Effect of Annealing Temperature and Time on Structure and Performance of Poly(vinyl)alcohol Nanocomposite Membranes
Anahid Sabetghadam, Toraj Mohammadi Research Centre for Membrane Separation Processes, Department of Chemical Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran
The importance of annealing process in tuning the morphological properties of membranes is obvious. In this study, poly(vinyl)alcohol as a continuous phase in nanocomposite membranes was crosslinked by mixed silanes. Tetraethylorthosilicate (TEOS) and Aminopropyltriethoxysilane (APTEOS) as two kinds of silane coupling agents go through cohydrolization and cocondensation reactions during sol–gel process to create nanoparticles in the polymer matrix. The condensation reaction is endothermic, which leads to formation of linking bonds between silica nanoparticles and polymer chains during dehydration process. This reaction is sensitive to temperature and time of annealing. The results showed that the PAT33 nanocomposite membrane performs much better separation than other prepared membranes. In this work, effects of annealing temperature and time on physicochemical properties of the prepared nanocomposite membranes and their pervaporation performance were investigated. POLYM. ENG. SCI., 50:2392–2399, 2010. ª 2010 Society of Plastics Engineers
INTRODUCTION According to type and intensity of interactions between organic and inorganic phases, organic–inorganic membrane materials can be further distinguished as two different classes. Many researchers continue to explore the ‘‘Class I’’ organic–inorganic nanocomposite membranes, in which organic and inorganic components interact through weak hydrogen bonding, van der Waals or electrostatic forces. This class of nanocomposite membranes is prepared by physically inserting inorganic particles, such as zeolites [1–3], carbon molecular sieve [4, 5], silica, [6, 7] and crystalline graphite flake [8] into polymeric membranes, which suffer from formation of nonselective voids at the organic–inorganic interface and serious agglomeration of the inorganic particles. To solve this Correspondence to: T. Mohammadi; e-mail:
[email protected] DOI 10.1002/pen.21763 Published online in Wiley Online Library (wileyonlinelibrary.com). C 2010 Society of Plastics Engineers V
problem, the ‘‘Class II’’ organic–inorganic nanocomposite membranes [9–13], in which organic and inorganic components are linked together through strong ionic/covalent bonding, have attracted more and more research interest. Because of better combination of organic moiety and inorganic moiety, the ‘‘Class II’’ organic–inorganic nanocomposite membranes have shown significantly improved separation properties in many cases. It is well known that the structural morphology of membranes is central to the separation efficiency of organic–inorganic nanocomposite membranes. Among many kinds of influencing factors, annealing is more and more recognized as an important step to control the morphology of prepared membranes [14]. In recent years, organic–inorganic nanocomposite membranes for pervaporation (PV) separation prepared from tetraethoxysilane (TEOS) have been reported [9, 11– 13]. The silica network is formed from self-condensation reaction of the hydrolyzed SiOH groups. Unfortunately, the fast reaction rate of TEOS self-condensation also brings about some inconveniences for preparation of the nanocomposite membranes [15]. To control the reaction rate and the morphology more conveniently, glycidyloxypropyltrimethoxysilane (GPTMS) was used to prepare PVA-GPTMS nanocomposite membranes by Jiang and coworkers [15]. When the nanocomposite membranes were annealed, water permselectivity increased with increasing annealing temperature and time. The annealing process promoted the dehydration-condensation reaction between PVA and silane coupling agents, and leaded to an enhanced water permselectivity of the nanocomposite membranes [9, 16]. Liu and coworkers prepared nanocomposite membranes by using aminopropyltriethoxysilane (APTEOS) as silane coupling agent. They investigated effects of annealing temperature and time on the morphology and performance of the nanocomposite membranes [17]. In this study, organic–inorganic nanocomposite, nanocomposite membranes composed of PVA and silica nanoparticles were prepared using in situ sol– gel method. Two kinds of silanes, TEOS, and APTEOS, were used for carrying out the hydrolysis and condensation reactions via the sol–gel method. Effects of annealing temPOLYMER ENGINEERING AND SCIENCE—-2010
TABLE 1. Annealing process parameters.
perature and time on crystallinity, morphology, and separation performance of the membranes were investigated.
Temperature of annealing for 8 h (8C)
Time of annealing at 1208C (h)
80 80
4 4
EXPERIMENTAL PVA PAT33
120 120
150 150
8 8
12 12
Materials Poly(vinyl)alcohol (PVA) with 98% hydrolization degree and molecular weight of 1,45,000 was purchased from Merck. Also TEOS, APTEOS, and hydrochloric acid (HCl) (1 N) were supplied by Merck and were used without any purification. Deionized water was used in this study as solvent.
MEMBRANE CHARACTERIZATION Contact Angle
Membrane Preparation PVA solution was prepared by complete dissolving of the polymer powder (5.0 g) in 100 mL of deionized water (18.0 MW), at a temperature of 908C under magnetic stirring. The PVA solution (5.0 wt%) was cooled down to room temperature (258C) with stirring. The pH value was adjusted to 2.0 with addition of HCl (1 N). Then, according to the polymer composition, TEOS and APTEOS were added to the solution gently with a certain mass ratio of 1:1 (APTEOS and TEOS to PVA). If pH value of the solution increased, it would be necessary to add more acid as catalyst to the solution to readjust it to 2 during the solution preparation process. The solution pH is of crucial importance on coupling the silanes to the polymers. Then the solution should be stirred for 12 h to complete hydrolization reaction until no phase separation occurred and the hydrolyzed silanes with OH group could be dispersed through the aqueous solution. Hydrolization reactions of TEOS and APTEOS are presented as follows: HCl=H2 O
SiðOCH2 CH3 Þ4 þ xH2 O ! SiðOHÞx ðOCH2 CH3 Þ4x þ x CH2 CH3 OH HCl=H2 O
H2 NðCH2 Þ3 SiðOCH2 CH3 Þ3 þ xH2 O !
H2 NðCH2 Þ3 SiðOHÞx ðOCH2 CH3 Þ3x þ x CH2 CH3 OH After stirring for 12 h, the solution would be casted on a plexiglass plate by a casting knife and dried for 48 h at 258C. Then the membranes would be pealed off and more dried at higher temperatures in an oven at 808C for 2 h and at 1508C for 8 h. To investigate the effect of relative APTEOS content on physical structure and PV performance of the nanocomposite membranes, it was varied as 0, 0.25, 0.33, 0.50, 0.75 and the resulting membranes were designated as PAT0, PAT25, PAT33, PAT50, and PAT75, respectively. Thickness of the prepared membranes was 35 6 1 lm, as measured by a digital micrometer (Mitutoyo, Model MDC-25SB). For annealing process the optimized nanocomposite membrane and PVA DOI 10.1002/pen
were selected for further investigation. According to Table 1, the annealing process is the membrane drying in three different temperature and time.
Contact angle between water and the membranes was directly measured using a contact angle measuring instrument [G10, KRUSS, Germany] to evaluate their hydrophilicity. Deionized water was used as the probe liquid in all the measurements. To minimize the experimental error, the contact angle was measured at five random locations for each sample at room temperature and then the average values were reported. Fourier Transforms Infrared (FTIR) Spectroscopic Analysis Fourier transform infrared (FTIR) spectra were obtained by an instrument [8400S SHIMADZU, Japan] equipped with attenuated total reflectance accessories (ATR). Scanning Electron Microscopic (SEM) Analysis Scanning electron micrographs of the nanocomposite membranes were taken to study their morphology by a scanning electron microscope (SEM) [Philips XL-30, Netherlands]. X ray Diffractometer (XRD) Analysis Physical structure of the PVA and the nanocomposite membranes was studied at room temperature using CAD4-PDP11/44 X-ray diffractometer (Enraf—Nonious, Netherlands). The dried membranes were mounted on the microscope slides; the membrane samples were scanned in the reflection mode at an angle 2y 20. Pervaporation Separation of Ethanol/Water Mixtures PV experiments were carried out at a temperature of 508C and using 15 and 25% water in ethanol solutions. Effective surface area of the nanocomposite membranes in contact with the feed was 36 cm2. Pressure in the permeate side was maintained at 12 mmHg. The feed was circulated in the set up for 1.5 h before starting the POLYMER ENGINEERING AND SCIENCE—-2010 2393
FIG. 2. Effect of APTEOS content on PV performance of PAT nanocomposite membranes (75% ethanol aqueous solution at 508C).
FIG. 1. PVA and fully crosslinked PVA chains.
experiments and permeate was collected in a liquid nitrogen cold trap. Concentration of permeate was measured using an Abbe’s refractometer (Nova-tech C10C, accuracy is 60.0003 units). Permeation properties of the nanocomposite membranes were characterized by permeation flux (Jp), separation factor (asep) and PV separation index (PSI), calculated using the following equations, respectively: JP ¼
WP At
(1)
PH2 O =PETOH FH2 O =FETOH
(2)
PSI ¼ JP ðasep 1Þ
(3)
asep ¼
where WP is the mass of permeate (kg); A is the membrane area in contact with the feed mixture (m2); t is the permeation time (h); PH2 O and PETOH are the mass fractions of water and ethanol in permeate and FH2 O and FETOH are the mass fractions of water and ethanol in the feed, respectively.
ing the membrane drying at room and higher temperatures. Cocondensation reaction is dehydration of the silanols which causes synthesis of silica nanoparticles. Also, it creates linking bonds between the silica nanoparticles and the PVA chains by dehydration of silanols and hydroxyl groups of PVA [15]. In other words, the PVA chains are crosslinked by silane coupling agents and this remarkably improves structural properties of the nanocomposite membranes. Figure 2 shows permeation flux and separation factor of the nanocomposite membranes. Increasing permeation flux can be attributed to the fact that the larger aminopropyl groups in APTEOS decreases the compactness of polymer chains [17]. On the other hand, if only TEOS is used as silane coupling agent, the polymer matrix compactness increases and permeation flux decrease significantly. In addition, separation factor increases sharply from PAT0 to PAT33 as compactness of the membranes increases and after that it decreases from PAT33 to PAT75. According to the results of contact angle measurement in Table 2, the highest hydrophilicity of the membranes is belonged to the PAT33. The reason for that may be due to the higher number of hydroxyl groups attached to the polymer-silica matrix in this membrane. In lower APTEOS content, hydroxyl groups of TEOS are going through condensation reaction with each other because its hydrolyzation and condensation reactions are
RESULTS AND DISCUSSION TABLE 2. Contact angle measurement.
Pervaporation Performance of the Nanocomposite Membranes Structure of the nanocomposite membranes prepared by mixing APTEOS and TEOS silane coupling agents is illustrated in Fig. 1. Silanol molecules are formed by cohydrolization of APTEOS and TEOS in aqueous solution. The silanols go through cocondensation reaction dur2394 POLYMER ENGINEERING AND SCIENCE—-2010
Membrane PVA PAT0 PAT20 PAT33 PAT50 PAT75
Contact angle (8) 66.8 73.9 69.7 64.8 68.5 72.6
DOI 10.1002/pen
FIG. 3. Effects of APTEOS content on sorption selectivity of PAT nanocomposite membranes.
so fast and also the extra APTEOS content decreases the hydrophilicity of the membranes [15, 18]. Increasing separation factor can also be attributed to the sorption properties of nanocomposite membranes. As illustrated in Fig. 3, with increasing the relative APTEOS content in the nanocomposite membranes, sorption selectivity increases from PAT0 to PAT33 and after that it decreases.
FIG. 5. Effects of annealing temperature on crosslinking bond formation of PVA membrane.
selected for further investigation of the effects of annealing temperature and time on their characteristics. Chemical Structure of the Annealed Membranes
Pervaporation separation index (PSI) of the nanocomposite membranes is illustrated in Fig. 4. As can be observed, PSI values increase with increasing the relative APTEOS content up to 33 (wt%). As both permeation flux and separation factor increase from PAT0 to PAT33 membranes, the highest PSI is for PAT33 membrane. As a result, PAT33 membrane can be recommended as an optimized prepared nanocomposite membrane. Consequently, the PAT33 and pure PVA membranes were
The ATR-IR spectra of PVA and PAT33 nanocomposite membranes in the range of 400–2000 cm21 were presented in Figs. 5–8. The absorption peak at 1000–1100 cm21 is assigned to the stretching vibration of CO groups in PVA. According to the ATR-IR analysis increasing the annealing temperature increases the absorption peak due to the more performed condensation reaction. The higher absorbance peak at 1000–1100 cm21 is related to the formation of siloxane (SiOSi) and (SiOC) bonds in PAT33 nanocomposite membrane. When hydroxyl groups of the PVA membrane go through condensation and crosslinking reactions, the resulting membrane absorbs more infrared rays and its transmission decreases. As it can be observed in Figs. 5 and 6, the
FIG. 4. Effects of APTEOS content on PSI of PAT nanocomposite membranes (75% ethanol aqueous solution at 508C).
FIG. 6. Effects of annealing temperature on crosslinking bond formation of PAT33 nanocomposite membrane.
Pervaporation Separation Index
DOI 10.1002/pen
POLYMER ENGINEERING AND SCIENCE—-2010 2395
FIG. 7. Effects of annealing time on crosslinking bond formation of PVA membrane.
peak of CO bond becomes wider after crosslinking reaction [18]. As illustrated in Figs. 7 and 8, with increasing the annealing time from 4 to 12 h, the intensity of absorption peaks increases. Higher intensity of the absorption peaks indicates that more SiOSi and SiOC bonds are formed in the polymer matrix [15]. As a result, it can be said that, more linking bonds between the polymer chains and the silica nanoparticles are formed.
FIG. 9. Effect of annealing temperature on crystallinity of PVA and PAT33 nanocomposite membranes.
In semicrystalline polymers, the amorphous region favors diffusion of all the components resulting in higher permeation fluxes, and the crystalline region, which is compact, enhances permeation of just smaller molecules resulting in higher permselectivities. PVA is a semicrystalline polymer; its crystallinity is generally about 65– 80% [17]. Figure 9 shows the XRD patterns of PVA and PAT33 nanocomposite membranes annealed at 80, 120, and 1508C for 8 h. As observed, the typical peak of PVA
and PAT33 nanocomposite membranes appears at 2y 20, and its intensity increases with increasing the annealing temperature due to more crosslinking reactions. In the annealing process, the dissociative hydroxyl groups in the PVA chains go through dehydration reaction resulting in more crystalline region. As observed in Fig. 10, the same trend is found for the annealing time, however its extent is less significant. This is due to the fact that the condensation reaction in the sol–gel process is endothermic, so increasing temperature effectively causes more crosslinking reactions of hydroxyl groups. In general, increasing annealing temperature and time lead to more linking bonds formation between the nanoparticles and the polymer chains in PAT33 nanocomposite membrane. These reactions increase crystallinity and chain packing in the membrane structure [17]. On the other hand, by comparing Figs. 9 and 10, it is revealed that peak intensity of PVA membrane is higher than that of PAT33 nanocomposite membrane in all cases, however according to Fig. 9 the difference between peak intensity of the two membranes is much more significant. It may relate to large
FIG. 8. Effects of annealing time on crosslinking bond formation of the PAT33 nanocomposite membrane.
FIG. 10. Effect of annealing time on crystallinity of PVA and PAT33 nanocomposite membranes.
Physical Structure of the Annealed Membranes
2396 POLYMER ENGINEERING AND SCIENCE—-2010
DOI 10.1002/pen
TABLE 3. The XRD peak area for different annealing temperature.
TABLE 4. The XRD peak area for different annealing time.
Annealed membrane
Peak area (m2)
Annealed membrane
Peak area (m2)
Annealed membrane
Peak area (m2)
Annealed membrane
Peak area (m2)
PVA (808C) PVA (1208C) PVA (1508C)
3034.5 3539.8 4896.4
PAT33 (808C) PAT33 (1208C) PAT33 (1508C)
945 1103.7 1178.4
PVA (4 h) PVA (8 h) PVA (12 h)
2833.3 3136.8 3539.8
PAT33 (4 h) PAT33 (8 h) PAT33 (12 h)
1178.4 1882.8 2020.2
number of reacted aminopropyl groups in APTEOS and this decreases the compactness, while increases the amorphous region in the PAT33 membrane [15]. It can thus be concluded that the effect of annealing temperature on crystallinity of PVA membrane is more significant than that of PAT33 nanocomposite membrane. The area under the XRD typical peak was calculated by multiplying the net height of peak in 2y. The typical peak for all the membranes is illustrated at 2y 25. The result of this calculation is presented in Tables 3 and 4. According to Tables 3 and 4, increasing the annealing temperature and time increases the area under the typical peak. Thus, these results confirm increasing of crystallinity quantitatively.
Scanning Electron Microscopy (SEM) Surface SEM images of the annealed PVA, PAT0 (without APTEOS), and PAT33 nanocomposite membranes are presented in Fig. 11. It can be observed that the size of synthesized silica nanoparticles in the polymer matrix is in the range of 80–250 nm in presence of two kinds of silanes. As also observed, the second silane coupling agent (APTEOS) improves the dispersion of synthesized silica nanoparticles on the surface of PAT33 nanocomposite membrane which annealed at 1508C for 8h. The condensation reactions of the TEOS are fast and APTEOS limits the rate of condensation reactions in the annealing process; thus smaller particles in the range of nanometer are created in the polymer matrix [15, 17].
Thermal Gravimetric Analysis (TGA) Thermal stability of the nanocomposite membranes was studied by TGA analysis. The TGA graphs of PVA
and PAT33 membranes with and without annealing are illustrated in Fig. 12. According to this Figure, thermal stability of PAT33 nanocomposite membranes increase with annealing process because the compatibility between the polymer chains and the nanoparticles increases due to the more crosslinking bonds formation, and as a result, the first degradation temperature postpones to higher temperature (1508C) [8]. According to Fig. 12, the PVA first and second degradations occur at lower temperatures in comparison with the PAT33 membranes with or without annealing. This observation revealed that crosslinking and silica phase creation in the PAT33 membrane increase thermal stability of the nanocomposite membranes.
Pervaporation Performance of the Annealed Membranes According to the ATR-IR and XRD characterization, it was found out that effects of annealing temperature and time on chemical and physical structures of the synthesized membranes are significant. Furthermore, it is well known that structural characteristics have important role on PV performance of the membranes [19]. The permeation properties of PAT33 nanocomposite and PVA membranes were investigated using 75 wt% ethanol aqueous solutions at 508C. Effects of annealing temperature on permeation properties of PVA and PAT33 nanocomposite membranes are illustrated in Fig. 13, respectively. As observed, annealing temperature decreases permeation flux and increases separation factor of both PVA and PAT33 nanocomposite membranes. These results confirm that annealing of the nanocomposite membranes causes them to become denser due to the dehydration and condensation reactions. Also, regarding the PVA membrane, hydroxyl groups of the
FIG. 11. The SEM photographs of (a) PVA, (b) PAT0, and (c) PAT33 membranes.
DOI 10.1002/pen
POLYMER ENGINEERING AND SCIENCE—-2010 2397
FIG. 15. Effect of annealing time on PV performance of PVA and PAT33 nanocomposite membrane. FIG. 12. TGA graphs of PVA and PAT33 membranes with and without annealing.
FIG. 13. Effect of annealing temperature on PV performance of PVA and PAT33 membrane.
FIG. 14. Effect of annealing temperature on PSI values of PAT33 nanocomposite and PVA membranes.
2398 POLYMER ENGINEERING AND SCIENCE—-2010
PVA chains go through dehydration reaction and more compact polymer chains are formed in the membrane matrix [17]. For better comparison, the PSI values were plotted as a function of annealing temperature to investigate the effect of annealing. According to Fig. 14, it can be concluded that the effect of annealing temperature on permeation performance of PAT33 nanocomposite membranes is more significant than that of PVA membrane. As illustrated in Fig. 15, the same trend is observed by increasing annealing time. The condensation reaction of silanols or hydroxyl groups of PVA more proceeds with increasing annealing time. As a result, more compact polymer chains and crosslinked networks are formed in the membrane structure and these cause separation factor of the membranes increases, while their permeation flux decreases [17]. Also, the PSI values as a function of annealing time of PVA and PAT33 nanocomposite membranes were presented in Fig. 16. It can be observed that
FIG. 16. Effect of annealing time on PSI values of PVA and PAT33 nanocomposite membranes.
DOI 10.1002/pen
TABLE 5. Comparison between this study and the other nanocomposite membranes [15, 17 and this study]. GPTMS-PVA [15]
APTEOS-PVA [17]
APTEOS/TEOS-PVA [This study]
Temperature(8C)
a
J (Kg/h m )
PSI
a
J (Kg/h m )
PSI
a
J (Kg/h m2)
PSI
80 120 150
25 50 55
0.15 0.13 0.07
3.6 6.37 3.78
118 140 200
0.135 0.105 0.06
15.79 14.59 11.94
45 97 270
0.819 0.705 0.577
36.03 67.68 155.21
2
the PSI values of PAT33 nanocomposite membrane are greater than those of PVA membrane. As indicated in Table 5, by comparing the results of this work with those of others, it can be stated that annealing temperature improves the PV performance of the nanocomposite membranes prepared by the sol–gel method [15, 17]. The best PV performance of the nanocomposite membranes prepared in this work is for the membrane annealed at 1508C for 8 h.
CONCLUSIONS Nanocomposite PAT33 membranes based on PVAAPTEOS/TEOS were prepared via in situ sol–gel reaction. In these nanocomposite membranes, compatibility between the polymer matrix and the silica nanoparticles improved and phase separation was prevented during the membrane formation. In this study, it was observed that condensation reaction of hydroxyl groups of PVA and alkoxysilane leads to restricting the polymer chains movement. So, it can be found out that crosslinking with alkoxysilane is the main reason for improvement of separation performance of the membranes. The effects of annealing temperature and time on physical and chemical structure of pure PVA and PAT33 nanocomposite membranes were investigated. Annealing process improved dehydration reaction in both of the PVA and PAT33 membranes. This process enhanced the crosslinking reactions. Furthermore, with increasing the annealing temperature and time, the amorphous region of pure PVA and PAT33 nanocomposite membranes decreased due to increasing the degree of crosslinking. It was found out that degree of crystallinity in pure PVA membrane is higher than that in PAT33. On the other hand, the PV performance of the prepared membranes was improved with increasing the annealing temperature and time. It was investigated that the effect of annealing process on PV performance of the PAT33 nanocomposite membrane was found more significant in comparison with that of the pure PVA membrane.
DOI 10.1002/pen
2
REFERENCES 1. H.J.C. Hennepe, D. Bargeman, M.H.V. Mulder, and C.A. Smolders, J. Membr. Sci., 35, 39 (1987). 2. W. Ji, S.K. Sikdar, and S.T. Hwang, J. Membr. Sci., 103, 243 (1995). 3. H. Wang, B.A. Holmberg, and Y. Yan, J. Mater. Chem., 12, 3640 (2002). 4. L. Li, Z. Xiao, Z. Zhang, and S. Tan, Chem. Eng. J., 97, 83 (2004). 5. F.B. Peng, Z.Y. Jiang, C.L. Hu, Y.Q. Wang, H.Q. Xu, and J.Q. Liu, Sep. Purif. Technol., 48, 229 (2006). 6. T.C. Merkel, Z. He, and I. Pinnau, Macromolecules, 36, 6844 (2003). 7. T.C. Merkel, B.D. Freeman, R.J. Spontak, Z. He, I. Pinnau, P. Meakin, and A.J. Hill, Chem. Mater., 15, 109 (2003). 8. F. Peng, L. Lu, C. Hu, H. Wu, and Z. Jiang, J. Membr. Sci., 259, 65 (2005). 9. T. Uragami, K. Okazaki, H. Matsugi, and T. Miyata, Macromolecules, 35, 9156 (2002). 10. Y.-L. Liu, Y.-H. Su, K.-R. Lee, and J.-Y. Lai, J. Membr. Sci., 251, 233 (2005). 11. S.S. Kulkarni, A.A. Kittur, M.I. Aralaguppi, and M.Y. Kariduraganavar, J. Appl. Polym. Sci., 94, 1304 (2004). 12. M.Y. Kariduraganavar, S.S. Kulkarni, and A.A. Kittur, J. Membr. Sci., 246, 83 (2005). 13. T. Uragami, T. Katayama, T. Miyata, H. Tamura, T. Shiraiwa, and A. Higuchi, Biomacromolecules, 5, 1567 (2004). 14. F. Peng, L. Lu, H. Sun, and Z. Jiang, J. Membr. Sci., 281, 600 (2006). 15. F. Peng, L. Lu, H. Sun, Y. Wang, H. Wu, and Z. Jiang, J. Membr. Sci., 275, 97 (2006). 16. T. Uragami, H. Matsugi, and T. Miyata, Macromolecules, 38, 8440 (2005). 17. Q. Zhang, Q. Liu, Z. Jiang, L. Ye, and X. Zhang, Micro. Meso. Mat., 110, 379 (2008). 18. R. Guo, C. Hu, F. Pan, H. Wu, and Z. Jiang, J. Membr. Sci., 281, 454 (2006). 19. M. Mulder, Basic Principles of Membrane Technology, Kluwer Academic Publishers, Netherlands (1991).
POLYMER ENGINEERING AND SCIENCE—-2010 2399
