THE SAME

RELATED APPLICATIONS

[0001] This application claims priority benefit of United States Provisional Application Serial Number 62/530,445 filed 10 July 2017; the contents of which are hereby incorporated by reference.

TECHNICAL FIELD

[0002] The present invention generally relates to a patterned microfilter membrane and a method for preparing a patterned microfilter membrane, and more specifically to a patterned microfilter membrane for water and wastewater filtration with high water flux and antifouling capabilities.

BACKGROUND

[0003] In the last few decades, there has been an increasing interest in membrane technologies for water and wastewater treatments due to their distinct advantages such as low energy consumption, ease of preparation and upscaling process, and a wide variety of polymers that can be used at a relatively low cost(l-2). However, the deposition of unwanted colloid, organic and inorganic particles on the membrane surface and inside the membrane pores causes severe water flux reduction(3). This is known as membrane fouling and it is one of the most critical issues that limits the widespread use of this technology. Recently, more attention has been placed on surface topography as an environmental promising approach to reduce membrane fouling. The idea involves applying patterns on the membrane surface, so that it increases the surface area of the membrane, which increases the water permeate. Moreover, it also adds some turbulence which prevents the deposition of fouling material on the membrane surface. Vogelaar et al. reported for the first time the use of a micromolding process to prepare patterned polymeric membranes, so-called "phase separation micromolding (Ρ8μΜ)"(4). Later on, this method opened avenues in the research community, enabling researchers to prepare and test different polymeric membranes(5-6).

[0004] The main disadvantage of Ρ8μΜ is that the location of the dense layer is at the unpattemed side of the membrane, since the patterned side is always blocked by the mold itself. However, Won et al. proposed a modified immersion precipitation method to relieve the formation of the dense layer on the unpattemed side of the membrane(7). Marouf et al. used nanoimprint lithography (NIL) to pattern commercial NF membranes, but results showed that this method affected the pore size/number of the membrane. Therefore, when the prepared membranes were tested in a cross-flow system with distilled deionized (DI) water, the flat membranes showed higher water flux compared with the patterned one. According to Darcy's law, an increase in the surface area leads to an increase in the water permeates through membrane(8). Accordingly, there is a need in the art for a way to produce patterned membranes with a thin dense layer over large structured surface area using a structure mold made of a non-solvent. SUMMARY OF THE INVENTION

[0005] A micro filtration membrane is provided that includes a patterned side and an un-patterned side and a dense layer located at the patterned side of the membrane. The membrane is useful for the treatment of water and wastewater. The membrane filters 15% more water than an un-patterned membrane and in some embodiments up to 800%. According to various embodiments, the membrane is formed of a polymer casting solution. The patterned side includes projecting features having a size of 1- 500μιη, which according to some embodiments include finger-like asymmetric structures.

[0006] A method of preparing the microfiltration membrane is also provided that includes providing a patterned non-solvent mold, applying a casting solution to the mold, and forming a dense layer at an interface of the non-solvent mold and the casting solution. According to embodiments, the non-solvent mold is a hydrogel mold and the casting solution is a polymer casting solution.

BRIEF DESCRIPTION OF THE DRAWINGS

[0007] The present invention is further detailed with respect to the following drawings that are intended to show certain aspects of the present of invention, but should not be construed as limit on the practice of the invention, wherein:

[0008] FIG. 1 is a Scanning Electron Microscope (SEM) image showing the internal structures of the mattered membrane prepared using a hydrogel mold according to the present invention; [0009] FIG. 2 is a SEM image showing the internal structures of the mattered membrane prepared using a hydrogel mold according to the present invention;

[0010] FIG. 3 is a schematic diagram showing a cross-flow filtration system;

[0011] FIG. 4 is a graph showing water permeate of the patterned and un-patterned membranes;

[0012] FIG. 5 is a SEM image showing a patterned membrane having ridges and valleys according to one embodiment of the present invention;

[0013] FIG. 6 is a SEM image showing the patterned membrane of FIG. 5;

[0014] FIG. 7 is a SEM image showing the internal structures of a ridge of FIGS. 5 and 6;

[0015] FIG. 8 is a SEM image showing the internal structures of a ridge of FIGS. 5 -7;

[0016] FIG. 9 is a SEM image showing the internal structures of a ridge of FIGS. 5- 8;

[0017] FIG. 10 is a SEM image showing the internal structures of a ridge of FIGS. 5-9.

[0018] FIG. 11 is a SEM image showing a patterned membrane having posts according to one embodiment of the present invention;

[0019] FIG. 12 is a SEM image showing the internal structures of a post of the patterned membrane of FIG. 11;

[0020] FIG. 13 is a SEM image showing a patterned membrane having other structures according to one embodiment of the present invention; [0021] FIG. 14 is a SEM image showing a patterned membrane having other structures according to one embodiment of the present invention; and

[0022] FIG. 15 is a SEM image showing a patterned membrane having other structures according to one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0023] The present invention has utility as a patterned micro filtration membrane and method for preparing the same for water and wastewater treatment.

[0024] The present invention provides a technique to develop patterned micro filtration (MF) membranes using hydrogel molds. A patterned hydrogel mold is replicated using a polymer casting solution. The hydrogel acts as the nonsolvent and since the place of the solution/nonsolvent interface defines the location of the dense layer, the inventive method produces a dense layer at the patterned side of the membrane. This technique develops micropatterned membranes with higher water flux and antifouling compared to the conventional ones.

[0025] Embodiments of the present disclosure provide a microfiltration membrane having a patterned side, an un-patterned side, and a dense layer located at the patterned side of the membrane. According to embodiments, the patterned side of the membrane includes a plurality of projecting features having a size of 1-500μιη. According to embodiments the projecting features may be finger-like asymmetric structures, a plurality of circular posts, a plurality of square projections, a plurality of rectangular projections, or a combination thereof. [0026] It is to be understood that in instances where a range of values are provided that the range is intended to encompass not only the end point values of the range but also intermediate values of the range as explicitly being included within the range and varying by the last significant figure of the range. By way of example, a recited range of from 1 to 4 is intended to include 1-2, 1-3, 2-4, 3-4, and 1-4.

[0027] The pattern of the projecting features on the patterned side of the membrane correspond in shape, position, and number to a pattern of shaping features formed on the membrane forming non-solvent mold. According to various embodiments, the patterned non-solvent mold the shaping features have a size of 1-500μιη. The shaping features may be finger-like asymmetric indents, a plurality of circular indents, a plurality of square indents, fractal patterns, a plurality of rectangular indents, or a combination thereof.

[0028] According to embodiments, an inventive microfiltration membrane is formed by providing a patterned non-solvent mold, applying a polymer casting solution to the mold, and forming a dense layer at an interface of the non-solvent mold and the polymer casting solution. According to embodiments, the polymer casting solution is a polymer of at least one of polyvinyl chloride (PVC), polyurethane, and acrylic dissolved or dispersed in a solution. According to embodiments, the method additionally includes drying the polymer casting solution after applying the polymer casting solution to the patterned non-solvent mold and removing the microfiltration membrane from the patterned non-solvent mold. [0029] It is to be understood that in instances where a range of values are provided that the range is intended to encompass not only the end point values of the range but also intermediate values of the range as explicitly being included within the range and varying by the last significant figure of the range. By way of example, a recited range of from 1 to 4 is intended to include 1-2, 1-3, 2-4, 3-4, and 1-4.

EXAMPLES

[0030] The preparation steps for an inventive patterned membranes using hydrogel molds illustratively includes a polymethylmethacrylate (PMMA) substrate being used to prepare ridge-pattern master mold using C02 laser cutter. The master mold can also be prepared using lithography techniques or 3D printing technology depending on the desired features and dimensions. Next, a hydrogel solution is prepared and cast on top of the patterned mold that comprises the desired structures and left to gel at room temperature before demolding. The hydrogel mold is then placed on a smooth substrate, and pressurized air is used to remove standing water from the patterned side. Without this step, some water drops will tend to remain between the patterns that can affect the quality of the replicated structures, resulting in a non-uniform membrane. The polymer solution is cast on the patterned side of the hydrogel mold using a film applicator. Thereafter, the solvent exchanges with the non-solvent at the film surface and due to the huge sudden gradient of the polymer chemical potential, this causes a net movement of the polymer towards the solvent/non-solvent interface. After the polymer concentration in the top layer of the cast film reaches a specific point, a skin layer forms on the surface and acts as a barrier for further non-solvent transport into the sub-layer of the polymer solution. The polymer solution beneath the skin layer has a lower polymer concentration, forming the finger-type structures in a two-step process; the initiation, and the propagation of fingers. The initiation of fingers forms at points where the skin layer fractures as a result of shrinkage stress and syneresis. Subsequently, the growth of the fingers occurs and propagates towards the bottom side of the membrane. Finally, the membrane is demolded from the hydrogel and is stored in distilled water for later usage.

[0031] Materials: Polyethersulfone (PES, BASF Ultrason E6020p, M _w =58 kDa), Ν,Ν-Dimethylacetamide (DMAc, Sigma-Aldrich), and polyvinylpyrrolidone (PVP, Sigma- Aldrich, M _w = 35 kDa) were used to prepare polymer casting solution. Agarose (Sigma-Aldrich, CAS number: 9012-36-6) and PVP were utilized to make hydrogel solution. Polymethylmethacrylate (PMMA, McMaster-Carr) was utilized to develop master molds.

[0032] Preparation of solutions: The membrane polymer solution was prepared by dissolving 15wt% PES and 2wt% PVP in 83wt% DMAc. The solution was placed on a magnetic stirrer overnight to produce a homogeneous polymer solution. The hydrogel solution was prepared with 5wt% agarose and lwt% PVP mixed with 94 wt% distilled water. The mixture was then placed in a microwave and heated up until boiling. Finally, the mixture was gently degassed to remove air bubbles. The PEG solution was prepared by dissolving 0.25 g of BSA powder in 1 L of distilled water and then stirred for 10 minutes. In each experiment, a fresh PEG solution was used. [0033] Preparing the master molds: The PMMA trenches molds were manufactured using a C0 ₂ laser cutter (VLS 3.5 Versa Laser). The posts and the crosses master mold were prepared using lithography techniques.

[0034] Characterization: For each membrane, a consistent filtration protocol was followed. All filtration experiments started by compacting the membrane with distilled water at 3 bar for 30 minutes. Then, the TMP lowered to 2 bar and PEG solution was used as a feed to the cross-flow system. After collecting 70 grams, a sample from the permeate was collected to measure the rejection. Data was recorded every 30 seconds using a balance and converted into LMH (L m ^"2 h ^"1 ) using Equation (1):

[0035] where J _w , Q, A, and At are the water permeate flux (L m ^"2 h ^"1 ), the measured amount of permeate (L), the surface area (m ² ), and the sampling time (h). For evaluating the rejection, samples from the feed and the permeate solutions were collected after one hour and analyzed using total organic carbon (TOC) analyzer to measure the concentration of PEG. The following equation was used to measure the PEG rejection:

[0036] where C _p and Cf are the concentrations of the BSA in the permeate and the feed, respectively. [0037] Field-emission scanning electron microscope (FESEM) was used to examine the surface and the cross-section of the membrane samples. Membrane samples were dried overnight and frozen in liquid nitrogen before fracturing. Prior to imaging, the samples were coated with a 2 nm gold layer using a Denton gold sputter unit. Surface wettability was measured using a contact angle measurement device (KRUSS - DSA100) where 1 of water was dropped on a dry membrane surface and the angle was recorded directly. The pre-wetting procedure was performed by applying a drop on the membrane surface and waiting for 5 minutes until the drop was completely absorbed, followed by dropping another water drop on the same spot and recording the contact angle.

[0038] Scanning Electron Microscope (SEM) images show that this method produces finger-like asymmetric structures that are typical for immersion precipitation, whereas the dense layer is located at the patterned side of the membrane, as shown in FIGS. 1-2.

[0039] Filtration experiments are conducted using the cross-flow filtration system as shown in FIG. 3. The feed water passes tangentially along the surface of the membrane, and due to the pressure difference across the membrane channel it drives the water to pass through the membrane. The solution that passes over the membrane surface and goes back to the feed tank is known as retentate, while the one that passes through the membrane is known as permeate. FIG.4 shows the results of testing the patterned and the un-patterned membranes using the cross-flow system with distilled water. It can be shown clearly that the patterned membrane provides higher water flux when compared with the un-patterned one. This is due to the increase in the surface area which is directly proportional with the water flux. The patterned membrane increases the water permeate by about 800%, compared to a conventional membrane.

[0040] FIGS. 5-15 show a formation of wide range of features in terms of size (1- 500μιη) and geometries on the membrane surface.

[0041] The surface chemistry of the hydrogel and conventionally produced membranes was examined using FTIR with absorption peaks of the inventive membranes mainly spread in the range of 700 to 1700 cm-1 which represents the characteristics bands of PES and PVP polymers: The PES peaks can be found in three main bands. Peaks located at 1105/1150 cm-1 and 1292/1320 cm-1 are due to the symmetric and asymmetric stretching vibrations of sulfone functional groups (Ar-S02- Ar), respectively. Peaks at 1240 cm-1 and 1486M580 wavelengths are due to the aromatic ether structures (Ar-O-Ar) and the aromatic ring groups, respectively. The existence of PVP is shown at peak 1662 cm-1 (vibration of amide groups C=0). Both types of membranes showed the same trend indicating that the hydrogel molding technique does not affect the chemical property of membranes.

[0042] The contact angle measurements for the dry samples of patterned-hydrogel, unpatterned-hydrogel and the conventional membranes were 43°, 58°, and 56°, respectively. In the case of pre-wetted samples, again the contact angle decreased to 52° for both the unpatterned-hydrogel and the conventional membranes and 32° for the patterned one. However, since hydrogel is not changing the chemical properties of the membrane surface, the reduction in the contact angle for the patterned membrane is probably due to the existence of the patterns which changes the physical property of the surface for Wenzel wetting.

[0043] The average pore size for the prepared membranes was calculated using Guerout-Elford-Ferry equation. The pore sizes of patterned-hydrogel, unpatterned hydrogel and conventional membranes were 57, 41, and 16 nm, respectively, which is in the range of UF membranes.

[0044] While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the described embodiments in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient roadmap for implementing the exemplary embodiment or exemplary embodiments. It should be understood that various changes may be made in the function and arrangement of elements without departing from the scope as set forth in the appended claims and the legal equivalents thereof.

CITED REFERENCES

[0045] (1) S. Hermans et al. Recent developments in thin film (nano) composite membranes for solvent resistant nanofiltration. Current Opinion in Chemical Engineering 8pp. 45-54. 2015.

[0046] (2) J. Mulder. Basic Principles of Membrane Technology 2012. [0047] (3) D. Rana and T. Matsuura. Surface modifications for antifouling membranes. Chem. Rev. 110(4), pp. 2448-2471. 2010.

[0048] (4) L. Vogelaar et al. Phase separation micromolding— Ρ8μΜ. Adv Mater 15(16), pp. 1385-1389. 2003.

[0049] (5) Y. Gencal, E. Durmaz and P. Culfaz-Emecen. Preparation of patterned microfiltration membranes and their performance in crossflow yeast filtration. J. Membr. Sci. 476pp. 224-233. 2015.

[0050] (6) M. Gironde's et al. Polymeric microsieves produced by phase separation micromolding. J. Membr. Sci. 283(1), pp. 411-424. 2006.

[0051] (7) Y. Won et al. Preparation and application of patterned membranes for wastewater treatment. Environ. Sci. Technol. 46(20), pp. 11021-11027. 2012.

[0052] (8) S. H. Maruf et al. Use of nanoimprinted surface patterns to mitigate colloidal deposition on ultrafiltration membranes. J. Membr. Sci. 428pp. 598-607. 2013.