the influence of peg molecular weight on morphologies and properties of pvdf asymmetric membranes

Chinese Journal of Polymer Science Vol. 26, No. 4, (2008), 405−414 Chinese Journal of Polymer Science ©2008 World Scientific

THE INFLUENCE OF PEG MOLECULAR WEIGHT ON MORPHOLOGIES AND PROPERTIES OF PVDF ASYMMETRIC MEMBRANES*

Dan-ying Zuoa**, You-yi Xub, Wei-lin Xua and Han-tao Zoua a Key Lab of Green Processing and Functionalization of New Textile Materials, Ministry of Education, Wuhan University of

Science and Technology, Hubei 430073, China b Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China

Abstract The preparation and properties of asymmetric poly(vinyldiene fluoride) (PVDF) membranes are described in this study. Membranes were prepared from a casting solution of PVDF, N,N-dimethylacetamide (DMAc) solvent and water-soluble poly(ethylene glycol) (PEG) additives by immersing them in water as coagulant medium. Experiments showed that when PEG molecular weight increased, the changes in the resultant membranes’ morphologies and properties showed a transition point at PEG6000. This indicated that PEG with a relatively low molecular weight was used as a pore-forming agent to enhance pure water flux and reduce solute rejection of membranes, but PEG was used as a pore-reducing agent with a further increment of PEG molecular weight to result in pure water flux decreasing and solute rejection increasing. Finally, combined with the precipitation rates of different membrane-forming systems, the membrane formation mechanism describing PEG mobility was discussed extensively basing on the length changes of PEG molecular chains and the affinity between PEG and casting solution. The results offered a better understanding of effects of PEG additives on membrane structure and properties. Keywords: Poly(vinyldiene fluoride); Poly(ethylene glycol); Components mutual diffusion; Phase separation; Asymmetric membrane.

INTRODUCTION

Since Loeb and Souriajian first introduced the phase inversion method[1], considerable progress has been made for understanding the formation mechanism of asymmetric membranes. In that technique, a thin film of polymer solution is cast on to a suitable substrate and subsequently immersed into a coagulation bath. The diffusion exchange of a solvent and a non-solvent results in the phase separation and the formation of membrane[2]. It usually turns out to be a characteristic morphology of asymmetric membranes showing a dense top layer, commonly recognized as the skin layer, and a more porous sublayer. The asymmetric membranes have been widely used for gas separation and liquid separation, because the skin layer plays a role of selective barrier film to the permeation of solute through the membrane, whereas the porous sublayer offers exclusively good mechanical strength.

Poly(vinylidene fluoride) (PVDF) is widely used as a membrane material since it exhibits excellent chemical resistance, good physical and thermal stability. It has been reported that the morphology and properties of PVDF asymmetric membranes could be adjusted by dissolving a third component as additives in the casting solutions consisting of a polymer and a solvent. As far as we know, polyvinylpyrrolidone (PVP)[3, 4], polyethylene glycol (PEG)[5], lithium chloride (LiCl)[6, 7], ethanol[8], polystyrene sulfonic acid[9], water[10], ethylene glycol[11, 12], glycerol and phosphoric acid[13] have been commonly used as the additives. Some researchers tried to explain the behavior of additives in terms of their physical and chemical characters such as

* This work was supported by the National Basic Research Program of China (No. 2003CB615705) and the National Natural Science Foundation of China (No. 50433010). ** Corresponding author: Dan-ying Zuo (左丹英), E-mail: [email protected]; [email protected] Received May 31, 2007; Revised July 30, 2007; Accepted August 2, 2007

D.Y. Zuo et al. 406

water-solubility, activity and surface tension. In general, these additives were considered as pore-forming agents. Then, these additives would enhance the membrane fluxes and reduce the rejection. In this research, PEG with different molecular weight was added to the PVDF casting solutions. The influences of PEG molecular weight on structure and properties of the resultant membranes were investigated. Membrane characterization revealed that for properties and morphology of the resultant membranes exited a turning point at a certain molecular weight of PEG. In other words, PEG could induce or suppress the macrovoid expansion for PVDF membrane-forming systems depending on its molecular weight. Thus, for a better understanding of the role played by these additives, the membrane formation mechanism was discussed extensively basing on PEG mobility due to the changes of the length of PEG molecular chains and the affinity between PEG and casting solutions.

EXPERIMENTAL

Materials PVDF (FR904, Mw = 2 × 106, Mn = 4.7 × 105) was supplied by Shanghai 3F Ltd. China and dried at 85°C for 2 h prior to use. N,N-dimethylacetamide (DMAc) was the reagent grade and used as the solvent. Five kinds of PEG (PEG200, PEG1000, PEG6000, PEG10000, PEG20000) were used as additives in PVDF/DMAc solutions, and their molecular weights were 200, 1000, 6000, 10000 and 20000, respectively. Distilled and deionized water was used as coagulant.

Membrane Preparation Membranes were prepared using the immersion precipitation method at 25°C. The compositions and viscosities of the casting solutions are shown in Table 1. Polymer solutions were cast into thin films (150 μm thick) on glass sheets, and then the solution films were immersed into a water bath immediately. After the precipitation was completed (5 min), the formed membranes were transferred into distilled water for 5 days. The dry membranes were obtained by soaking with ethanol and hexane for 24 h respectively and dried at 50°C sequentially.

Table 1. The compositions and viscosities of PVDF/PEG/DMAc solutions

Membrane Casting solution DMAc/PEG/PVDF (in mass ratio) Molecular weight of PEG Viscosity (Pa s)

A 81/5/14 200 8.80 B 81/5/14 1000 8.95 C 81/5/14 6000 9.15 D 81/5/14 10000 9.30 E 81/5/14 20000 9.60

Light Transmittance Measurements Precipitation rate in immersing stage for various systems was determined by a light transmittance instrument through recording the variation of transmitted light intensity with time lasting[14, 15]. The light intensity profiles were plotted as a function of time.

Membrane Characterization The morphologies in membrane cross-sections and surfaces were inspected on a scanning electronic micrograph (SIRion200 FESEM, Netherlands). The membranes were fractured in liquid nitrogen and coated with gold under vacuum before test. After the membranes were dried at 80°C for 2 h, the existence of residual PEG on the surface of dried membranes was tested by attenuated total reflection-Fourier transform infrared (ATR-FTIR) spectroscopy (Bruker Vector 22 FT-IR spectrometer, Germany).

The overall porosity was estimated by measuring the area (A), mass (Wm) and thickness (D) of membranes and the density (ρp) of the polymer[16]. After membranes were dried in air for 48 h and put in vacuum oven at 25°C for 48 h, the mass and the thickness were measured with an electronic balance and a thickness gauge as well as SEM. The percentage porosity was calculated as follows:

The Influence of PEG Molecular Weight on Morphologies and Properties of PVDF Asymmetric Membranes 407

( ) %100)/(

%Porosity pm ×−

=DAWDA ρ

(1)

Measurements of pure water flux and solute rejection were performed using an ultrafiltration cell with an effective membrane area of 32.15 cm2 (SCM-300, China) at (25 ± 3)°C. For pure water flux measurements, the compressed distilled water was used as permeate. Membranes were initially pressurized with distilled water at 0.15 MPa for 0.5 h to compact membranes for getting a constant flux. And then the steady water fluxes were measured at 0.1 MPa.

For solute rejection experiments, the feed solution was prepared by dissolving bovine serum albumin (BSA, M = 67000) in the buffer solution of K2HPO4-KH2PO4 with 7.4 of pH value. The concentration of solute in the feed was taken to be 200 mg/L, and the transmembrane pressure was employed under 0.1 MPa with vigorous agitation (600 r/min). After the permeating flux reached a stable constant value (ca. 0.5 h after operation), samples of permeate were collected for subsequent spectroscopic analysis by an ultraviolet spectrometer (UV-1601, Japan) at 280 nm. The experimentally obtained rejection, R, is defined as

%100)/1( fP ×−= ccR (2)

where cp and cf are the BSA concentration in the permeate and in the feed, respectively.

RESULTS AND DISCUSSION

Membrane Permeation Properties and Morphologies Permeability and structure for PVDF membranes with different PEG additives are shown in Table 2. With an increment in PEG molecular weight, water flux, membrane thickness and porosity initially increased then went through a maximum, finally declined. Conversely, BSA rejection initially decreased, then went through a minimum, finally rose a little. This turning point was 6000. These membranes prepared under above conditions are found to have good ultrafiltration properties.

Table 2. Properties of membranes prepared from PVDF/PEG/DMAc/water

systems with PEG additive of different molecular weight Membrane Water flux (L/(m2 h)) BSA rejection (%) Thickness (μm) Porosity (%)

A 64.01 92.6 39 71.49 B 85.98 86.8 42 76.72 C 143.31 80.4 56 82.47 D 121.02 83.4 42 79.32 E 74.52 87.7 40 69.04

In order to understand these results, the cross-sections and the upper surface of membranes were carefully

studied with SEM. Figure 1 shows the SEM micrographs of membranes cast from 14 wt% PVDF solutions using PEG additives with different molecular weight. Although membranes A−E all showed the characteristics of an asymmetric membrane composing of a skin layer near the top surface and a porous supporting solid matrix, PEG with different molecular weight exerted an obvious effect on membrane surface and cross-section. The size and number of pore on the membrane upper surface increased when the molecular weight of PEG additive increased from 200 to 6000. However, with a further increase in PEG molecular weight to 20000, the pore size and pore number of the upper surface declined. The variation trend of macrovoid growth in the membrane cross-section was similar to that of the pores on the membrane upper surface. The finger-like cavities grew gradually with PEG molecular weight increasing from 200 to 6000. The finger-like pores expanded in breadth and length towards the membrane bottom when PEG molecular weight increased to 6000. At the same time, the wall of fingerlike macrovoids changed from a dense to porous morphology, what’s more, the sponge-like structure underneath the finger-like pores became more porous and interconnected, as seen from Figs. 2(A, B, C). However, when PEG molecular weight exceeded 6000, the finger-like pores were suppressed and the sponge-like structures were developed although the macrovoids wall presented a more porous structure as shown in Figs. 2(D-T, E-T).

D.Y. Zuo et al. 408

Fig. 1 SEM photographs of PVDF membranes A−E with (1) cross-section and (2) upper surface


Fig. 2 Magnification of the cross section SEM photomicrographs of membranes A−E T) Top part; B) Bottom part

It could be concluded from SEM observations of membranes that the membrane morphologies and

structure agreed well with permeation results that depended on not only the overall membrane morphologies but also especially, the upper surface of membranes. Generally, more pores on membrane surface and the better interconnectivity inside membrane would contribute to enhancing pure water flux and reducing solute rejection[17]. When PEG molecular weight increased from 200 to 20000, the pore size and pore number on membrane upper surface initially increased, then declined slightly. Correspondingly, pure water fluxes increased initially, and then decreased, and BSA rejections reduced initially, then rose. The membrane thickness and

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porosity also depended on the membrane morphology and structure. The sufficient development of macrovoids and interconnectivity leads to the increase of the membrane thickness and porosity. As PEG molecular weight increased from 200 to 20000, the macrovoids were developed initially and then were suppressed. So the thickness and porosity of membranes increased initially, and then decreased.

It could be deduced from the above analysis that PEG acted as a pore-forming agent when it had a lower molecular weight, whereas PEG could suppress the growth of finger-like macrovoids with the further molecular weight increasing.

Precipitation Processes An interesting characteristic was the precipitation rate during membrane formation. The results of light transmission experiments during the precipitation of a 14 wt% PVDF solution with PEG of various molecular weights are shown in Fig. 3. For each profile, the time at which light intensity began to rapidly decrease was identified as the onset point of precipitation. For all membrane-forming systems no change of light transmittance was observed at the first stage, and this time length is defined as the delayed time. It appeared that the so-called delayed precipitation took place in all the immersion cases. What’s more, the delayed time became longer from 7.04 s to 12.83 s with PEG molecular weight increasing. In other words, the precipitation rate became slower with PEG molecular weight augmenting. It has been generally accepted as a common rule that membranes prepared with slow precipitation rates often show sponge-pore morphology in membrane cross-section[1, 18]. This implied that the PVDF membranes with PEG additives in the casting solution had a tendency toward suppressing the formation of large pores by decelerating precipitation. However, from Fig. 1, the finger-like pores in cross-sections were existed in membranes A−E. The precipitation rate can be related to the effect of the PEG additives on the diffusivity of different components in the casting solution and coagulant media[19, 20]. Thus membrane formation mechanism may be changed and will be described in the next section.

Fig. 3 Light transmission curves with different molecular weight of PEG additive

Membrane Formation Mechanism In order to understand the formation mechanism of asymmetric membranes, it is convenient to analyze the membrane as a two-layer structure: the top layer and the sublayer[21].

The Formation of the Membrane Top Layer The formation of the top layer went through two stages: the precipitation process (ca. 5 min) and the soaking process (ca. 5 days). On the one hand, an increment in PEG molecular weight enhanced the viscosity of the casting solution. On the other hand, the higher the PEG molecular weight, the longer the PEG molecule chain length, and then the poorer the mobility of PEG molecules[22]. Consequently, these two factors would hamper component exchanges during the precipitation process, and then lead to precipitation rate reducing, which was consistent with the result of the light transmittance experiments, i.e., the delayed time of phase separation for the


five membrane-forming systems changed from 7.04 s to 12.83 s with PEG molecular weight increasing. A viscosity enhancement of casting solutions and a decrease in the PEG mobility also made it more difficult for PEG molecules to diffuse from the casting solution into the coagulation bath. Hence, some PEG molecules could remain in the renascent membrane after a precipitation process of 5 min. Moreover, the higher PEG molecular weight, the more the amount of residual PEG. Many studies also referred to the residual polymeric additives in the membranes[23, 24]. In fact, during the formation of the top layer, the phase separation of a polymer/PEG/solvent/coagulant membrane-forming system involved demixing of the polymer components. PVDF molecules could be considered as immobile because of their very high molecular weight, whereas PEG additives with a low molecular weight may have better mobility. Thus PEG molecules could move to the lean polymer phase of membrane surface along with DMAc outflow. Therefore, aggregated PEG molecules dispersed uniformly on the membrane surface. That is to say, during the precipitation process the formed top surface should not be porous, but be relatively dense with a great deal of aggregated residual PEG. However, it was the aggregated PEG dispersed in membrane surface that resulted in the pore formation of membrane surface at the soaking stage of membrane preparation.

During the soaking stage of membranes (ca. 5 days), the pore formation mechanism on membrane surface proposed by Young et al. could explain the top surface morphology of these resultant membranes[25, 26], i.e. during soaking of membranes water-soluble additives were dissolved out to create pores in membranes. Soaking in water for 5 days further leached the residual DMAc and aggregated PEG out of the membranes, so the pores formed on membrane surface. Since the volume of the aggregated PEG became larger with the PEG molecular weight increasing, the formed pores on membrane surface also should enlarge slightly. However, when PEG10000 and PEG20000 with the high molecular weights were used as additive, they possessed the poorer mobility compared with other PEG with the lower molecular weight. Accordingly, the amount of them moving from membrane middle into membrane surface became less, and the volume and number of aggregated EG molecules became smaller on the membrane surface, and then the pore size and number on membrane surface began to reduce. The final membranes were measured by FT-IR in order to examine if there was residual PEG in the membranes. The FT-IR result was shown in Fig. 4. It was obvious that the two characteristic absorption peaks of PEG (O―H: 2885 cm−1, C―O: 1110 cm−1) did not appeared. All characteristic absorption peaks corresponded to those of PVDF. This suggested that all PEG in the membrane top layer had been leached out after membranes were soaked for 5 days.

Fig. 4 ATR-FTIR spectra of the surfaces of membranes

A−E

The Formation of Membrane Sublayer The membrane sublayer mainly depends on the precipitation process and the top layer structure. During precipitation process, the formation of top layer poses an additional barrier to mass transfer between the sublayer

D.Y. Zuo et al. 412

and the coagulation bath. When the top layer was relatively compact, it would increase the mass transfer resistance of solvent from the sublayer to the coagulation bath. Accordingly, outflow rate of solvent from the sublayer declines, whereas, coagulant still diffuses through the top layer into the membrane sublayer and works on its neighboring solvent. And then the lean-polymer phase and rich-polymer phase occur. The nuclei of lean-polymer phase grow to form pores when coagulant water diffuses continually into the nuclei to induce neighboring solvent to diffuse into them. From the above mechanism of pore formation in the sublayer, it can be conclude that the initiation of macrovoids originated from nuclei of lean-polymer phase in the immersed casting solution, and the growth of macrovoids depends on the circumstances of casting solution in front of the lean-polymer phase nuclei[27]. If the composition in front of nuclei remains stable for a relatively long period, then macrovoid will expand from the freshly formed nuclei of the diluted phase. Whereas a sponge-structure sublayer is formed when the composition at the nuclei front becomes instable, and new nuclei are being generated in front of the existing ones. Macrovoids expand as a result of diffusion flow. In this work, as observed from membrane SEM photos, with PEG molecular weight increasing, the fingerlike macrovoids in the membrane sublayer were initially developed, then suppressed. Thus it can be seen that the existence of PEG evidently affected the mutual diffusion among components in the process of macrovoids formation. Moreover, phase behavior of the casting solution in the sublayer also related to demixing of PVDF and PEG blends as mentioned in previous section. For simplicity, we returned to a pseudo ternary system containing casting solution (PVDF and DMAc), PEG additive and coagulant water. Now, we discuss how PEG mobility may explain the macrovoids growth based on the mechanism mentioned above. In general, the mobility of PEG molecules along with solvent outflow becomes poorer with lengthening PEG molecular chains and decreasing affinity between PEG and casting solutions. At first, it is well known that PEG molecular chains become longer with PEG molecular weight increasing. Then we designed one experiment to measure the affinity between casting solutions and PEG with different molecular weight.

A specific amount of PEG with different molecular weight was added to 10 g casting solution composed of 1.4 g PVDF and 8.6 g DMAc. The mixture was agitated at 70°C until a clear homogeneous solution was obtained. Then the pellucid solution was stored in a water bath at 25°C for 6−8 h. If the mixture maintained a clear homogeneous solution, then PEG was added sequentially until gelation in the casting solution emerged. The PEG amount that made systems begin to phase separation was defined as the gelation value. For PEG with different molecular weight, the gelation values are represented in Table 3.

Table 3. The gelation values of PEG with different molecular weight for PVDF/DMAc casting solutions

Additive PEG200 PEG1000 PEG6000 PEG10000 PEG20000 Gelation value (g) 5.64 3.63 2.31 1.51 0.98

The lower the gelation value is, the poorer the affinity between PEG and casting solution is. That is to say,

PEG mobility would be improved with PEG molecular weight increasing. So an increment in PEG molecular weight had two contrary effects on PEG mobility. There was a trade-off between these two effects. Now, combined with the morphologies of the resultant membranes, the effects of PEG on the macrovoid growth were discussed. During macrovoids growth in the sublayer, when the molecular weight of PEG increased from 200 to 6000, the affinity between casting solutions and PEG became poorer, which facilitated casting solutions to induce phase separation and PEG to apart from casting solutions to enter into the nuclei of lean polymer phase. This made the casting solution in front of nuclei become stable and favored the nuclei to expend to the larger fingerlike macrovoids. Here, compared with a decrease of PEG molecule mobility due to molecular chain lengthening, the PEG easier separation from casting solutions because of a decrease in affinity between PEG and casting solutions was a leading factor. But while PEG molecular weight increased further to 6000, a decrease of PEG molecule mobility due to molecular chain lengthening played a key role in influencing PEG molecule diffusion. In this way, it was comparatively difficult that PEG10000 and PEG20000 molecules followed DMAc outflow to diffuse into the nuclei. At the same time, DMAc still diffused into nuclei. Thus casting solution in


front of nuclei became instable and was induced to demixing. Accordingly the new nuclei of lean polymer phase were yielded, and the previous nuclei could not grow further. As a result, when the molecular weight of PEG additives increased to 10000, the development of fingerlike macrovoids was suppressed.

The above mechanism of PVDF asymmetric membrane formation with PEG as additives could interpret the effect of PEG additives on the precipitation rate, morphologies and permeability of the resultant membranes.

CONCLUSIONS

The results of the present work suggested that the addition of PEG with different molecular weight could change the PVDF membrane structure and properties. A mechanism considering the influence of PEG molecular weight on the mobility of PEG molecules and the affinity between PEG and casting solution was proposed to investigate the formation of the top layer and sublayer of membranes. The formation of top layer went through the precipitation process and the soaking process. During precipitation process, because of the addition of PEG with increasing molecular weight, a viscosity enhancement of casting solution and a decrease in PEG mobility made it more difficult for PEG molecules to diffuse from casting solution into coagulation bath. As a consequence, the delayed phase separation occurred in all the immersion cases, and the formed top surface should be relatively dense with a great deal of aggregated residual PEG. During the soaking process, the aggregated PEG on membrane surface could be dissolved into water, so the porous membrane surface formed. As for the formation of sublayers, the macrovoid growth or suppression was controlled by the trade-off between PEG molecular chain length and the affinity between PEG and casting solution. When PEG molecular weight increased from 200 to 6000, the improved PEG mobility because of a decrease in affinity between PEG and casting solutions was a leading factor. This made it easier for PEG to apart from casting solutions and to enter into the nuclei of lean polymer phase. Consequently, the casting solution in front of nuclei became stable and favored the nuclei to expend to the larger fingerlike pores. However, when PEG molecular weight increased further to 10000 and 20000, a decrease of PEG molecule mobility due to molecular chain lengthening played a key role in preventing PEG molecule from following DMAc outflow to diffuse into the nuclei. Thus casting solution in front of nuclei became instable and was induced to demixing. Accordingly the new nuclei were yielded, and the freshly formed nuclei could not grow further to form the fingerlike pores, then structure of sponge-like pore increased. REFERENCES 1 Leob, G.S. and Sourirajan, S., Dav. Chem. Ser., 1963, 38: 117 2 Mulder, M., “Basic Principle of Membrane Technology”, Kluer Academic Publishers, Dordrecht, 1992, p.50 3 Wang, D.L., Li, K. and Teo, W.K., J. Membr. Sci., 1999, 163: 211 4 Deshmukh, S.P. and Li, K., J. Membr. Sci., 1998, 150: 75 5 Lu, Y., Chen, H.L and Li, B.G., Acta Polymerica Sinica(in Chinese), 2002, (5): 656 6 Tomaszewska, W., Desalination, 1996, 104: 1 7 Bottino, A., Capannelli, G., Munari, S. and Turturro, A., Desalination, 1988, 68: 167 8 Shih, H.C., Yeh, Y.S. and Yasuda, H., J. Membr. Sci., 1990, 50: 299 9 Uragami, T., Fujimoto, M. and Sugihara, M., Desalination, 1980, 34: 311 10 Khayet, M. and Takeshi, M., Ind. Eng. Chem. Res., 2001, 40: 57 11 Khayet, M., Feng, C.Y., Khulbe, K.C. and Matsuura, T., Polymer, 2002, 42: 3879 12 Khayet, M., Feng, C.Y., Khulbe, K.C. and Matsuura, T., Desalination, 2002, 148: 321 13 Benzinger, W.D. and Robinson, D.N., 1983, U.S. Pat., 4384047 14 Machado, P.S.T., Habert, A.C. and Borges, C.P., J. Membr. Sci., 1999, 155: 171 15 Zuo, D.Y., Zhu, B.K., Cao, J.H. and Xu, Y.Y, Chinese J. Polym. Sci., 2006, 24(3): 281

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the influence of peg molecular weight on morphologies and properties of pvdf asymmetric membranes

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