Technical Field

The present invention relates to a porous membrane fabrication system. In particular, the system is for the continuous fabrication of porous membrane.

Background

With increasing application-driven demands, new classes of porous membranes with uniform pore architectures and straight pore channels are increasingly adopted as compared to membranes which have random pore morphology and tortuous pathways. Most of the methods of forming such porous membranes involve mold-based imprint lithography and replica molding techniques which involve replicating highly ordered pattern from a prefabricated mold onto a target material at elevated temperature and pressure or under ultraviolet radiation. These approaches, however, typically leave behind thin residual layer due to incomplete polymer resin displacement by the mold protrusions. The residual layer removal typically requires additional etching procedures which are prohibitively complex and laborious. Other fabrication methods include photolithography/laser interference lithography or sequential writing/machining methods such as electron-beam lithography and focus ion beam milling but their adoption are also limited by process repetition/complexity and low throughput which result in high cost.

Another problem is that most membrane production methods involving lithographic templates are batch processes and are therefore unable to be scaled up. Membrane fabrication beyond the bench scale remains a challenge due to the complex, hard-to- implement processing steps. There is therefore a need for an improved method and system for fabricating a porous membrane.

Summary of the invention

The present invention seeks to address these problems, and/or to provide an improved. In general terms, the invention relates to a roll-to-roll system for continuous fabrication of a porous membrane. The system may be scaled up, thereby improving production throughput and reproducibility as compared to batch systems. The system also enables the fabrication of porous membranes having pore architectures and surface textures which are otherwise difficult to fabricate.

According to a first aspect, the present invention provides a system for continuous fabrication of porous membrane, the system comprising: an unloading unit onto which a flexible web substrate is wound;

a roller unit comprising slots configured to fix a mold template, wherein the mold template is for contacting the web substrate;

a deposition head for depositing a curing material between the mold template and the web substrate when the mold template and the web substrate are in contact;

a curing unit to cure the curing material to form a web supported porous membrane;

a separator to separate the web supported porous membrane from the mold template; and

a gripper unit configured to advance the web substrate unwound from the unloading unit along a web substrate transport path towards the roller unit and for gripping and pulling the web supported porous membrane after the web supported porous membrane has been separated from the mold template.

The web substrate may be any suitable web substrate. For example, the web substrate may comprise a polymer. In particular, the web substrate may comprise, but is not limited to, polyethylene terephthalate (PET), polycarbonate (PC), Ethylene tetrafluoroethylene (ETFE), or a combination thereof. Even more in particular, the web substrate may comprise PET.

According to a particular aspect, the system may further comprise at least one steering roller configured to support and/or align the web substrate along the web substrate transport path. The system may further comprise a control system configured to control tension of the web substrate along the web transport path. In particular, the control system may comprise a first load cell configured to measure a tension of the web substrate at a first tension zone along the web substrate transport path. According to a particular aspect, the first tension zone may be between the unloading unit and the roller unit, and closer to the unloading unit than the roller unit.

The control system may comprise a second load cell configured to measure a tension of the web substrate at a second tension zone along the web substrate transport path. According to a particular aspect, the second tension zone may be between the unloading unit and the roller unit, and closer to the roller unit than the unloading unit.

The tension of the web substrate may be controlled along the web transport path by adjustment of rotation speed of the roller unit and pull force of the gripper unit. In particular, the rotation speed of the roller unit and the pull force of the gripper unit may control the unwinding speed of the web substrate from the unloading unit.

According to a particular aspect, the curing unit may be a UV curing unit.

According to a second aspect, the present invention provides a method of continuously fabricating a porous membrane using a system according to the first aspect, the method comprising: unwinding a flexible web substrate from an unloading unit along a web substrate transport path;

advancing the web substrate along the web substrate transport path towards a roller unit;

- contacting the web substrate with a mold template, the mold template being fixed to slots on the roller unit;

depositing a curing material between the mold template and the web substrate; and

curing the curing material to form a web supported porous membrane.

The web substrate may be any suitable web substrate. In particular, the web substrate may be as described above in relation to the first aspect.

The mold template may be any suitable mold template for the purposes of the present invention. For example, the mold template may comprise an elastomeric polymer. In particular, the mold template may comprise, but is not limited to polydimethylsiloxane (PDMS).

According to a particular aspect, the mold template may comprise a patterned surface. The patterned surface may be any suitable pattern desired to be imprinted on the web substrate supported membrane.

The curing material deposited on the mold template during the depositing may be any suitable curing material. For example, the curing material may comprise a curable resin. In particular, the curing material may comprise, but is not limited to: polyurethane, PDMS, or a combination thereof. The curing may comprise any suitable form of curing. According to a particular aspect, the curing comprises UV curing. The UV curing may be ay a suitable UV dose. For example, the UV curing may be at a UV dose of 600-1000 mJ/cm ² .

The method may further comprise separating the web supported porous membrane from the mold template, to thereby obtain the web supported porous membrane. The method may further comprise transferring the porous membrane from the web substrate to a target substrate.

The porous membrane formed from the method may be any suitable membrane. For example, the porous membrane formed from the method may be: an ordered porous membrane, a surface patterned porous membrane or a hierarchical porous membrane. In particular, the membrane may have a pore diameter of £ 200 pm.

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 shows a schematic representation of the system according to one embodiment of the present invention;

Figure 2 shows a schematic representation of a roller unit and mold template according to one embodiment of the present invention; Figure 3 shows a schematic representation of a part of the method according to one embodiment of the present invention;

Figures 4(a) and (b) show the photograph and planar SEM and Figure 4(c) shows the perspective SEM of a membrane formed from the method according to one embodiment of the present invention;

Figure 5 shows the optical microscopy top view images and scanning electron microscopy 45° perspective images of membranes fabricated as a function of UV intensity and central roller rotation speed (production throughput);

Figures 6(a) and (b) show planar optical and perspective scanning electron microscopy images, respectively, showing membranes with non-porous pores and slanted pore channels due to sub-optimum web tension parameters and Figures 6(c) and (d) show planar optical and perspective scanning electron microscopy images, respectively, showing membranes with through-holes with straight pore channels formed with optimised web tension, throughput and UV dose parameters; Figure 7 shows PDMS mold stability and membrane pattern uniformity in which Figures 7(a) to (d) show the perspective SEM images showing PDMS molds after 2, 18, 30 and 50 fabrication cycles, respectively, Figure 7(e) shows a photograph of a membrane segment and the corresponding optical microscopy planar images of the membrane across five regions (labelled (i)-(v)); Figures 8(a) and (b) show a schematic representation of the fabrication of ordered porous membranes with surface protrusions, using a patterned web substrate with recessed pattern, as detailed in Figure 8(b), Figure 8(c) shows the planar SEM image of a surface patterned membrane (550nm, pillar), Figure 8(d) shows the magnified SEM image of membranes with surface protrusions of various length scales from (i) 2 mhi to (ii) 550 nm to (iii) 200 nm;

Figures 9 (a) and (b) show PDMS micropillar mold template with sub-micrometer recessed structure at base, Figure 9(c) shows the schematic representation of the fabrication of double-side patterned microporous membranes by a second molding process utilising the PDMS micropillar mold and another patterned web substrate, Figures 9(d) and (e) show the pattern on membranes formed on the top and bottom surfaces, respectively;

Figure 10(a) shows a schematic representation of the fabrication of hierarchical porous membrane comprising multiscale pore size, Figures 10(b), (c) and (d) shows the scanning electron microscopy (SEM) images of the architecture of a hierarchical porous membrane with uniform 550 nm and 20 mhi pore sizes, Figure 10(e) shows the planar SEM images (i-iii) showing the distortion of sub-micrometer membrane pore size with increasing applied pressure/web tension; and

Figure 11(a) shows a photograph and Figure 11 (b) shows the perspective SEM image of a free-standing membrane after peeling off from the web substrate.

Detailed Description

As explained above, there is a need for an improved system for the continuous fabrication of membranes as compared to the batch production systems of the prior art.

Membranes play important roles in many processes from cellular level up to industrial scales but majority of membranes have random pore morphology and tortuous paths which may not meet the requirements of increasing number of applications. In particular, recent demands in purification, cell biology, and stencilling applications are driving the development of new classes of membranes with very uniform pore architectures (size, shape, depth, order, density) and straight pore channels. Generally, the present invention relates to a roll-to-roll system suitable for continuous fabrication of porous membranes, including ordered porous membranes. The system of the present invention may also enable fabrication of membranes with multilevel, multiscale and precise architectures down to the nanoscale.

In particular, the system of the present invention comprises a series of flexible and reusable patterned molds, and allows all fabrication steps to be automated concurrently with precise positioning and reproducible contact force. The system takes into consideration: (i) the methodologies and force required to bring a mold and web substrate into conformal contact to create the capillaries for filling with curing material; (ii) the surface properties of the network capillaries (substrate and mold) that promotes curing material capillary filling and subsequent mold separation and membrane transfer; (iii) the optimum mold pattern design to achieve good balance between curing material capillary filling time, fabrication throughput and the resulting membrane pore size. There is also provided a method of continuously fabricating porous membranes using the system of the present invention. According to a first aspect, the present invention provides a system for continuous fabrication of porous membrane, the system comprising: an unloading unit onto which a flexible web substrate is wound;

a roller unit comprising slots configured to fix a mold template, wherein the mold template is for contacting the web substrate;

a deposition head for depositing a curing material between the mold template and the web substrate when the mold template and the web substrate are in contact;

a curing unit to cure the curing material to form a web supported porous membrane;

a separator to separate the web supported porous membrane from the mold template; and

a gripper unit configured to advance the web substrate unwound from the unloading unit along a web substrate transport path towards the roller unit and for gripping and pulling the web supported porous membrane after the web supported porous membrane has been separated from the mold template.

According to a particular aspect, the system may further comprise at least one steering roller configured to support and/or align the web substrate along the web substrate transport path.

A particular embodiment of the system of the present invention is as shown in Figure 1. Figure 1 shows a roll-to-roll system 100 comprising an unloading unit 102 onto which a flexible web substrate 104 is wound. The unloading unit 102 feeds a web substrate along a web substrate transport path. In particular, the web substrate is fed along a web substrate transport path towards a roller unit 106. The web substrate path may comprise at least one steering roller 110 to support and/or align the web substrate as the web substrate advances along the web substrate transport path. The web substrate may be any suitable web substrate. For example, the web substrate may comprise a polymer. In particular, the web substrate may comprise, but is not limited to, polyethylene terephthalate (PET), polycarbonate (PC), Ethylene tetrafluoroethylene (ETFE), or a combination thereof. Even more in particular, the web substrate may comprise PET. According to a particular aspect, the web substrate may be a patterned web substrate.

The web substrate may have any suitable dimensions. For example, the web substrate may be 30 mm wide and 50 mhi thick.

The roller unit 106 may be configured to enable attachment of mold templates. At the roller unit 106, the web substrate makes contact with a patterned side of the mold template before being fixed to a gripper unit 118 at the opposite side of the roller unit 106. The gripper unit 118 may be configured to forward the web substrate 104 unwound from the unloading unit 102 along a web substrate transport path towards the roller unit 106 and for gripping and pulling the web supported porous membrane after the web supported porous membrane has been separated from the mold template.

In the system 100, it is important to ensure that the web substrate is clean throughout the process, and particularly before the web substrate contacts the mold template. Accordingly, there may be provided an ionising air knife (now shown) to eliminate dust particles and statics on the web substrate before the web substrate makes contact with the mold template.

According to a particular aspect, the system 100 may comprise a control system (not shown) configured to control tension of the web substrate along the web transport path. The control system may comprise a first load cell (now shown) configured to measure a tension of the web substrate at a first tension zone 120 along the web substrate transport path. The control system may comprise a second load cell (not shown) configured to measure a tension of the web substrate at a second tension zone 122 along the web substrate transport path.

The first tension zone 120 and the second tension zone 122 may be between the unloading unit 102 and the roller unit 106. In particular, the first tension zone 120 may be in closer proximity to the unloading unit 102 as compared to the roller unit 106. The second tension zone 122 may be in closer proximity to the roller unit 106 as compared to the unloading unit 102.

According to a particular aspect, the tension of the web substrate may be controlled along the web transport path by adjustment of rotation speed of the roller unit 106 and pull force of the gripper unit 118. In particular, the rotation speed of the roller unit 106 and the pull force of the gripper unit 118 may control the unwinding speed of the web substrate from the unloading unit 102.

In particular, each of the first load cell and the second load cell may measure and feedback the tension of the web substrate to the control system. The control system may maintain the tension of the web substrate at a pre-determined set-point. Controlling and adjusting the rotation speed of the roller unit 106 and a pull force of the gripper unit 118, which may in turn be synchronised with the speed at which the web substrate is unwound from the unloading unit 102, maintains the tension of the web substrate within the system 100. The control system may comprise a sensor for detecting the web substrate edge position. The sensor may be an ultrasonic sensor. In particular, the web substrate may be aligned vertically by the at least one steering roller 110. For example, if the sensor detects that the web substrate is not in position within a ±0.1 mm accuracy as compared to a pre-determined set point, the control system will cause the at least one steering roller 110 to adjust the alignment of the web substrate.

The roller unit 106 may have a suitable circumference. The circumference of the roller unit 106 may be divided into a plurality of slots 108 as shown in Figure 1 and further shown in Figure 2. The circumference of the roller unit 106 may be sufficiently large so as not to cause any curvature-induced length-scale deviation to the mold pattern features.

The mold template may be any suitable mold template. For example, the mold template may comprise features to enable the mold template to fix within the slots 108 of the roller unit 106. The features may be any suitable features. For example, the feature may be a step feature at the sides of the mold template, as shown in Figure 2, to allow mounting of the mold template by mechanical fixation to the mold slots 108 of the roller unit 106. The mold template may comprise a built-in curing material dispense area, as shown in Figure 2. Further, the mold template may be patterned. The pattern may be any suitable pattern. The mold template may be prepared by any suitable method known in the art. For example, the design features of the mold template and the pattern features of the mold template may be replicated from a master template.

The mold template may be of a suitable thickness. In particular, the thickness of the mold template may cushion any long-range surface waviness present at the surface of the slot 108 and prevent it from being transmitted into the pattern surface. According to a particular aspect, the thickness of the mold template may be ³ 5 mm. The mold template may be fixed within the slots 108 of the roller unit 106 such that the patterned surface of the mold template faces outwardly in order to contact the web substrate. Additional fixation force may be provided via vacuum suction through grooves engraved on the slots 108, as shown in Figure 2. The tension of the web substrate around the roller unit 106 may be adjusted by the control system to ensure that the web substrate makes conformal contact with the patterned mold template when the system 100 is in use.

The deposition head 112 may be any deposition head 112 suitable for depositing curing material between the mold template and the web substrate after the mold template and web substrate have been contacted. For example, the deposition head 112 may comprise a syringe and piston arrangement. The advantage of such an arrangement is that it would allow changing the curing material to be deposited in a shorter duration.

According to a particular aspect, the mold template and the web substrate may adopt a vertical orientation. In this way, curing material may be dispensed between the mold template and the web substrate from a deposition head 112 in a downward filling direction. For example, the curing material may be deposited to capillaries formed between the mold template and the web substrate.

The curing unit 114 may be any suitable curing unit capable of curing the curing material deposited between the mold template and the web substrate. For example, the curing may be by UV. According to a particular aspect, the curing may be by UV.

Therefore, the curing unit 114 may be a UV curing unit capable of providing a UV source. The curing unit 114 may be positioned downstream the roller unit 106 so that there is sufficient time for the curing material to be deposited.

The separator 116 may be any suitable separator configured to separate the web supported porous membrane from the mold template. In particular, the separator 116 may be a deflection roller.

The gripper unit 118 may comprise two alternating gripping arms. One of the gripping arms first grips and pulls the web substrate for a certain length and at pre-set web tension and speed (up to 100 mm/min), while the other gripping arm then takes over the grip to prevent web tension loss, before the membrane is cut and collected from the former gripping arm.

According to a second aspect, the present invention provides a method of continuously fabricating a porous membrane using a system according to the first aspect, the method comprising: unwinding a flexible web substrate from an unloading unit along a web substrate transport path;

advancing the web substrate along the web substrate transport path towards a roller unit;

contacting the web substrate with a mold template, the mold template being fixed to slots on the roller unit;

- depositing a curing material between the mold template and the web substrate; and

curing the curing material to form a web supported porous membrane.

The method may further comprise separating the web supported porous membrane from the mold template, to thereby obtain the web supported porous membrane. The method may further comprise transferring the porous membrane from the web substrate to a target substrate. Figure 3 shows a schematic representation of the contacting and depositing (Figure 3(a)), curing (Figure 3(b)) and separating (Figure 3(c)). The web substrate may be any suitable web substrate. In particular, the web substrate may be as described above. The web substrate may be one to which a cured membrane may preferential attach to during the separating of the web supported membrane from the mold template without the need of an adhesion promoter coating on the surface of the web substrate.

The mold template may be any suitable mold template for the purposes of the present invention. For example, the mold template may comprise an elastomeric polymer. In particular, the mold template may comprise, but is not limited to polydimethylsiloxane (PDMS).

According to a particular aspect, the mold template may comprise a patterned surface. The patterned surface may be any suitable pattern desired to be imprinted on the web substrate supported membrane.

The choice of PDMS as mold material may be advantageous in that: it faithfully replicates the features of the master template; its elastomeric and compliant character enables good contact with the web substrate during curing material capillary filling; its relatively low surface energy allows easy separation from cured curing material yet does not retard curing material capillary flow.

The curing material deposited between the mold template and the web substrate during the depositing may be any suitable curing material. For example, the curing material may comprise a curable resin. The curing material may be any suitable curing material which has a low viscosity. The viscosity of the curing material may be £ 400 cP. In particular, the curing material may comprise, but is not limited to: polyurethane, PDMS, or a combination thereof. Even more in particular, the curing material may be a UV- curable curing material.

The curing material may comprise additives. For example, the additives may be for tuning the viscosity and/or surface properties of the curing material. In particular, the additives may be, but not limited to, 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl acrylate (DFHA).

The curing may comprise any suitable form of curing. According to a particular aspect, the curing comprises UV curing. The UV curing may be ay a suitable UV dose. For example, the UV curing may be at a UV dose of 600-1000 mJ/cm ² . According to a particular aspect, at position (i) as shown in Figure 1 , a curing material such as, but not limited to, polyurethane (for example polyurethane NOA73 (Norland Products Inc.)) may be continuously deposited by deposition head 112 onto the designated area along a mold template which is in contact with the continuous web substrate feed (Figure 3(a)). The deposition head 112 may be mounted on an X-Y-Z translation stage which may allow fine adjustments to the dispense location. The deposition head 112 may be connected to a control system so that the volume of curing material being deposited by the deposition head 112 may be controlled.

The depositing may be synchronised to the rotation speed of the roller unit 106, such that the depositing of the curing material may be controlled. In particular, the curing material may be deposited to a selected mold template fixed to the roller unit 106.

Following the depositing, the curing material may be cured by the curing unit 114 (Figure 3(b)), thereby forming a porous membrane on the web substrate. Accordingly, the membrane is supported on the web substrate. The curing unit 114 may be a UV curing unit. In particular, the curing unit 114 may be a UV LED system. The curing unit 114 may have an irradiation window and a suitable irradiance.

Following curing, the web supported membrane may be separated from the mold template (Figure 3(c)) and may exit the roller unit 106 at position (iv) of Figure 1. The separation angle of the membrane against the mold template may be £15° with a separator to minimise stress build-up on the mold template pattern such as the micropillars, and the likelihood of pattern deformation upon separation. The separated web supported membrane may be continuously pulled by the gripper unit 118.

Following the separation, the mold template may return to position (i) at the deposition head 112 for the next fabrication cycle. Successful separation between the mold template and the cured membrane is paramount because any residue on the surface of the mold template would affect subsequent membrane fabrication cycles.

The porous membrane formed from the method may be any suitable membrane. For example, the porous membrane formed from the method may be: an ordered porous membrane, a surface patterned porous membrane or a hierarchical porous membrane. An example of the membrane formed from the method of the present invention is shown in Figure 4. Figures 4(a), (b) and (c), respectively, show a photograph, a planar and cross sectional scanning electron microscopy (SEM) image of a membrane with 20 mhi^ίqGhbΐbG cylindrical pores fabricated by the system and method as described above. Other membrane pore shape, size and thickness may be obtained by utilising mold templates with corresponding pattern features.

The fabricated membranes may be characterised as a function of UV dose i.e. UV intensity ^c UV exposure time. The latter may also vary with the rotation speed of the roller unit 106. For example, low UV dose (i.e. £ 400 mJ/cm ² ) may result in pore shape distortions and residual layer formation due to resin reflow. The optimum UV dose may be about 700 mJ/cm ² which yields uniform pore shape and straight pore channels. The membrane pore morphologies with different UV dose can be found in Figure 5.

The tension of the web substrate may be a crucial fabrication parameter which ensures a conformal yet non-excessive mold template contact with the web substrate. For example, for a PDMS mold template comprising pillar structures, the pillar structures were unaffected with web tension up to 0.2 kg, above which they bent and resulted in non-straight pore channels (see Figure 6).

The stability of the PDMS mold template and the resulting membrane morphology may be assessed by optical microscopy and SEM with increasing fabrication cycles. Selected small regions with missing pillars (< 1% of total pattern area) were observed after 50 fabrication cycles (see Figure 7). However, there were no discernible differences in terms of membrane pore shape, size and uniformity with increasing fabrication cycles. About 500 membrane segments (each 80 ^c 20 mm ² ) were produced before mold replacement was required. This shows the method and system of the present invention to be durable and its ability to produce membranes which are reproducible with high integrity.

The present system and method may also be used for fabricating membranes with pattern and pore features with multilevel and multiscale (hierarchical) designs. For the purposes of the present invention, the membrane formed from the method and system of the present invention may be a porous membrane including, but not limited to, an ordered porous membrane, a surface patterned porous membrane, or a hierarchical porous membrane. For the purposes of the present invention, an ordered porous membrane may be defined as a membrane comprising 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. The porous membrane formed may comprise through-holes extending all the way from the top surface of the membrane to the bottom surface of the membrane. The porous membrane may comprise pores of any suitable diameter. For example, the average diameter of the pores of the porous membrane may be £ 200 mhi. In particular, the average diameter may be 100 nm-200 mhi, 200 nm-100 mhi, 300 nm-50 mhi, 400 nm-10 mhi, 500 nm-1000 nm, 600-900 nm, 700-800 nm.

According to a particular aspect, the pores of the membrane formed 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.

An example of the membrane formed from the method of the present invention and system 100 is shown in Figures 8(a) and (b) which shows the ordered microporous membranes incorporated with surface protrusions. In particular, the web substrate may have a recessed pattern in close contact with the mold template comprising micropillars. As a result, the curing material may be prevented from flowing underneath the contacted area between the patterned web substrate and the micropillars of the mold template, ensuring that the web substrate surface texture may be replicated only at the membrane surface but not at the pores, as shown in Figure 8(b). A planar SEM image of the surface patterned membrane is shown in Figure 8(c). Ordered microporous membranes with various surface protrusions may also be fabricated, as shown in the magnified SEM image in Figure 8(d). The surface protrusions may have a suitable size. For example, the average size of the surface protrusion may be £ 5 mhi. In particular, the average size of the surface protrusion may be 200-5000 nm, 300- 2000 nm, 400-1000 nm, 500-900 nm, 550-800 nm, 600-700 nm. By replica molding the surface patterned membranes, micropillar molds with recessed structures at the base may be obtained, as shown in Figures 9(a) and (b). A second capillary molding process may then be carried out with such hierarchical micropillar mold and another patterned web substrate, yielding a double-side patterned microporous membrane as shown in Figure 9 (c), (d) and (e). In the prior art methods, patterning each side of a membrane may only be imprinted sequentially. However, with the method and system of the present invention, not only is patterning of both sides in a single filling step achieved, but also there is flexibility to customise surface textures and functions on both sides by merely combining the corresponding pattern features on the mold template and patterned web substrate.

According to a particular aspect, the membrane formed may be a hierarchical membrane comprising two or more layers of membrane. Each of the two or more membrane layers may have a suitable pore size, order, narrow size distribution and thickness. Each of the two of more layers of membrane may comprise a different pore size, thereby forming a hierarchical membrane. In particular, each layer of membrane may have a narrow pore size distribution of different range from the other layers of membranes. Such a hierarchical membrane may enable optimised efficiency in applications such as filtration and carbon dioxide capture, just to name a few.

By contacting a micropillar mold template with a web substrate carrying a sub- micrometer protruding pattern, hierarchically structured membrane comprising both ordered microporous and nanoporous membranes arranged in a two-level architecture may be obtained, as shown schematically in Figure 10(a). The inset details the resin filling at both levels, firstly at the micrometer scale capillaries: around the micropillars of the mold template (highlighted by arrows in inset) and subsequently at the sub- micrometer capillaries: around the web sub-micrometer pillars in contact with the micropillars of the mold template. The curing material may flow into the sub-micrometer capillaries from all directions around the circumference of the micropillars. Examples of a hierarchical porous membrane with uniform 20 mhi and 550 nm pore sizes are shown in Figure 10(b)-(d). The SEM images detail the architecture of the membrane, comprising a 40^m-thick ordered microporous membrane and a ~500-nm-thick nanoporous membrane on top. Accordingly, the method of the present invention greatly simplifies the fabrication process for hierarchical porous membranes because it negates the need for multilevel photolithography or the need to fabricate each hierarchy separately before overlaying one on top of the other. Accordingly, the present invention enables membranes at different hierarchies to be formed simultaneously, resulting in a mechanically robust integrated structure made of a single material.

The effect of increasing applied pressure/web tension on the pore size distortion of the ultrathin nanoporous membrane is shown in Figure 10(e)(i-iii). Delicate contact force is thus required to ensure good fabrication fidelity.

Another key fabrication factor is the ability to overcome the slow filling rate of curing material in capillaries with very small cross-sectional dimensions. The filling rate of micrometer scale capillaries is of the order of cm/min while in comparison, the filling of sub-micrometer capillaries is of the order of tens of mGh/hiίh, which is prohibitively slow especially over a long (cm) filling distance. Hierarchical capillaries that incorporate both micrometer and sub-micrometer dimensions arranged in a two-level architecture enable the overall resin filling rate to be increased by orders of magnitude. This may be attributed to the small and isolated sub-micrometer capillaries footprint areas, each -tens of mhi ² , which the curing material can easily reach as the micrometer scale capillaries are filled. The flow of curing material in the hierarchical capillaries is highlighted in the inset of Figure 10(a). The micrometer scale capillaries essentially act as spacers which help to alleviate the geometric restriction on resin capillary filling at sub-micrometer lengthscale, thereby greatly extends the fabrication resolution and speed. The filling rate of the curing material of submicrometer capillaries is of the order pm/min, which may be slow over a long cm distance. By having hierarchical capillaries that incorporate both micrometer and submicrometer dimensions arranged in a two- level architecture, the overall curing material filling rate may be increased by orders of magnitude. In this way, the membrane fabrication resolution of the capillary-driven molding process down to the previously unattainable submicrometer length scale may be achieved.

High filling rate of curing material may also be achieved by using patterned mold template and web substrate with high surface energy and curing material formulations with low surface tension and viscosity. The use of high surface energy mold template and web substrate however, could potentially hinder the separating of the web supported porous membrane from the mold template and subsequent transfer of the hierarchical porous membranes from the patterned web substrate which has higher surface area to a target substrate. In this case, low surface energy web substrate materials such as ethylene tetrafluoroethylene (ETFE) may be used to facilitate defect- free membrane transfer.

The mold template may be coated with anti-sticking silane coating to reduce the mold- membrane adhesion but this would come at the expense of reduced filling rate of curing material which may be driven by interfacial energies. In order to circumvent this, UV-curable fluorinated agent may be added to curing material to lower its viscosity and surface tension in order to restore high filling rate (~cm/min) when for example ETFE web substrate and silanised mold template are used. Combining mold and web with different pattern length-scales and surface energies allow large-scale fabrication of a variety of membrane architectures, as well as facilitate defect-free membrane transfer to various target substrates tailored for specific applications. Figure 11 (a) shows a flexible but robust free-standing porous membrane with multilevel hierarchy after peeling off the web substrate following the fabrication using the method and system of the present invention. Figure 11(b) shows the perspective SEM image of the membrane. On the one hand, the top hierarchy provides the seemingly fragile but increasingly relevant nanoscale membrane. On the other hand, the underlying microscale membrane bestows the necessary geometrical reinforcement to the nanoporous membrane on top. As a result, the overall mechanical strength of the membrane is greatly improved for defect-free demolding, handling and implementation.

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.