B

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Fig. 7 . SEM photomicrograph of cross-section of aromatic polyamide-hydrazide RO membranes prepared from casting solution without LiNO3 at 70 °C and for various durations of evaporation time . (A) 3, (B) 8 and (C) 60 minutes.

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hydrazide membranes and found the influence also to be remarkable . In Fig . 8(A) and 8(B) needle-like structure characteristics appeared . These samples were precipitated in water and CH-OH . With these gelating agents the precipitation rates were made higher . In Fig. 8(C) and 8(D), sponge-like structure characteristics appeared . These samples were precipitated by gelating agents with low precipitation rates i .e . glycerine and acetone. It is clear from these results that the choice of the proper gelating agent is also an important factor that cannot be neglected in improving the performance of thermal evaporation aromation polyamide-hydrazide membranes .



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CONCLUSIONS

1 . The morphological characteristics of RO membranes have been examined by optical microscopy in collaboration with the electron microscopy method . This method is advantageous in correlating an overall picture with the fine structural details . The method of handling the sample of SEM with CPD may avoid the contraction of membranes caused by surface tension in phase change processes and may better preserve the fine structure of the membranes . 2 . The presence of miscelles is a significant structural characteristic of aromatic polyamide type asymmetric RO membranes formed from heated and evaporated casting solutions . 3 . There are different types of local defects on the surface of some membranes . These defects may be roughly divided into two types, i .e. penetrating defects and non-penetrating defects . The former leads to the serious lowering of rejection and the latter on the whole does not significantly affect the rejection . Penetrating defects must be completely removed . 4 . Usually there are higher rejections for these membranes, their surfaces are properly dense without penetrating detects and have some orderly arrangement of larger aggregates than general micelles . 5 . Four typical structures in cross-section have been found in our aromatic polyamide-hydrazide asymmetric membranes prepared by thermal evaporation, i .e. finger-like, sponge-like, needle-like and pseudo-poreless . The relationship of these four typical structures with performances of the membrane is partly different from CA membranes and NOMEX membranes prepared without thermal evaporation . In our membranes the needle-like pore structure in cross-section is an important structural characteristic . Membranes with such structure have a good performance . 6 . The solvent retained in casting solution, the presence of salt and the salt content and the gelating agent all have marked influence on the structure of aromatic polyamide-type asymmetric RO membranes formed with evaporation at moderate temperatures . These factors must be considered scrupulously .

ACKNOWLEDGMENTS

The authors gratefully acknowledge the continuous support and advice of Associate Professor Gu Chang Li, the head of the laboratory . Special thanks are extended to Liu Hui Min and Lu Hong Rui for carrying out part of the optical and the electron microscopy, to Jiang Xijie, Zhang Shui Zhi, Zhen Chang Tu and Zhu Wen Gang for synthesizing the membrane materials, to Shao Shou Ian, Liu Gui Xiang and Zhu Rui Zhi for casting the membranes and determining their performance .



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