Patent Application
20190022595
The present invention relates to a method of forming thin film through-hole membranes.
Through-hole membranes are extensively used in purification processes, cell biology studies and biomedical applications. Conventional polymeric membranes, fabricated by phase inversion, electrospinning, and track etching are relatively thick and have micron-sized pores with random placement and tortuous paths. On the other hand, flexible thin membranes with high porosity, small tortuosity and spatially ordered, monodisperse pores would better suit the applications' sharp size selectivity and high flux requirements but the fabrication of such membranes typically require photolithography processes; or direct writing and micromachining methods such as electron-beam lithography and focus ion beam milling which are limited by their intrinsic cost, process repetition/complexity and low throughput capability.
Examples of current methods which allow duplication of ordered surface pattern from pre-fabricated mold directly onto target material by mechanical contact and 3-D material displacement via squeeze flow and capillary action are nanoimprint lithography (NIL) and soft lithography. However, the problem with these methods is that the replica molding often leaves a thin residual layer under the mold protrusions, whose removal requires complex and laborious post fabrication processes such as reactive ion etching (RIE) and/or chemical etching, thereby leading to increased fabrication costs.
There is therefore a need for an improved method of forming thin film through-hole membranes.
The present invention seeks to address these problems, and/or to provide an improved method of forming thin film through-hole membranes.
In general terms, the invention relates to a simple and cost-effective method which does not require any delicate laborious post-fabrication steps such as dry and/or wet etching steps in forming submicrometer thin film through-hole membranes. Further, the method also enables free. In particular, the method is based on capillary force driven mold-based lithography. The method of the present invention also allows rapid and clean transfer of the formed membrane from a patterning substrate to a target substrate.
According to a first aspect, the present invention provides a method of forming a thin film through-hole membrane comprising:
According to a particular aspect, the thin film through-hole membrane formed from the method comprises ordered and uniform-sized pores.
The thin film may comprise a thermoplastic polymer. Any suitable thermoplastic polymer may be used for the present invention. For example, the thermoplastic polymer may be selected from the group consisting of: polystyrene (PS), poly(methyl methacrylate) (PMMA), polyether block amide and combinations thereof. In particular, the thin film may comprise PS.
The sacrificial layer may comprise any suitable water-soluble polymer. According to a particular aspect, the water-soluble polymer comprised in the sacrificial layer may have a glass transition temperature higher than the pre-determined temperature. For example, the water-soluble polymer may be selected from the group consisting of: poly(sodium 4-styrenesulfonate) (PSS), acryloyl morpholine (ACMO), polyvinylpyrrolidone (PVP), and combinations thereof. In particular, the sacrificial layer may comprise PSS.
The patterned mold may comprise any suitable polymer. For example, the patterned mold may comprise an elastomeric polymer. In particular, the elastomeric polymer may be selected from the group consisting of: polydimethylsiloxane (PDMS), polyurethane acrylate (PUA), and combinations thereof. Even more in particular, the elastomeric polymer may be PDMS.
The patterning structure comprising the patterning substrate, the sacrificial layer and the thin film may be formed by any suitable method. In particular, the sacrificial layer and the thin film may be sequentially provided on the surface of the patterning substrate by spin coating, aerosol spraying, doctor blading or dip coating.
The thin film comprised in the patterning structure may have a suitable thickness. According to a particular aspect, the thickness of the thin film is less than the pillar height of the patterned mold. For example, the thin film may have a thickness of <1 μm. In particular, the thin film as provided in the patterning structure may have a thickness of 50-900 nm, 75-875 nm, 100-850 nm, 150-800 nm, 200-750 nm, 250-700 nm, 300-650 nm, 350-600 nm, 400-550 nm, 450-500 nm. Even more in particular, the thickness of the thin film may be 100-500 nm.
The sacrificial layer comprised in the patterning structure may have a suitable thickness. For example, the thickness of the sacrificial layer may be 50-200 nm. In particular, the thickness of the sacrificial layer may be 50-200 nm, 75-175 nm, 100-150 nm, 125-140 nm, 130-135 nm. Even more in particular, the thickness of the sacrificial layer may be about 150 nm.
The imprinting may be by any suitable method. For example, the imprinting may be by capillary force lithography (CFL). The imprinting may be carried out under suitable conditions such as a pre-determined temperature and pre-determined period of time. For example, the pre-determined temperature may be any suitable temperature for the purposes of the present invention. In particular, the pre-determined temperature may be a temperature which is above the glass transition temperature of the thin film.
During the imprinting, the contact angle of the thin film polymer melt on the surface of the patterned mold may be a suitable angle. In particular, the contact angle may be <90°.
According to a particular aspect, the method may further comprise treating the patterned mold with plasma prior to the imprinting. The treating may be carried out under conditions suitable for the purposes of the present invention.
The contacting may be carried out under suitable conditions. For example, the contacting may comprise contacting the patterning structure with water at room temperature.
The method may further comprise transferring the released thin film through-hole membrane onto a surface of a target substrate.
A second aspect of the present invention provides a thin film through-hole membrane prepared from the method of the first aspect.
According to a third aspect, there is provided a hierarchical membrane for graded filtration comprising at least one thin film through-hole membranes prepared from the method of the first aspect. In particular, each of the thin film through-hole membranes comprised in the hierarchical membrane comprises a different pore size. Even more in particular, the hierarchical membrane may be comprised in a membrane housing module.
In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings:
As explained above, there is a need for improved method of forming thin film through-hole membranes. Particulate respirator products made of charged polypropylene micro-fibres with randomly distributed inter-fibre distances generally have a wide pore size range and rough fibrous surface which leads to low nanoscale selectivity and low wettability, respectively. In addition, loose fibre compactness and tortuous pore path induced surface fouling also represent major issues which will greatly affect membrane long term performance and stability. There is therefore a need for membranes with narrow pore size distribution for optimised filtration efficiency.
The present invention provides a method of fabricating a thin film through-hole polymeric membrane with uniform and tuneable pore size down to the sub-100 nm region which is simple without involving delicate chemistries and laborious post-fabrication steps such as dry and/or wet etching. In this way, the method avoids the problems associated with etching processes. The method is also simple and scalable. Further, the membrane formed from the method may be easily detached from the substrate after patterning. Accordingly, the thin film through-hole membrane may be transferred with high fidelity onto various target substrates without defects.
According to a first aspect, there is provided a method of forming a thin film through-hole membrane comprising:
A method 100 of forming a thin film through-hole membrane and subsequently transferring the thin film through-hole membrane from one substrate to another substrate may generally comprise the steps as shown in
Step 102 comprises providing a sacrificial layer on a patterning substrate. The sacrificial layer is a layer which may be easily provided on a surface of a substrate as well as a layer which is able to rapidly dissolve upon contacting water at room temperature instead of requiring chemical etchants. In particular, the sacrificial layer may comprise a water-soluble polymer. Any suitable water-soluble polymer may be used for providing the sacrificial layer. In particular, the water-soluble polymer comprised in the sacrificial layer may have a glass transition temperature higher than the pre-determined temperature. For example, the water-soluble polymer may be selected from the group consisting of: poly(sodium 4-styrenesulfonate) (PSS), acryloyl morpholine (ACMO), polyvinylpyrrolidone (PVP), and combinations thereof. According to a particular embodiment, the sacrificial layer comprises PSS.
The sacrificial layer may have a suitable thickness. For example, the thickness of the sacrificial layer may be 50-200 nm. In particular, the thickness of the sacrificial layer may be 50-200 nm, 75-175 nm, 100-150 nm, 125-140 nm, 130-135 nm. According to a particular embodiment, the thickness of the sacrificial layer may be 150 nm.
The sacrificial layer may be provided on the surface of the patterning substrate by any suitable method. For example, the sacrificial layer may be provided on the surface of the patterning substrate by, but not limited to, spin coating, aerosol spraying, doctor blading or dip coating, or a combination thereof. According to a particular embodiment, the sacrificial layer is provided by spin coating.
Once the sacrificial layer is provided on the surface of the patterning substrate, a thin film is provided on the sacrificial layer to form a patterning structure. Accordingly, step 104 comprises providing a thin film on the sacrificial layer. The thin film may be of any suitable material which may form a through-hole membrane. For example, the thin film may comprise a thermoplastic polymer. Any suitable thermoplastic polymer may be used for the purposes of the present invention. The thermoplastic polymer may be any suitable polymer which a low surface tension. Examples of the thermoplastic polymer include, but are not limited to: polystyrene (PS), poly(methyl methacrylate) (PMMA), polyether block amide, and combinations thereof. According to a particular embodiment, the thin film may comprise PS.
The thin film may have a suitable thickness. For example, the thin film may have a thickness of <1 μm. In particular, the thin film may have a thickness of 50-900 nm, 75-875 nm, 100-850 nm, 150-800 nm, 200-750 nm, 250-700 nm, 300-650 nm, 350-600 nm, 400-550 nm, 450-500 nm. According to a particular embodiment, the thickness of the thin film may be 100-500 nm.
The thin film may be provided on the sacrificial layer by any suitable method. For example, the thin film may be provided on the sacrificial layer by, but not limited to, spin coating, aerosol spraying, doctor blading or dip coating, or a combination thereof. According to a particular embodiment, the thin film is provided by spin coating.
The patterning substrate may be any suitable substrate. In particular, the patterning substrate may be any suitable substrate on which the sacrificial layer and thin film may be provided. The selection of the patterning substrate may differ depending on the sacrificial layer and the thin film to be provided on the surface of the patterning substrate. For example, the patterning substrate may comprise glass or silicon. In particular, a person skilled in the art would understand which substrate to use as a patterning substrate depending on the sacrificial layer and thin film to be provided. According to a particular embodiment, the patterning substrate comprises silicon.
Step 106 comprises imprinting the thin film with a patterned mold to form a thin film through-hole membrane on the patterning structure. The patterned mold may have a pillar height which is more than the thickness of the thin film provided on the patterning structure.
The patterned mold may comprise any suitable polymer. The patterned mold may comprise any suitable polymer which is rigid enough to preserve the mechanical stability of small mold features while simultaneously being flexible enough to provide good conformal contact when contacted with the thin film. For example, the patterned mold may comprise an elastomeric polymer. The elastomeric polymer may be any suitable polymer which has a high surface energy. In particular, the elastometic polymer may be selected from the group consisting of, but not limited to: polydimethylsiloxane (PDMS), polyurethane acrylate (PUA), and combinations thereof. According to a particular embodiment, the elastomeric polymer may be PDMS. According to another particular embodiment, the elastomeric polymer may be PUA. In particular, a patterned mold comprising PUA is preferred for imprinting smaller pores. Even more in particular, a patterned mold comprising PUA is preferred for imprinting sub-500 nm sized pores on the thin film. An example of a thin film through-hole membrane formed using a patterned mold comprising PUA is shown in
The patterned mold may have any suitable pattern and structure. For example, the patterned mold may be a cylindrical pillar structure, wherein each pillar has a diameter of about 0.55 μm.
The imprinting may be by any suitable method. For example, the imprinting may be by capillary force lithography (CFL). The imprinting may be carried out under suitable conditions. The suitable conditions may comprise a pre-determined temperature and pre-determined period of time. The pre-determined temperature may be any suitable temperature for the purposes of the present invention. According to a particular embodiment, the pre-determined temperature may be a temperature which is above the glass transition temperature of the thin film, but lower than the glass transition temperature of the sacrificial layer. In particular, the pre-determined temperature may be about 120-140° C.
In particular, the imprinting may comprise bringing the patterned mold to conformal contact with the thin film on the patterning structure followed by thermal annealing to facilitate membrane imprint by CFL. As shown in
The use of the elastomeric patterned mold in CFL allows constant conformal contact between the patterned mold and the thin film, thereby ensuring uniform imprinting. Such uniform imprinting together with the thin film thickness being less than the pillar height of the patterned mold enables CFL patterning to span from the surface of the thin film which contacts the patterned mold to the surface of the thin film in contact with the sacrificial layer. In particular, capillary induced Laplace pressure drives the CFL, leading to spontaneous polymer melt filling into cavities along the contours of the confining patterned mold when thermal annealing is at a temperature above the glass transition temperature of the thin film. As a result of the CFL patterning during the imprinting, the imprint perforates the thin film to form a thin film through-hole membrane.
The pre-determined period of time (t) to form the thin film through-hole membrane during imprinting by capillary filling of the polymer of the thin film to a certain depth (z) is a factor of the capillary system and the polymer flow of the thin film. This is exemplified by Equation (1).
In particular, the capillary system comprises factors such as the size of the pattern of the patterned mold (R) and the air permeability of the patterned mold (d, Pe). The polymer flow of the thin film comprises factors such as thickness of the thin film (h0), molecular weight, temperature, viscosity (η), and thin film-patterned mold wettability. Among these factors, the mold wettability is important as when the contact angle of the thin film melt on the surface of the patterned mold (θ) exceeds 90°, the capillary force for the CFL is negative and the liquid does not spontaneously fill through the capillary for patterning to occur on the thin film. This correlates with high polymer melt surface tension (γp) and/or low mold surface energy (γm). Accordingly, during the imprinting, the contact angle of the thin film polymer melt on the surface of the patterned mold may be a suitable angle. In particular, the contact angle may be <90°.
The thin film through-hole membrane formed from the imprinting comprises ordered and uniform-sized pores. The pores formed may have any suitable shape. For example, the pores may be spherical, oval, rod, and the like. The shape of the pores formed may be dependent on the conditions of the imprinting. For example, formation of oval shaped pores may be attributed to controlled dewetting of the thin film around the edges of cylindrical pillars of the patterned mold as the mold approaches the patterning substrate, thereby resulting in pore openings with distinct, noncircular morphology. If the thin film beneath the cylindrical pillars of the patterned mold are fully dewetted, the pores may be circular shaped. At elevated temperature and prolonged process time during the imprinting, the pillars of the patterned mold may collapse during the CFL, thereby forming rod shaped pores. Specific shapes of pores may be more suitable for certain applications. For example, elongated pores may be more suitable in lowering fouling tendency in filtration membranes. If spherical pores are desired at elevated temperatures and longer pre-determined period of time, the pillar deflection or collapse may be avoided by modifying the surface of the patterned mold.
Accordingly, the method 100 may optionally further comprise treating the patterned mold with plasma prior to the imprinting of step 106. The treating may be carried out under conditions suitable for the purposes of the present invention. The plasma exposure results in a thin silica-like surface layer having a high elastic modulus being formed on the surface of the patterned mold. The surface layer formed may impart higher mechanical stability and may minimise the pillar deflection during the imprinting at elevated temperatures and prolonged pre-determined period of time.
The treating of the patterned mold with plasma may also increase the surface energy of the patterned mold which enhances the capillary flow of the moderately hydrophilic polymer comprised in the patterned mold during CFL.
For the purposes of the present invention, the thin film through-hole membrane having an ordered array of pores refers to an array of pores having a systematic arrangement. For example, the pore array may be such that there are a pre-determined number of rows and columns of pores, each row and column having a pre-determined number of pores. The pores in each row and/or column may be the same or different. An ordered array of pores may also be taken to comprise pores arranged in a non-random manner. For example, each pore may be spaced equidistant from one another.
According to another particular aspect, the thin film through-hole membrane formed from the imprinting may comprise asymmetric pore channels. For the purposes of the present invention, asymmetric pore channels may be defined as channels which may be consist of a first shape on one side of the membrane and a second shape on the opposite side of the membrane. For example, the asymmetric channels may comprise spherical pore shape on the top side of the membrane and non-spherical pore shape on the bottom side, or spherical pores of different pore sizes on both sides of the membrane.
The pores may have any suitable size. The size of the pores formed may be dependent on the conditions of the imprinting. Pore size may be measured by (optical or electron) microscopy. Pore size of each pore refers to the average pore diameter. According to a particular aspect, the pores of the thin film through-hole membrane may have a substantially uniform pore size. For example, at least about 80% of the pores have a uniform pore size. In particular, at least about: 90%, 95%, 98% or 100% of the pores have a uniform pore size. The average size of each pore may be 0.08-0.4 μm. For example, the average size of each pore may be 0.1-0.35 μm, 0.15-0.3 μm, 0.2-0.25 μm.
In order to release the thin film through-hole membrane, the method comprises a step 108 of contacting the patterning structure comprising the thin film through-hole membrane with water. The contacting may be under suitable conditions. For example, the contacting may be at room temperature. The water may also be at room temperature. During the contacting, the sacrificial layer may dissolve when contacted with water since the sacrificial layer comprises a water soluble polymer, thereby releasing the thin film through-hole membrane from the patterning structure. The released thin film through-hole membrane may be a free-standing thin film through-hole membrane.
The advantage of the sacrificial layer is that despite the thermal annealing during the imprinting of step 106 which enhances the adhesion between the membrane and the patterning substrate, providing the sacrificial layer which comprises a water soluble polymer enables the sacrificial layer to be dissolved when the patterning structure with the thin film through-hole membrane is contacted with water. In this way, the thin film through-hole membrane is released from the patterning structure and patterning substrate without requiring chemical etchants which may damage the integrity of the membrane. Further, the sacrificial layer provides a solvent resistant surface for direct formation of the thin film on the sacrificial layer by any suitable method, such as spin coating. The sacrificial layer is also thermally stable such that it is neither imprinted nor intermixed with the adjacent thin film during the imprinting of step 106.
The method 100 further comprises a step 110 of transferring the released thin film through-hole membrane onto a surface of a target substrate. The target substrate may be any suitable substrate. For example, the target substrate may be a substrate having a complex surface such as a patterned, flexible, non-planar, or curved surface. The method of the present invention enables a film to be easily provided on a target substrate comprising a complex surface. Depositing and patterning a thin film directly on a target substrate with a complex surface would otherwise be difficult using conventional processing steps. The target substrate may be a porous substrate.
According to a particular embodiment, a method of forming the thin film through-hole membrane on a patterning substrate and subsequently transferring the thin film through-hole membrane to a target substrate is shown in
The advantage of the method of the present invention is that none of the steps involves peeling or other deformation that may cause warping, stretching or bending of the thin film or the formed thin film through-hole membrane which would lead to the damage and fracture of the membrane. The method of the present invention therefore provides a reproducible and versatile method to form and subsequently transfer with high integrity and defect-free thin film through-hole membranes.
The method of the present invention may also be applied for repeated layering of thin film through-hole membranes on the target substrate by repeating the method for a number of times as required by the number of layers desired on the target substrate.
A second aspect of the present invention provides a thin film through-hole membrane prepared from the method described above. Examples of thin film through-hole membranes are shown in
Membranes with nanoscale thickness are advantageous because fluid transport across the membrane scales inversely with membrane thickness. However, such membranes may not be robust enough without suitable mechanical support. Accordingly, the present invention provides a hierarchical membrane for graded filtration comprising at least one thin film through-hole membrane prepared according to the method 100. The hierarchical membrane may also comprise an underlying microporous mechanical support layer. Each of the thin film through-hole membrane and the microporous mechanical layer may have a suitable pore size, order, narrow size distribution and thickness. An example of a hierarchical membrane is shown in
According to one particular embodiment, the hierarchical membrane of the present invention may comprise a first thin film through-hole membrane prepared according to the method 100 and a second through-hole membrane with thickness and ordered pores in the micrometer range which may be used as a support layer for the thin film through-hole membrane. In particular, the first thin film through-hole membrane may be transferred onto the second through-hole membrane to form the hierarchical membrane.
The second membrane may be prepared by any suitable method. For example, the second membrane may be prepared by micro-molding in capillaries (MIMIC) using methods described in the art. For example, the first thin film through-hole membrane may comprise a thermoplastic polymer and the second membrane may comprise a ubiquitous ultraviolet (UV) curable resin. The thermoplastic polymer may be as described above. The UV curable resin may be any suitable UV curable resin such as, but not limited to, perfluoropolyether (PFPE), PUA, optical adhesives such as NOA, or a combination thereof.
According to one particular embodiment, the hierarchical membrane comprises at least two thin film through-hole membranes. The at least two thin film through-hole membranes may be prepared according to the method 100. Each of the thin film through-hole membranes comprised in the hierarchical membrane may comprise a different pore size.
The hierarchical membrane according to the present invention may be used in various applications. For example, the hierarchical membrane may be used in high selectivity filtration, stencil patterning, cell culture platforms, (bio)analytical and preparative microfluidic devices, and size and shape-selective membrane modules for product purification/fractionation and for environmental remediation (water and air).
The hierarchical membrane may be comprised in a membrane module. An example is shown in
The membrane module may be combined with one or more membrane modules comprising hierarchical membranes of different pore sizes to form a membrane module housing.
Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention.
Having now generally described the invention, the same will be more readily understood through reference to the following examples which are provided by way of illustration, and are not intended to be limiting.
Materials
Thermoplastic polymers like polystyrene (PS) (BP Chemicals) was precipitated with excess methanol and vacuum dried before use. Poly(sodium 4-styrenesulfonate) (PSS) (Sigma-Aldrich) and ultraviolet (UV) light curable polymers like NOA73 (Norland Products Inc.) and perfluoropolyether (PFPE) based resin (MD-700, Solvay) and photoinitiator (2-Hydroxyl-2methylpropiophenone, Sigma-Aldrich) were used as received. Polyurethane acrylate (PUA) resin was formulated by mixing aliphatic urethane acrylate in tripropyleneglycol diacrylate (Ebecryl E265); trifunctional acrylate modulator: trimethylolpropane ethoxy triacrylate (TMPEOTA); photoinitiators Darocur 1173 and Irgacure 184. Polydimethylsiloxane (Sylgard 184, Dow Corning) working molds were replicated from photolithographically prepared silicon master molds having complementary relief structure.
Preparation of Thin Film Through-Hole Membranes
Capillary force lithography (CFL) was used to prepare the thin film through-hole membranes. Polymer solutions were prepared, stirred and filtered with 0.45 μm polytetrafluoroethylene (PTFE) syringe filters before use. First PSS, followed by polystyrene thin films were sequentially spin coated (3000 rpm, 30 s) onto cleaned glass or silicon substrates from PSS-deionised water (5 wt %) and polymer-toluene (2.5-6 wt %) solutions. The replicated patterned PDMS mold was then conformally placed onto the polymer-PSS bilayer for CFL above polymer's glass transition temperature. Selected PDMS molds were plasma treated in air (30 W, PDC-002, Harrick Plasma) and used immediately. Thin (≤1.5 mm-thick) PDMS mold minimised thermal stress build-up and ensured continual conformal contact during CFL at elevated temperature.
After PDMS demolding, the membrane-PSS bilayer sample edges were scored with blade before sliding it into DI water bath at a small angle. Upon contacting water, PSS sacrificial layer promotes interfacial water diffusion between membrane and substrate, the membrane thus separates from substrate and floats on the water bath surface for transfer. The transferred membranes were sandwiched between two pieces of aluminium sheets with pre-cut windows and sealed together with epoxy resin, before transferring onto custom module holder.
The method also allows the membrane to be inversely transferred, if required. Specifically, a flexible backing layer was placed onto the membrane top surface, a few drops of DI water was then dispensed at the edges which selectively diffuse into PSS sacrificial layer, facilitating membrane separation from the substrate and attachment to the backing layer, exposing the initially buried membrane bottom surface.
In order to ascertain that the imprint spanned the entire polymer thin film thickness (h0) i.e. membrane pores were open-through, membrane pore openings at both top and bottom surfaces were verified using SEM as shown in
Preparation of Hierarchical Membranes
A hierarchical membrane comprising a thin film through-hole membrane as prepared in Example 1 is placed on a support membrane. The support membrane had thickness and ordered pores in the micrometer range and was fabricated by another capillary and mold-based lithography method, namely micromolding in capillaries (MIMIC). In particular, the support membrane was prepared by UV curing of molded liquid prepolymer carried out using a 400 W metal halide flood light with λ=250-650 nm and 75 mW/cm2 at 12 cm sample-to-light distance (UVR400/600, Epoxy & Equipment Technology).
The hierarchical membrane as prepared in Example 2 was tested for its high selectivity filtration.
The filtration experiments were carried out using a custom designed dead-end test cell. Polystyrene latex beads were purchased (Sigma-Aldrich) and reconstituted by adding filtered deionised water to form suspensions with different concentration. The filtration efficiency and particle size distribution were measured by analysing feed and filtrate streams using UV-visible spectrometer (USB4000, Ocean Optics) and DLS (NanoBrook Omni, Brookhaven Instruments), respectively. Scattering angle 90° was used for all DLS measurements. The solutions, especially the feed are diluted to prevent multiple scattering and viscosity effects for accurate particle size measurement.
Two liquid filtration experiments were designed to test and demonstrate the membrane's unique capabilities. Firstly, as a high selectivity size-exclusion-based sieve to discriminate species with small size difference (see schematic in
For the first filtration experiment, feed stream consists of binary latex particle mixtures of 0.3 μm and 0.6 μm suspended in DI water. The feed is filtered at pressure drop of 80-100 kPa with a hierarchical membrane having cut-off pore size (0.45 μm) between the size of both particle populations. SEM image of the membrane surface after filtration (
In the second filtration experiment, the feed stream comprised only 0.3 μm particles. Filtration was performed with membrane having asymmetric pore size at top (˜0.4 μm) and bottom (˜0.25 μm) surface. The photograph in
As each membrane has well controlled pore size and high size selectivity, having the membranes working in tandem can yield higher combined filtration performance. To do that, a proof-of-concept multi-membrane filtration cell was designed. The 3D-printed filtration cell has multiple slots for the membrane modules (see
The filtration performance of the hierarchical membrane was evaluated with an experimental setup that generates polydisperse smoke aerosol as shown in
Thick tobacco residue was captured by the filter when the test membrane was absent. Between a fibre-based surgical mask and the hierarchical membrane, considerably less tar was transmitted through the membrane module comprising the hierarchical membrane comprising stacked 20 μm and 0.35 μm grade membranes as seen in
| Number | Date | Country | Kind |
|---|---|---|---|
| 10201600134R | Jan 2016 | SG | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/SG2017/050010 | 1/9/2017 | WO | 00 |
