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Influence of rupture strength of interfacially polymerized thin-film structure on the performance of polyamide composite membranes
Journal of Membrane Science 198 (2002) 63–74
Influence of rupture strength of interfacially polymerized thin-film structure on the performance of polyamide composite membranes Il Juhn Roh∗ Department of Mechanical Engineering, University of Colorado at Boulder, Boulder, CO 80309-0427, USA Received 22 May 2001; received in revised form 6 August 2001; accepted 9 August 2001
Abstract The permeation properties of a thin-film composite (TFC) membrane depend upon the material properties as well as the structural properties of the polymer forming the active layer. Membranes with the active layers prepared with 1,3,5-benzentricarbonyl chloride (TMC) and aliphatic diamines including dimethylenediamine (DMDA), 1,6-hexamethylenediamine (HMDA), and 1,9-nonamethylenediamine (NMDA) exhibit inferior performance compared to membranes with active layers composed of aromatic diamines including 1,3-benzendiamine (MPDA) and 1,4-benzendiamine (PPDA). It is also observed that the water flux for these membranes decreases as the length of the methylene chain increases due to decreasing hydrophilicity. Furthermore, because of the low rupture strength of the thin-films that form the active layer, the salt rejection also decreases with increasing methylene chain length. The membranes prepared with MPDA and various acyl chlorides including 1,6-hexamethylenedicarbonyl chloride (SC), 1,3-benzenedicarbonyl chloride (IPC), and 1,4-benzene dicarbonyl chloride (TPC) have low rupture strength and poor performance characteristics except for the membrane having network structure, TMC. It is observed that while hydrophilicity has a small effect on the permeation performance of the thin-films its effect of the rupture strength is large. Membranes with weak rupture strength evidence low salt rejection. Hence, the permeation performance of composite membranes with thin-films having weak mechanical strength at high operating pressures depends upon not only the physicochemical properties of the active material including the chemical properties, but also the mechanical strength of the polymer comprising the thin-film. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Composite membranes; Reverse osmosis; Skin; Permeation performance; Mechanical strength
1. Introduction High permeability and selectivity are desirable properties for reverse osmosis (RO) membranes. To possess these properties, the active layer of the membrane should be very thin and hydrophilic in nature. Thin-film-composite (TFC) structure membranes fab∗ Fax: +1-303-492-3498. E-mail addresses: [email protected], [email protected] (I.J. Roh).
ricated via interfacial polymerization (IP) meet this demand [1,2]. The active layer of a TFC is a thin-film which controls the passage of the solvent (water) and solutes. Using IP, various polymers were synthesized to form the active layer of the composite membranes [1–9]. The thin-film polymers synthesized via IP can have varying physicochemical properties and mechanical strengths based on the types of monomers. For optimum permeation performance, it is necessary to understand the effect of the chemical/molecular
0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 6 3 0 - 5
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Fig. 1. Basic structure of a composite reverse osmosis membrane.
structural properties as well as the mechanical properties of the active layer on its permeation characteristics. The most successful active layer polymers in TFC membranes are extremely thin with a cross-linked structure. Hence, they are insoluble in common solvents, thereby bringing about sampling problems for characterization with conventional analytical techniques. Due to this difficulty, numerous researchers have sought to optimize the active layer performance by experimenting with a variety of membrane formation schemes and polymers [10–12]. In such experiments, the approach has been inferential, since the active layer is not directly studied. In fact, research related to the direct characterization of the active layer to elucidate the effect of the polymer’s intrinsic material properties as well as its physicochemical and mechanical properties on the permeation performance is rarely found in the literature [13–15]. The structural characteristics of a TFC RO membrane are shown in Fig. 1. A TFC RO membrane consists of important two structural components: one is a highly porous support layer that comprises almost the entire thickness and provides the required mechanical strength. This layer has relatively large pores on its surface. The other is an active layer or the barrier layer. The barrier layer is a very thin-film that bridges and overcoats the surface pores of the porous support. Owing to this structural characteristic, the barrier layer should be mechanically strong to withstand the high pressures typically encountered in RO. If the thin barrier layer does not possess sufficient mechanical strength to withstand the high hydraulic pressures dur-
ing RO operation, it could partially tear, especially in the large pores at the surface of the support layer [13]. Any such “defect” in the thin active film would lead to a bulk flow of brine, which would contaminate the permeated solution [16]. Hence, it is of great interest to elucidate the effect of the mechanical strength of the active layer on the permeation performance. In the present article, the contribution of the mechanical strength to overall permeation performance as well as the measurement of the chemical properties of the thin-films based on the monomer structure are described. 2. Experimental 2.1. Materials For the interfacial synthesis of the active thin-films, diamines and acyl chlorides were used. The acyl chlorides included 1,6-hexamethylenedicarbonyl chloride (SC, Aldrich, 97%), 1,3-benzenedicarbonyl chloride (IPC, Aldrich, 98%), 1,4-benzene dicarbonyl chloride (TPC, Aldrich, 99%), and 1,3,5-benzenetricarbonyl chloride (TMC, Aldrich, 98%). Before use for thin-film synthesis: SC, IPC, TMC were distilled under vacuum at 150, 170, and 160◦ C, respectively. The TPC was sublimated under vacuum at 50◦ C. The purified acyl chlorides were stored in a vacuum desiccator containing calcium chloride to prevent hydrolysis of the acyl chloride. The diamines used were 1,2-dimethylenediamine (DMDA) (Aldrich, >99.5%), 1,6-hexamethylenedia-
I.J. Roh / Journal of Membrane Science 198 (2002) 63–74
mine (HMDA) (Aldrich, 98%), 1,9-nonamethylenediamine (NMDA) (Aldrich, 98%), 1,3-benzendiamine (MPDA) (Aldrich, 99%), and 1,4-benzendiamine (PPDA, Aldrich, 99%). HMDA, NMDA, MPDA were distilled under vacuum at 180, 200, and 170◦ C, respectively. The PPDA was sublimated under vacuum at 50◦ C, and the DMDA was used without further purification. The purified diamines were filled in a dark bottle with N2 gas and stored in a refrigerator. The solvent for TMC, n-hexane, was dried by shaking with magnesium sulfate and then distilling. Before using purified n-hexane within 1–2 days, it was stored in a tightly closed bottle with Molecular Sieve A® to prevent dissolution of water. For the preparation of the diamine solution, RO purified and deionized water was used. 2.2. Synthesis of polyamides In order to study systematically on the effect of diamine and the acyl chloride structures, two series of IP experiments were conducted. In series I, MPDA was fixed as the amine monomer and a variety of acyl chloride monomers including SC, IPC, TPC, and TMC were tested. In series II, the full range of amines was studied including DMDA, HMDA, NMDA, PPDA, and MPDA, but only one acyl chloride, TMC, was fixed as the acyl chloride monomer. The active layer polyamides were synthesized via IP of the 0.5% (w/v) aqueous diamines solution and the 0.1% (w/v) n-hexane acyl chlorides solution. The reaction was conducted using an unstirred non-dispersion method for 2 min at 20◦ C. The thin-film polyamides formed at the solution interface were retrieved and precipitated in excess of acetone. The solid thin-film was filtered off, and the washed with acidic water, methanol, and fresh water to remove the unreacted monomer remnants and occluded salt. The pure solid polymers were dried under vacuum at room temperature and then used with no further post-treatment. The chemical structures of these polyamides are believed to be as shown in Table 1. 2.3. Preparation and characterization of TFC membrane TFC membranes were prepared via a two-step process: first, an asymmetric support polysulfone
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membrane was fabricated, and then the thin-film active layer was deposited on its surface. The asymmetric support layers were made as described elsewhere [17]. The thin-film active layers were prepared by in situ IP on the support layer using an unstirred non-dispersion method described in an earlier paper [13]. The concentrations of diamine and acyl chloride were fixed at 0.5 and 0.1% (w/v), respectively. The fabricated TFC membranes were used for permeation testing with no further post-treatment. Performance measurements of composite RO membranes were carried out at 3.03 MPa with 2000 ppm NaCl solution. The permeate and feed salt concentrations were measured using a standardized digital conductivity meter (YSI Co., model 32). The salt rejection (R) is given by Cs R (%) = 1 − × 100 (1) Cs where Cs is the salt concentration of the permeate stream and Cs the salt concentration of the feed. To minimize experimental error, the membrane permeabilities were measured from three samples and averaged. 2.4. Contact angle measurements In order to study the hydrophilicity of the active thin-films, contact angle measurements were made using an image analyzer system (PAS PX-380). Sets of liquid droplets of approximately 3 l volume were placed on IP films and observed through a video camera. The dimensions of the liquid droplets were measured approximately 10 s after placing the droplets on the IP film. The images of the liquid droplets were captured on videotape and the liquid droplets were magnified before the measuring the contact angle (θ). To minimize experimental error, the contact angle was measured six times for each sample and then averaged. The surface tension of dispersion and polar force for polymers can be obtained from the following equation [18]: cos θ =
4(γsd γLd /(γsd + γLd )) γL p p p p 4(γs γL /(γs + γL )) + −1 γL
(2)
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Table 1 Chemical structure of polyamides Chemical structure of monomers Diamines Series I
Series II
Chemical structure of polymers Acyl chlorides
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Table 1 (Continued) Chemical structure of monomers Diamines
Chemical structure of polymers Acyl chlorides
where θ is the contact angle of the liquid, γ L , γLd p and γL are total surface tension, dispersion force and polar force of surface tension for a standard liquid, p respectively; γsd and γs are dispersion force and polar force of surface tension for the polymer. Using the contact angle values with the reference liquids as shown in Table 2 and the surface components of the reference liquids, the components of surface tension for the thin-film polymer can be determined. 2.5. Measurement of water sorption In order to measure the concentrations of dissolved water in the active layer (C w ), the purified and weight-measured polymer was saturated with
2000 ppm NaCl aqueous solution for 7 day. The saturated polymer was retrieved from the NaCl solution, and the excess solution present at the surface of polymer was removed with a filter paper.
Table 2 Dispersion and polar force components of surface tension (dyn/cm) of reference liquids (20◦ C)a p
Reference liquids
γLd
γL
γL
Water Glycerol Formamide
29.1 37.4 35.1
43.7 26.0 23.1
72.8 63.4 58.2
a Superscripts d and p refer the dispersion component and polar component, respectively.
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Then the NaCl-saturated polymer was vacuum-dried at room temperature for 3 days. The dried and weight-measured samples were put in a chamber, which was saturated with moisture. After 1 day, the samples were again weighed in a microbalance, and the amount of absorbed water was calculated. To minimize experimental error, nine replicates were used.
resulting pressure-deformation behavior monitored. The rupture strength (SR ) of the thin-films can be obtained from the pressure and the drop-radius at the instant of IP film rupture via PDMA. An earlier paper discusses the details of the PDMA technique [13].
3. Results and discussion 2.6. Mechanical strength measurements 3.1. Permeation performance In order to study the mechanical resistance to rupture, the active layer thin-films when subjected to the high hydraulic RO operation pressure, and the stress in the IP films at the instant of rupture (rupture strength) was measured. To this end, a promising technique for directly measuring the mechanical properties of unsupported IP films was reported by Greenberg and coworkers as the “pendant drop mechanical analysis (PDMA)” [19]. PDMA was applied to measure the non-normalized structural parameter or rupture strength (SR ) of the unsupported thin-films [13]. PDMA first requires the formation of an IP film over the surface of a drop of diamine monomer solution when it is contacted by a solution containing acyl chloride monomer. This film-covered drop can then be pressurized in a controlled fashion and the
3.1.1. Effect of acyl chloride structure The different mechanical and physicochemical properties of linear as well as network structures prepared using di/tri-functional aromatic and aliphatic acyl chloride monomers were studied. Acyl chlorides used in series I result in different chemical structures in the final thin-film polymer as seen in Table 1. The di-functional acyl chlorides including SC, IPC, and TPC result in a linear structure, whereas the tri-functional acyl chloride, TMC results in a network structure. SC results in an aromatic (amine)–aliphatic (acyl chloride) structure, but the polymers prepared using aromatic acyl chlorides including IPC, TPC, and TMC have a fully aromatic structure. Fig. 2 shows the permeation performance in series I. The
Fig. 2. Effect of acyl chloride structures on the permeation performance. Note that the performance of membrane made by the reaction of MPDA (0.5% (w/v)) with various acyl chlorides (0.1% (w/v)) were obtained by 3.03 MPa, 2000 ppm NaCl solution at 25◦ C ( : Jw ; : R).
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Fig. 3. Effect of amine monomer structures on the permeation performance. Note that the performance of membrane made by the reaction of TMC (0.1% (w/v)) with various diamines (0.5% (w/v)) were obtained by 3.03 MPa, 2000 ppm NaCl solution at 25◦ C ( : Jw ; : R)
thin active films evidencing linear structures (SC, IPC, and TPC) had low salt rejection while network structure (TMC) gave higher salt rejection. Thin active films with the aromatic–aliphatic linear structure (SC) showed high water fluxes while fully aromatic linear structures (IPC and TPC) resulted in lower water fluxes. In summary, the thin active films with aromatic–aliphatic linear structure resulted in the best water flux while the thin-films with completely aromatic network structures resulted in the best salt rejection as seen in Fig. 2. These results imply that the permeation properties of the thin-films prepared from series I are related to the structural properties of the thin-film polymers. 3.1.2. Effect of amine structure The permeation performances for the composite membranes obtained from series II are shown in Fig. 3. All the thin-films obtained from this series have a cross-linked network structure. The thin-films prepared with aliphatic diamines including DMDA, HMDA, and NMDA have an aliphatic–aromatic network structure. However, the thin-films synthesized with aromatic diamines including PPDA and MPDA have fully aromatic network structures as shown in Table 1. All of these network structure polyamides had a relatively high salt rejection compared to the
linear structure polyamides as shown in Fig. 2. In the case of the membranes having aliphatic–aromatic network structures, the water flux and the salt rejection decrease with increasing length of the aliphatic methylene chain. On the other hand, the thin-films with a fully aromatic network structures evidence high salt rejection and relatively high water flux compared to the membranes having aliphatic–aromatic network structures. These permeation behaviors imply that the permeation properties of the thin-films are affected not only by the chemical properties, but also by the structure of the active layer polymer. As previously described, since the structural characteristics of a TFC membrane, the permeation performance of the TFC membrane can be affected by the chemical properties as well as by the mechanical strength of the active layer. In order to evaluate the factors affecting the permeation performance, the chemical properties as well as the mechanical strength of the active layer polymer are examined in the following paragraphs. 3.2. Effect of acyl chloride structure Fig. 4 shows the rupture strength of the active thin-films for series I. As previously noted, the rupture strength (SR ) is the stress in the IP films with
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Fig. 4. Dependence of the rupture strength of active layer of membrane on the acyl chloride structures. Note that the active layer of membrane were made by the reaction of MPDA (0.5% (w/v)) with various acyl chloride monomers (0.1% (w/v)).
specific thickness at the instant of rupture. Thus, it depends upon the rupture stress and the thickness of the active films [13], and represents a structural as well as a material characteristic of the active polymer. Meanwhile, the rupture of the active layer polymer in RO was affected by the various factors including the rupture strength and strain of the active layer polymer, the surface pore size of the support, and the applied pressure in RO. According to previous literature, the rupture strength of the active layer affecting the salt rejection is 5 kPa mm and the water flux is 3 kPa mm [13]. That is, if the rupture strength of the active layer is below 5 kPa mm, the salt rejection is affected by the bulk flow of brine. Furthermore, if the rupture strength of the active layer has below 3 kPa mm, the bulk flow of brine affect the water flux as well as salt rejection. Note that the comparison of the membrane performance is based on the same experimental conditions as described in a previous article [13]. The thin-films with linear structure prepared using diacyl chlorides (HDMC, IPC, and TPC) have low rupture strength (<2 kPa mm). On the other hand, thin-film with network structures prepared with a triacyl chloride (TMC) has high rupture strength (>5 kPa mm). It implies that the SR has a dependence upon the polymer structure, i.e. whether it is a lin-
ear or a cross-linked network. Thus, the permeation characteristics of the linear structure polyamides with low SR present the results of the chemical properties as well as the bulk flow of brine due to a partial break-up of the barrier layer. However, the permeation performance of the network polyamide having higher SR (>5 kPa mm) yield their inherent material properties. In order to assess the effect of the likely factors affecting permeation performance, the chemical properties of the active layer were studied in the polar p force of surface tension (γs ) and the concentration of dissolved water in the membrane (C w ). The polar force of surface tension was evaluated as a possible measure of the hydrophilicity of the thin active film. Fig. 5 shows the polar force of surp face tension (γs ) and the concentration of dissolved water (C w ) for membranes obtained from series I. In these polyamides, the components affecting the polar force are an amide bond, an amine and carboxylic acid end-group, and a free carboxylic acid group in network structure as seen in Table 1. The free carboxylic acid group would result in unreacted acyl chloride groups, which can easily be converted to the corresponding carboxylic acids through hydration. The amine and carboxylic acid groups are more hydrophilic than the amide bonds in the polyamide.
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Fig. 5. Dependence of the rupture strength of active layer of membrane on the amine monomer structures. Note that the active layer of membrane were made by the reaction of TMC (0.1% (w/v)) with various diamine monomers (0.5% (w/v)).
The presence of these end-groups depends upon presumably the degree of polymerization, which, in turn depends upon the reactivity and the structure of the monomers [20]. The thin-films made from various acyl chlorides showed different polar forces. The polar forces of the thin polyamide films with the fully aromatic linear structure are lower compared to the polyamide having aromatic–aliphatic linear structure. This behavior is also identified by the concentration of dissolved water in the membrane (C w ), which depends upon the hydrophilicity of the active layer. The p C w and γs showed similar behaviors. The water flux of the membranes with fully aromatic structures showed the dependence upon the polar force and the water content of the thin-film as shown in Figs. 2 and 5. However, if the polar force alone is considered, membranes with fully aromatic linear structures (IPC and TPC) show relatively high water fluxes compared with membranes having fully aromatic network structures (TMC). Also, the polyamide films with aromatic–aliphatic linear structures (SC) show unexpectedly high water fluxes. Although, this thin-film has high polar forces, they exhibit very high water fluxes compared with membranes made using aromatic acyl chlorides. This hints at some other influential factor causing high fluxes.
The salt rejection is related to the ratio of the water flux and the salt flux as shown in Eq. (1). Thus, high salt rejection implies that the water flux is relatively higher compared with the salt flux. The thin-films obtained from series I exhibited complicated salt rejection properties. As mentioned earlier, the salt rejection for series I seems to depend upon the structural properties of the thin-film. Membranes with aromatic–aliphatic linear structures have low salt rejections, which implies high water flux and also high salt flux. Among the membranes prepared with aromatic acyl chlorides, the thin-films with linear structures evidence low salt rejections, but network structures result in very high salt rejections. Hence, the salt rejection depends upon the polymer structure, which affects the rupture strength of the thin-films. The membranes for series I showed some dependence of water flux on hydrophilicity of the thin-films, but no dependence of the salt rejection. These permeation properties are a result of the effect of the rupture strength of the thin-films. Therefore, low salt rejection of the thin-films with linear structure, and the low salt rejection and the high water flux of membranes made with MPDA and SC are due to the effect of the rupture strength of the thinfilms.
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Fig. 6. Dependence of the polar force of surface tension and the concentration of dissolved water in the active layer on the acyl chloride structures. Note that the active layer of membrane were made by the reaction of MPDA (0.5% (w/v)) with various acyl chloride monomers p (0.1% (w/v)) ( : γs ; : Cw ).
3.3. Effect of amine structure Fig. 6 shows the rupture strength of the active layers for series II. Owing to their cross-linked network structure, these polyamides have a higher rupture strength compared to the polymers with linear structures (see Fig. 4). The active layers with fully aromatic network structures (PPDA and MPDA) have high rupture strength (>5 kPa mm). Here, because of the high rupture strength, bulk flow of brine originating from tearing of the active layer is avoided, and hence, the permeation performance corresponds to the intrinsic material property of the active layer. On the other hand, the active layers composed of the aliphatic–aromatic network structures (DMDA, HMDA, and NMDA) have a rupture strength close to 3 kPa mm. Here, the salt rejection is affected by the bulk flow of brine. However, because SR is around 3 kPa mm, the effect on the water flux is not prominent. Fig. 7 shows the polar force of surface tension and the concentration of dissolved water for membranes obtained from series II. The polar forces for the thin-films made from aliphatic diamines decrease as the methylene chain of the aliphatic amines increases in length. It implies that the effect of the content
of methylene chain in the polymer structure on the hydrophilicity is presumably prominent than that of end-group including amine and carboxylic acid in aliphatic diamines. Hence, the polar force of surface tension decreases with increasing methylene chain length. This behavior is also observed for the concentration of dissolved water in the membrane (C w ). The C w of the thin-films made from aliphatic diamines decrease as the methylene chain of the aliphatic amines increases in length. The water flux of these memp branes decreases as γs and C w decrease as shown in Figs. 3 and 7. As expected, for these membranes the permeation depends on the hydrophilicity, and p hence is sensitive to γs and C w . The fully aromatic polyamides (PPDA and MPDA) have higher C w and p γs compared to the polyamide composed of the similar hydrocarbon number (HMDA). This difference could be due to the effect of polymer structure. A detailed study on the effect of aromatic diamines on permeation performance will be discussed in another paper [21]. The salt rejection of the membranes made from aliphatic diamines decreases as the methylene chain length increases. The salt rejection and the water flux show a similar trend. That is, membranes with long
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Fig. 7. Dependence of the polar force of surface tension on the acyl chloride monomers. Note that the active layer of membrane were p made by the reaction of TMC (0.1% (w/v)) with various diamine monomers (0.5% (w/v)) ( : γs ; : Cw ).
methylene chains have low water fluxes, but relatively high salt fluxes. On the other hand, membranes prepared with aromatic diamines have excellent salt rejections compare with the membranes prepared from aliphatic diamines. These results also imply that the permeation performance of composite membranes depends upon the chemical properties as well as the mechanical strength of the active layer. In summary, the active layer polymers possessing sufficient mechanical strength to withstand the high hydraulic pressures during RO operation showed relatively good correspondence between the permeation performance and the chemical properties. However, the permeation characteristics of the active layer polymers with weak rupture strength were affected not only by the chemical properties but also the bulk flow of brine due to a partial break-up of the barrier layer.
4. Conclusions Membranes prepared with MPDA and various acyl chlorides have very different permeation performance and physicochemical properties. The thin-films with linear structures prepared using MPDA and di-functional acyl chlorides including SC, IPC, and TPC show low rupture strength while thin-film prepared with a tri-functional acyl chloride, TMC, result
in a network structure with high rupture strength. The permeation performance of composite membranes with low rupture strength active layers depends upon the rupture strength as well as the chemical properties (hydrophilicity) of the thin-films. The linear structure with low rupture strength has low salt rejections. Membranes prepared with TMC and various diamines have comparatively high rupture strength due to a network structure. These membranes also exhibited high salt rejections and evidenced a relatively good relationship between the chemical properties of the thin-film and the permeation performance of the composite membrane. Membranes composed of fully aromatic network structure have higher rupture strengths, higher salt rejections and comparatively higher water flux than the membranes consisting of aliphatic–aromatic network structures. Also, the methylene chain length of aliphatic diamines affects the water flux. The water flux is observed to decrease as the methylene chain length of the aliphatic diamines increases, which in turn decreases the hydrophilicity of the thin-films. These results indicate that the permeation properties of the composite membranes without adequate mechanical strength not only depend upon the chemical properties including hydrophilicity of the thin-film polymer, but also on the mechanical strength of the thin-films.
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Acknowledgements The authors gratefully acknowledge the valuable comments and suggestions made by Dr. Vivek P. Khare and Senthilkumar Ramaswamy. References [1] L.T. Rozelle, J.E. Cadotte, K.E. Cobian, C.V. Kopp, Jr., Nonpolysaccharide membranes for reverse osmosis, in: S. Sourirajan (Ed.), NS-100 Membranes for Reverse Osmosis and Synthetic Membranes, National Research Council Canada, Ottawa, Canada, 1977, p. 249. [2] J.E. Cadotte, Interfacially synthesized reverse osmosis membrane, US Patent 4,277,344 (1981). [3] P. Eriksson, Water and salt transport through two type of polyamide composite membranes, J. Membr. Sci. 36 (1988) 297. [4] T. Uemura, Y. Himeshima, M. Kurihara, Interfacially synthesized reverse osmosis membrane, US Patent 4,761,234 (1988). [5] S.A. Sundet, S.D. Arthur, D. Campos, T.J. Eckman, R.G. Brown, Aromatic/cycloaliphatic polyamide membrane, Desalination 64 (1987) 259. [6] R.E. Kesting, The four tiers of structure in integrally skinned phase inversion membrane and their relevance to the various separation regimes, J. Appl. Polym. Sci. 41 (1990) 2739. [7] K. Kimmerle, H. Strathmann, Analysis of the structuredetermining process of phase inversion membranes, Desalination 79 (1990) 283. [8] R. Cheng, J. Glater, J.B. Neethling, M.K. Stenstrom, The effects of small halocarbones on RO membrane performance, Desalination 85 (1991) 33. [9] N.V. Desai, R. Ranggarajan, A.V. Rao, D.K. Garg, B.V. Ankleschwaria, M.H. Mehta, Preliminary investigation of thin-film composite reverse osmosis membranes developed from SAN as support membranes, J. Membr. Sci. 71 (1992) 201.
[10] J.E. Cadotte, L.T. Rozelle, In situ formed condensation polymers for reverse osmosis membranes, NTIS Report PB-229337, November 1972. [11] R.J. Petersen, P.K. Eriksson, J.E. Cadotte, Highly permeable reverse osmosis membranes, in: T. Saegusa, T. Higashimura, A. Abe (Eds.), Frontiers of Macromolecular Science, Blackwell Scientific Publishers, London, 1989, p. 511. [12] M. Kurihara, T. Uemura, Y. Nakagawa, T. Tonomura, The thin-film composite low pressure reverse osmosis membranes, Desalination 54 (1985) 75. [13] I.J. Roh, J.-J. Kim, S.Y. Park, Mechanical properties and reverse osmosis performances of interfacially polymerized polyamide thin-films, J. Membr. Sci. 197 (1–2) (2001). [14] S.-Y. Kwak, Relationship of relaxation property to reverse osmosis permeability in aromatic polyamide thin-film-composite membranes, Polymer 40 (1999) 6361– 6368. [15] K.J. Mysels, W. Wrasidlo, Strength of interfacial polymerization films, Langmuir 7 (1991) 3052. [16] L.E. Applegate, C.R. Antonson, in: H.K. Lonsdale, H.E. Podall (Eds.), Reverse Osmosis and Membrane Research, Plenum Press, New York, 1972. [17] I.J. Roh, S.Y. Park, J.J. Kim, C.K. Kim, The effects of polyamide molecular structure on the performance of reverse osmosis membrane, J. Polym. Sci.: Part B 36 (11) (1998) 1821. [18] S. Wu, Calculation of interfacial tension in polymer systems, Polym. Sci. C34 (1971) 19. [19] A.R. Greenberg, V.P. Khare, W.B. Krantz, Development of a technique for the in situ measurement of the mechanical properties of ultra-thin interfacially polymerized films, in: Proceedings of the Material Research Society Symposium, Vol. 356, Material Research Society, 1995, pp. 541–546. [20] I.J. Roh, Effect of molecular structure and chemical/physical properties of interfacially polymerized polyamide thin-films on reverse osmosis characteristics, dissertation, Seoul National University, Korea, 1998. [21] I.J. Roh, Effect of aromatic diamines on the network structure and permeation performance of interfacially polymerized polyamide thin-films, Polymer, submitted.
Influence of rupture strength of interfacially polymerized thin-film structure on the performance of polyamide composite membranes Il Juhn Roh∗ Department of Mechanical Engineering, University of Colorado at Boulder, Boulder, CO 80309-0427, USA Received 22 May 2001; received in revised form 6 August 2001; accepted 9 August 2001
Abstract The permeation properties of a thin-film composite (TFC) membrane depend upon the material properties as well as the structural properties of the polymer forming the active layer. Membranes with the active layers prepared with 1,3,5-benzentricarbonyl chloride (TMC) and aliphatic diamines including dimethylenediamine (DMDA), 1,6-hexamethylenediamine (HMDA), and 1,9-nonamethylenediamine (NMDA) exhibit inferior performance compared to membranes with active layers composed of aromatic diamines including 1,3-benzendiamine (MPDA) and 1,4-benzendiamine (PPDA). It is also observed that the water flux for these membranes decreases as the length of the methylene chain increases due to decreasing hydrophilicity. Furthermore, because of the low rupture strength of the thin-films that form the active layer, the salt rejection also decreases with increasing methylene chain length. The membranes prepared with MPDA and various acyl chlorides including 1,6-hexamethylenedicarbonyl chloride (SC), 1,3-benzenedicarbonyl chloride (IPC), and 1,4-benzene dicarbonyl chloride (TPC) have low rupture strength and poor performance characteristics except for the membrane having network structure, TMC. It is observed that while hydrophilicity has a small effect on the permeation performance of the thin-films its effect of the rupture strength is large. Membranes with weak rupture strength evidence low salt rejection. Hence, the permeation performance of composite membranes with thin-films having weak mechanical strength at high operating pressures depends upon not only the physicochemical properties of the active material including the chemical properties, but also the mechanical strength of the polymer comprising the thin-film. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Composite membranes; Reverse osmosis; Skin; Permeation performance; Mechanical strength
1. Introduction High permeability and selectivity are desirable properties for reverse osmosis (RO) membranes. To possess these properties, the active layer of the membrane should be very thin and hydrophilic in nature. Thin-film-composite (TFC) structure membranes fab∗ Fax: +1-303-492-3498. E-mail addresses: [email protected], [email protected] (I.J. Roh).
ricated via interfacial polymerization (IP) meet this demand [1,2]. The active layer of a TFC is a thin-film which controls the passage of the solvent (water) and solutes. Using IP, various polymers were synthesized to form the active layer of the composite membranes [1–9]. The thin-film polymers synthesized via IP can have varying physicochemical properties and mechanical strengths based on the types of monomers. For optimum permeation performance, it is necessary to understand the effect of the chemical/molecular
0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 0 1 ) 0 0 6 3 0 - 5
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Fig. 1. Basic structure of a composite reverse osmosis membrane.
structural properties as well as the mechanical properties of the active layer on its permeation characteristics. The most successful active layer polymers in TFC membranes are extremely thin with a cross-linked structure. Hence, they are insoluble in common solvents, thereby bringing about sampling problems for characterization with conventional analytical techniques. Due to this difficulty, numerous researchers have sought to optimize the active layer performance by experimenting with a variety of membrane formation schemes and polymers [10–12]. In such experiments, the approach has been inferential, since the active layer is not directly studied. In fact, research related to the direct characterization of the active layer to elucidate the effect of the polymer’s intrinsic material properties as well as its physicochemical and mechanical properties on the permeation performance is rarely found in the literature [13–15]. The structural characteristics of a TFC RO membrane are shown in Fig. 1. A TFC RO membrane consists of important two structural components: one is a highly porous support layer that comprises almost the entire thickness and provides the required mechanical strength. This layer has relatively large pores on its surface. The other is an active layer or the barrier layer. The barrier layer is a very thin-film that bridges and overcoats the surface pores of the porous support. Owing to this structural characteristic, the barrier layer should be mechanically strong to withstand the high pressures typically encountered in RO. If the thin barrier layer does not possess sufficient mechanical strength to withstand the high hydraulic pressures dur-
ing RO operation, it could partially tear, especially in the large pores at the surface of the support layer [13]. Any such “defect” in the thin active film would lead to a bulk flow of brine, which would contaminate the permeated solution [16]. Hence, it is of great interest to elucidate the effect of the mechanical strength of the active layer on the permeation performance. In the present article, the contribution of the mechanical strength to overall permeation performance as well as the measurement of the chemical properties of the thin-films based on the monomer structure are described. 2. Experimental 2.1. Materials For the interfacial synthesis of the active thin-films, diamines and acyl chlorides were used. The acyl chlorides included 1,6-hexamethylenedicarbonyl chloride (SC, Aldrich, 97%), 1,3-benzenedicarbonyl chloride (IPC, Aldrich, 98%), 1,4-benzene dicarbonyl chloride (TPC, Aldrich, 99%), and 1,3,5-benzenetricarbonyl chloride (TMC, Aldrich, 98%). Before use for thin-film synthesis: SC, IPC, TMC were distilled under vacuum at 150, 170, and 160◦ C, respectively. The TPC was sublimated under vacuum at 50◦ C. The purified acyl chlorides were stored in a vacuum desiccator containing calcium chloride to prevent hydrolysis of the acyl chloride. The diamines used were 1,2-dimethylenediamine (DMDA) (Aldrich, >99.5%), 1,6-hexamethylenedia-
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mine (HMDA) (Aldrich, 98%), 1,9-nonamethylenediamine (NMDA) (Aldrich, 98%), 1,3-benzendiamine (MPDA) (Aldrich, 99%), and 1,4-benzendiamine (PPDA, Aldrich, 99%). HMDA, NMDA, MPDA were distilled under vacuum at 180, 200, and 170◦ C, respectively. The PPDA was sublimated under vacuum at 50◦ C, and the DMDA was used without further purification. The purified diamines were filled in a dark bottle with N2 gas and stored in a refrigerator. The solvent for TMC, n-hexane, was dried by shaking with magnesium sulfate and then distilling. Before using purified n-hexane within 1–2 days, it was stored in a tightly closed bottle with Molecular Sieve A® to prevent dissolution of water. For the preparation of the diamine solution, RO purified and deionized water was used. 2.2. Synthesis of polyamides In order to study systematically on the effect of diamine and the acyl chloride structures, two series of IP experiments were conducted. In series I, MPDA was fixed as the amine monomer and a variety of acyl chloride monomers including SC, IPC, TPC, and TMC were tested. In series II, the full range of amines was studied including DMDA, HMDA, NMDA, PPDA, and MPDA, but only one acyl chloride, TMC, was fixed as the acyl chloride monomer. The active layer polyamides were synthesized via IP of the 0.5% (w/v) aqueous diamines solution and the 0.1% (w/v) n-hexane acyl chlorides solution. The reaction was conducted using an unstirred non-dispersion method for 2 min at 20◦ C. The thin-film polyamides formed at the solution interface were retrieved and precipitated in excess of acetone. The solid thin-film was filtered off, and the washed with acidic water, methanol, and fresh water to remove the unreacted monomer remnants and occluded salt. The pure solid polymers were dried under vacuum at room temperature and then used with no further post-treatment. The chemical structures of these polyamides are believed to be as shown in Table 1. 2.3. Preparation and characterization of TFC membrane TFC membranes were prepared via a two-step process: first, an asymmetric support polysulfone
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membrane was fabricated, and then the thin-film active layer was deposited on its surface. The asymmetric support layers were made as described elsewhere [17]. The thin-film active layers were prepared by in situ IP on the support layer using an unstirred non-dispersion method described in an earlier paper [13]. The concentrations of diamine and acyl chloride were fixed at 0.5 and 0.1% (w/v), respectively. The fabricated TFC membranes were used for permeation testing with no further post-treatment. Performance measurements of composite RO membranes were carried out at 3.03 MPa with 2000 ppm NaCl solution. The permeate and feed salt concentrations were measured using a standardized digital conductivity meter (YSI Co., model 32). The salt rejection (R) is given by Cs R (%) = 1 − × 100 (1) Cs where Cs is the salt concentration of the permeate stream and Cs the salt concentration of the feed. To minimize experimental error, the membrane permeabilities were measured from three samples and averaged. 2.4. Contact angle measurements In order to study the hydrophilicity of the active thin-films, contact angle measurements were made using an image analyzer system (PAS PX-380). Sets of liquid droplets of approximately 3 l volume were placed on IP films and observed through a video camera. The dimensions of the liquid droplets were measured approximately 10 s after placing the droplets on the IP film. The images of the liquid droplets were captured on videotape and the liquid droplets were magnified before the measuring the contact angle (θ). To minimize experimental error, the contact angle was measured six times for each sample and then averaged. The surface tension of dispersion and polar force for polymers can be obtained from the following equation [18]: cos θ =
4(γsd γLd /(γsd + γLd )) γL p p p p 4(γs γL /(γs + γL )) + −1 γL
(2)
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Table 1 Chemical structure of polyamides Chemical structure of monomers Diamines Series I
Series II
Chemical structure of polymers Acyl chlorides
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Table 1 (Continued) Chemical structure of monomers Diamines
Chemical structure of polymers Acyl chlorides
where θ is the contact angle of the liquid, γ L , γLd p and γL are total surface tension, dispersion force and polar force of surface tension for a standard liquid, p respectively; γsd and γs are dispersion force and polar force of surface tension for the polymer. Using the contact angle values with the reference liquids as shown in Table 2 and the surface components of the reference liquids, the components of surface tension for the thin-film polymer can be determined. 2.5. Measurement of water sorption In order to measure the concentrations of dissolved water in the active layer (C w ), the purified and weight-measured polymer was saturated with
2000 ppm NaCl aqueous solution for 7 day. The saturated polymer was retrieved from the NaCl solution, and the excess solution present at the surface of polymer was removed with a filter paper.
Table 2 Dispersion and polar force components of surface tension (dyn/cm) of reference liquids (20◦ C)a p
Reference liquids
γLd
γL
γL
Water Glycerol Formamide
29.1 37.4 35.1
43.7 26.0 23.1
72.8 63.4 58.2
a Superscripts d and p refer the dispersion component and polar component, respectively.
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Then the NaCl-saturated polymer was vacuum-dried at room temperature for 3 days. The dried and weight-measured samples were put in a chamber, which was saturated with moisture. After 1 day, the samples were again weighed in a microbalance, and the amount of absorbed water was calculated. To minimize experimental error, nine replicates were used.
resulting pressure-deformation behavior monitored. The rupture strength (SR ) of the thin-films can be obtained from the pressure and the drop-radius at the instant of IP film rupture via PDMA. An earlier paper discusses the details of the PDMA technique [13].
3. Results and discussion 2.6. Mechanical strength measurements 3.1. Permeation performance In order to study the mechanical resistance to rupture, the active layer thin-films when subjected to the high hydraulic RO operation pressure, and the stress in the IP films at the instant of rupture (rupture strength) was measured. To this end, a promising technique for directly measuring the mechanical properties of unsupported IP films was reported by Greenberg and coworkers as the “pendant drop mechanical analysis (PDMA)” [19]. PDMA was applied to measure the non-normalized structural parameter or rupture strength (SR ) of the unsupported thin-films [13]. PDMA first requires the formation of an IP film over the surface of a drop of diamine monomer solution when it is contacted by a solution containing acyl chloride monomer. This film-covered drop can then be pressurized in a controlled fashion and the
3.1.1. Effect of acyl chloride structure The different mechanical and physicochemical properties of linear as well as network structures prepared using di/tri-functional aromatic and aliphatic acyl chloride monomers were studied. Acyl chlorides used in series I result in different chemical structures in the final thin-film polymer as seen in Table 1. The di-functional acyl chlorides including SC, IPC, and TPC result in a linear structure, whereas the tri-functional acyl chloride, TMC results in a network structure. SC results in an aromatic (amine)–aliphatic (acyl chloride) structure, but the polymers prepared using aromatic acyl chlorides including IPC, TPC, and TMC have a fully aromatic structure. Fig. 2 shows the permeation performance in series I. The
Fig. 2. Effect of acyl chloride structures on the permeation performance. Note that the performance of membrane made by the reaction of MPDA (0.5% (w/v)) with various acyl chlorides (0.1% (w/v)) were obtained by 3.03 MPa, 2000 ppm NaCl solution at 25◦ C ( : Jw ; : R).
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Fig. 3. Effect of amine monomer structures on the permeation performance. Note that the performance of membrane made by the reaction of TMC (0.1% (w/v)) with various diamines (0.5% (w/v)) were obtained by 3.03 MPa, 2000 ppm NaCl solution at 25◦ C ( : Jw ; : R)
thin active films evidencing linear structures (SC, IPC, and TPC) had low salt rejection while network structure (TMC) gave higher salt rejection. Thin active films with the aromatic–aliphatic linear structure (SC) showed high water fluxes while fully aromatic linear structures (IPC and TPC) resulted in lower water fluxes. In summary, the thin active films with aromatic–aliphatic linear structure resulted in the best water flux while the thin-films with completely aromatic network structures resulted in the best salt rejection as seen in Fig. 2. These results imply that the permeation properties of the thin-films prepared from series I are related to the structural properties of the thin-film polymers. 3.1.2. Effect of amine structure The permeation performances for the composite membranes obtained from series II are shown in Fig. 3. All the thin-films obtained from this series have a cross-linked network structure. The thin-films prepared with aliphatic diamines including DMDA, HMDA, and NMDA have an aliphatic–aromatic network structure. However, the thin-films synthesized with aromatic diamines including PPDA and MPDA have fully aromatic network structures as shown in Table 1. All of these network structure polyamides had a relatively high salt rejection compared to the
linear structure polyamides as shown in Fig. 2. In the case of the membranes having aliphatic–aromatic network structures, the water flux and the salt rejection decrease with increasing length of the aliphatic methylene chain. On the other hand, the thin-films with a fully aromatic network structures evidence high salt rejection and relatively high water flux compared to the membranes having aliphatic–aromatic network structures. These permeation behaviors imply that the permeation properties of the thin-films are affected not only by the chemical properties, but also by the structure of the active layer polymer. As previously described, since the structural characteristics of a TFC membrane, the permeation performance of the TFC membrane can be affected by the chemical properties as well as by the mechanical strength of the active layer. In order to evaluate the factors affecting the permeation performance, the chemical properties as well as the mechanical strength of the active layer polymer are examined in the following paragraphs. 3.2. Effect of acyl chloride structure Fig. 4 shows the rupture strength of the active thin-films for series I. As previously noted, the rupture strength (SR ) is the stress in the IP films with
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Fig. 4. Dependence of the rupture strength of active layer of membrane on the acyl chloride structures. Note that the active layer of membrane were made by the reaction of MPDA (0.5% (w/v)) with various acyl chloride monomers (0.1% (w/v)).
specific thickness at the instant of rupture. Thus, it depends upon the rupture stress and the thickness of the active films [13], and represents a structural as well as a material characteristic of the active polymer. Meanwhile, the rupture of the active layer polymer in RO was affected by the various factors including the rupture strength and strain of the active layer polymer, the surface pore size of the support, and the applied pressure in RO. According to previous literature, the rupture strength of the active layer affecting the salt rejection is 5 kPa mm and the water flux is 3 kPa mm [13]. That is, if the rupture strength of the active layer is below 5 kPa mm, the salt rejection is affected by the bulk flow of brine. Furthermore, if the rupture strength of the active layer has below 3 kPa mm, the bulk flow of brine affect the water flux as well as salt rejection. Note that the comparison of the membrane performance is based on the same experimental conditions as described in a previous article [13]. The thin-films with linear structure prepared using diacyl chlorides (HDMC, IPC, and TPC) have low rupture strength (<2 kPa mm). On the other hand, thin-film with network structures prepared with a triacyl chloride (TMC) has high rupture strength (>5 kPa mm). It implies that the SR has a dependence upon the polymer structure, i.e. whether it is a lin-
ear or a cross-linked network. Thus, the permeation characteristics of the linear structure polyamides with low SR present the results of the chemical properties as well as the bulk flow of brine due to a partial break-up of the barrier layer. However, the permeation performance of the network polyamide having higher SR (>5 kPa mm) yield their inherent material properties. In order to assess the effect of the likely factors affecting permeation performance, the chemical properties of the active layer were studied in the polar p force of surface tension (γs ) and the concentration of dissolved water in the membrane (C w ). The polar force of surface tension was evaluated as a possible measure of the hydrophilicity of the thin active film. Fig. 5 shows the polar force of surp face tension (γs ) and the concentration of dissolved water (C w ) for membranes obtained from series I. In these polyamides, the components affecting the polar force are an amide bond, an amine and carboxylic acid end-group, and a free carboxylic acid group in network structure as seen in Table 1. The free carboxylic acid group would result in unreacted acyl chloride groups, which can easily be converted to the corresponding carboxylic acids through hydration. The amine and carboxylic acid groups are more hydrophilic than the amide bonds in the polyamide.
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Fig. 5. Dependence of the rupture strength of active layer of membrane on the amine monomer structures. Note that the active layer of membrane were made by the reaction of TMC (0.1% (w/v)) with various diamine monomers (0.5% (w/v)).
The presence of these end-groups depends upon presumably the degree of polymerization, which, in turn depends upon the reactivity and the structure of the monomers [20]. The thin-films made from various acyl chlorides showed different polar forces. The polar forces of the thin polyamide films with the fully aromatic linear structure are lower compared to the polyamide having aromatic–aliphatic linear structure. This behavior is also identified by the concentration of dissolved water in the membrane (C w ), which depends upon the hydrophilicity of the active layer. The p C w and γs showed similar behaviors. The water flux of the membranes with fully aromatic structures showed the dependence upon the polar force and the water content of the thin-film as shown in Figs. 2 and 5. However, if the polar force alone is considered, membranes with fully aromatic linear structures (IPC and TPC) show relatively high water fluxes compared with membranes having fully aromatic network structures (TMC). Also, the polyamide films with aromatic–aliphatic linear structures (SC) show unexpectedly high water fluxes. Although, this thin-film has high polar forces, they exhibit very high water fluxes compared with membranes made using aromatic acyl chlorides. This hints at some other influential factor causing high fluxes.
The salt rejection is related to the ratio of the water flux and the salt flux as shown in Eq. (1). Thus, high salt rejection implies that the water flux is relatively higher compared with the salt flux. The thin-films obtained from series I exhibited complicated salt rejection properties. As mentioned earlier, the salt rejection for series I seems to depend upon the structural properties of the thin-film. Membranes with aromatic–aliphatic linear structures have low salt rejections, which implies high water flux and also high salt flux. Among the membranes prepared with aromatic acyl chlorides, the thin-films with linear structures evidence low salt rejections, but network structures result in very high salt rejections. Hence, the salt rejection depends upon the polymer structure, which affects the rupture strength of the thin-films. The membranes for series I showed some dependence of water flux on hydrophilicity of the thin-films, but no dependence of the salt rejection. These permeation properties are a result of the effect of the rupture strength of the thin-films. Therefore, low salt rejection of the thin-films with linear structure, and the low salt rejection and the high water flux of membranes made with MPDA and SC are due to the effect of the rupture strength of the thinfilms.
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Fig. 6. Dependence of the polar force of surface tension and the concentration of dissolved water in the active layer on the acyl chloride structures. Note that the active layer of membrane were made by the reaction of MPDA (0.5% (w/v)) with various acyl chloride monomers p (0.1% (w/v)) ( : γs ; : Cw ).
3.3. Effect of amine structure Fig. 6 shows the rupture strength of the active layers for series II. Owing to their cross-linked network structure, these polyamides have a higher rupture strength compared to the polymers with linear structures (see Fig. 4). The active layers with fully aromatic network structures (PPDA and MPDA) have high rupture strength (>5 kPa mm). Here, because of the high rupture strength, bulk flow of brine originating from tearing of the active layer is avoided, and hence, the permeation performance corresponds to the intrinsic material property of the active layer. On the other hand, the active layers composed of the aliphatic–aromatic network structures (DMDA, HMDA, and NMDA) have a rupture strength close to 3 kPa mm. Here, the salt rejection is affected by the bulk flow of brine. However, because SR is around 3 kPa mm, the effect on the water flux is not prominent. Fig. 7 shows the polar force of surface tension and the concentration of dissolved water for membranes obtained from series II. The polar forces for the thin-films made from aliphatic diamines decrease as the methylene chain of the aliphatic amines increases in length. It implies that the effect of the content
of methylene chain in the polymer structure on the hydrophilicity is presumably prominent than that of end-group including amine and carboxylic acid in aliphatic diamines. Hence, the polar force of surface tension decreases with increasing methylene chain length. This behavior is also observed for the concentration of dissolved water in the membrane (C w ). The C w of the thin-films made from aliphatic diamines decrease as the methylene chain of the aliphatic amines increases in length. The water flux of these memp branes decreases as γs and C w decrease as shown in Figs. 3 and 7. As expected, for these membranes the permeation depends on the hydrophilicity, and p hence is sensitive to γs and C w . The fully aromatic polyamides (PPDA and MPDA) have higher C w and p γs compared to the polyamide composed of the similar hydrocarbon number (HMDA). This difference could be due to the effect of polymer structure. A detailed study on the effect of aromatic diamines on permeation performance will be discussed in another paper [21]. The salt rejection of the membranes made from aliphatic diamines decreases as the methylene chain length increases. The salt rejection and the water flux show a similar trend. That is, membranes with long
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Fig. 7. Dependence of the polar force of surface tension on the acyl chloride monomers. Note that the active layer of membrane were p made by the reaction of TMC (0.1% (w/v)) with various diamine monomers (0.5% (w/v)) ( : γs ; : Cw ).
methylene chains have low water fluxes, but relatively high salt fluxes. On the other hand, membranes prepared with aromatic diamines have excellent salt rejections compare with the membranes prepared from aliphatic diamines. These results also imply that the permeation performance of composite membranes depends upon the chemical properties as well as the mechanical strength of the active layer. In summary, the active layer polymers possessing sufficient mechanical strength to withstand the high hydraulic pressures during RO operation showed relatively good correspondence between the permeation performance and the chemical properties. However, the permeation characteristics of the active layer polymers with weak rupture strength were affected not only by the chemical properties but also the bulk flow of brine due to a partial break-up of the barrier layer.
4. Conclusions Membranes prepared with MPDA and various acyl chlorides have very different permeation performance and physicochemical properties. The thin-films with linear structures prepared using MPDA and di-functional acyl chlorides including SC, IPC, and TPC show low rupture strength while thin-film prepared with a tri-functional acyl chloride, TMC, result
in a network structure with high rupture strength. The permeation performance of composite membranes with low rupture strength active layers depends upon the rupture strength as well as the chemical properties (hydrophilicity) of the thin-films. The linear structure with low rupture strength has low salt rejections. Membranes prepared with TMC and various diamines have comparatively high rupture strength due to a network structure. These membranes also exhibited high salt rejections and evidenced a relatively good relationship between the chemical properties of the thin-film and the permeation performance of the composite membrane. Membranes composed of fully aromatic network structure have higher rupture strengths, higher salt rejections and comparatively higher water flux than the membranes consisting of aliphatic–aromatic network structures. Also, the methylene chain length of aliphatic diamines affects the water flux. The water flux is observed to decrease as the methylene chain length of the aliphatic diamines increases, which in turn decreases the hydrophilicity of the thin-films. These results indicate that the permeation properties of the composite membranes without adequate mechanical strength not only depend upon the chemical properties including hydrophilicity of the thin-film polymer, but also on the mechanical strength of the thin-films.
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