Separation membranes

Abstract

A process for the preparation of ultrafiltration and microfiltration polymeric flat sheet separation membranes is disclosed, the process comprising a unidirectional cooling step. Membranes prepared according to the process exhibit numerous advantages over ultrafiltration and microfiltration membranes prepared via conventional processes. In particular, the membranes prepared by the present process exhibit remarkable pure water flux, superior mechanical properties and increased anti-fouling characteristics. Also disclosed are particular PVDF ultrafiltration and microfiltration membranes having improved flux, mechanical and anti-fouling properties.

Claims

1. A process for the preparation of a flat sheet polymeric membrane having an average pore size of 0.01-10 μm, the process comprising the steps of: a) providing a polymeric dope solution comprising a polymer and a first solvent, wherein: i) the polymer is poly(vinylidene fluoride), poly(ethersulfone) or cellulose acetate and the first solvent is dimethyl sulfoxide, or ii) the polymer is cellulose acetate and the first solvent is acetic acid; b) casting the polymeric dope solution onto a substrate to form a cast polymeric film, the cast polymeric film having a first surface being in contact with the substrate, and a second surface disposed opposite the first surface; c) subjecting the cast polymeric film to a cooling means, the cooling means being provided at a temperature that is 5-120° C. below the melting temperature of the first solvent; and d) contacting the cooled cast polymeric film resulting from step c) with a second solvent, the second solvent being provided at a temperature below the melting point of the first solvent, and the second solvent being miscible with the first solvent; wherein during step c) only one of the first and second surfaces of the cast polymeric film is subjected to the cooling means, and wherein step d) removes the first solvent in a frozen state from the cast polymeric film.

2. The process of claim 1, wherein the polymer is poly(vinylidene fluoride) or poly(ethersulfone) and the first solvent is dimethyl sulfoxide.

3. The process of claim 1, wherein the polymer is poly(vinylidene fluoride) the first solvent is dimethyl sulfoxide.

4. The process of claim 1, wherein the dope polymeric solution comprises 10-30 wt % of the polymer.

5. The process of claim 1, wherein step b) comprises casting the polymeric dope solution onto a substrate to form a cast polymeric film having an average thickness of 0.05-2 mm.

6. The process of claim 1, wherein the cooling means is provided at a temperature that is 15-100° C. below the melting temperature of the first solvent.

7. The process of claim 1, wherein step c) comprises subjecting the first surface of the cast polymeric film to the cooling means.

8. The process of claim 1, wherein the substrate is sheet-like, having a first surface being in contact with the cast polymeric film, and a second surface disposed opposite the first surface.

9. The process of claim 8, wherein step c) comprises subjecting the second surface of the substrate to the cooling means.

10. The process of claim 1, wherein the substrate is glass or is metalli.

11. The process of claim 1, wherein the substrate has an average thickness of 2-15 mm.

12. The process of claim 1, wherein the cooling means is a sheet-like structure.

13. The process of claim 1, wherein the polymeric dope solution further comprises at least one dopant suitable for increasing the hydrophilicity of the membrane.

14. The process of claim 13, wherein the weight ratio of polymer to dopant within the polymeric dope solution is 2:1 to 8:1.

15. The process of claim 1, wherein: i) the flat sheet polymeric membrane has an average pore size of 0.01-1.5 μm; ii) the cooling means in step c) is a sheet-like structure or a liquid, and is provided at a temperature that is 40-80° C. below the melting temperature of the first solvent; and iii) step c) comprises subjecting only the first surface of the cast polymeric film to the cooling means by applying the cooling means to the second surface of the substrate.

16. The process of claim 3, wherein: i) the flat sheet polymeric membrane has an average pore size of 0.01-1.5 μm; ii) in step b), the substrate has a thickness of 2-20 mm and the polymeric dope solution is cast onto the substrate to form a cast polymeric film having a thickness of 0.05-2 mm; iii) the cooling means in step c) is a plate formed from a metal or glass, and is provided at a temperature that is 40-80° C. below the melting temperature of the first solvent; and iv) step c) comprises subjecting only the first surface of the cast polymeric film to the cooling means by applying the cooling means to the second surface of the substrate.

17. The process of claim 1, wherein the second solvent is water.

18. The process of claim 1, wherein the cooling means is provided at a temperature of −18 to −50° C.

19. The process of claim 13, wherein the at least one dopant is poly(ethylene glycol).

Description

EXAMPLES

(1) One or more examples of the invention will now be described, for the purpose of illustration only, with reference to the accompanying figures:

(2) FIG. 1: The setting of initial conditions for the calculation of thermal conduction under the circumstance of unidirectional cooling with a pre-cooled plate.

(3) FIG. 2: The setting of initial conditions for the calculation of thermal conduction under the circumstance of unidirectional cooling with a pre-cooled liquid.

(4) FIG. 3: Schematic showing the membrane preparation process.

(5) FIG. 4: PVDF membranes prepared by the combined crystallisation and diffusion method with different cooling rates. (A) Temperature change in the polymer solution at the position 10 μm away from the cooling interface; the left of the slash is the material of the cold plate, and the right is the material of the casting plate. The brown dashed line shows the melting point of the solvent DMSO. (B) Temperature profile of the polymer solution from the cold end after cooling for 1 s. (C) Cross-sectional SEM image of the Glass/Glass sample. (D-F) Cold-end surface and (G-I) top layer cross-sectional SEM images of the Glass/Glass, Glass/Al, Al/Al sample, respectively. (J) Hexane and (K) Liquid Nitrogen sample's surface SEM image.

(6) FIG. 5: SEM images of the CCD Al/Al NMP PVDF membrane. (A) Cross-sectional overview; (B, C) close view showing the separation layer; (D) high-magnification image at the separation layer; (E) surface of the separation layer; (F) surface of the back side.

(7) FIG. 6: SEM images of the CCD Al/Al DMAc PVDF membrane. (A) Cross-sectional overview; (B) close view showing the separation layer; (C) surface of the separation layer; (D) surface of the back side.

(8) FIG. 7: Pore size distribution of PVDF membranes measured by the gas-liquid displacement method. (A) CCD Glass/Glass 1 mm sample; (B) CCD Glass/Al 1 mm sample; (C) CCD Al/Al 1 mm sample; (D) NIPS DMSO 1 mm sample; (E) NIPS DMSO 0.5 mm sample; (F) NIPS NMP 0.3 mm sample.

(9) FIG. 8: CCD Al/Al PVDF membranes prepared with different casting thicknesses. (A) Temperature change in the polymer solution at the position 10 μm away from the cooling interface. (B) Temperature profile of the polymer solution from the cold end after cooling for 1 s. (C-F) Cross-sectional SEM image of the separation layer of the 1 mm, 0.5 mm, 0.3 mm and 0.1 mm sample, respectively.

(10) FIG. 9: SEM images of the CCD Al/Al PVDF membrane prepared with a 0.3 mm casting thickness. (A) cross-sectional overview; (B) pores on the surface; (C) pore structure in the separation layer; (D) cross-sectional view of inter-connected micro-channels at the back side and (E) opened micro-channels on the back side.

(11) FIG. 10: SEM images of the NIPS DMSO PVDF membrane prepared with a 0.3 mm casting thickness. (A) cross-sectional overview; (B) top surface; (C) close cross-sectional view of the top layer; (D) close cross-sectional view of the back side and (E) the back surface.

(12) FIG. 11: SEM images of the NIPS NMP (A-D) and NIPS DMAc (E-H) PVDF membranes prepared with a 0.3 mm casting thickness. (A, E) cross-sectional overview; (B, F) close cross-sectional view of the top layer; (C, G) top surface; (D, H) the back surface.

(13) FIG. 12: BSA fouling test result of a CCD Al/Al 1.0 mm PVDF membrane.

(14) FIG. 13: FT-IR spectra of the CCD and NIPS membranes. The blue, red and black arrows point to the characteristic peaks of the α, β and γ phase PVDF, respectively. NIPS membranes all show intensive peaks of α phase, together with peaks from β and γ phase. But for the CCD membranes, all peaks of α phase disappeared.

(15) FIG. 14: XRD patterns of the CCD and NIPS membranes.

(16) FIG. 15: DSC results of the CCD membranes. (A) Al/Al 1.0 mm, crystallinity 59%; (B) Al/Al 0.5 mm, crystallinity 62%; (C) Glass/Al 1.0 mm, crystallinity 62%; (D) Glass/Glass 1.0 mm, crystallinity 64%.

(17) FIG. 16: SEM image of the CCD Al/Al 1.0 mm PVDF membrane. The image shows the cross section view close to the back side, which clearly depicted tightly connected PVDF grains and the grain boundary.

(18) FIG. 17: CCD Al/Al 1.0 mm PVDF membrane and NIPS DMSO 1.0 mm PVDF before and after high-pressure gas-liquid displacement measurements that reached 35 bar. (A) Untested and (B) tested CCD membrane; (C) untested and (D, E) tested NIPS membrane. The tested NIPS membrane was obviously compressed and the lower part of the finger-like macrovoids collapsed.

(19) FIG. 18: Mercury intrusion porosimetry results of CCD Glass/Al 1.0 mm membrane and Al/Al 1.0 mm membrane. (A) Cumulative intrusion volume vs. pore size; (B) incremental intrusion volume vs. pore size; (C) incremental intrusion volume vs. pore size within small pore size range.

(20) FIG. 19: SEM images of a CCD Al/Al 0.3 mm PVDF membrane after the abrasion test for 2 weeks. (A) The cross-sectional overview; (B) a cross-sectional view close to the separation layer; (C) high magnification image of the top separation layer; (D) overview of the membrane surface; and (E) high magnification image of the surface.

(21) FIG. 20: SEM images of a NIPS DMSO 0.3 mm PVDF membrane after the abrasion test for 2 weeks. (A) The cross-sectional overview; (B) a cross-sectional view close to the separation layer; (C) overview of the membrane surface; and (D) a closer view on the surface.

(22) FIG. 21: SEM images of a NIPS NMP 0.3 mm PVDF membrane after the abrasion test for 2 weeks. (A) The cross-sectional overview; (B) a cross-sectional view close to the separation layer; (C) high magnification image of the top separation layer; (D) overview of the membrane surface; (E) a closer view on the surface; and (F) high magnification image of the surface.

(23) FIG. 22: SEM images of a NIPS DMAc 0.3 mm PVDF membrane after the abrasion test for 2 weeks. (A) The cross-sectional overview; (B) a cross-sectional view close to the separation layer; (C) high magnification image of the top separation layer; (D) overview of the membrane surface; (E) a closer view on the surface; and (F) high magnification image of the surface.

(24) FIG. 23: Cross-sectional overview SEM images of the 1 mm thick CCD PES membranes prepared by using four different combinations of casting plate and cooling plate.

(25) FIG. 24: SEM images of close cross-sectional view (left) and surface (right) of the separation layers of the 1 mm thick CCD PES membranes prepared by using different combinations of casting plate and cooling plate.

MATERIALS AND METHODS

(26) Chemicals

(27) Commercial polyvinylidene fluoride (PVDF, Kynar® K-761, Mw=440,000 Da, ρ=1.79 g/cm.sup.3) was purchased from Elf Atochem and was dried at 80° C. for 24 hours before use. Poly(ethylene glycol) (PEG-400, Mw=400 kDa), bovine serum albumin (BSA), DMSO (DMSO), dimethylacetamide (DMAc), N-methyl-2-pyrrolidone (NMP), ethanol, hexane, SiC, were purchased from Sigma-Aldrich, UK and were used as received.

(28) Scanning Electron Microscopy (SEM)

(29) The morphologies of the membrane samples including separation layer, supporting layer and cross section were observed by SEM (LEO Gemini 1525 FEGSEM, Tokyo, Japan). The wet membranes were first immersed in ethanol for 30 minutes in order to replace all the water inside the pores, and then were fractured in liquid nitrogen to obtain the cross-sectional samples. The prepared samples were coated with gold of 10 nm thickness using an Emitech K550 ion sputtering device prior to SEM observation

(30) Fourier Transform Infrared Spectroscopy (FT-IR)

(31) The structure of PVDF membranes was analysed by using a Fourier Transform Infra-Red (FTIR) spectrometer (Perkin Elmer, Spectrum One equipped with an attenuated total reflection (ATR) attachment). The samples were placed on the sample holder and all spectra were recorded in the wavenumber range of 4000-600 cm.sup.−1 by accumulating 8 scans at a resolution of 2 cm.sup.−1.

(32) Differential Scanning Calorimetry (DSC)

(33) The melting behaviour of each membrane sample was characterized by DSC (Pyris-1, Perkin Elmer, Beaconsfield, UK) and was used to determine the percentage crystallinity of PVDF in the membranes. The samples were heated from 20° C. to 220° C. at 10° C./min. The percentage crystallinity of PVDF in each membrane sample was calculated by the equation shown below:

(34) $\%\mathrm{Crystallinity}=\frac{\Delta \mathrm{Hm}}{\Delta {\mathrm{Hm}}^0}\times 100\%$
where ΔHm is the heat associated with melting (fusion) of the membrane and is obtained from the DSC thermograms, ΔHm.sup.0 is the heat of melting if the polymer was 100% crystalline and is 104.7 J/g for PVDF.
X-Ray Diffraction (XRD)

(35) The crystalline structure of all the membrane samples were determined using an X-ray diffractometer (X'Pert PRO Diffractometer, PANalytical) with a source intensity of 40 kV/40 mA. All the samples were characterised in the scanning range of 5°<2θ<50°.

(36) Water Contact Angle Measurement

(37) The surface hydrophilicity of the membranes was studied by measuring the water contact angle on the surfaces of both separation layer and supporting layer. The measurement was conducted at room temperature using a ramé-hart Standard Goniometer (Model 250, ramé-hart instrument co., USA). In the test, 4 μL of deionised water droplets were deposited onto the membrane surfaces of each sample. The image was taken and the contact angle was measured from shape analysis by DROPimage Advanced software. At least five independent readings were taken at different sites on each membrane sample and the average value was reported.

(38) Pure Water Permeation Test

(39) In order to evaluate the membrane permeability, pure water permeation tests were conducted using a 300 mL dead-end filtration cell (HP4750 Stirred Cell, Sterlitech Corporation, USA). The PVDF membrane samples prepared by the CCD method were tested directly at 1 bar without any pre-treatment such as membrane compaction at higher pressure. This is because the membrane samples prepared by such method possessed excellent mechanical strength and could withstand at high pressure without any flux decline being observed. On the other hand, the membrane samples prepared by the conventional NIPS method were compacted at a pressure of 2 bar for 30 min prior to sample collection at 1 bar. The permeance of the membrane was calculated based on the equation shown below:

(40) $J=\frac{V}{A\times t}$
where J is the water flux, V is the permeate volume, A is the effective membrane area, t is the time of permeate collection.
BSA Fouling Test

(41) The fouling test was conducted using a cross-flow filtration cell (CF042 Crossflow Assembly, Sterlitech Corporation, USA) and BSA was employed as a model protein to investigate the fouling resistance of the membrane samples. In the test, 1.0 g/L BSA aqueous solution was circulated through the feed side of the filtration cell, and the weight of permeate was recorded by a computer in real time.

(42) Gas-Liquid Displacement Porosimetry

(43) In this work, the membrane pore size and pore size distribution were characterized by the gas-liquid displacement method using POROLUX 1000 (POROMETER nv, Belgium). The wet membrane was cut into certain size and wetted with a specific wetting liquid, POREFIL™ (POROMETER nv, Belgium, surface tension of 16 mN/m). In the test, the pressure of the testing gas N.sub.2 was increased from 0 to 34.5 bar step by step to replace the wetting liquid inside the membrane pores. At each step, both the pressure and the flow have to be stabilized within ±1% for 2 s before the data was recorded. The relevant pore size corresponding to each operating pressure can be calculated based on the Young-Laplace equation:

(44) $d=\frac{4\gamma \cos \theta }{\Delta P}$
where d is the diameter of the pores behaving as gas paths and contributing to the gas flow at each operating pressure; γ is the surface tension of the wetting liquid, which is 16 mN/m; θ is the contact angle of the wetting liquid on the membrane surface, which is 0°; ΔP is the specific operating pressure.

(45) Only the neck size of open pores could be measured using this method and for each sample, the mean flow pore (MFP) diameter and pore size flow distribution were obtained.

(46) Mercury Intrusion Porosimetry

(47) For some samples, mercury intrusion data were also collected at absolute pressure ranging between 1.38×10.sup.3 and 2.28×10.sup.8 Pa (0.2-33,500 psia) (Micromeritics Autopore IV) with an equilibration time of 10 s and assuming a mercury contact angle of 130°. The flat sheet membranes were cut into sections of approximately 4 mm in diameter prior to mercury intrusion analysis.

(48) Mechanical Testing

(49) Mechanical properties of the membranes were tested according to American Society for Testing and Materials (ASTM) D882 using a tensile testing machine, Lloyd EZ 50. The samples were cut into 10 mm wide parts and the thickness was measured with micrometre. Each sample was initially fixed at a gauge length of 50 mm and was then stretched at a constant rate of 10 mm/min; the corresponding tensile force was recorded by a transducer. The elongation ratio and tensile strength at the breaking point and Young's modulus were measured. At least five samples were tested for each membrane and the averaged value was recorded.

(50) Abrasion Testing

(51) In this work, the abrasion test was carried out using the same dead-end filtration cell as in the water permeation test. A 2000 mg/L silicon carbide suspension was prepared and used to simulate the accelerated abrasion condition in wastewater treatment. The wet membrane sample was placed in the filtration cell, and 300 mL of the SiC suspension was filled and then stirred at 400 RPM for 2 weeks. Subsequently, the membrane sample was washed under ultrasound for 10 min to remove all debris worn away from the membrane during the test. Then the change in the membrane structure was observed using SEM.

(52) Calculation of Temperature Profile and Cooling Rate

(53) Transient temperature profiles across casting polymer films and the cooling rates at fixed positions in the casting film were calculated by the commonly used finite difference method with the explicit scheme. The 1-D heat conduction models for the scenarios involved in this research were set as below.

(54) 1) Cooling with a pre-cooled plate

(55) The setting of the initial conditions for the calculation of thermal conduction and temperature profile under the circumstance of cooling with a pre-cooled cold plate is illustrated in FIG. 1

(56) The boundary of the cold plate was fixed at −30° C., as it was continuously cooled by a freezer. A 20 mm air gap was used to allow the temperature of the top surface of the polymer casting film to change, and the boundary was fixed at 20° C. The thickness of the air gap has a little influence on the final temperature of the top surface of the polymer film, but basically produces no difference within the time of interest.

(57) Heat conduction in the layers of different materials was deemed as heat diffusion along 1-D grids, on which points with an interval (Δx) of 5 μm were used to solve the heat conduction equation numerically:

(58) $\frac{\partial T}{\partial t}+\kappa \frac{{\partial }^{2}T}{\partial x^2}$
where T is the temperature, t is the time, x is the distance and K is the thermal diffusivity. The thermal diffusivity k is defined as

(59) $\kappa =\frac{k}{\rho c_p}$
where ρ is density, c.sub.p heat capacity and k thermal conductivity.

(60) Inside each homogeneous material layer, heat conduction is calculated by

(61) $T_i^{n+1}=T_i^n+\kappa \Delta t\left(\frac{T_{i+1}^n-2T_i^n+T_{i-1}^n}{{\left(\Delta x\right)}^2}\right)$
and at the interface between different material layers, heat conduction is calculated by

(62) $T_i^{n+1}=T_i^n+\frac{{\kappa }_{i-1}\Delta t}{{\left(\Delta x\right)}^2}{T}_{i-1}^n-\left(\frac{{\kappa }_{i-1}\Delta t}{{\left(\Delta x\right)}^2}+\frac{{\kappa }_i\Delta t}{{\left(\Delta x\right)}^2}\right)T_i^n+\frac{{\kappa }_i\Delta t}{{\left(\Delta x\right)}^2}{T}_{i+1}^n$
Here i is the position on the grid, and n is the number of the time step Δt, which is set
2kΔt≤(Δx).sup.2
to meet the criteria of stability for the calculation.
2) Cooling by immersing into liquid nitrogen or pre-cooled hexane

(63) In these two cases, same algorithm was used, but the grids and boundary conditions were different, as shown in FIG. 2.

(64) To simplify the calculation and to keep the calculating time within a manageable duration, and also due to the lack of available literatures, it is assumed that the thermal diffusivities in all layers are constant within the temperature range of interest, and it does not change in the casting film even if the film is turned from liquid to solid. This will of course lead some inaccuracy but will not alter the trends shown in the results. That is because within such a relatively short temperature range (from 20° C. to −30° C., except the case of liquid nitrogen, in which the lower temperature range from −30° C. to −196° C. is no longer interested), the change in the thermal diffusivity is usually very small. And for common solvents and polymer, the thermal diffusivity usually does not change significantly when phase transformation happens. The thickness change of the casting film during the cooling process was also not taken into account, since the change was small and should not lead significant effects to the temperature profiles.

Example 1—Membrane Preparation

(65) PVDF flat sheet membranes with highly asymmetric and self-assembled ordered structure were produced by a combined crystallization and diffusion (CCD) method. The PVDF dope solutions were prepared by dissolving PVDF powder (20 wt. %) in DMSO at 80° C. or in NMP and DMAc at room temperature, and then was left in the oven at 80° C. overnight to remove bubbles. The dope solution was then casted on a casting plate of certain thickness and was then unidirectionally cooled to a pre-determined temperature in two ways. Referring to FIG. 3, unidirectional cooling method 1 involves transferring the casting plate onto a pre-cooled cold plate (−30° C., detected by an infrared thermometer) on a freezing board. Unidirectional cooling method 2 involves immersing the casting plate into a pre-cooled liquid such as hexane (−15° C., detected by an infrared thermometer) or liquid nitrogen (−196° C.). For method 1, the materials of the cold and casting plates were aluminium or glass to realize different thermal conductions and thus different cooling rates; for method 2, a 1 cm thick glass casting plate was used to ensure fast cooling from only one side. After complete quenching, the frozen casting film was immersed in iced water to leach the solvent out. The water was changed regularly to remove the residual solvent. Apart from the materials of the plates, the casting thickness was varied in order to investigate their effects on the membrane morphology and properties.

(66) Modified PVDF membranes were also prepared with the same method, except the polymer solution is blended with PEG-400, with the PEG:PVDF:DMSO mass ratio of 1:4:16.

(67) Conventional non-solvent induced phase separation (NIPS) method was employed to prepare PVDF membranes as the control samples using DMSO, DMAc and NMP as the solvent and deionized water as the non-solvent. The polymer solution was cast on a glass plate at room temperature and then immediately immersed into water bath. The fabricated membrane was then kept in deionised water, which was changed frequently to remove the residual solvent before all the characterisations.

(68) The preparation conditions of each sample are summarised in Table 1 below:

(69) TABLE-US-00001 TABLE 1 Casting conditions for the preparation of flat sheet membranes according to CCD and NIPS procedures Casting Casting Cold plate/Cooling Sample solvent Thickness Plate Condition CCD pure Glass/Glass DMSO 1 mm 6 mm Glass 6 mm Glass plate at −30° C. PVDF Glass/Al DMSO 1 mm 6 mm Al 6 mm Glass plate at −30° C. membranes Al/Al 1.0 mm DMSO 1 mm 6 mm Al 6 mm Al plate at −30° C. Al/Al 0.5 mm DMSO 0.5 mm 6 mm Al 6 mm Al plate at −30° C. Al/Al 0.3 mm DMSO 0.3 mm 6 mm Al 6 mm Al plate at −30° C. Al/Al 0.1 mm DMSO 0.1 mm 6 mm Al 6 mm Al plate at −30° C. Hexane DMSO 1 mm 10 mm Glass Hexane bath at −15° C. Liquid N.sub.2 DMSO 1 mm 10 mm Glass Liquid nitrogen (−196° C.) Al/Al NMP NMP 1 mm 6 mm Al 6 mm Al plate at −30° C. Al/Al DMAc DMAc 1 mm 6 mm Al 6 mm Al plate at −30° C. CCD Glass/Al DMSO 1 mm 6 mm Al 6 mm Glass plate at −30° C. modified Al/Al 0.3 mm DMSO 0.3 mm 6 mm Al 6 mm Al plate at −30° C. PVDF-PEG membranes NIPS pure DMSO 1.0 mm DMSO 1.0 mm 6 mm Glass N/A PVDF DMSO 0.5 mm DMSO 0.5 mm 6 mm Glass N/A membranes DMSO 0.3 mm DMSO 0.3 mm 6 mm Glass N/A DMAc 0.3 mm DMAc 0.3 mm 6 mm Glass N/A NMP 0.3 mm NMP 0.3 mm 6 mm Glass N/A

Example 2—Membrane Structure and Properties

(70) Effect of Cooling Profile

(71) FIG. 4(A) shows the calculated temperature changes at the position of 10 μm from the cold-end interface of the polymer film, and FIG. 4(B) shows the temperature profiles within 200 μm from the cold end of the cast films after cooling for 1 second.

(72) FIG. 4(C) gives a cross-sectional scanning electron microscopy (SEM) image of the final membrane prepared by using a glass cold plate and a glass casting plate (denoted as Glass/Glass). The prepared membrane has an asymmetric structure with gradually opened micro-channels, which have been commonly observed in freeze-drying process when unidirectional cooling is applied due to the Mullins-Sekerka instability (8, 14, 15). In addition, pores smaller than 1 μm can be seen at the cold end edge and surface (FIG. 4(D & G)). With a faster cooling rate realised by employing a glass cold plate and an aluminium casting plate (denoted as Glass/Al), the pore size however can be quickly reduced to about 100 nm, as shown in FIG. 4(E & H). With an even faster cooling rate, realised by an aluminium cold plate and an aluminium casting plate (denoted as Al/Al), the pore size can be dramatically reduced to 30-50 nm, as shown in FIG. 4(F& I).

(73) Apparently in the case of Glass/Glass and Glass/Al samples, the slow cooling rates would have produced bigger initial DMSO crystallites than the Al/Al sample, and the temperature gradients in the cold-end region are also much less steep than the latter case, which would have resulted in slower polymer diffusions. Both factors would contribute to the formation of bigger pore size in Glass/Glass and Glass/Al samples, and it is difficult to tell which factor is more important during the membrane formation process.

(74) However, those cases using pre-cooled hexane and liquid nitrogen clearly show the importance of polymer diffusion. When immersed into the hexane and liquid nitrogen, the polymer film underwent much faster cooling than the Al/Al case, and the initial DMSO crystallites in principle should be smaller than the Al/Al case. However, the pore sizes in both cases are even bigger than the Glass/Glass case, as shown in FIG. 4(J) and 1(K). In both cases, although a temperature gradient similar to or larger than the Al/Al case was applied, the polymer film at the cold end (10 μm from the cooling interface, for example) was cooled down to less than 7° C. within 0.001 s and the polymer film would be frozen almost instantly, leaving virtually no time for polymer to diffuse. Taking the diffusivity of polymer solute in liquid DMSO as 1×10.sup.−9 m.sup.2/s, which is a typical value for polymer diffusion in solvents, the maximum distance that the polymer can travel within 0.001 s could be calculated to be only 1.4 μm, implying there was essentially no meaningful polymer diffusion occurred. As a consequence, the initial DMSO crystallites were allowed to agglomerate to form bigger grains resulting in big pores in the final membranes.

(75) Effect of Solvent

(76) If the sequence of phase separation and solvent crystallisation is altered, different results are obtained. In two other Al/Al cooling cases (−30° C.), N-methyl-2-pyrrolidone (NMP) and dimethylacetamide (DMAc) were used as the solvent, whose melting points are considerably lower than room temperature (−24° C. and −20° C., respectively), and deliberately let phase separation happen prior to the crystallisation of the solvent due to the reduction of solvent power. In both cases, due to the precipitated solid polymer blocks that lead steric hindrance effect to lateral solvent crystallisation, the formation of micro-channels was prevented, and the membranes have a quite homogenous structure, which are commonly observed in TIPS membranes. In both membranes, a thick, dense separation layer was formed at the cold side due to prolonged diffusion times. Since the dense separation layer was formed before solvent crystallisation, no solvent crystallites are expected to present in this layer, and in fact no visible pores were found in this layer under high-resolution SEM (FIGS. 5 & 6).

(77) Referring to FIG. 5, the NMP Al/Al membrane shows a sponge-like structure in the supporting layer, and no micro-channels were formed. This is due to the low melting point at −24° C., which is 46° C. lower than the room temperature. Within this large temperature difference, the PVDF solution had phase separated long before the crystallisation of NMP, and the formed polymer blocks would prohibit the formation of micro-channels during later NMP crystallisation. However, polymer diffusion from the hotter part to the cold end still happens, and due to the longer available time for diffusion, the denser separation layer in the NMP Al/Al membrane is much thicker than the DMSO Al/Al membrane, reached 5 μm. Since the formation of the separation layer is earlier than NMP crystallisation, no NMP crystallites presented in the separation layer and therefore there were no nano-scale pores from NMP crystallites formed. This thick and dense separation layer showed a very low pure water flux of 6.5 LMH bar.sup.−1, and no pores larger than 18.6 nm were detected with gas-liquid displacement porosimetry.

(78) Referring to FIG. 6, the DMAc (melting point at −20° C.) Al/Al membrane also shows a homogeneous but porous supporting layer and a denser top layer. This membrane broke apart when the crystallisation of DMAc was finished and only debris were obtained, which may be due to the damaging shape of DMAc crystal grains that cuts the membrane. The SEM images show some deep cracks formed at the air side of the membrane. Permeation characteristics such as pore size and pure water flux were therefore not obtained for this membrane.

(79) Pore Size Distribution and Flux

(80) The CCD pure PVDF membranes made from DMSO solution show very narrow pore size distributions (FIG. 7) and superior permeation performances compared with NIPS membranes. FIG. 7 shows typical pore size distributions of CCD PVDF membranes and NIPS PVDF membranes. All measurements used same pressure steps to ensure fair comparisons. FIG. 7(A) shows a CCD Glass/Glass 1 mm sample, which has a sharp peak at 334 nm with a percent flow of 97.0%; FIG. 7(B) shows a CCD Glass/Al 1 mm sample, which has a sharp peak at 103 nm with a percent flow of 98.3%; and FIG. 7(C) shows a CCD Al/Al 1 mm sample that has a peak at 40 nm with a percent flow of 72.0%. On the other hand, the membranes prepared by the NIPS method showed much broaden pore size distributions. The NIPS DMSO 1 mm sample showed a maximum percent flow of 18.9% at 38 nm (FIG. 7(D)), the NIPS DMSO 0.5 mm sample showed a maximum percent flow of 16.4% at 48 nm (FIG. 7(E)), and the NIPS NMP 0.3 mm sample showed a maximum percent flow of only 8.4% at 57 nm (FIG. 7(F)). The NIPS DMAc samples could not be measured, either due to the extremely low pore numbers, or because the pores are smaller than the limit of the equipment (18 nm).

(81) Table 2 summarises the permeation characteristics of these membranes and compare them with some commercial membranes from major manufacturers.

(82) TABLE-US-00002 TABLE 2 Permeation characteristics of CCD PVDF membranes, NIPS PVDF membranes and some commercial PVDF membranes Membrane Pure water flux Pore size material Membrane Type (LMH bar.sup.−1) (nm) Pure PVDF CCD membranes .sup.a Glass/Glass 1.0 mm 10998 ± 407  345 ± 26 Glass/Al 1.0 mm 5017 ± 547 119 ± 10 Al/Al 1.0 mm 861 ± 78 45 ± 3 Al/Al 0.5 mm 570 ± 37 29 ± 3 Al/Al 0.3 mm 608 ± 82 30 ± 9 Al/Al 0.1 mm 486 ± 28  38 ± 11 NIPS membranes .sup.a DMSO 1.0 mm  6.9 ± 3.4 35 ± 7 DMSO 0.5 mm  6.1 ± 1.2 45 ± 9 DMSO 0.3 mm  9.3 ± 5.8  54 ± 12 NMP 0.3 mm  2.3 ± 2.6  61 ± 11 DMAc 0.3 mm  2.7 ± 0.5 <18  Modified PVDF PVDF-PEG CCD Glass/Al 1.0 mm 6649 ± 675 162 ± 1  membrane .sup.a Al/Al 0.3 mm 1384 ± 112 38 ± 2 Commercial DOW 40-120 30 membranes .sup.b QUA  20 40 KOCH PURON ™ 100 30 GE ZeeWeed 1500 135 20 TORAY 30(MBR conditions) 80 Pall >3000.sup.  200  Pall >8200.sup.  450  TriSep TM10  90 200  Hydranautics HYDRAcap ® 34-110 80 .sup.a Sample names are ended with casting thickness; pore sizes were determined by the gas-liquid displacement method. .sup.b Pore sizes are nominal pore sizes provided by the manufacturer; water fluxes were converted from product brochure of membrane modules, but operation pressures and other conditions are unclear.

(83) Table 2 shows that the pure water permeation fluxes of the CCD membranes are substantially higher than the commercial membranes. Especially for the MF membranes with pore sizes of 119 nm and 345 nm, the pure water flux reached stunning 5017 and 10998 LMH bar.sup.−1, respectively. Considering the fact that most commercial membranes use modified PVDF to increase surface hydrophilicity, to make fair comparison, pure PVDF membranes were also prepared using polymer solutions of same PVDF concentration but via conventional NIPS method. The NIPS pure PVDF membranes showed pure water fluxes of less than 10 LMH bar.sup.−1 (Table 2), which are two orders of magnitude lower than the CCD Al/Al membrane with similar pore size.

(84) Separate from the data presented in Table 2, a CCD PVDF membrane prepared from a DMSO dope solution cast on a 6 mm thick glass substrate at a thickness of 0.5 mm and unidirectionally cooled in hexane provided at −12° C. had an average pore size of 950 nm±120 nm and a pure water flux of 14062±638 LMH/bar.

(85) Casting Thickness

(86) A very interesting correlation between the permeation flux and membrane thickness was found in the CCD Al/Al membranes, i.e. the flux increases as the thickness increases. Table 2 also gives the permeation characteristics of the Al/Al membranes with different casting thicknesses. It can be seen that the pore size does not show significant changes with the thickness, but the flux increase from 485.7 LMH bar.sup.−1 for the 100 μm membrane gradually to 860.6 LMH bar.sup.−1 for the 1 mm membrane. This trend can be attributed to different PVDF diffusion rates during the unidirectional cooling. It can be calculated that by changing the thickness of casting film, the cooling rate at the cold end was almost not affected during the time of interest (FIG. 8(A)). However, the temperature gradient increases when the thickness reduces (FIG. 8(B)), which would provide a larger driving force for the polymer solute to diffuse to the cold end and thus form a denser and thicker separation layer, leading to a slower permeation flux. The changes in the thickness and the density of the separation layer are clearly revealed by SEM images (FIG. 8(C-F)), which agree very well with the trends of the pure water flux and the temperature gradient. As a comparison, such a trend has not been observed in PVDF membranes also using DMSO as the solvent but via the conventional NIPS method.

(87) Modification of Membranes

(88) The permeation flux of the CCD membranes can be further enhanced by improving the hydrophilicity of the pores, since pure PVDF is widely considered to be a hydrophobic material (1, 2). Table 2 above also lists the permeation characteristics of two typical modified CCD PVDF microfiltration and ultrafiltration membranes, which were improved simply by blending a hydrophilic polymer polyethylene glycol (PEG) with PVDF. The improvement is especially evident for the Al/Al ultrafiltration membrane whose pure water permeation flux increased from 500-600 to about 1400 LMH bar.sup.−1 after modification, whilst the pore size was kept unchanged. Both the modified and unmodified CCD membranes showed significantly higher fluxes than commercial PVDF membranes (which normally use modified PVDF) with similar pore size, as depicted in Table 2, suggesting that the CCD PVDF membranes have great potential to replace existing commercial membranes.

(89) Membrane Microstructure

(90) To explore why the CCD method brings such high permeability to the PVDF membranes, the structural features of the CCD pure PVDF membranes were compared with those of the NIPS pure PVDF membranes. FIG. 9 shows clearly the structural features of a CCD Al/Al PVDF membrane prepared with 0.3 mm casting thickness. In this membrane, a thin separation layer is supported by numerous very well arranged micro-channels whose size gradually increases from the separation layer (FIG. 9(A)). The cross section image of the membrane shows clearly a number of tortuous pores in the separation layer, and intensively scattered pores on membrane surface (FIG. 9(B, C)). Furthermore, the supporting layer of the CCD membranes is composed of fully opened and oriented but inter-connected micro-channels, which actually give negligible resistances to water permeation (FIG. 9(D, E)).

(91) By comparison, the NIPS membranes show typical asymmetric structures with a skinned top layer supported by a region of finger-like voids and then a sponge-like layer (FIGS. 10 & 11). Although the skinned top layer of the NIPS membranes is thinner compared to the CCD membranes, only few pores on membrane surface can be observed within a scanned area under SEM (FIGS. 10 & 11), implying a very low surface porosity. On the other hand, the CCD membrane has a very porous separation layer.

(92) Fouling

(93) The NIPS membrane has a largely closed backside and back surface (FIGS. 10 & 11), which would not only contribute to the total transport resistance, but also tends to intensify fouling problem because foulant would accumulate in the supporting layer in real applications. Such foulant accumulation in the supporting layer however can be avoided in the CCD membranes.

(94) Fouling tests with a 1 g L.sup.−1 bovine serum albumin (BSA) solution reveal that the CCD Al/Al 1.0 mm membranes have improved resistance to fouling, which showed a slow and gradual decline of the permeation flux to 33% of the initial value after being tested for 24 hours (FIG. 12). As a comparison, pure PVDF membranes made by NIPS or TIPS underwent severe fouling very quickly with similar testing conditions (16-19), and frequent cleaning steps are required to restore the productivity of the membranes (Table 3).

(95) TABLE-US-00003 TABLE 3 Comparison of cross-flow BSA fouling test results for pure PVDF membranes made by NIPS, TIPS and CCD methods. Membrane preparation Ref method Experiment Results (1) TIPS Pure PVDF, BSA flux decreased 50 nm from ~80 LMH to pore size, ~27 LMH (33.7%) in 2 h. (2) NIPS Bare PVDF, BSA flux decreased to 10%. 20 nm (Unknown testing time; pore size, at 0.05 m.sup.3 BSA 1 g/L permeated (3) NIPS Pure PVDF, BSA flux decreased BSA to 33.5% in 1 h. 1 g/L. pore size (4) NIPS Pure PVDF, BSA flux decreased pore size to <10% in 2.5 h. unknown, This CCD Pure PVDF, BSA flux decreased work Al/Al 45 nm, to 64% in 1 h, 59% BSA 1 g/L in 2 h, and
Mechanical Properties

(96) Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and differential scanning calorimetry (DSC) results show that the CCD pure PVDF membranes are composed of β-phase and γ-phase PVDF crystallites with a crystallinity of around 60% (FIG. 13-15). The tightly connected PVDF grains and grain boundaries are clearly shown under SEM, especially in the supporting layer of the membranes (FIG. 16). This is distinct from the NIPS membranes, in which a large portion of α-phase PVDF presents (FIGS. 13 & 14).

(97) Due to the high crystallinity of PVDF and the absence of α-phase (20-22), the CCD membranes are more rigid compared with conventional NIPS membranes, which is reflected by low elongation ratios at the breaking point (<50% as listed in Table 4, while it is often higher than 100% for commercial NIPS PVDF membranes). The mechanical properties of the CCD membranes are shown in Table 4 below:

(98) TABLE-US-00004 TABLE 4 Mechanical properties of the CCD membranes Al/ Glass/ Glass/ Sample name Al 1.0 mm Al 1.0 mm Glass 1.0 mm Maximum Load (N) 12.0 ± 3.0  9.0 ± 2.8  8.0 ± 0.8 Elongation at Maximum 47.8 ± 5.9  26.3 ± 4.9  16.4 ± 6.8 Load (%) Tensile Strength (Mpa) 2.4 ± 0.6 1.9 ± 0.6  1.9 ± 0.2 Young's Modulus (Mpa) 68.1 ± 12.8 75.2 ± 13.1 66.5 ± 9.2 Maximum water speed* 23.6 20.1 18.8 (m/s)

(99) It is noted that with a faster cooling rate used during the membrane fabrication process, the membrane shows better mechanical properties: higher fracture load, longer elongation and higher tensile stress. It is reasonable to attribute this trend to the microstructural change in the membrane due to the different cooling rates: with a faster cooling rate, the CCD membrane has smaller micro-channels, and the number of the micro-channel would be larger (this assumption agrees with the proposed membrane formation mechanism and is confirmed by SEM images). With smaller but more micro-channels, the stress would be better distributed in the membrane and the energy would be easier to be dissipated by deformation, and fatal damages would be less likely to happen.

(100) Furthermore, since the CCD membranes show increased fluxes when the thickness increases, excellent mechanical properties and high fluxes can be obtained simultaneously. For example, the CCD Al/Al membranes with 1 mm casting thickness and 1 cm width showed a 12 Newton fracture tensile force in the tensile test (Table 4). This means that such a flat-sheet membrane of dimensions 1×2 m.sup.2 (width×length) can handle the drag force produced by flowing water along the length direction with a speed of 23.6 m s.sup.−1, which is much higher than the practical flow speeds used in real applications (normally less than 6 m s.sup.−1).

(101) Pressure Resistance

(102) Together with the unique structure of very well oriented micro-channels and gradually changed pore size, the high rigidity helps the CCD membrane to resist high pressures. As an example, the CCD Al/Al membrane with 1 mm casting thickness tested under a high pressure at 35 bar was able to maintain their original thickness (FIG. 17(A,B)). On the contrary, the NIPS membranes were severely compressed after the same test and the thickness was reduced to ⅔ of the original value (FIG. 17(C-E)). The CCD membranes can even withstand high-pressure mercury intrusion porosimetry tests, whereby NIPS membranes cannot handle.

(103) The mercury intrusion results of the Al/Al and Glass/Al membranes show typical cumulative intrusion volume-pore size curves (FIG. 18(A)) similar to those rigid pore structures such as in ceramic membranes, with an overall porosity of about 75-76% and a broaden pore size distribution from around 20 μm to less than 0.1 μm. The gradually increased intrusion volume reflects the gradual change in the pore size from the backside to the top separation layer in the CCD membranes. As expected, the incremental intrusion data (FIG. 18(B)) of the Al/Al membrane reveals a smaller pore size (11 μm) than the Glass/Al membrane (17 μm) at which intrusion starts, which correspond to the openings of the micro-channels on the backside. The average pore size of the Al/Al membrane is also smaller than the Glass/Al membrane. Closer observation of the incremental intrusion results (FIG. 18(C)) shows that the Al/Al membrane has higher pore volume at the pore size range of less than 100 nm than the Glass/Al membrane. These results agree very well with SEM images and gas-liquid displacement porosimetry results, and also agree with the prediction of membrane structure based on the cooling rate.

(104) Abrasion Resistance

(105) The CCD membranes have also shown excellent resistance to abrasion, which is commonly found in practice that shortens the membrane lifetime. After an accelerated abrasion test, the CCD Al/Al membrane could maintain its original pore structure (FIG. 19), whereas NIPS membranes were severely damaged (FIG. 20-22).

(106) With the experimental method employed for the abrasion tests, it is known that the most severe damages occur at the centre part of the membrane (5), therefore all SEM images given here were taken from the centre of the membranes for fair comparison.

(107) For the CCD Al/Al 0.3 mm membrane, it essentially kept the original pore structure in the separation layer and the whole membrane structure after the abrasion test. Although some extent of wearing can be found on the membrane surface, where debris were observed (FIG. 19(D)), the pore size on the surface and in the separation layer were not affected (FIG. 19(C, E)). In the FIG. 19(C), it can be seen that the thickness of the separation layer did not change compared with the untested same type of membrane shown in FIG. 9(C), meaning that the wearing of the membrane under such accelerated test was very slight.

(108) For the NIPS DMSO 0.3 mm membrane, the top separation layer was completely destroyed after the abrasion test. From the SEM images, it can be seen that there were only debris remained at the top layer and the separation layer was gone. For the NIPS NMP 0.3 mm membrane, the extent of wearing is less than the DMSO sample and the top layer still remains, but the top layer has been largely deformed and the pore structure has been completely altered. Big holes appear on the membrane surface and the surface microstructure has become very rough with apparent worn parts. The NIPS DMAc 0.3 mm membrane was the least damaged sample among the NIPS samples after the abrasion test. The top layer remained almost unchanged after the test, but some debris can be seen on the surface. However, in high magnification SEM images, it is clear that intensive cracks start to appear on the membrane surface after the test, which would change the pore size and ruin the selectivity of the membrane.

Example 3a—Preliminary Data for PES Membranes

(109) According to the protocol described in Example 1, poly(ether sulfone) CCD membranes were prepared from a 20 wt % PES solution, which was cast at a thickness of 500 μm onto Al, and then unidirectionally cooled on a pre-cooled Al plate. The pure water flux of the membrane was determined to be 167±17 LMH bar.sup.−1.

(110) The membrane has very smaller pores, which are smaller than 18.6 nm and were not detectable with the porometer. Further work will be needed to determine the actual pore size and the structure of the PES membranes, and to optimise the parameters to improve flux

Example 3b—Extended Data for PES Membranes

(111) Materials

(112) Commercial polyethersulfone (PES, Radel® A300, ρ=1.37 g/cm.sup.3) was purchased from Ameco Performance, USA. DMSO was purchased from Sigma-Aldrich, UK. All the chemicals were used as received.

(113) Membrane Preparation

(114) PES/DMSO membranes with different thicknesses were prepared using combined solvent crystallisation and polymer diffusion (CCD) method described in Example 1. Different material combinations for the casting and cooling plates were chosen to achieve different cooling rates and hence to illustrate the effect of cooling conditions on membrane structure and performance. The different combinations employed in this work for cooling/casting plates were copper/copper, aluminium/aluminium, glass/aluminium and glass/glass. The copper plates were 3 mm thick, whereas the aluminium and glass plates were both 6 mm thick.

(115) The dope solution used was 15 wt. % PES/DMSO solution. It was prepared by dissolving 150 g of PES resin in 850 g of DMSO solvent at room temperature. The solution was then left in the oven whose temperature was maintained at 80° C. to remove all the bubbles in the solution. The dope solution was then cast onto the casting plate with the desired thickness using a casting knife. After that, the casting plate was contacted with a cooling plate whose temperature was set and maintained to be −30° C. After the membrane was completely frozen, it was then immersed into iced water to leach out the solvent crystallites. The conditions of each membrane prepared are summarised in Table 5.

(116) TABLE-US-00005 TABLE 5 Casting conditions for the preparation of flat sheet membranes Casting Casting Sample Solvent Thickness Plate Cooling Plate Al/Al* 1.0 mm DMSO 1.0 mm Aluminium Aluminium at −30° C. Al/Al 0.7 mm DMSO 0.7 mm Aluminium Aluminium at −30° C. Al/Al 0.5 mm DMSO 0.5 mm Aluminium Aluminium at −30° C. Al/Al 0.3 mm DMSO 0.3 mm Aluminium Aluminium at −30° C. Cu/Cu 1.0 mm DMSO 1.0 mm Copper Copper at −30° C. Glass/Al 1.0 mm DMSO 1.0 mm Aluminium Glass at −30° C. Glass/Glass 1.0 mm DMSO 1.0 mm Glass Glass at −30° C. *Materials of the sequence cold plate/casting plate. Al stands for aluminium and Cu stands for copper.
Results

(117) FIG. 23 summarises the SEM images of the cross-sectional overview of the 1 mm thick CCD PES membranes prepared by four sets of casting plate and cooling plate. It can be seen that in all cases, a thin separation layer is supported by numerous well-arranged micro-channels whose size gradually increases from the separation layer. The structures are very similar to the CCD PVDF membranes described in Example 2.

(118) FIG. 24 gives detailed structure of the separating layer of the 1 mm thick CCD PES membranes. Generally speaking, smaller pores should be formed under faster cooling rates, which is in agreement with the CCD PVDF membranes described in Example 2. However, when the cooling rate is too fast, as shown in the Cu/Cu case, the polymer solution film at the cold end was cooled down so quickly that the polymer solution would be frozen almost instantly. Therefore, there was less time for polymer diffusion to take place and the DMSO crystals at the colder side would agglomerate to form bigger grains, resulting in bigger pores than the Al/Al case in the final membranes.

(119) To test the filtration performance of the membrane samples, pure water fluxes and pore sizes of prepared CCD PES membranes were measured and the results are tabulated in Table 6. All membranes showed very high water permeation flux. It is evident that the water permeation results and mean flow pore sizes of CCD membranes concur with the structure as seen in FIG. 24. For instance, the CCD Glass/Glass PES membrane has the largest pore size of 664 nm and showed the highest flux of 11,930 LHM bar.sup.−1, and the Al/Al PES membrane has the smallest pore size of 28 nm and showed the lowest water flux of 2,286 LHM bar.sup.−1.

(120) TABLE-US-00006 TABLE 6 Permeation characteristics of 1 mm thick CCD PES membranes Membrane Material Membrane Type/Ref. PWF MFP Size Pure PES CCD membranes .sup.a (LMH bar.sup.−1) (nm) Glass/Glass 1.0 mm 11,930 ± 199 664 ± 67 Glass/Al 1.0 mm  6,328 ± 300 121 ± 17 Al/Al 1.0 mm 2,286 ± 29 28 ± 1 Cu/Cu 1.0 mm  5,813 ± 409 87 ± 2 .sup.a Sample names are ended with casting thickness; pore sizes were determined by the gas-liquid displacement method.

(121) The effect of varying casting thickness on the structure and performance of the prepared CCD Al/Al PES membranes was also investigated. Table 7 shows the pure water fluxes and mean flow pore sizes of membranes prepared with various casting thickness. It can be seen clearly that as the casting thickness was decreased from 1.0 to 0.3 mm, the water flux reduced from approximately 2,286 to 1,619 LMH bar.sup.−1. The pore size also decreased when casting thickness decreased, especially for casting thickness of 0.5 and 0.3 mm whose pore sizes were so small that could not be tested using the porometer. This trend is again in agreement with the trend of the CCD Al/Al PVDF membranes described in Example 2.

(122) TABLE-US-00007 TABLE 7 Pure water fluxes and mean flow pore sizes of CCD Al/Al membranes prepared with various casting thickness Casting Thickness (mm) PWF (LMH bar.sup.−1) MFP Size (nm) 1.0 2,286 ± 29 29 ± 1 0.7  1,563 ± 174 23 ± 2 0.5 1,708 ± 33 <18.6 0.3 1,619 ± 52 <18.6

(123) While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended claims.

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