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Development of aromatic polyamide membranes for pervaporation and vapor permeation
Journal of Membrane Science 164 (2000) 241–249
Development of aromatic polyamide membranes for pervaporation and vapor permeation Min-Yu Teng a , Kueir-Rarn Lee a,∗ , Shu-Chin Fan b , Der-Jang Liaw c , James Huang d , Juin-Yih Lai b b
a Department of Chemical Engineering, Nanya Junior College of Technology, Chung Li, 32034, Taiwan Membrane Research Laboratory, Department of Chemical Engineering, Chung Yuan University, Chung Li, 32023, Taiwan c Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, 10772, Taiwan d Yeu Ming Tai Chemical Industrial Co. Ltd., P.O. BOX 46-196, Taichung Industrial Park, Taichung, 407, Taiwan
Received 18 February 1999; received in revised form 11 May 1999; accepted 7 June 1999
Abstract The pervaporation and vapor permeation performances of a series of fluorine-containing aromatic polyamide membranes for various alcohol mixtures were investigated. Compared with pervaporation, vapor permeation effectively increases the permselectivity of water. The solubility of alcohol in an aromatic membrane is higher than that of water, but the diffusivity of water across the membrane is higher than that of the alcohols. In addition, the relationship between the polymer structure and pervaporation performance was obtained. It was found that the permeation rate can be increased by the introduction of bulky substituted groups and arylene ether groups into the polymer backbone. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Vapor permeation; Fluorine-containing aromatic polyamide; Membrane
1. Introduction Liquid mixtures cannot be separated or concentrated by a distillation method such as: azeotropic mixtures, close-boiling point mixtures, isomers, and heat-sensitive mixtures. Pervaporation separation processes offer potentially more economical alternatives for the above difficult separation mixtures. Basically, transport in pervaporation separates an upstream liquid mixture from downstream permeants in the gaseous state with a reduced pressure. Aliphatic polyamide membranes have been regarded as promising membrane materials as a result of their excellent strength ∗ Corresponding author. Fax: +886-3-4563672. E-mail address: [email protected] (K.-R. Lee).
and commercial availability. Many researchers have focused their attention on improving the polyamide membrane separation performance, including: polymer blending, chemical grafting, plasma grafting, and ␥-ray irradiation [1–7]. Nevertheless, according to the solution diffusion model [8], the permselectivity of a membrane must be attributable to solubility and diffusivity. Thus, the efficiency of the pervaporation process depends mainly on the intrinsic properties of the polymers used to prepare the membrane. From this, the development of novel polyamide pervaporation membranes with good separation performance is exceedingly important. Since aromatic polyamide possesses good mechanical properties and chemical resistance, it can be used in many membrane separation processes such as gas separation and pervaporation [9–10]. How-
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ever, the aromatic polyamide is generally difficult to fabricate into membranes because it is difficult to dissolve in most organic solvents. Several researchers reported that the solubility of the aromatic polymer improves with the introduction of flexible links to the polymer backbone [11–13]. We have reported previously that a serious of fluorine-containing aromatic polyamides were soluble in polar solvents resulting from the introduction of the bulky group into the polymer backbone [9–10]. In addition, the effect of the aromatic polyamide structure on pervaporation performances has also been investigated. The permeation rate increases when the polymer backbone contains a bulky pendant group. Therefore, many efforts have been made to reduce polymer packing density and to enhance the specific volume. The purpose of this work is to study the effects of diamine structure on the aromatic polyamide properties. In addition, the effects of feed composition, the molar volume of alcohols, and the feed solution temperature on the pervaporation and vapor permeation performances of the prepared aromatic polyamide membranes were investigated.
2. Experimental
enyl phosphite, 1.8 ml of pyridine, and 6 ml of N-methyl-2-pyrrolidinone (NMP) was heated with stirring at 100◦ C for 3 h. After cooling, the reaction mixture was poured into large amount of methanol with constant stirring, producing a stringy precipitate that was washed thoroughly with methanol and hot water, collected on a filter, and dried at 100◦ C under vacuum. The IR spectrum (film) exhibited absorptions around 3300 cm−1 (N–H) and 1650 cm−1 (C=O). The synthetic route of the aromatic polyamide is indicated in Scheme 1.
2.3. Membrane preparation The polyamide membrane was prepared from a casting solution containing 10 wt% of polyamide in N,N-dimethylacetamide(DMAc). The membranes were formed by casting the solution onto a glass plate to a pre-determined thickness using a Gardener knife at room temperature. The glass plate was then heated at 70◦ C for 1 h. The average thickness of the membranes is about 25–30 m.
2.4. Specific volume measurements
2.1. Materials Three types of fluorine-containing polyamides were prepared by direct polymerization of 4,4-hexafluoroisopropylidenedibenzoic acid with various diamines, i.e. 1,4-bis(4-aminophenoxy)benzene), 1,4-bis(4-aminophenoxy)2-tert-butylbenzene and 1,4-bis(4-aminophenoxy)2,5-di-tert-butylbenzene. 1,4-bis(4-aminophenoxy)benzene was supplied from Wakayama Seika Co. Ltd. and used without further purification. The diamines, i.e. 1,4-bis(4-aminophenoxy)2-tert-butylbenzene and 1,4-bis(4-aminophenoxy)2,5-di-tertbutylbenzene, were prepared as described previously [14]. All reagent-grade chemicals were directly used without further purification. Water was de-ionized and distilled. 2.2. Polymerization A mixture of 2.5 mmol of diamin , 2.5 mmol of diacid, 0.60 g of calcium chloride, 1.8 ml of triph-
The specific volume was measured by using a micromeritrics Accupyc 1330 Pycnometer. This instrument measures the volume of the solid by the gas displacement method.
2.5. Characterization Elemental analysis was made(Perkin-Elmer 2400 instrument). Wide-angle X-ray diffraction scans were generated by a Philips model PW 1710 diffractometer using Ni-filtered Cu K␣ radiation(40 kV, 30 mA). Fourier transform infrared(FTIR) spectra were recorded on a Jasco FT/IR-7000). Thermogravimetric data were obtained on a Du Pont 2200 in flowing nitrogen(60 cm3 /min) at a heating rate of 20◦ C/min. Differential scanning calorimetry analysis was performed on a Perkin-Elmer DSC-7 differential scanning calorimeter in flowing nitrogen(60 cm3 /min) at a heating rate of 20◦ C/min.
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243
Scheme 1.
2.6. Sorption measurement The membranes were immersed in alcohol–water mixtures for 24 h at 25◦ C. They were subsequently blotted between tissue paper to remove excess solvent and placed in the left tube of a twin tube set-up. The system was evacuated while the left tube was heated with hot water and the right tube was cooled in liquid nitrogen. The composition of the condensed liquid in the right tube was determined by GC. 2.7. Pervaporation and vapor permeation measurement A traditional pervaporation process [7] was used. In pervaporation (Scheme 2(a)), the feed solution is in direct contact with the membrane. The effective area was 10.2 cm2 . The permeation rate was determined by measuring the weight of the permeate. The compositions of the feed solution and the permeate were measured by gas chromatography (GC China Chromatography 8700 T). The experiment of vapor permeation was carried out by using the same apparatus as pervaporation, except that the feed solution is not in contact with the membrane. The feed solution was vaporized first and then permeated through the membrane. It should be noted that, to prevent the feed liquid being in contact with the membrane, we turned the permeation cell upside down as shown in Scheme 2(b) and adjusted the flow rate of the feed carefully. In addition, to verify that the liquid feed was not in contact
with the membrane, we attached a filter paper to the membrane surface facing the feed solution. After the vapor permeation experiment, we found that the filter paper remained dry, confirming that the liquid feed was not in direct contact with the membrane. The separation factor. αA/B = (YA /YB )/(XA /XB ) Where XA , XB and YA , YB are the weight fractions of A and B in the feed and the permeate (A being the more permeative species), respectively. In vapor permeation, XA and XB are the weight fractions of water and alcohol vapors in the feed, and YA and YB are the weight fractions of the water and alcohol in the permeate.
3. Results and discussion 3.1. Properties of synthesized aromatic polyamides The properties of the synthesized aromatic polyamides are shown in Table 1. The inherent viscosities and densities of the aromatic polyamides are higher than 0.67 dl/g and 1.650 g/cm3 , respectively. The molecular weight is high enough, judging from the high viscosity of the polymer and toughness of the membrane. In addition, the glass transition temperature of these polymers was in the range of 215–262◦ C. The glass transition temperature decreases with increasing volume of the substituted group in the polymer backbone. Compared with the result achieved
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Scheme 2. Table 1 Properties of synthesized aromatic polyamides Polyamide
ηinh (dl/g)
Density (g/cm3 )
Tg (◦ C)
T10% (◦ C)
Char yield (%)
Crystallinity
Amide I Amide II Amide III
0.67 1.12 0.89
1.872 1.769 1.650
262 220 215
431 420 415
22 20 13
Amorphous Amorphous Amorphous
by Koros et al. [15], an opposite phenomenon was obtained. These observations might be due to the fact that the 2,5-di-tert-butylbenzene group has two bulky pendent groups which could result in increasing steric hindrance. The groups increase the space between
the polymer chains, and therefore the free volume making easier the rotational movements of the main chain segments. X-ray diffraction measurements of the polymers indicate that all aromatic polyamides are amorphous. This could be explained by the fact
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Table 2 Pervaporation and sorption properties of amide I membranes for alcohol–water mixturesa Alcohol
Total sorption (g/g)
Alcohol sorption (g/g)
Water sorption (g/g)
α pv
α sop
Permeation rate (g/m2 h)
Methanol Ethanol n-propanol t-butanol
0.10 0.15 0.22 0.28
0.027 0.054 0.105 0.149
0.073 0.096 0.115 0.131
6.1 10.7 46.2 1799.6
24.4 16.2 9.8 7.8
586 474 271 195
a Alcohol
in feed: 90 wt%; α pv : selectivity of pervaporation; α sop : selectivity of sorption.
that 2-5-di-tert-butylbenzene and hexafluorupane both have bulky pendent groups which can result in significant steric hindrance. That is, the packing density of polymer chains decreases during the membrane formation process, resulting in an increase in the specific volume. Moreover, it can be seen that the specific volume of the aromatic polyamides follows the order: amide III > amide II > amide I. From the viewpoint of molecular structure, the polymer with a larger substituted group (amide III), which gives a higher barrier to chain rotation in the polyamide membranes, may also inhibit local segmental motion more easily. Thus, it can be deduced that the polymer packing density is lower when a larger pendent group is introduced into the polymer backbone. The TGA results indicate that the decomposition temperature in nitrogen, at which 10% loss of mass was observed, for the polymers was in the range of 415 – 431◦ C. Moreover, the char yield at 800◦ C in nitrogen for all polymers exceeds 13%. Thus, the aromatic polyamides possess satisfactory thermal stability.
shape of alcohol. The separation factor was found to depend on the molecular length for this linear alcohol series; it was also found that the permeation rate of t-butanol is lower than that of n-propanol, which may be due to the steric hindrance of the former being higher than that of the latter. Table 2 also shows that the sorption selectivity of alcohol decreases in going from methanol to t-butanol. Compared with the pervaporation results, an opposite trend was obtained in sorption results. The interaction between permeants and polymer membranes can be used to further explain the above phenomena. The difference in the solubility parameters between the polymer membrane and alcohol for different alcohols are listed in Table 3. The difference in solubility parameters between the polymer membrane and alcohols follows the order of t-butanol < n-propanol < ethanol < methanol. Thus, the degree of swelling of the larger size alcohol is higher than that of the smaller size alcohol. These results completely support the data shown in Table 2; that is, the larger size alcohol has higher affinity for the membrane than the smaller size alcohol.
3.2. Effect of various alcohols on the pervaporation separation performances
3.3. Comparison of the pervaporation and vapor permeation performances of the aromatic polyamide membranes
To investigate the effects of solubility and diffusivity on the membrane permselectivity, sorption and pervaporation experiments for amide I membranes were made. The pervaporation and sorption properties of the amide I membranes for alcohol–water mixtures are shown in Table 2. The data show that an increase in the number of carbon atoms in alcohol results in an increase in the separation factor for pervaporation and a decrease in the separation factor for sorption, but gives a decrease in the permeation rate for pervaporation and an increase in total sorption. These results can be explained by the molecular size and
The performances of aqueous ethanol solutions through the amide I membrane by pervaporation and vapor permeation are shown in Table 4. Water is predominantly permeated through the amide I membranes in both methods. It is also shown in Table 4 that the permeation rate increases with an increase of ethanol concentration in the feed solution. Additionally, the permeation rate of vapor permeation is lower than that of pervaporation. These phenomena might be due to the fact that the membrane is in direct contact the feed solution and swollen for the
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Table 3 Effect of the difference between the solubility parameter of membrane and alcohol on the degree of swelling and the composition of the solution adsorbed in amideI membrane at 25◦ C Alcohol solution (90 wt%)
δ membr. −δ alcohol a
Degree of swelling (%)
Alcohol in membrane (wt%)
Methanol Ethanol n-propanol t-butanol
6.5 4.7 3.9 2.6
9.9 14.6 22.5 28.1
26.9 35.7 47.8 53.3
a␦
= 7.9 was predicted by Hoftyzer and Van Krevalen method.
membr
Table 4 Comparison of the pervaporation and vapor permeation performances of the amide I membranes at 25◦ C Ethanol concentration in feed (wt%)
Pervaporation
Vapor permeation
Permeation rate (g/m2 h)
H2 O in permeate (wt%)
Permeation rate (g/m2 h)
H2 O in permeate (wt%)
90 70 50 30 10
474 422 362 305 293
54.5 68.0 74.7 90.4 96.5
382 351 326 300 285
97.7 98.4 99.2 99.5 99.8
pervaporation process but is not in contact with the liquid feed for the vapor permeation process. Thus, the diffusivity of the permeating species in vapor permeation is lower than that in pervaporation because the swelling effect caused by the liquid feed is less obvious in vapor permeation. Moreover, the water concentration in permeate of amide I membranes in pervaporation is lower than those in vapor permeation. For example, the 385 separation factor with the 382 g/m2 h permeation rate of amide I membranes in vapor permeation can be obtained for a 90 wt% ethanol concentration. The permeation rate and separation factor are 474 g/m2 h and 10.7, respectively, for the pervaporation process under the same operating condition (see Table 4). This suggests that the vapor permeation method is effective in increasing the water permselectivity for the aqueous ethanol solution. 3.4. Effect of ethanol concentration in the feed on the permeation rate of vapor permeation for different aromatic polyamide membranes The effect of composition of feed on the permeation rate of vapor permeation for a series aromatic polyamide membranes is shown in Fig. 1. As the feed ethanol concentration increases, the permeation rate increases accordingly for all the aromatic polyamide membranes. These results might be due to the plas-
ticizing effect of ethanol. Generally, hydrophobic membranes have a stronger interaction with alcohol than with water. The degree of swelling of aromatic polyamide membranes also increases with increasing ethanol concentration, as shown in Fig. 2. When the ethanol concentration in the feed is higher, the amorphous region of the membranes is more swollen. Hence, the polymer chain in the swollen region becomes more flexible and the energy required for diffusive transport also decreases, resulting in permeation rate increases with feed ethanol concentration increases. Moreover, the increase of the molecular volume of the substituted group in the polymer backbone raises the permeation rate. These phenomena might be due to the fact that the aromatic polyamide (amide III) has a bulky segment (2,5-di-tert-butylbenzene) in the polymer backbone which could result in increasing steric hindrance. Thus, the free volume of the amide III membrane is higher than those of the amide I and amide II membranes. These results agree well with the results from the data of specific volume, shown in Fig. 3. 3.5. Effect of feed solution temperature on the pervaporation and vapor permeation performances The effect of feed solution temperature on permeation rates and separation factors for vapor
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Fig. 1. Effect of feed composition on the permeation rate of vapor permeation at 25◦ . (䊉) amide I (N) amide II (䊏) amide III.
Fig. 2. Effect of feed ethanol concentration on the degree of swelling for the amide I membrane at 25◦ .
Fig. 3. Effect of the polyamide structure on the specific volume at 25◦ .
permeation of 90 wt% aqueous ethanol solutions through aromatic polyamide membranes are shown in Fig. 4. It shows that the permeation rate increases and the separation factor decreases as feed solution temperature increases. From these phenomena it may
be assumed that the increases of swelling of the membrane matrix at higher temperature results in increased polymer segmental motions. Additionally, the partial pressure of ethanol in the vapor phase increases with increasing the feed solution temperature, resulting
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Fig. 4. Effect of the feed solution temperature on the vapor permeation performances. (䊉) separation factor, (䊊) permeation rate: amide I. (N) separation factor, (4) permeation rate: amide II. (䊏) separation factor, (䊐) permeation rate: amide III.
in the membrane further swelling. This facilitates the transport of ethanol molecules along with water, thereby reducing the separation factor. Moreover, compared with the amide I and amide II membranes, the permeation rates of a 90 wt% aqueous alcohol solution through the amide III membranes have a higher value, thus the pervaporation separation through the amide III membranes are more temperature-sensitive than that through the amide I and amide II membranes. Furthermore, the effect of feed solution temperature on the separation performances of pervaporation and vapor permeation through the amide I membranes are shown in Fig. 5. It shows that the separation factor of pervaporation is nearly constant irrespective of the feed solution temperature, but the separation factor of vapor permeation decreases drastically. These results can be illustrated as that the partial pressure of ethanol in the vapor phase increases with increasing feed solution temperature for the vapor permeation process, resulting in the liquid zone of the amide I membrane increases [16]. Thus, the transport of ethanol molecules along with water results in decreased separation factor. Nevertheless, no evident difference in the separation factor was found during the pervaporation process. These phenomena might be due to the fact that the individual permeation rate of water and ethanol through the amide membranes
Fig. 5. Effect of the feed solution temperature on the separation performances of pervaporation and vapor permeation for the amide I membrane. (䊏) permeation rate, (䊐) separation factor: pervaporation. (䊉) permeation rate, (䊊) separation factor: vapor permeation.
increased proportionately. Thus the separation factors were not appreciably different.
4. Conclusion The aromatic polyamide membranes with higher specific volume can be prepared by the introduction of a bulky group into the polymer backbone. Compared with pervaporation, vapor permeation effectively increases the permselectivity of water. The solubility of alcohol is higher than that of the water. Both the interaction between the permeant and membrane and the plasticizing effect of permeant on the membrane could significantly alter the permeation and separation properties of prepared membranes. In addition, the permeation rate can be increased by the introduction of bulky substituted groups into the polymer backbone.
References [1] Y.F. Xu, R.Y.M. Huang, Pervaporation separation of acetic acid–water mixtures using modified membranes. I. Blended polyacrylic acid (PAA)-nylon-6 membrane, J. Appl. Polym. Sci. 36 (1988) 1121. [2] R.Y.M. Huang, J.-W. Rhim, Prediction of pervaporation separation characteristics for methanol-pentane- nylon-6/ PAA blended membrane, J. Membr. Sci. 71 (1992) 211.
M.-Y. Teng et al. / Journal of Membrane Science 164 (2000) 241–249 [3] R.Y.M. Huang, M.M. Young and J.W. Rhim, Pervaporation separation of ethanol–water mixtures using nylon-4 and its blended membrane, Proc. 2nd Int. Conf. Sep. Sci. Technol., American Chemical Society, Washington, DC, 1 (1980) 74. [4] R.Y.M. Huang, A. Moreira, R. Notarfonzo, Y.F. Xu, Pervaporation separation of acetic acid–water mixtures using modified membranes. I. Blended polyacrylic acid (PAA)-nylon-6 membrane, J. Appl. Polym. Sci. 35 (1988) 1191. [5] R.Y.M. Huang, Y.F. Xu, Pervaporation separation of acetic acid–water mixtures using modified membranes. II. (␥-ray induced grafted polyacrylic acid (PAA)-nylon-6 membrane, J. Membr. Sci. 43 (1989) 143. [6] J.Y. Lai, R.Y. Chen, K.R. Lee, Polyvinyl alcohol ␥-ray grafted nylon 4 membrane for pervaporation and evapopermeation, Sep. Sci. Technol. 28 (1993) 1437. [7] K.R. Lee, R.Y. Chen, J.Y. Lai, Plasma deposition of vinyl acetate onto nylon 4 membrane for pervaporation and evapomeation separation of aqueous alcohol mixtures, J. Membr. Sci. 75 (1992) 171. [8] M.N. Pinho, Mass transfer in radiation–grafted pervaporation membrane, J. Membr. Sci. 54 (1990) 131. [9] K.R. Lee, D.J. Liaw, B.Y. Liaw, J.Y. Lai, Selective separation of water from aqueous alcohol solution through fluorine-containing aromatic polyamide membranes by pervaporation, J. Membr. Sci. 131 (1997) 249. [10] K.R. Lee, D.J. Liaw, B.Y. Liaw, J.Y. Lai, Sorption and transport properties of gases in fluoro-containing aromatic polyamide membrane, Eur. Polym. J. 34(5/6) (1998) 665.
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[11] Y. Saegusa, M. Kuriki, A. Kawai, S. Nakamura, Preparation and characterization of fluorine-containing aromatic condensation polymers. I. Preparation and characterization of fluorine-containing polycarbonate and copolycarbonate by two-phase transfer-catalyzed polycondensation of 2,2-bis(4-hydroxyphenyl)-1,1,1,3,3,3-hexafluor-opropan and/ or 2,2-bis(4-hydroxyphenyl) propane with trichloromethyl chloroformate, J. Polym. Sci. Part A: Polym. Chem. 28 (1990) 3327. [12] J.W. Park, M. Lee, J.W. Liu, S.D. Kim, J.Y. Chang, S.B. Rhee, Synthesis and characterization of soluble alternation aromatic copolyimides, Macromolecules 27 (1994) 3459. [13] T. Matsuura, Y. Hasuda, S. Nishi, N. Yamada, Polyimide derived from 2, 20 -bis(trifluoromethyl) -4,40 -(diaminobiphenyl). 1. Synthesis and characterization of polyimides with 2,2-bis (3,4-dicarbozyphenyl) hexafluoropropane dianhydride or pyromellitic dianhydride, Macromolecules 24 (1991) 5001. [14] K.R. Lee, Y.C. Chen, D.J. Liaw and J.Y. Lai, Novel aromatic polyamide membrane: Gas transport and sorption properties, J. Membr. Sci., (1998), submitted in revised form. [15] W.J. Koros, M.R. Coleman, D.R.B. Walker, Controlled permeability polymer membrane, Annu. Rev. Mater. Sci. 22 (1991) 47. [16] A. Mochizuki, S. Amiya, Y. Sato, H. Ogawara, S. Yamaskita, Pervaporation separation of water/ethanol mixtures through polysaccharide membrane. IV. The relationship between the permselectivity of alginic acid membrane and its solid state structure., J. Appl. Polym. Sci. 40 (1990) 385.
Development of aromatic polyamide membranes for pervaporation and vapor permeation Min-Yu Teng a , Kueir-Rarn Lee a,∗ , Shu-Chin Fan b , Der-Jang Liaw c , James Huang d , Juin-Yih Lai b b
a Department of Chemical Engineering, Nanya Junior College of Technology, Chung Li, 32034, Taiwan Membrane Research Laboratory, Department of Chemical Engineering, Chung Yuan University, Chung Li, 32023, Taiwan c Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei, 10772, Taiwan d Yeu Ming Tai Chemical Industrial Co. Ltd., P.O. BOX 46-196, Taichung Industrial Park, Taichung, 407, Taiwan
Received 18 February 1999; received in revised form 11 May 1999; accepted 7 June 1999
Abstract The pervaporation and vapor permeation performances of a series of fluorine-containing aromatic polyamide membranes for various alcohol mixtures were investigated. Compared with pervaporation, vapor permeation effectively increases the permselectivity of water. The solubility of alcohol in an aromatic membrane is higher than that of water, but the diffusivity of water across the membrane is higher than that of the alcohols. In addition, the relationship between the polymer structure and pervaporation performance was obtained. It was found that the permeation rate can be increased by the introduction of bulky substituted groups and arylene ether groups into the polymer backbone. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Pervaporation; Vapor permeation; Fluorine-containing aromatic polyamide; Membrane
1. Introduction Liquid mixtures cannot be separated or concentrated by a distillation method such as: azeotropic mixtures, close-boiling point mixtures, isomers, and heat-sensitive mixtures. Pervaporation separation processes offer potentially more economical alternatives for the above difficult separation mixtures. Basically, transport in pervaporation separates an upstream liquid mixture from downstream permeants in the gaseous state with a reduced pressure. Aliphatic polyamide membranes have been regarded as promising membrane materials as a result of their excellent strength ∗ Corresponding author. Fax: +886-3-4563672. E-mail address: [email protected] (K.-R. Lee).
and commercial availability. Many researchers have focused their attention on improving the polyamide membrane separation performance, including: polymer blending, chemical grafting, plasma grafting, and ␥-ray irradiation [1–7]. Nevertheless, according to the solution diffusion model [8], the permselectivity of a membrane must be attributable to solubility and diffusivity. Thus, the efficiency of the pervaporation process depends mainly on the intrinsic properties of the polymers used to prepare the membrane. From this, the development of novel polyamide pervaporation membranes with good separation performance is exceedingly important. Since aromatic polyamide possesses good mechanical properties and chemical resistance, it can be used in many membrane separation processes such as gas separation and pervaporation [9–10]. How-
0376-7388/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 2 0 6 - 9
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M.-Y. Teng et al. / Journal of Membrane Science 164 (2000) 241–249
ever, the aromatic polyamide is generally difficult to fabricate into membranes because it is difficult to dissolve in most organic solvents. Several researchers reported that the solubility of the aromatic polymer improves with the introduction of flexible links to the polymer backbone [11–13]. We have reported previously that a serious of fluorine-containing aromatic polyamides were soluble in polar solvents resulting from the introduction of the bulky group into the polymer backbone [9–10]. In addition, the effect of the aromatic polyamide structure on pervaporation performances has also been investigated. The permeation rate increases when the polymer backbone contains a bulky pendant group. Therefore, many efforts have been made to reduce polymer packing density and to enhance the specific volume. The purpose of this work is to study the effects of diamine structure on the aromatic polyamide properties. In addition, the effects of feed composition, the molar volume of alcohols, and the feed solution temperature on the pervaporation and vapor permeation performances of the prepared aromatic polyamide membranes were investigated.
2. Experimental
enyl phosphite, 1.8 ml of pyridine, and 6 ml of N-methyl-2-pyrrolidinone (NMP) was heated with stirring at 100◦ C for 3 h. After cooling, the reaction mixture was poured into large amount of methanol with constant stirring, producing a stringy precipitate that was washed thoroughly with methanol and hot water, collected on a filter, and dried at 100◦ C under vacuum. The IR spectrum (film) exhibited absorptions around 3300 cm−1 (N–H) and 1650 cm−1 (C=O). The synthetic route of the aromatic polyamide is indicated in Scheme 1.
2.3. Membrane preparation The polyamide membrane was prepared from a casting solution containing 10 wt% of polyamide in N,N-dimethylacetamide(DMAc). The membranes were formed by casting the solution onto a glass plate to a pre-determined thickness using a Gardener knife at room temperature. The glass plate was then heated at 70◦ C for 1 h. The average thickness of the membranes is about 25–30 m.
2.4. Specific volume measurements
2.1. Materials Three types of fluorine-containing polyamides were prepared by direct polymerization of 4,4-hexafluoroisopropylidenedibenzoic acid with various diamines, i.e. 1,4-bis(4-aminophenoxy)benzene), 1,4-bis(4-aminophenoxy)2-tert-butylbenzene and 1,4-bis(4-aminophenoxy)2,5-di-tert-butylbenzene. 1,4-bis(4-aminophenoxy)benzene was supplied from Wakayama Seika Co. Ltd. and used without further purification. The diamines, i.e. 1,4-bis(4-aminophenoxy)2-tert-butylbenzene and 1,4-bis(4-aminophenoxy)2,5-di-tertbutylbenzene, were prepared as described previously [14]. All reagent-grade chemicals were directly used without further purification. Water was de-ionized and distilled. 2.2. Polymerization A mixture of 2.5 mmol of diamin , 2.5 mmol of diacid, 0.60 g of calcium chloride, 1.8 ml of triph-
The specific volume was measured by using a micromeritrics Accupyc 1330 Pycnometer. This instrument measures the volume of the solid by the gas displacement method.
2.5. Characterization Elemental analysis was made(Perkin-Elmer 2400 instrument). Wide-angle X-ray diffraction scans were generated by a Philips model PW 1710 diffractometer using Ni-filtered Cu K␣ radiation(40 kV, 30 mA). Fourier transform infrared(FTIR) spectra were recorded on a Jasco FT/IR-7000). Thermogravimetric data were obtained on a Du Pont 2200 in flowing nitrogen(60 cm3 /min) at a heating rate of 20◦ C/min. Differential scanning calorimetry analysis was performed on a Perkin-Elmer DSC-7 differential scanning calorimeter in flowing nitrogen(60 cm3 /min) at a heating rate of 20◦ C/min.
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Scheme 1.
2.6. Sorption measurement The membranes were immersed in alcohol–water mixtures for 24 h at 25◦ C. They were subsequently blotted between tissue paper to remove excess solvent and placed in the left tube of a twin tube set-up. The system was evacuated while the left tube was heated with hot water and the right tube was cooled in liquid nitrogen. The composition of the condensed liquid in the right tube was determined by GC. 2.7. Pervaporation and vapor permeation measurement A traditional pervaporation process [7] was used. In pervaporation (Scheme 2(a)), the feed solution is in direct contact with the membrane. The effective area was 10.2 cm2 . The permeation rate was determined by measuring the weight of the permeate. The compositions of the feed solution and the permeate were measured by gas chromatography (GC China Chromatography 8700 T). The experiment of vapor permeation was carried out by using the same apparatus as pervaporation, except that the feed solution is not in contact with the membrane. The feed solution was vaporized first and then permeated through the membrane. It should be noted that, to prevent the feed liquid being in contact with the membrane, we turned the permeation cell upside down as shown in Scheme 2(b) and adjusted the flow rate of the feed carefully. In addition, to verify that the liquid feed was not in contact
with the membrane, we attached a filter paper to the membrane surface facing the feed solution. After the vapor permeation experiment, we found that the filter paper remained dry, confirming that the liquid feed was not in direct contact with the membrane. The separation factor. αA/B = (YA /YB )/(XA /XB ) Where XA , XB and YA , YB are the weight fractions of A and B in the feed and the permeate (A being the more permeative species), respectively. In vapor permeation, XA and XB are the weight fractions of water and alcohol vapors in the feed, and YA and YB are the weight fractions of the water and alcohol in the permeate.
3. Results and discussion 3.1. Properties of synthesized aromatic polyamides The properties of the synthesized aromatic polyamides are shown in Table 1. The inherent viscosities and densities of the aromatic polyamides are higher than 0.67 dl/g and 1.650 g/cm3 , respectively. The molecular weight is high enough, judging from the high viscosity of the polymer and toughness of the membrane. In addition, the glass transition temperature of these polymers was in the range of 215–262◦ C. The glass transition temperature decreases with increasing volume of the substituted group in the polymer backbone. Compared with the result achieved
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Scheme 2. Table 1 Properties of synthesized aromatic polyamides Polyamide
ηinh (dl/g)
Density (g/cm3 )
Tg (◦ C)
T10% (◦ C)
Char yield (%)
Crystallinity
Amide I Amide II Amide III
0.67 1.12 0.89
1.872 1.769 1.650
262 220 215
431 420 415
22 20 13
Amorphous Amorphous Amorphous
by Koros et al. [15], an opposite phenomenon was obtained. These observations might be due to the fact that the 2,5-di-tert-butylbenzene group has two bulky pendent groups which could result in increasing steric hindrance. The groups increase the space between
the polymer chains, and therefore the free volume making easier the rotational movements of the main chain segments. X-ray diffraction measurements of the polymers indicate that all aromatic polyamides are amorphous. This could be explained by the fact
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Table 2 Pervaporation and sorption properties of amide I membranes for alcohol–water mixturesa Alcohol
Total sorption (g/g)
Alcohol sorption (g/g)
Water sorption (g/g)
α pv
α sop
Permeation rate (g/m2 h)
Methanol Ethanol n-propanol t-butanol
0.10 0.15 0.22 0.28
0.027 0.054 0.105 0.149
0.073 0.096 0.115 0.131
6.1 10.7 46.2 1799.6
24.4 16.2 9.8 7.8
586 474 271 195
a Alcohol
in feed: 90 wt%; α pv : selectivity of pervaporation; α sop : selectivity of sorption.
that 2-5-di-tert-butylbenzene and hexafluorupane both have bulky pendent groups which can result in significant steric hindrance. That is, the packing density of polymer chains decreases during the membrane formation process, resulting in an increase in the specific volume. Moreover, it can be seen that the specific volume of the aromatic polyamides follows the order: amide III > amide II > amide I. From the viewpoint of molecular structure, the polymer with a larger substituted group (amide III), which gives a higher barrier to chain rotation in the polyamide membranes, may also inhibit local segmental motion more easily. Thus, it can be deduced that the polymer packing density is lower when a larger pendent group is introduced into the polymer backbone. The TGA results indicate that the decomposition temperature in nitrogen, at which 10% loss of mass was observed, for the polymers was in the range of 415 – 431◦ C. Moreover, the char yield at 800◦ C in nitrogen for all polymers exceeds 13%. Thus, the aromatic polyamides possess satisfactory thermal stability.
shape of alcohol. The separation factor was found to depend on the molecular length for this linear alcohol series; it was also found that the permeation rate of t-butanol is lower than that of n-propanol, which may be due to the steric hindrance of the former being higher than that of the latter. Table 2 also shows that the sorption selectivity of alcohol decreases in going from methanol to t-butanol. Compared with the pervaporation results, an opposite trend was obtained in sorption results. The interaction between permeants and polymer membranes can be used to further explain the above phenomena. The difference in the solubility parameters between the polymer membrane and alcohol for different alcohols are listed in Table 3. The difference in solubility parameters between the polymer membrane and alcohols follows the order of t-butanol < n-propanol < ethanol < methanol. Thus, the degree of swelling of the larger size alcohol is higher than that of the smaller size alcohol. These results completely support the data shown in Table 2; that is, the larger size alcohol has higher affinity for the membrane than the smaller size alcohol.
3.2. Effect of various alcohols on the pervaporation separation performances
3.3. Comparison of the pervaporation and vapor permeation performances of the aromatic polyamide membranes
To investigate the effects of solubility and diffusivity on the membrane permselectivity, sorption and pervaporation experiments for amide I membranes were made. The pervaporation and sorption properties of the amide I membranes for alcohol–water mixtures are shown in Table 2. The data show that an increase in the number of carbon atoms in alcohol results in an increase in the separation factor for pervaporation and a decrease in the separation factor for sorption, but gives a decrease in the permeation rate for pervaporation and an increase in total sorption. These results can be explained by the molecular size and
The performances of aqueous ethanol solutions through the amide I membrane by pervaporation and vapor permeation are shown in Table 4. Water is predominantly permeated through the amide I membranes in both methods. It is also shown in Table 4 that the permeation rate increases with an increase of ethanol concentration in the feed solution. Additionally, the permeation rate of vapor permeation is lower than that of pervaporation. These phenomena might be due to the fact that the membrane is in direct contact the feed solution and swollen for the
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Table 3 Effect of the difference between the solubility parameter of membrane and alcohol on the degree of swelling and the composition of the solution adsorbed in amideI membrane at 25◦ C Alcohol solution (90 wt%)
δ membr. −δ alcohol a
Degree of swelling (%)
Alcohol in membrane (wt%)
Methanol Ethanol n-propanol t-butanol
6.5 4.7 3.9 2.6
9.9 14.6 22.5 28.1
26.9 35.7 47.8 53.3
a␦
= 7.9 was predicted by Hoftyzer and Van Krevalen method.
membr
Table 4 Comparison of the pervaporation and vapor permeation performances of the amide I membranes at 25◦ C Ethanol concentration in feed (wt%)
Pervaporation
Vapor permeation
Permeation rate (g/m2 h)
H2 O in permeate (wt%)
Permeation rate (g/m2 h)
H2 O in permeate (wt%)
90 70 50 30 10
474 422 362 305 293
54.5 68.0 74.7 90.4 96.5
382 351 326 300 285
97.7 98.4 99.2 99.5 99.8
pervaporation process but is not in contact with the liquid feed for the vapor permeation process. Thus, the diffusivity of the permeating species in vapor permeation is lower than that in pervaporation because the swelling effect caused by the liquid feed is less obvious in vapor permeation. Moreover, the water concentration in permeate of amide I membranes in pervaporation is lower than those in vapor permeation. For example, the 385 separation factor with the 382 g/m2 h permeation rate of amide I membranes in vapor permeation can be obtained for a 90 wt% ethanol concentration. The permeation rate and separation factor are 474 g/m2 h and 10.7, respectively, for the pervaporation process under the same operating condition (see Table 4). This suggests that the vapor permeation method is effective in increasing the water permselectivity for the aqueous ethanol solution. 3.4. Effect of ethanol concentration in the feed on the permeation rate of vapor permeation for different aromatic polyamide membranes The effect of composition of feed on the permeation rate of vapor permeation for a series aromatic polyamide membranes is shown in Fig. 1. As the feed ethanol concentration increases, the permeation rate increases accordingly for all the aromatic polyamide membranes. These results might be due to the plas-
ticizing effect of ethanol. Generally, hydrophobic membranes have a stronger interaction with alcohol than with water. The degree of swelling of aromatic polyamide membranes also increases with increasing ethanol concentration, as shown in Fig. 2. When the ethanol concentration in the feed is higher, the amorphous region of the membranes is more swollen. Hence, the polymer chain in the swollen region becomes more flexible and the energy required for diffusive transport also decreases, resulting in permeation rate increases with feed ethanol concentration increases. Moreover, the increase of the molecular volume of the substituted group in the polymer backbone raises the permeation rate. These phenomena might be due to the fact that the aromatic polyamide (amide III) has a bulky segment (2,5-di-tert-butylbenzene) in the polymer backbone which could result in increasing steric hindrance. Thus, the free volume of the amide III membrane is higher than those of the amide I and amide II membranes. These results agree well with the results from the data of specific volume, shown in Fig. 3. 3.5. Effect of feed solution temperature on the pervaporation and vapor permeation performances The effect of feed solution temperature on permeation rates and separation factors for vapor
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Fig. 1. Effect of feed composition on the permeation rate of vapor permeation at 25◦ . (䊉) amide I (N) amide II (䊏) amide III.
Fig. 2. Effect of feed ethanol concentration on the degree of swelling for the amide I membrane at 25◦ .
Fig. 3. Effect of the polyamide structure on the specific volume at 25◦ .
permeation of 90 wt% aqueous ethanol solutions through aromatic polyamide membranes are shown in Fig. 4. It shows that the permeation rate increases and the separation factor decreases as feed solution temperature increases. From these phenomena it may
be assumed that the increases of swelling of the membrane matrix at higher temperature results in increased polymer segmental motions. Additionally, the partial pressure of ethanol in the vapor phase increases with increasing the feed solution temperature, resulting
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Fig. 4. Effect of the feed solution temperature on the vapor permeation performances. (䊉) separation factor, (䊊) permeation rate: amide I. (N) separation factor, (4) permeation rate: amide II. (䊏) separation factor, (䊐) permeation rate: amide III.
in the membrane further swelling. This facilitates the transport of ethanol molecules along with water, thereby reducing the separation factor. Moreover, compared with the amide I and amide II membranes, the permeation rates of a 90 wt% aqueous alcohol solution through the amide III membranes have a higher value, thus the pervaporation separation through the amide III membranes are more temperature-sensitive than that through the amide I and amide II membranes. Furthermore, the effect of feed solution temperature on the separation performances of pervaporation and vapor permeation through the amide I membranes are shown in Fig. 5. It shows that the separation factor of pervaporation is nearly constant irrespective of the feed solution temperature, but the separation factor of vapor permeation decreases drastically. These results can be illustrated as that the partial pressure of ethanol in the vapor phase increases with increasing feed solution temperature for the vapor permeation process, resulting in the liquid zone of the amide I membrane increases [16]. Thus, the transport of ethanol molecules along with water results in decreased separation factor. Nevertheless, no evident difference in the separation factor was found during the pervaporation process. These phenomena might be due to the fact that the individual permeation rate of water and ethanol through the amide membranes
Fig. 5. Effect of the feed solution temperature on the separation performances of pervaporation and vapor permeation for the amide I membrane. (䊏) permeation rate, (䊐) separation factor: pervaporation. (䊉) permeation rate, (䊊) separation factor: vapor permeation.
increased proportionately. Thus the separation factors were not appreciably different.
4. Conclusion The aromatic polyamide membranes with higher specific volume can be prepared by the introduction of a bulky group into the polymer backbone. Compared with pervaporation, vapor permeation effectively increases the permselectivity of water. The solubility of alcohol is higher than that of the water. Both the interaction between the permeant and membrane and the plasticizing effect of permeant on the membrane could significantly alter the permeation and separation properties of prepared membranes. In addition, the permeation rate can be increased by the introduction of bulky substituted groups into the polymer backbone.
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