Technical Field

The present invention relates to a composite membrane. In particular, the composite membrane is a porous composite membrane.

Background

Control of fluid and energy flow in porous medium is limited by the lack of control in the design of porous structures, particularly in their periodicity and connectivity. Consequently, the lack of flow control results in large variants in pressure gradient. In porous membranes, performance relies heavily on the pore architectures. Generally, in fluid flow applications, reducing pore sizes will enhance filtration efficiency but it will come at the expense of high operating pressure; reducing membrane thickness will improve fabrication feasibility but will also compromise membrane mechanical integrity. To keep the pressure drop low and the filtration efficiency above a desired level, a compromise is usually selected among these parameters. This is applicable to traditional fibrous based membranes which are fabricated by sequentially electrospinning each fibre into the form of a mat. As a result, the pore sizes of the fibrous membranes are wide-ranging with their porosity and tortuosity intrinsically difficult to control. On the other hand, lithographically fabricated isoporous membranes provide a through thickness, well-defined pore features of isoporous membranes which present a unique opportunity to design the flow transport across membrane with a degree of accuracy.

Accordingly, composite membranes have been formed in which multilayer membranes comprising electrospun membranes and isoporous membranes have been combined to form multilayer porous medium. However, the effective porosity of the isoporous membrane comprised in the composite membrane is severely compromised, resulting in high pressure drops across the membrane when in use.

There is therefore a need for an improved composite membrane.

Summary of the invention The present invention seeks to address these problems, and/or to provide an improved composite membrane. According to a first aspect, the present invention provides a composite membrane comprising a selective layer and a support layer, the selective layer and the support layer separated by an array of structures provided therebetween, wherein each structure comprised in the array of structures may have an average height of £ 20 mhi and an average width of £ 10 mhi.

According to a particular aspect, the array of structures may be provided: on a surface of the selective layer facing the support layer, on a surface of the support layer facing the selective layer, or a combination thereof.

The array of structures may comprise any suitable structures. For example, the array of structures may comprise three-dimensional structures. According to a particular aspect, the array of structures may comprise microscale structures, nanoscale structures or a combination thereof. In particular, the array of structures may comprise an ordered array of structures.

The selective layer comprised in the composite membrane may be any suitable selective layer. For example, the selective layer may be a porous layer. In particular, the selective layer may have uniform sized pores.

The selective layer may have a suitable thickness. In particular, the selective layer may have a thickness of £ 1000 nm.

The selective layer may be formed of any suitable material. According to a particular aspect, the selective layer may be formed of a polymer. For example, the polymer may comprise, but is not limited to: polystyrene, polymethylmethacrylate, polycarbonate, polyacrylonitrile, polypropylene, UV curable thermoset polymer, or polymer blends thereof.

The support layer comprised in the composite membrane may be any suitable support layer. For example, the support layer may be a porous layer. The support layer may have a suitable thickness. In particular, the support layer may have a thickness of £

100 mGP.

The support layer may be formed of any suitable material. According to a particular aspect, the support layer may be formed of a polymer. For example, the polymer may comprise an ultraviolet (UV) curable polymer. According to a particular aspect, the composite membrane may be a hierarchical composite membrane.

Brief Description of the Drawings

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:

Figure 1(a) shows a schematic representation of a composite membrane without an array of structures, Figure 1(b) shows a scanning electron microscopy (SEM) image of the membrane of Figure 1(a), Figure 1(c) shows a schematic representation of a composite membrane with an array of structures according to one embodiment of the present invention, and Figure 1(d) shows a SEM image of the membrane of Figure 1(c);

Figure 2(a) shows the pressure drop of membranes according to embodiments of the present invention as a function of the height of the structure comprised in the array of structure, the insert images highlight the structures with different height formed on the support layer, without the selective layer. Figure 2(b) shows the schematic representation of the experimental setup for the filtration performance measurements and pressure drop measurements, and Figure 2(c) shows the pressure drop of a membrane according to one embodiment of the present invention as a function of the air velocity; and

Figure 3(a) shows the particulate matter (PM) number concentration before and after filtration and the resulting PM removal efficiency of a membrane according to one embodiment of the present invention, Figure 3(b) shows the air velocity dependence of PM removal efficiencies of the membrane for PM0.2 to PM2; Figure 3(c) shows the PM concentration dependence of PM removal efficiencies of the membrane from PM0.2 to PM2 and Figure 3(d) shows a longer term PM removal efficiency of the membrane.

Detailed Description

As explained above, there is a need for an improved composite membrane compared to composite membranes available in the prior art. In general, the present invention provides a composite membrane comprising an array of structures to systematically connect various porous media. The array of structures may control the separation between layers of porous media to achieve better flow distributions, thereby enabling precise control of pressure, thermal and particle concentration gradient to be achieved.

According to a first aspect, the present invention provides a composite membrane comprising a selective layer and a support layer, the selective layer and the support layer separated by an array of structures provided therebetween, wherein each structure comprised in the array of structures comprises an average height of £ 20 mhi and an average width of £ 10 mhi.

The array of structures may be free-standing or embedded or formed on the selective layer and/or the support layer. According to one particular aspect, the array of structures may be provided on a surface of the selective layer facing the support layer. According to another particular aspect, the array of structures may be provided on a surface of the support layer facing the selective layer. According to yet another particular aspect, the array of structures may be provided on both a surface of the selective layer facing the support layer and on a surface of the support layer facing the selective layer. The advantage of the array of structures being embedded or formed on the selective layer and/or the support layer as compared to a free-standing array of structures provided between the selective layer and the support layer is that the assembly of the composite membrane may be simplified and shortened with lesser handling steps. Further, it would make for a more seamless connection between the layers and the array of structures, thereby minimising interfacial failure. Another advantage is that the array of structures may be formed from the same polymer as the layer on which the array of structure is formed on/embedded in, thereby enabling a mechanically robust integrated structure.

The array of structures allow the selective layer and the support layer to be separated, thereby creating an air gap between the selective layer and the support layer. This allows airflow across the composite membrane to be greatly enhanced as the porosity in each of the selective layer and the support layer is maximally realised instead of being obstructed by the adjacent layer when the selective layer and the support layer are formed without providing any array of structures between the selective layer and support layer in prior art hierarchical composite membranes. In prior art hierarchical composite membranes comprising a selective layer and a support layer without an array of structures provided therebetween as shown schematically in Figure 1(a), there is direct overlapping of the selective layer and the support layer and therefore the porosity of the selective layer is compromised. A perspective scanning electron microscopy (SEM) image of such a hierarchical composite membrane is shown in Figure 1(b). For example, directly overlapping the selective layer and the support layer, each with 10% porosity, will yield an effective porosity of 1%, resulting in a very high operating pressure drop across the composite membrane, thereby severely undermining the benefits of a hierarchical and/or multi-layer composite membrane.

A schematic representation and SEM image of an embodiment of the composite membrane of the present invention is as shown in Figure 1(c) and Figure 1(d), respectively.

The array of structures may comprise any suitable structure. Each structure comprised in the array of structures may have any suitable geometric shape. For example, the array of structures may comprise three-dimensional structures. The array of structures may comprise structures which are circular, rectangular, square, or a combination thereof. According to a particular aspect, the array of structures may comprise pillars. In particular, the array of structures may comprise square pillars. Each structure comprised in the array of structures may have any suitable size. For example, the array of structures may comprise a suitable height and width. For the purposes of the present invention, height may be the average height of the structures comprised in the array of structures. In particular, the height of the structure may be the distance between the selective layer and the support layer. For the purposes of the present invention, width may be the average width of the structures comprised in the array of structures.

According to a particular aspect, each structure comprised in the array of structures may comprise an average height of £ 20 mhi. In particular, the average height may be 0.1-20 mGh, 0.2-18 mhΐ, 0.5-15 mhΐ, 1-12 mhΐ, 2-10 mhΐ, 3-9 mhΐ, 4-8 mhΐ, 5-7 mhΐ, 5.5-6 mGP. Even more in particular, the average height may be £ 5 mhi. Each structure comprised in the array of structures may have a suitable width. For example, the width of each structure comprised in the array of structures may be based on the pore diameter of the membrane. According to a particular aspect, each structure comprised in the array of structures may comprise an average width of £ 10 mhi. In particular, the average width may be 0.1-10 mhi, 0.2-9 mhi, 0.5-8 mhi, 1-7 mhi, 2-6 mhi, 3-5 mGh, 4-4.5 mGP.

Each structure comprised in the array of structures may have a suitable aspect ratio. For the purposes of the present invention, the aspect ratio of the structure comprised in the array of structures is defined as the ratio of the height to width of the structure. The aspect ratio of the structure comprised in the array of structure may be any aspect ratio suitable to ensure that the array of structures do not collapse. For example, the aspect ratio may be a maximum of 1:2. In particular, the aspect ratio may be 1:0.2, 1:0.5, 1:0.7, 1:1, 1:1.2, 1:1.3, 1:1.5, 1:1.7, 1:2.

Each structure comprised in the array of structures may be suitably spaced from one another.

The structures comprised in the array of structures may be microscale structures, nanoscale structures or a combination thereof. In particular, the array of structures may comprise an ordered array of structures. For the purposes of the present invention, the array of structures having an ordered array of structures refers to an array of structures having a systematic arrangement. For example, the array of structures may be such that there are a pre-determined number of rows and columns of structures, each row and column having a pre-determined number of structures. The structures in each row and/or column may be the same or different. An ordered array of structures may also be taken to comprise structures arranged in a non-random manner. For example, each structure may be spaced equidistant from one another.

The selective layer may be any suitable selective layer. For example, the selective layer may be a porous selective layer. The selective layer may comprise random or uniform pores. According to a particular aspect, the selective layer may comprise uniform pores having 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. In particular, the selective layer may be an isoporous selective layer.

The selective layer may have a suitable thickness. The selective layer may have a thickness of £ 20 mhi. For example, the selective layer may have a thickness of 50 nm- 18 mGh, 75 nm-15 mhi, 100 nm-10 mhi, 150 nm-5 mhi, 200-1000 nm, 250-950 nm, 300-

900 nm, 350-850 nm, 400-800 nm, 450-750 nm, 500-700 nm, 550-650 nm, 600-625 nm. In particular, the selective layer may have a thickness of £ 1000 nm. Even more in particular, the thickness of the selective layer may be 100-500 nm.

The selective layer may be formed of any suitable material. According to a particular aspect, the selective layer may be formed of a polymer. For example, the polymer may comprise, but is not limited to: polystyrene (PS), polymethylmethacrylate (PMMA), polycarbonate (PC), polyacrylonitrile (PAN), polypropylene (PP), UV curable thermoset polymer, or polymer blends thereof.

The support layer may be any suitable support layer. For example, the support layer may be a porous support layer. The support layer may comprise random or uniform pores. According to a particular aspect, the support layer may comprise uniform pores having 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 support layer may have a suitable thickness. For example, the support layer may have a thickness of £ 100 mhi. In particular, the support layer may have a thickness of 1-100 mGh, 5-95 mhΐ, 10-90 mhΐ, 15-85 mhΐ, 20-80 mhΐ, 25-75 mhΐ, 30-70 mhΐ, 35-65 mhΐ, 40-60 mGh, 45-55 mhΐ, 47-50.

The support layer may be formed of any suitable material. According to a particular aspect, the support layer may be formed of a polymer. For example, the polymer may comprise, but is not limited to: ultraviolet (UV) curable polymer.

The composite membrane may be a hierarchical membrane. In particular, the selective layer may comprise pores of a smaller size as compared to the pores of the support layer, thereby forming a hierarchical membrane. The composite membrane of the present invention provides several advantages. In particular, the composite membrane comprising the array of structures results in at least a 86% pressure drop across the membrane while retaining filtration efficiency.

The configuration of the structures comprised in the array of structures also determine the extent of the reduction in pressure drop across the composite membrane as well as the effect on flow pattern. For example, the pressure drop of a composite membrane comprising a selective layer and a 20 mhi thick support layer with structures having a height of 2 mhi may be about 1800 Pa. The pressure drop of the same composite membrane with structures having a height of 11 mhi and a diameter of 5 mhi is about 800 Pa. As a control, the pressure drop of the composite membrane without any structures is about 6000 Pa. This shows that the provision of the array of structures greatly reduces the pressure drop across the membrane. This is shown in Figure 2(a) in which the results are obtained using the experimental setup shown in Figure 2(b). With structures of increasing height, the pressure drop of the composite membrane may decrease by at least about 86% for H=5 mhi, compared to those without an array of structures. The density of the structures comprised in the array of structures affects the pressure drop to a lesser extent as compared to the height of the structures comprised in the array of structures.

The composite membrane according to the present invention may be used in any suitable application. For example, the composite membrane may be for used in, but not limited to, particulate matter removal, high purification separation, energy management, and the like.

According to a particular aspect, the composite membrane may be for use in particulate matter (PM) removal. For example, Figure 2(c) provides the experimental and simulated results on the pressure drop of the composite membrane comprising an array of structures according to one embodiment of the present invention at different air velocity. The good agreement between the experimental and simulated values shows that the selective layer as supported by the array of structures, has sufficient mechanical strength to withstand the macroscopic impacts during filtration. Thus, the array of structures not only preserve the mechanical durability of the selective layer during filtration, but also greatly enhance the air flow across the composite membranes, as the porosity of the selective layer is maximally utilised due to the air gap formed by the provision of the array of structures between the selective layer and the support layer, instead of being obstructed by the support layer in the case of hierarchical membranes without an array of structures.

The filtration performance of the composite membrane according to one embodiment of the present invention may be demonstrated based on the extent of particulate matter (PM) removal from polluted air analogous to those on a hazy day. The filtration experiments were mostly carried out at 7 cm/s. Figure 3(a) shows the PM number concentration before and after filtration, and the resulting PM removal efficiencies of membranes as categorised by PM size from PM0.2-PM2. The filtration efficiencies for PM2 to PM0.3 was very high >95% while for PM0.2 it was relatively lower at about 85%.

The PM size where onset of filtration efficiency reduction was observed commensurated well with the pore size of the selective layer which was about 0.5 mhi, as characterised with SEM. Furthermore, the filtration efficiency of PMs bigger (smaller) than the pore size of membrane was unaffected (affected) by increasing air flowrate, as seen in Figure 3(b), and average inflow concentration of PM, as seen in Figure 3(c). A 15 hour measurement of the PM removal efficiency of the composite membrane was also carried out to assess its long-term filtration performance and the results are shown in Figure 3(d). It can be seen that the composite membrane can maintain its high filtration efficiency for different PM size throughout the duration of the test, thus corroborating that it has the mechanical resilience required for a wide range of filtration applications.

According to another particular aspect, the composite membrane may be for use in ultra-high size selective purification of technologically and physiologically relevant micro-nanoparticles and (bio) macromolecules such as exosomes and cancer calls.

Yet another example in which the composite membrane may be used is in increasing the thermal efficiency. For example, the thermal efficiency may be improved as a consequence of the synergistic effects of increased convective heat transfer arising from improved flow efficiency, and enhanced conductive heat transfer resulting from the increased surface area of the interconnected array of structures. Accordingly, the composite membrane may be incorporated in solar chimneys, collectors, heat exchangers, heaters, thermal energy saving units used in solar systems, roof cooling, insulation and thermal storage systems. The array of structures may be fabricated and integrated with screens, shelters, filters, porous ceramic and porous baffles that may be used for controlling the thermal energy of building infrastructures and electronic devices. The present invention also provides a method of forming the composite membrane described above. In particular, the method may comprise forming a selective layer and subsequently transferring the selective layer onto a support layer comprising an array of structures.

The selective layer may be formed by any suitable method. For example, the selective layer may be formed by capillary force lithography (CFL) (Suh KY et al, 2001, Adv Mater, 13:1386-1389).

The support layer may be formed from any suitable method. For example, the support layer may be formed from the method as described in WO 2019/190404. In particular, the support layer may comprise an array of structures formed on a surface of the support layer.

Subsequently, the selective layer may be transferred onto the surface of the support layer comprising the array of structures. The selective layer may be transferred by any suitable method. In particular, the selective layer may be transferred by the method as described in WO 2017/119850. 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.