WO2019022668A1

WO2019022668A1 - Polyacrylonitrile membranes, methods and uses thereof

Info

Publication number: WO2019022668A1
Application number: PCT/SG2018/050370
Authority: WO
Inventors: Hui Min THAM; Kai Yu Wang; Tai-Shung Chung
Original assignee: National University Of Singapore
Priority date: 2017-07-26
Filing date: 2018-07-26
Publication date: 2019-01-31

Abstract

The present disclosure relates to polyacrylonitrile (PAN) membranes and uses thereof. In a specific embodiment, PAN hollow fibers were prepared by spinning a dope composition comprising PAN and polyvinylpyrrolidone (PVP) and further crosslinking the PAN using hydrazine monohydrate to obtain a PAN hollow fiber membrane with minimal macrovoids in the cross sectional area for use in organic solvent filtration (OSN). The present disclosure also relates to methods of forming the presently disclosed PAN membrane.

Description

POLYACRYLONITRILE MEMBRANES, METHODS AND

USES THEREOF

FIELD

The present disclosure relates to polyacrylonitrile (PAN) membranes and uses thereof. The present disclosure also relates to methods of forming the presently disclosed PAN membrane.

BACKGROUND

Organic solvent nanofiltration (OSN) is a membrane -based process for molecular separation in organic media. Compared to other separation processes like distillation and crystallization, OSN possesses the advantages of being a continuous process that requires lower energy consumption during operation while having mild operating temperatures. Given these characteristics, OSN is thus particularly suited for use in the pharmaceuticals and fine chemicals industries where significant amounts of organic solvents are used and where products are often thermally unstable.

However, despite the advantages of OSN, there are not many commercially available OSN membranes to date. This is because there are still unresolved challenges abound, ranging from membrane swelling and leaching, to low solvent permeances.

Current OSN research is dedicated towards the fabrication of flat sheet membranes. These OSN membranes make use of an additional selective layer to achieve the desired separation. This ranges from the use of thin film composite membranes involving interfacial polymerization to coating of an additional material atop a supporting substrate. Although these methods represent viable means to modify a membrane's properties, care must be taken that all layers of such composite membranes are solvent-resistant. Additionally, the different layers should swell to the same extent in various solvents to prevent delamination. For example, one such known membrane which is used in non- aqueous filtration comprises a thin-film composite membrane which requires the application of an additional layer as the selective layer on top of the porous support. In a further example, another known membrane comprises at least two layers of film-forming polymers coated thereon, a first layer of monomeric or polymeric diazonium salts which have been reacted with themselves and with a difunctional compound, and a second layer, which is chemically bonded to the first one, of a cross-linked, ionically charged hydrophilic polymer. These literature examples show that the desired selectivity of membranes in filtration can only be achieved upon the formation of at least two layers adjacent to each other. Such membranes are often complex to make and these production methods may also not be favourable for scaling up. In this regard, there is a need for an OSN membrane that favours scaling up for use in industrial processes.

Accordingly, it is generally desirable to have an OSN membrane that can overcome or ameliorate at least one of the above mentioned problems.

SUMMARY OF INVENTION

The present invention relates to polyacrylonitrile (PAN) membranes, methods of preparation and uses. In particular, the present invention relates to PAN membranes for use in, but not limited to, organic solvent filtration (OSN). The inventors have found that by mixing the PAN polymer with a polymer additive, the formation of macrovoids can be minimized or eliminated. By subjecting the PAN polymers of the present invention to specific stretching conditions, the formation of macrovoids can further be minimized or eliminated, which is advantageous for use in filtration. It has been further found that by forming the PAN membrane as a hollow fiber, good pure water permeance, and good pure ethanol permeance can be obtained. Further, the membrane prepared by the presently disclosed PAN hollow fibers is capable of retaining solutes up to a size of 2 nm and/or with a molecular weight cut-off of 200-1,000 g mol^-1 in a variety of organic solvents. In a first aspect, the present invention discloses a PAN membrane comprising:

a) a PAN polymer at about 75 wt% to about 90 wt% based on combined amount of PAN polymer and polymer additive; and

b) polymer additive at about 12 wt% to about 23 wt% based on combined amount of PAN polymer and polymer additive;

wherein less than about 8% of the cross sectional area of the PAN membrane comprises macrovoids; and

wherein the PAN polymer is cross linked by an amine cross linker.

In an embodiment, the PAN membrane is a PAN hollow fiber membrane. In an embodiment, the PAN polymer is selected from a PAN homopolymer having a weight- average molecular weight of about 30,000 g mol^-1 to about 250,000 g mol^-1, copolymer PAN-methyl acrylate, PAN-methyl methacrylate and a combination thereof.

In an embodiment, the polymer additive has a weight-average molecular weight between about 10,000 g mol^-1 to about 1,300,000 g mol^-1.

In a second aspect, the present invention discloses a polyacrylonitrile (PAN) hollow fiber membrane comprising:

a) a PAN polymer at about 79 wt% to about 86 wt% based on combined amount of PAN polymer and polymer additive; and

b) a polymer additive, the polymer additive is polyvinylpyrrolidone at about 14 wt% to about 21 wt% based on combined amount of PAN polymer and polymer additive;

wherein less than about 8% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoid; and

wherein the PAN polymer is cross linked by hydrazine monohydrate.

In an embodiment, and with specific reference to the second aspect, the PAN polymer is selected from PAN homopolymer, having a weight- average molecular weight of about 30,000 to about 250,000 g mol^-1, copolymer PAN-methyl acrylate or PAN-methyl methacrylate. In another embodiment, and with specific reference to the second aspect, the PAN polymer and polymer additive are subjected to a stretch in a direction of about 50% more to about 100% more than its original dimension in that direction so that less than about 8% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoid.

In another embodiment, and with specific reference to the second aspect, the PAN polymer and polymer additive are subjected to a take-up speed of about 50% more to about 100% more than its free fall speed so that less than about 8% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoid.

In a third aspect, the present invention discloses a method of forming a PAN membrane, comprising the steps of:

a) providing a dope composition comprising a PAN polymer and a polymer additive; b) extruding the dope composition to form a first extruded polymer;

c) stretching the first extruded polymer in a direction to a range of about 40% more to about 110% more than its original dimension in that direction to form a second polymer; and

d) cros slinking the PAN polymer in the second polymer with an amine cross linker to form the PAN membrane;

wherein the PAN polymer is about 13 wt% to about 19 wt% of the dope composition; wherein the polymer additive is about 1 wt% to about 6 wt% of the dope composition; and wherein less than about 8% of the cross sectional area of the PAN membrane comprises macrovoid. In an embodiment, when the PAN membrane is formed as a hollow fiber, the method further comprises the step of providing a bore fluid substantially adjacent to the dope composition prior to step (b).

In a fourth aspect, the present invention provides a method of forming a polyacrylonitrile (PAN) hollow fiber membrane, comprising the steps of:

a) providing a dope composition comprising a PAN polymer and polyvinylpyrrolidone;

b) extruding the dope composition to form a first extruded polymer;

c) stretching the first extruded polymer in a direction to a range of about 50% more to about 100% more than its original dimension in that direction to form a second polymer; and

d) crosslinking the PAN polymer in the second polymer with hydrazine monohydrate to form the PAN hollow fiber membrane;

wherein the PAN polymer is about 15 wt% to about 17 wt% of the dope composition; wherein polyvinylpyrrolidone is about 2 wt% to about 4 wt% of the dope composition; and wherein less than about 8% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoid.

In an embodiment, and with specific reference to the fourth aspect, the polyvinylpyrrolidone has a weight- average molecular weight of about 10,000 g mol^-1 to about 1,300,000 g mol^-1.

In another embodiment, and with specific reference to the fourth aspect, the PAN polymer is selected from PAN homopolymer, having a weight-average molecular weight of about 30,000 to about 250,000 g mol^-1, copolymer PAN-methyl acrylate or PAN-methyl methacrylate.

In another embodiment, and with specific reference to the fourth aspect, the step of stretching the first extruded polymer comprises taking-up the first extruded polymer on a take up drum at a speed in a range of about 50% more to about 100% more than its original free fall speed to form a second polymer.

In a fifth aspect, the present invention discloses a PAN membrane formed by the method disclosed herein. BRIEF DESCRIPTION OF DRAWINGS

Figure 1 illustrates FESEM images of the cross-sections of free-fall hollow fiber membranes spun with dope (a) A, (b) B and (c) C as per the spinning conditions listed in Table 1.

Figure 2 illustrates FESEM images of dope C hollow fiber membranes spun with different take-up speeds (a) free-fall, (b) 50% higher take-up speed and (c) 100% higher take-up speed.

Figure 3 illustrates (a) PWP/MWCO and (b) pore size distribution of unmodified PAN hollow fiber membranes.

Figure 4 illustrates (a) PWP and pore size distributions of PAN hollow fiber membranes cross-linked for (b) 8 h, (c) 14 h and (d) 18 h.

Figure 5 illustrates FESEM images of the cross-sections of hollow fiber membranes cross- linked for 18 h spun with different take-up speeds (a) free-fall, (b) 50% higher take-up speed and (c) 100% higher take-up speed.

Figure 6 illustrates FESEM images of dope C hollow fiber membranes spun at a take-up speed 100% higher than the free-fall one and then cross-linked for 18 h.

Figure 7 illustrates XPS spectra of (a) unmodified hollow fiber membranes and (b) hollow fiber membranes heated in ethanol for 8h.

Figure 8 illustrates (a) C Is (b) O Is and (c) N Is XPS spectra of hollow fiber membranes cross-linked for 8 h.

Figure 9 illustrates C Is and N Is XPS spectra of hollow fiber membranes cross-linked for (a, b) 14 h and (c, d) 18 h.

DETAILED DESCRIPTION

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by those of ordinary skill in the art to which the invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, preferred methods and materials are described. For the purposes of the present invention, the following terms are defined below.

As used herein, "polyacrylonitrile" or "PAN" is a vinyl polymer, and a derivative of the acrylate family of polymers. It is made from the monomer acrylonitrile and can be polymerised by free radical vinyl polymerization. PAN is a synthetic, semicrystalline organic polymer, with the linear formula (¾Η₃Ν)_n. Though it is thermoplastic, it does not melt under normal conditions. It degrades before melting. More commonly used are PAN copolymers made from mixtures of other monomers with acrylonitrile as the main monomer. For example, monomers of vinyl chloride, styrene and/or butadiene can be added to acrylonitrile to form PAN copolymers. Accordingly, PAN homopolymer and PAN copolymers are within the scope of PAN as used herein to describe the present invention. In particular, PAN homopolymer, having a weight- average molecular weight Mw 30,000 to 250,000; copolymer PAN-methyl acrylate, PAN-methyl methacrylate may be used. The term "polymer additive" refers to a substance that is added to a polymer to modify its properties. Such substance is usually added at a lower weight percentage than the polymer itself, and can be any kind or molecular, polymeric, inorganic or organic substance. For example, plasticizers can be used to lower the glass transition temperature of the polymer, fillers can be used to make it cheaper, and oily components can be used to improve its rheology. The polymer additive as used in this present invention provides for a polymer cross sectional area with minimal or no macrovoids.

The term "membrane" as used herein refers to a polymeric material which is porous, for use in an application that utilises this property. Such membranes are usually permeable to certain selective entities when subjected to, for example, a pressure and/or concentration gradient. Such membranes can be used in membrane technology, which relies on physical forces (and optionally without heat or at cold conditions) to separating gases or liquids from a mixture. The skilled person would be aware that the selection of polymeric membrane is not trivial and has to have appropriate characteristics for the intended application. For example, in the case of biotechnology applications, the polymeric membrane has to offer a low binding affinity for separated molecules. In the case of waste water treatment, the membrane has to withstand the harsh conditions. In this regard, the polymeric membrane can for example be assessed in terms of its chains rigidity, chain interactions, stereo-regularity, and polarity of its functional groups. The term "hollow fiber membrane" refers to a membrane in the form of a hollow fiber; i.e. the core of the fiber is hollow while the fiber is a semi-permeable barrier. The skilled person would know that to form a hollow fiber, a spinneret is used. The spinneret is a device containing a needle through which solvent is extruded and an annulus through which a polymer solution is extruded. As the polymer is extruded through the annulus of the spinneret, it retains a hollow cylindrical shape. As the polymer exits the spinneret, it solidifies into a membrane through a process known as phase inversion. Extrusion of the polymer, polymer additive and solvent through the spinneret can be accomplished either through the use of gas-extrusion or a metered pump. The average pore diameter and pore distribution are measurable via porosimetry. Pore diameter can also be measured via evapoporometry, in which evaporation of 2-propanol through the pores of a membrane is related to pore-size via the Kelvin equation. Scanning electron microscopy or transmission electron microscopy can be used to yield a qualitative perspective of pore size.

The term "dope composition" refers to a composition comprising a PAN polymer and a polymer additive before forming the PAN membrane. The "dope" is the polymer additive. In this regard, the polymer additive is added as an amount which is less than the PAN polymer. When the dope composition is processed, a PAN membrane is formed.

The term "macrovoid" as would be understood by the person skilled in the art, refers to defects that can be found in a polymer membrane. Macrovoids may, for example, be encountered in phase inversion as well as in other solution-cast membranes. Such macrovoids are in the micrometre size range, and can appear as teardrop or elliptical shape voids.

As used herein, 'ultrafiltration' refers to a variety of membrane filtration in which forces like pressure or concentration gradients lead to a separation through a semipermeable membrane. Accordingly, suspended solids and solutes of high molecular weight are retained in the so-called retentate, while solvents and low molecular weight solutes pass through the membrane in the permeate (filtrate). This separation process is generally capable of purifying and/or concentrating entities of about 10³ - 10⁶ Da. Membranes for use in ultrafiltration usually can have a pore size (diameter) of about 0.01 μιη.

As used herein, 'ultrafiltration' refers to a filtration based on size exclusion or particle capture. Nanofiltration method can utilise a membrane that have nanometer sized through- pores. Nanofiltration membranes can have pore sizes (diameters) of about 1-10 nm typically < 2 nm.

Organic solvent nanofiltration usually utilizes a polymer membrane comprising polymers such as polyimides (PI) and polybenzimidazole (PBI). While these polymers are deemed as excellent solvent-resistant materials, Pis and PBI are often associated with the formation of nanofiltration-level integrally-skinned asymmetric membranes. These polymers are often used in combination with PAN, wherein PAN is being used as a filtration support for these membranes. PI and PBI are also expensive and accordingly are not commercially viable. In this regard, the inventors have found that PAN, when specifically formed as disclosed herein, can function as both a filtration membrane and a filtration support. Accordingly, no further coatings or additional layers are required. In particular, it was found that cross- linking of PAN membrane and the use of polymer additives give improved solvent resistance and/or nanofiltration properties without resorting to additional complicated measures such as interfacial polymerization or coating. Advantageously, PAN is relatively lower priced than other polymers and has inherent solvent resistance. Accordingly, the PAN membranes of the present invention may be used directly as nanofiltration-level membranes, i.e. without requiring additional processing steps in addition to its formation as disclosed herein.

One problem of the art is that membranes need to be able to withstand high pressures for use in, for example, filtration or nanofiltration. Without wanting to be bound by theory, the inventors have found that in order to produce a membrane capable of withstanding pressures typically used in nanofiltration (such as operating pressure of about 0.5 x 10⁶ Pa to about 2 x 10⁶ Pa), morphologies comprising a sponge-like cross section with minimal or no macrovoids are desired. In an embodiment, the PAN membrane comprises a PAN polymer, the PAN polymer is a PAN homopolymer with a weight- average molecular weight (M_w) of 200,000 g mol^-1. In another embodiment, the PAN polymer is selected from a PAN homopolymer having a weight- average molecular weight (M_w) of about 30,000 to about 250,000 g mol^-1, copolymer PAN-methyl acrylate, PAN-methyl methacrylate and a combination thereof. In another embodiment, the PAN polymer is selected from a PAN homopolymer having a weight- average molecular weight M_w of about 30,000 to about 250,000 g mol^-1, copolymer PAN-methyl acrylate and PAN-methyl methacrylate.

In an embodiment, the PAN membrane comprises a PAN polymer at about 75 wt% to about 90 wt% based on combined amount of PAN polymer and polymer additive. In another embodiment, the PAN polymer is about 76 wt% to about 89 wt%; about 77 wt% to about 88 wt%; about 78 wt% to about 87 wt%; or about 79 wt% to about 86 wt%. In another embodiment, the PAN polymer is about 75 wt%; about 77 wt%; about 79 wt%; about 81 wt%; about 83 wt%; about 85 wt%; about 86 wt%; about 88 wt%; or about 90 wt%.

Polymer additives can be added to adjust the micro structure and pore size of the PAN membrane. This is believed to be due to phase inversion when preparing the PAN membrane. Advantageously, the inventors have found that the use of polymer additives such as polyvinylpyrrolidone can reduce macrovoid formation. For example, it is believed that the addition of polyvinylpyrrolidone controls the flowability of the dope composition and prevents non-solvent penetration into the dope composition. In this regard, with respect to polymer additives, both the molecular weight and the amount of additive added are vital in suppressing macrovoid formation. In an embodiment, the polymer additive is polyvinylpyrrolidone (PVP). It would be appreciated that the PVP can be selected from any of the commercially available PVP. For example, PVP K30 can be used. Additionally, the polymer additive should preferably have weight- average M_w between about 10,000 to about 1,300,000; about 15,000 to about 1,200,000; about 20,000 to about 1,100,000; about 25,000 to about 1,000,000; about 30,000 to about 900,000; about 35,000 to about 800,000; about 40,000 to about 700,000; about 45,000 to about 600,000; or about 50,000 to about 500,000. The skilled person would know that PVP K30 has a weight- average M_w of about 40,000 g mol^-1. In this regard, in an embodiment, the polymer additive has a weight- average molecular weight of about 40,000 g mol^-1. In another embodiment, the polymer additive is PVP with a weight - average molecular weight of about 40,000 g mol^-1. In another embodiment, the polymer additive is selected from PEO (polyethylene oxide) and PVA (polyvinyl alcohol).

In an embodiment, the PAN membrane comprises a polymer additive at about 12 wt% to about 23 wt% based on combined amount of PAN polymer and polymer additive. In another embodiment, the polymer additive is about 13 wt% to about 23 wt%; about 13 wt% to about 22 wt%; about 14 wt% to about 22 wt%; about 14 wt% to about 21 wt%; about 15 wt% to about 21 wt%; about 15 wt% to about 20 wt%; about 16 wt% to about 20 wt%; or about 16 wt% to about 19 wt%. In another embodiment, the polymer additive is about 12 wt%; about 13 wt%; about 14 wt%; about 15 wt%; about 16 wt%; about 17 wt%; about 18 wt%; about 19 wt%; about 20 wt%; about 21 wt%; about 22 wt%; or about 23 wt%.

The inventors have found that crosslinking provides additional advantages to the PAN membrane. Without wanting to be bound by theory, it is believed that cross-linking can result in densification of the selective layer as well as pore shrinkage caused by cross - linkers pulling the polymer chains closer together. Even more advantageous is if the cross- linking occurred throughout the membrane and not only on surface. An added advantage can be found if the cross linker used is simple to apply, easy availability and low price. It was found that after crosslinking, PAN membrane advantageously showed improved solvent resistance and nanofiltration properties without resorting to additional complicated measures such as interfacial polymerization or coating. Such PAN membranes can be used as cost-effective OSN membranes.

Accordingly, in an embodiment, the PAN polymer in the second polymer is cross linked by an amine cross linker to form the PAN membrane. In an embodiment, the amine cross linker is hydrazine monohydrate.

As will be further discussed below, the inventors have found that stretching the PAN membrane during its formation results in a PAN membrane that is free of or at least has a minimal amount of macrovoids. Accordingly, in an embodiment, the PAN polymer and polymer additive are subjected to a stretch in a direction of about 40% more to about 110% more than its original dimension in that direction. This stretch can be an elongation force along its length. In another embodiment, the PAN polymer and polymer additive are subjected to a take-up speed of about 40% more to about 110% more than its free fall speed. The increase in dimension or speed can be about 50% more to about 100% more, or can be about 40% more, about 50% more, about 60% more, about 70% more, about 80% more, about 90% more, about 100% more or about 110% more.

Figure 2, for example, shows the cross sectional area of the PAN membrane with and without macrovoids before and after the stretch. Figure 2a shows the cross sectional area if the stretch is not performed on the PAN membrane. It can be observed that about 8% of the cross sectional area comprises macrovoids. Figure 2b shows the cross sectional area when the PAN membrane is subjected to about 50% stretch (or to about 50% increase in take-up speed). In this case, the cross sectional area comprising macrovoids decreases to about 5%. Figure 2c shows the cross sectional area when PAN membrane is subjected to about 100% stretch (or to about 100% increase in take-up speed). The cross sectional area comprising macrovoids decreases to about 0%. Accordingly, in an embodiment, the cross sectional area of the PAN membrane is free of macrovoid. In another embodiment, less than about 8%, about 7.5%, about 7%, about 6.5%, about 6%, about 5.5%, about 5%, about 4.5%, about 4%, about 3.5%, about 3%, about 2.5%, about 2%, about 1.5%, about 1%, about 0.5% or about 0.1% of the cross sectional area of the PAN membrane comprises macrovoids.

In an embodiment, the ratio of PAN polymer to polymer additive based on combined amount of PAN polymer and polymer additive is about 2 to about 13. In another embodiment, the ratio is about 2.5 to about 10, about 3 to about 7. In another embodiment, the ratio is about 2, about 2.5, about 3, about 3.5, about 3.75, about 4, about 4.25, about 4.5, about 4.75, about 5, about 5.5, about 6, about 7, about 8, about 9, about 10, about 11, about 12, or about 13. In another embodiment, the ratio is selected from about 3.75, about 4.25 or about 6. Accordingly, in an embodiment, the PAN membrane comprising:

wherein less than about 8% of the cross sectional area of the PAN membrane comprises macrovoid;

wherein the PAN polymer is cross linked by an amine cross linker; and

wherein the ratio of the PAN polymer to polymer additive based on combined amount of

PAN polymer and polymer additive is about 2 to about 13.

Compared to flat sheet membranes, the inventors have found that it is further advantageous to have the membrane extruded as a hollow fiber. It is believed that hollow fiber membranes have the further advantages of possessing a larger surface area per unit membrane volume and a self-supporting structure that does not require additional backing materials. In this regard, it was found that a hollow fiber OSN membrane without the need of additional backing materials or layers represents a most simple and elegant strategy. Furthermore, such simple designs favour scaling up. In this regard, the inventors have found that the properties of the membrane (such as average pore diameter and membrane thickness) can be tuned by changing the dimensions of the spinneret, temperature and composition of "dope" (polymer) and "bore" (solvent) solutions, length of air gap (for dry- jet wet spinning), temperature and composition of the coagulant, as well as the speed at which produced fiber is collected by a motorized spool. The ratio of outer diameter to inner diameter (OD:ID) is as a result influenced by these factors.

In an embodiment, the PAN membrane is formed as a hollow fiber. In another embodiment, the PAN membrane is a PAN hollow fiber membrane. In this regard, the skilled person would understand that 'hollow fiber' refers to a tube like structure.

Apart from the morphology, the inventors have further found that a PAN hollow fiber membrane with sufficiently large outer diameter to inner diameter ratio (OD:ID) is also advantageous. This is believed to be due to a thicker fiber wall which will provide greater mechanical strength. However, it has been noted that a critical membrane thickness exists, above which macrovoids will begin to form. Hence, a careful balance between both factors is required. In an embodiment, the outer diameter is about 800 μιη to about 900 μιη. In another embodiment, the outer diameter is about 810 μιη to about 890 μιη, about 820 μιη to about 880 μιη, about 820 μιη to about 870 μιη, or about 820 μιη to about 860 μιη. In another embodiment, the outer diameter is about 810 μιη, about 820 μιη, about 830 μιη, about 840 μιη, about 850 μιη, about 860 μιη, about 870 μιη, about 880 μιη, about 890 μιη, or about 900 μηι. In an embodiment, the inner diameter is about 400 μιη to about 500 μιη. In another embodiment, the inner diameter is about 410 μιη to about 490 μιη, about 420 μιη to about 480 μιη, about 420 μιη to about 470 μιη, or about 430 μιη to about 470 μιη. In another embodiment, the inner diameter is about 410 μιη, about 420 μιη, about 430 μιη, about 440 μιη, about 450 μιη, about 460 μιη, about 470 μιη, about 480 μιη, about 490 μιη, or about 500 μιη. In an embodiment, the OD:ID ratio is about 1.5 to about 2. In another embodiment, the OD:ID ratio is about 1.6 to about 2; about 1.6 to about 1.9; or about 1.7 to about 1.9. In another embodiment, the OD:ID ratio is about 1.5; about 1.6; about 1.7; about 1.8; about 1.9 or about 2.

The above mentioned PAN membrane and/or PAN hollow fiber membrane has a sufficiently high permeance that is sufficient for, for example, ultrafiltration and/or organic solvent nanofiltration. The inventors have further found that cross-linking resulted in a further reduction of permeance. Without wanting to be bound by theory, it is believed that the polymer chains rearrange to give rise to a denser membrane structure.

As mentioned above, the PAN polymer is crosslinked in the PAN membrane. With the addition of the cross linker (for example hydrazine monohydrate), it is believed that the following reaction occurs:

In this scheme, cross-linking of the nitrile groups on PAN by hydrazine occur via a nucleophilic addition reaction. However, given the presence of water in the system derived from hydrazine monohydrate, alkaline hydrolysis of the nitrile groups to form carboxylic acids may also occur. Such reaction is further believed to aid in improving the solvent resistance of PAN membrane (and/or PAN hollow fiber membrane). It is believed that intramolecular hydrogenated naphthyridine-type cyclic structures would be formed along the PAN polymer chains after treatment with both base and heat. The resulting highly conjugated structure obtained is responsible for the yellowish-red tone observed in PAN samples that have underwent such treatments. Compounds containing such cyclic structures were found to have poor solubility in strong polar aprotic solvents such as dimethylformamide (DMF) with decreases in solubility being noted even before visible color changes appear. For example, in cross-linked PAN hollow fibers, a yellowish tint was observed that may possibly be attributed to the formation of the conjugated cyclic structures. Additionally, a small peak corresponding to C=N-C at a binding energy of 398.3 eV can be seen in XPS studies. Hence, these structures may also be responsible for improving the solvent resistance of PAN membrane and/or PAN hollow fiber membranes.

Accordingly, in an embodiment, the PAN membrane and/or PAN hollow fiber membrane has a XPS binding energy peak at about 286 eV to about 287 eV. In another embodiment, the XPS binding energy peak comprises a bimodal distribution at about 286 eV to about 287 eV. In another embodiment, the XPS binding energy peak is at about 286.3 eV. This is believed to be attributed to the formation of the N-C=N moiety with a binding energy of approximately 286.3 eV and a fall in the C≡N peak intensity. In another embodiment, the XPS binding energy peak is at about 399 eV to about 400 eV. In another embodiment, the XPS binding energy peak is at about 399.5 eV. This is believed to be attributed to the formation of the cross-linked moieties C-N-N-C and C=N-H at a binding energy of around 399.3 eV following a decrease in the intensity of the C≡N peak.

The PAN membrane and/or PAN hollow fiber membrane are able to reject dyes of small molecular weights (i.e. the dye is prevented from entering the lumen side of the hollow fiber in an Outer selective' hollow fiber membrane or prevented from exiting the lumen of the hollow fiber in an 'inner selective' hollow fiber membrane). Such molecules includes, but are not limited to, dyes and PEG. It should be noted that membranes behave differently in different solvents and accordingly MWCO determined in one solvent need not coincide with that determined in another solvent. Further, the mathematical model used to determine the MWCO in the aqueous system is subject to its own set of assumptions that naturally leads to inaccuracies. Lastly, the shape of the solute molecules may also play a role in affecting its permeability across the membrane. For example, PEG molecules are generally linear molecules and may slip through the membrane pores more easily compared to the more sterically bulky dyes. As such, dye molecules are more easily rejected than PEG molecules of comparable molecular weights. As used herein, "dye" is a substance that is soluble in the solvent it is in. It is used to impart colour by absorbing and/or re-emitting light of a certain wavelength. In this sense, coloured dyes absorb light in the visible wavelength and hence is observed as having a specific colour. Fluorescence dye or fluorophore absorbs light energy of a specific wavelength and re-emits light at a longer wavelength, usually in the visible range. Such are included within the scope of this definition.

In an embodiment, the PAN membrane and/or PAN hollow fiber membrane has a rejection of dyes with a molecular weight of more than 550 g mol ¹. In another embodiment, the PAN membrane and/or PAN hollow fiber membrane has a rejection of dyes with a molecular weight of more than 600 g mol^-1. In another embodiment, the PAN membrane and/or PAN hollow fiber membrane can reject dyes such as Rose Bengal, Brilliant Blue R and Remazol Brilliant Blue R. In another embodiment, the rejection is more than about 90%, about 91%, about 92%, about 93%, about 94%, about 95%, about 96%, about 97%, about 98%, about 99%, about 99.5% or about 99.9%. In another embodiment, the PAN hollow fiber membrane has a rejection of Remazol Brilliant Blue R of at least 98%. In another embodiment, the PAN hollow fiber membrane has a rejection of Remazol Brilliant Blue R of at least 99%. In another embodiment, the PAN hollow fiber has a rejection of Remazol Brilliant Blue R of at least 99.5%. In another embodiment, the PAN hollow fiber membrane has a rejection of Remazol Brilliant Blue R of at least 99.9%. It is believed that since ethanol has a significantly lower dielectric constant than water, the charges on Remazol Brilliant Blue R are well shielded in ethanol. Accordingly, the rejection of Remazol Brilliant Blue R is mainly attributed to its steric effect and not charge. Although a very poor rejection for Methylene Blue was obtained using the same cross-linked hollow fibers, this may suggest a fairly sharp pore size distribution.

The molecular weight cut off (MWCO) can be determined using a series of PEG dissolved in DI water. MWCO refers to the lowest molecular weight solute or molecule in which at least 80% (or preferably at least 90%) of the solute or molecule is retained by the membrane. In an embodiment, the PAN membrane and/or PAN hollow fiber membrane has a PEG MWCO of about 2,500 g mol^-1. In another embodiment, the PEG MWCO is about 2,000 g mol^-1, about 1,800 g mol^-1 or about 1,600 g mol^-1.

In an embodiment, the PAN membrane and/or PAN hollow fiber membrane remains insoluble in solvents such as N-methylpyrrolidone and dimethylformamide. In another embodiment, the PAN membrane and/or PAN hollow fiber membrane remains insoluble in solvents such as acetone, ethyl acetate, hexane, tetrahydrofuran, chloroform, and alcohol solvents such as methanol, ethanol, propanol, isopropanol, 2-butanol, n-butanol, isobutanol, tert-butanol, 1-pentanol, 2-pentanol, 3-pentanol, 2-methylbutanol. In another embodiment, the PAN membrane and/or PAN hollow fiber membrane remains insoluble in solvents for at least two months. In contrast, unmodified membranes (not crosslinked) dissolved completely within minutes.

In an embodiment, the PAN membrane and/or PAN hollow fiber membrane has a median pore diameter of less than about 2 nm. In another embodiment, the median pore diameter is less than about 1.8 nm, about 1.6 nm, about 1.4 nm, about 1.2 nm or about 1 nm.

The PAN membrane and/or PAN hollow fiber membrane as formed and crosslinked using the method disclosed herein has a pure water permeance of less than about 25 L m --2 h--1 bar-

1. In another embodiment, the pure water permeance is less than about 22 L m --2 h--1 bar--1 , 20 L m^-2 h^-1 bar^-1, 15 L m^-2 h^-1 bar^-1, 12 L m^-2 h^-1 bar^-1, 10 L m^-2 h^-1 bar^-1, 9 L m^-2 h^-1 bar^-1 or 8 L m --2 h --1 bar --1. In an embodiment, the PAN membrane and/or PAN hollow fiber membrane has a pure ethanol permeance is less than about 8 L m --2 h --1 bar --1. In another embodiment, the pure ethanol permeance is less than about 7 L m --2 h--1 bar --1 , 6.5 L m --2 h--1 bar^-1, 6 L m^-2 h^-1 bar^-1, 5.5 L m^-2 h^-1 bar^-1, 5 L m^-2 h^-1 bar^-1, 4.5 L m^-2 h^-1 bar^-1, 4 L m^-2 h^-1 bar^-1, 3.5 L m^-2 h^-1 bar^-1, 3 L m^-2 h^-1 bar^-1 or 2.5 L m^-2 h^-1 bar^-1.

In an embodiment, the PAN hollow fiber membrane comprising:

wherein polyvinylpyrrolidone has a weight-average molecular weight of about 10,000 g mol^-1 to about 1,300,000 g mol^-1;

wherein the PAN polymer is cross linked by hydrazine monohydrate.

In another embodiment, the PAN hollow fiber membrane comprising:

wherein the PAN polymer is selected from PAN homopolymer, having a weight- average molecular weight of about 30,000 to about 250,000 g mol^-1, copolymer PAN-methyl acrylate or PAN-methyl methacrylate;

wherein polyvinylpyrrolidone has a weight-average molecular weight of about 10,000 g mol^-1 to about 1,300,000 g mol^-1; and

wherein the PAN polymer is cross linked by hydrazine monohydrate.

In another embodiment, the PAN hollow fiber membrane comprising:

a) a PAN polymer, the PAN polymer is a PAN homopolymer at about 79 wt% to about 86 wt% based on combined amount of PAN polymer and polymer additive; and b) a polymer additive, the polymer additive is polyvinylpyrrolidone at about 14 wt% to about 21 wt% based on combined amount of PAN polymer and polymer additive;

wherein the PAN homopolymer has a weight-average molecular weight of about 200,000 g mol^-1;

wherein polyvinylpyrrolidone has a weight-average molecular weight of about 40,000 g mol^-1; and

wherein less than about 8% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoid; and wherein the PAN polymer is cross linked by hydrazine monohydrate.

In another embodiment, the PAN hollow fiber membrane comprising:

a) a PAN polymer, the PAN polymer is a PAN homopolymer at about 79 wt% to about 86 wt% based on combined amount of PAN polymer and polymer additive;

b) a polymer additive, the polymer additive is polyvinylpyrrolidone at about 14 wt% to about 21 wt% based on combined amount of PAN polymer and polymer additive; and wherein the PAN homopolymer has a weight-average molecular weight of about 200,000 g mol^-1;

wherein polyvinylpyrrolidone has a weight-average molecular weight of about 40,000 g mol^-1;

wherein the ratio of the PAN polymer to polymer additive based on combined amount of PAN polymer and polymer additive is about 3.75, about 4.25 or about 6; and

wherein the PAN polymer is cross linked by hydrazine monohydrate.

In another embodiment, the PAN hollow fiber membrane comprising:

wherein the ratio of the PAN polymer to polymer additive based on combined amount of PAN polymer and polymer additive is about 3.75, about 4.25 or about 6;

wherein less than about 8% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoid;

wherein the PAN polymer is cross linked by hydrazine monohydrate; and wherein the PAN hollow fiber membrane has a pure water permeance of less than about 10 L m^-2 h^-1 bar^-1.

In another embodiment, the PAN hollow fiber membrane comprising:

wherein the PAN polymer is cross linked by hydrazine monohydrate; and

wherein the PAN hollow fiber membrane has a pure water permeance of less than about 10

L m --2 h--1 bar--1 and a pure ethanol permeance of less than about 3 L m --2 h--1 bar--1.

In an embodiment, and with specific reference to the second aspect, the PAN polymer and polymer additive are subjected to a stretch in a direction of about 50% more or about 100% more than its original dimension in that direction so that less than about 8% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoids. In another embodiment, and with specific reference to the second aspect, the PAN polymer and polymer additive are subjected to a take-up speed of about 50% more or about 100% more than its free fall speed so that less than about 8% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoids. In an embodiment, the PAN hollow fiber membrane can withstand a pressure of at least about 2.5 x 10⁶ Pa without collapsing. In another embodiment, the crosslinked PAN hollow fiber membrane can withstand a pressure of at least about 2.5 x 10⁶ Pa without collapsing. In another embodiment, the pressure is at least about 2.4 x 10⁶ Pa, at least about 2.3 x 10⁶ Pa, at least about 2.2 x 10⁶ Pa, at least about 2.1 x 10⁶ Pa, at least about 2 x 10⁶ Pa, at least about 1.8 x 10⁶ Pa, at least about 1.6 x 10⁶ Pa, at least about 1.4 x 10⁶ Pa, at least about 1.2 x 10⁶ Pa, at least about 1 x 10⁶ Pa, at least about 0.8 x 10⁶ Pa or at least about 0.5 x 10⁶ Pa.

As mentioned above, the inventors have found that the properties of the membrane (such as average pore diameter and membrane thickness) can be tuned by changing the dimensions of the spinneret, temperature and composition of "dope" (polymer) and "bore" (solvent) solutions, length of air gap (for dry-jet wet spinning), temperature and composition of the coagulant, as well as the speed at which produced fiber is collected by a motorized spool. Accordingly, the present invention also discloses a method of forming a polyacrylonitrile (PAN) membrane. The method comprises a step of providing a dope composition comprising a PAN polymer and a polymer additive. The PAN polymer and polymer additive are as mentioned herein.

In an embodiment, the dope composition is provided as a liquid mixture. In another embodiment, the dope composition further comprises a solvent. In another embodiment, the solvent is a polar aprotic solvent. In another embodiment, the solvent is selected from DMSO, DMF, NMP, DM Ac and a combination thereof. In another embodiment, the solvent is DMSO.

In an embodiment, the dope composition is provided as a liquid mixture and comprises a solvent at about 75 wt% to about 90 wt% of the dope composition. In another embodiment, the solvent is about 76 wt% to about 89 wt%; about 77 wt% to about 88 wt%; about 78 wt% to about 87 wt%; about 79 wt% to about 86 wt%; about 79 wt% to about 85 wt%; about 79 wt% to about 84 wt%; or about 79 wt% to about 83 wt%. In another embodiment, the solvent is about 75 wt%; about 76 wt%; about 77 wt; about 78 wt%; about 79 wt%; about 80 wt%; about 81 wt%; about 82 wt%; about 83 wt%; about 84 wt%; about 85 wt%; about 86 wt%; about 87 wt%; about 88 wt%; about 89 wt%; or about 90 wt%. In another embodiment, the solvent is about 79 wt%; about 81 wt% or about 82.5 wt%.

The inventors have found that by subjecting the dope composition to extrusion and stretching, a PAN membrane substantially free of macrovoid can be obtained. Accordingly, the method comprises a step of extruding the dope composition. The extrusion can be influenced by the dope composition viscosity as well as the flow rate. It was found that varying the viscosity of the dope composition can assist in reducing the formation of macrovoids, by varying the amount of PAN and/or polymer additive. For example, viscosity can be further increased in the dope composition by increasing the PAN concentration, polymer additive concentration or both the PAN and polymer additive concentration.

As mentioned, the inventors have found that the use of a sufficiently high polymer concentration in the dope composition can reduce macrovoid formation. In an embodiment, the concentration of PAN polymer in the dope composition is about 13 wt% to about 19 wt% of the dope composition. In another embodiment, the concentration of PAN polymer is about 13.5 wt% to about 18.5 wt%, about 14 wt% to about 18 wt%, about 14.5 wt% to about 17.5 wt%, or about 15 wt% to about 17 wt% of the dope composition. In another embodiment, the concentration of PAN polymer is about 13 wt%, about 13.5 wt%, about 14 wt%, about 14.5 wt%, about 15 wt%, about 15.5 wt%, about 16 wt%, about 16.5 wt%, about 17 wt%, about 17.5 wt%, about 18 wt%, about 18.5 wt% or about 19 wt% of the dope composition.

In an embodiment, the concentration of polymer additive in the dope composition is about 1 wt% to about 6 wt%. In another embodiment, the concentration of polymer additive is 1.5 wt% to about 6 wt%, about 2 wt% to about 5 wt%, or about 2.5 wt% to about 5 wt%. In another embodiment, the concentration of polymer additive is about 1 wt%, about 1.5 wt%, about 2 wt%, about 2.5 wt%, about 3 wt%, about 3.5 wt%, about 4 wt%, about 4.5 wt%, about 5 wt%, about 5.5 wt% or about 6 wt%.

In an embodiment, the combined polymer concentration (PAN polymer and polymer additive) in the dope composition is about 14 wt% to about 25 wt% of the dope composition. In another embodiment, the combined polymer concentration is about 15 wt% to about 24 wt%, about 16 wt% to about 23 wt%, about 17 wt% to about 22 wt%, or about 17 wt% to about 21 wt% of the dope composition. In another embodiment, the combined polymer concentration is about 15 wt%, about 16 wt%, about 17 wt%, about 17.5 wt%, about 18 wt%, about 19 wt%, about 20 wt%, about 21 wt%, about 22 wt%, about 23 wt%, about 24 wt%, or about 25 wt% of the dope composition.

In an embodiment, the ratio of PAN polymer to polymer additive in the dope composition is about 2 to about 13. In another embodiment, the ratio is about 2.5 to about 10, about 3 to about 7. In another embodiment, the ratio is about 2, about 2.5, about 3, about 3.5, about 3.75, about 4, about 4.25, about 4.5, about 4.75, about 5, about 5.5, about 6, about 7, about 8, about 9, about 10, about 11, about 12, or about 13. In another embodiment, the ratio is selected from about 3.75, about 4.25 or about 6.

In an embodiment, the dope composition is extruded with a flow rate of about 3 mL/min to about 6 mL/min to form a first extruded polymer (step b). In another embodiment, the flow rate is about 3.5 mL/min to about 5.5 mL/min. In another embodiment, the flow rate is about 4 mL/min to about 5 mL/min. In another embodiment, the flow rate is selected from about 3 mL/min, about 3.5 mL/min, about 4 mL/min, about 4.5 mL/min, about 5 mL/min, about 5.5 mL/min and about 6 mL/min. In another embodiment, the flow rate is selected from about 4 mL/min, about 4.5 mL/min and about 5 mL/min.

In an embodiment, the dope composition is maintained at a temperature of about 40°C to about 80°C, about 45°C to about 75°C, about 50°C to about 70°C, or about 55°C to about 65°C. In another embodiment, the temperature is about 40°C, about 45°C, about 50°C, about 55°C, about 60°C, about 65°C, about 70°C, about 75°C or about 80°C.

In an embodiment, the air gap is about 1 cm to about 5 cm. In another embodiment, the air gap is about 1.5 cm to about 4.5 cm, about 1.5 cm to about 4 cm, about 1.5 cm to about 3.5 cm, about 1.5 cm to about 3 cm, or about 1.5 cm to about 2.5 cm. In another embodiment, the air gap is about 2 cm.

The extruded polymer is allowed to enter a bath of coagulation solvent. The purpose of this is to coagulate the polymer, changing the polymer from a liquid or semi-liquid state to a solid state. The coagulation solvent can be any solvent which is incompatible with PAN but which is compatible with the polymer additive. In this regard, the coagulation solvent is a non-solvent for PAN but a solvent for the polymer additive. It is believed that this assist the leaching out of the polymer additive from the pores of the membrane. For example, water can be used. In an embodiment, the temperature of the bath is about 0°C to about 10°C, about 1°C to about 9°C, about 2°C to about 8°C, about 3°C to about 7°C, about 4°C to about 7°C, about 5°C to about 7°C or about 6°C to about 7°C.

As mentioned above, in addition to PAN polymer concentrations and polymeric additives, stretching by taking-up the extruded polymer on a drum or spool with a certain take-up speed after the extrusion step may produce membrane with reduced number and size of macrovoid formation. Without wanting to be bound by theory, the inventors believe that this may be attributed to the fact that the higher take-up speed may bring about better packing and alignment of polymer chains. This, in turn, retards the penetration of the external coagulant and results in delayed demixing and hence less and/or smaller macrovoid formation. With respect to hollow fiber membrane, it is also believed that another factor retarding the intrusion of the external coagulant may be the sudden shrinkage of fiber dimension caused by elongational stretching at higher take-up speeds. Due to the shrinkage, solvents within the nascent fiber may be forced radially outwards, hence countering the intrusion of external coagulants. Additionally, the elongational stresses induced by the higher take-up speed may also create extra phase instability, thus facilitating spinnodal decomposition across the nascent membranes. As a result of the above factors, when applied to hollow fiber membranes (see description below), hollow fiber membranes with smaller diameters, sponge-like morphologies and reduced number (and size) of macrovoids are obtained with an increment of the take-up speed. Accordingly, in an embodiment, the extruding step results in the PAN membrane being substantially free of macrovoids. In another embodiment, the stretching step results in the PAN membrane being substantially free of macrovoids. In another embodiment, the extruding and stretching steps result in the PAN membrane being substantially free of macrovoids.

The method comprises the step of stretching the first extruded polymer in a direction to a range of about 40% more to about 110% more than its original dimension in that direction to form a second polymer. The stretch can be about 50% more to about 100% more, about 50% more to about 90% more, about 50% more to about 80% more, or about 50% more to about 70% more of the original dimension. The stretch can be about 50% more, about 60% more, about 70% more, about 80% more, about 90% more or about 100% more of the original dimension. Preferentially, the stretch is about 50% more or about 100% more of the original dimension. The original dimension can be its original free fall dimension, i.e. the dimension of the polymer resulting only due to gravity.

In an embodiment, the stretching of the first extruded polymer is effected by varying its take-up speed on the take-up drum or spool. In this regard, the method can alternatively comprise a step of taking up (and hence stretching) the first extruded polymer on a take-up drum at a take-up speed of about 40% more to about 110% more than the free fall speed of the first extruded polymer to form a second polymer. The free fall speed refers to the speed in which the extruded polymer is falling due to gravity. By taking up the extruded polymer faster than its free fall speed, the extruded polymer is thus subjected to a stretching force (elongation vector). This step is preferably performed after the step of extruding the dope composition. The take-up speed can be about 50% more to about 100% more, about 50% more to about 90% more, about 50% more to about 80% more, or about 50% more to about 70% more than the free fall speed. The take-up speed can be about 50% more, about 60% more, about 70% more, about 80% more, about 90% more or about 100% more than the free fall speed. Preferentially, the take-up speed is about 50% more or about 100% more than the free fall speed. In an embodiment, the free fall speed is about 6 m/min to about 9 m/min. In another embodiment, the free fall speed is about 6 m/min, about 7.4 m/min or about 8.8 m/min. Accordingly, the take-up speed is about 8 m/min, about 9 m/min, about 10 m/min, about 11 m/min, about 12 m/min, about 13 m/min, about 14 m/min, about 15 m/min, about 16 m/min, about 17 m/min, or about 18 m/min. In an embodiment, the pure water permeance (PWP) decreases as the take-up speed (stretching) increases. In another embodiment, the MWCO decreases as the take-up speed (stretching) increases. In another embodiment, both the PWP and MWCO decreases as the take-up speed (stretching) increases. In another embodiment, the pore size distribution decreases as the take-up speed (stretching) increases (i.e. pore size distribution becomes narrower with a smaller median pore size as the take-up speed rises).

These are in line with the theory that a higher take-up speed (or stretching) results in better alignment of polymer chains. This is because when a higher take-up speed is applied during the spinning process, the polymer chains within the dope solution will undergo an elongation-induced alignment which results in a more monodisperse interstitial chain spacing between the polymer molecules. Consequently, the selective outer-layer of the hollow fiber membranes gives smaller pore sizes, improved rejection and lower PWP as the take-up speed increases. As a result, smoother surfaces with less porous defects are observed for hollow fibers spun with a higher take-up speed than those obtained under free-fall conditions.

The PAN membrane can be crosslinked in a solvent which is able to dissolve/solubilize, or at least disperse the cross linker. In an embodiment, the PAN membrane is crosslinked in about 25 v/v% solution of hydrazine monohydrate in ethanol. In another embodiment, the crosslinking is performed with hydrazine monohydrate, of about 15 v/v% to about 40 v/v% in ethanol; about 20 v/v% to about 35 v/v%; or about 25 v/v% to about 30 v/v%. In another embodiment, the concentration of hydrazine monohydrate in ethanol is about 15 v/v%; about 20 v/v%; about 25 v/v%; about 30 v/v%; about 35 v/v%; or about 40 v/v%.

In an embodiment, the PAN membrane is crosslinked at a temperature of about 50°C to about 100°C. In another embodiment, the temperature is about 60°C to about 90°C, or about 65°C to about 80°C. In another embodiment, the temperature is about 50°C, about 60°C, about 65°C, about 70°C, about 75°C, about 80°C, about 90°C or about 100°C. In an embodiment, the PAN membrane is crosslinked for duration of about 6 h to about 56 h. In another embodiment, the duration is about 7 h to about 52 h, or about 8 h to about 48 h. In another embodiment, the duration is about 6 h, about 7 h, about 8 h, about 9 h, about 10 h, about 11 h, about 12 h, about 14 h, about 16 h, about 18 h, about 20 h, about 24 h, about 28 h, about 36 h, about 48 h, about 52 h, or about 56 h.

In an embodiment, the PAN membrane is formed as a hollow fiber. In this regard, the skilled person would understand that 'hollow fiber' refers to a tube like structure. Accordingly, the PAN membrane is a PAN hollow fiber membrane. The skilled person would understand that to form a hollow fiber, a spinneret can be used. In this regard, the method further comprises the step of providing a bore fluid substantially adjacent to the dope composition prior to step (b). In another embodiment, the method of forming a PAN hollow fiber membrane comprises the steps of:

a) providing a dope composition comprising a PAN polymer and a polymer additive; b) providing a bore fluid substantially adjacent to the dope composition;

c) extruding the dope composition with the bore fluid to form a first extruded polymer;

d) stretching the first extruded polymer in a direction to a range of about 140% to about 210% of its original dimension to form a second polymer; and

e) crosslinking the PAN polymer in the second polymer with an amine cross linker to form the poly aery lonitrile membrane;

wherein the polyacrylonitrile is about 13 wt% to about 19 wt% of the dope composition; wherein the polymer additive is about 1 wt% to about 6 wt% of the dope composition; and wherein less than about 8% of the cross sectional area of the PAN membrane comprises macro void.

The inventors have found that by using a bore fluid, phase inversion may also take place at the lumen side once the dope composition comes into contact with the bore fluid upon extrusion. It was observed that this directly influences the morphology seen at the inner surface of the hollow fibre. Further, by changing the composition of the bore fluid through varying the relative compositions solvent/non- solvent (e.g. water) allows the alteration of the morphology and properties of the hollow fibre by influencing the phase inversion process. In this regard, water can be a non-solvent in the bore fluid. In an embodiment, the bore fluid is selected from DMF, NMP, DMAc, DMSO, water or a combination thereof. In another embodiment, the bore fluid is a combination of DMSO and water. In another embodiment, the bore fluid has a flow rate of about 1 mL/min to about 3 mL/min, or about 1.5 mL/min to about 2.5 mL/min. In another embodiment, the bore fluid has a flow rate of about 1 mL/min, about 1.5 mL/min, about 2 mL/min, about 2.5 mL/min or about 3 mL/min. With respect to hollow fiber membrane, the dope composition is flowing at a flow rate and the bore fluid is flowing at a different flow rate. The inventors have found that the flow rates are to be calibrated based on the spinneret dimensions, so that hollow fiber membrane of suitable dimensions can be obtained and/or formed. For example, by using a larger annulus, a larger flow rate is needed. The inner surface of the fiber can be subjected to different conditions to the external surface of the fiber. The inner and outer surfaces of the hollow fiber membrane can be subjected to different conditions by exposing the outer surface, for example, to a lower temperature (for example, cold water coagulant bath) than the inner surface (for example, room temperature bore fluid). In another embodiment, the inner surface is exposed to a bore fluid made of a mixture of solvent and water (non- solvent) compared to the outer surface which is exposed to a non-solvent fluid (water). Advantageously, inward-pointed macrovoids were eliminated and a mostly macrovoid-free morphology was obtained.

In an embodiment, the dope composition has a relative flow rate to the bore fluid of about 1 mL/min to about 6 mL/min. In another embodiment, the relative flow rate is about 1.5 mL/min to about 5 mL/min, about 2 mL/min to about 4 mL/min, or about 2.5 mL/min to about 3.5 mL/min. In another embodiment, the relative flow rate is about 1 mL/min, about 1.5 mL/min, about 2 mL/min, about 2.5 mL/min, about 3 mL/min, about 3.5 mL/min, about 4 mL/min, about 4.5 mL/min, about 5 mL/min, about 5.5 mL/min, or about 6 mL/min. The OD:ID ratio can be increased by increasing the dope viscosity. Increasing the dope viscosity can in turn reduce the number of macrovoids. As mentioned above, this may be performed by increasing the PAN and/or polymer additive amount. For example, the PVP K30 amount (and hence wt%) can be increased. The OD:ID ratio can alternatively be increased by reducing the bore flow rate. The OD:ID ratio can also be varied by changing the dimensions of the spinneret. The skilled person would understand that for the OD:ID ratio to be used in any meaningful comparison, either the OD or ID must be fixed to allow for a comparison of differences in wall thickness between different hollow fibers. By using the method as disclosed herein, macrovoid can be eliminated, or at least minimized. In an embodiment, the cross sectional area of the PAN hollow fiber membrane is free of macrovoid. In another embodiment, less than about 8%, about 7.5%, about 7%, about 6.5%, about 6%, about 5.5%, about 5%, about 4.5%, about 4%, about 3.5%, about 3%, about 2.5%, about 2%, about 1.5%, about 1%, about 0.5% or about 0.1% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoid.

In an embodiment, the present invention provides a method of forming a polyacrylonitrile (PAN) hollow fiber membrane, comprising the steps of:

a) providing a dope composition comprising a PAN polymer and polyvinylpyrrolidone, the PAN polymer selected from PAN homopolymer, having a weight- average molecular weight of about 30,000 to about 250,000 g mol^-1, copolymer PAN-methyl acrylate or PAN-methyl methacrylate;

b) extruding the dope composition to form a first extruded polymer;

wherein the PAN polymer is about 15 wt% to about 17 wt% of the dope composition; wherein polyvinylpyrrolidone is about 2 wt% to about 4 wt% of the dope composition; wherein polyvinylpyrrolidone has a weight-average molecular weight of about 10,000 g mol^-1 to about 1,300,000 g mol^-1; and

wherein less than about 8% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoid. In an embodiment, the present invention provides a method of forming a polyacrylonitrile (PAN) hollow fiber membrane, comprising the steps of:

a) providing a dope composition comprising a PAN homopolymer having a weight- average molecular weight of about 200,000 g mol^-1 and polyvinylpyrrolidone having a weight- average molecular weight of about 40,000 g mol^-1;

b) extruding the dope composition to form a first extruded polymer;

wherein the PAN homopolymer is about 15 wt% to about 17 wt% of the dope composition; wherein polyvinylpyrrolidone is about 2 wt% to about 4 wt% of the dope composition; and wherein less than about 8% of the cross sectional area of the PAN membrane comprises macrovoid.

In an embodiment, and with specific reference to the fourth aspect, the step of stretching the first extruded polymer is stretching the first extruded polymer in a direction to a range of about 50% more or about 100% more than its original dimension in that direction to form a second polymer.

a) providing a dope composition comprising a PAN polymer and polyvinylpyrrolidone, the PAN polymer selected from PAN homopolymer, having a weight- average molecular weight of about 30,000 to about 250,000 g mol^-1, copolymer PAN-methyl acrylate or PAN-methyl methacrylate; b) extruding the dope composition to form a first extruded polymer;

c) taking-up the first extruded polymer on a take up drum at a speed in a range of about 50% more to about 100% more than its original free fall speed to form a second polymer; and

wherein less than about 8% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoid.

b) extruding the dope composition to form a first extruded polymer;

wherein the PAN homopolymer is about 15 wt% to about 17 wt% of the dope composition; wherein polyvinylpyrrolidone is about 2 wt% to about 4 wt% of the dope composition; and wherein less than about 8% of the cross sectional area of the PAN membrane comprises macrovoid. In an embodiment, the present invention provides a method of forming a polyacrylonitrile (PAN) hollow fiber membrane, comprising the steps of: a) providing a dope composition comprising a PAN homopolymer having a weight- average molecular weight of about 200,000 g mol^-1 and polyvinylpyrrolidone having a weight- average molecular weight of about 40,000 g mol^-1;

b) providing a bore fluid substantially adjacent to the dope composition;

d) taking-up the first extruded polymer on a take up drum at a speed of about 50% more to about 100% more than its original free fall speed to form a second polymer; and e) crosslinking the PAN polymer in the second polymer with hydrazine monohydrate to form the PAN hollow fiber membrane;

wherein the PAN homopolymer is about 15 wt% to about 17 wt% of the dope composition; wherein polyvinylpyrrolidone is about 2 wt% to about 4 wt% of the dope composition; wherein polyvinylpyrrolidone has a weight-average molecular weight of about 10,000 g mol^-1 to about 1,300,000 g mol^-1; and

wherein less than about 8% of the cross sectional area of the PAN membrane comprises macro void.

In an embodiment, and with specific reference to the fourth aspect, the step of stretching the first extruded polymer comprises taking-up the first extruded polymer on a take up drum at a speed in a range of about 50% more or about 100% more than its original free fall speed to form a second polymer.

In an embodiment, a PAN membrane or PAN hollow fiber membrane is formed by the method disclosed herein. In another embodiment, a PAN membrane or PAN hollow fiber membrane is formed by a method comprising the steps of:

a) providing a dope composition comprising a PAN polymer and polyvinylpyrrolidone, b) extruding the dope composition to form a first extruded polymer;

c) stretching the first extruded polymer in a direction to a range of about 40% more to about 110% more than its original dimension in that direction to form a second polymer; and d) crosslinking the PAN polymer in the second polymer with hydrazine monohydrate to form the PAN membrane or PAN hollow fiber membrane;

wherein the PAN polymer is about 15 wt% to about 17 wt% of the dope composition; wherein polyvinylpyrrolidone is about 2 wt% to about 4 wt% of the dope composition; and wherein less than about 8% of the cross sectional area of the PAN membrane or PAN hollow fiber membrane comprises macrovoid.

In another embodiment, a PAN membrane or PAN hollow fiber membrane is formed by a method comprising the steps of:

a) providing a dope composition comprising a PAN homopolymer having a weight- average molecular weight of about 200,000 g mol ¹ and polyvinylpyrrolidone having a weight- average molecular weight of about 40,000 g mol^-1;

b) extruding the dope composition to form a first extruded polymer;

c) taking-up the first extruded polymer on a take up drum at a speed of about 50% more or about 100% more than its original free fall speed to form a second polymer; and d) crosslinking the PAN polymer in the second polymer with hydrazine monohydrate to form the PAN membrane or PAN hollow fiber membrane;

Those skilled in the art will appreciate that the invention described herein in susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications which fall within the spirit and scope. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations of any two or more of said steps or features.

Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" and "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates.

Certain embodiments of the invention will now be described with reference to the following examples which are intended for the purpose of illustration only and are not intended to limit the scope of the generality hereinbefore described.

Examples

The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that that prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. Materials

Polyacrylonitrile (PAN, M_w = 200,000 g mol^-1) was from Dolan GmbH. More generally, the following PAN may be utilized: PAN homopolymer having a weight- average molecular weight M_w 30,000 to 250,000; copolymer PAN-methyl acrylate, PAN-methyl methacrylate. Dimethylsulfoxide (DMSO, >99.9%, Sigma-Aldrich) was used as the solvent for preparing the dope and bore fluid (other suitable solvents can be selected from the group of aprotic nonpolar solvents, such as DMSO, DMF, NMP, DMAc etc. or their mixtures) while polyvinylpyrrolidone K30 (PVP K30, Sigma-Aldrich) was employed as an additive in the dope solution to adjust the micro structure and pore size during the phase inversion (additives should preferably have a weight- average Mw between 10,000 and 1300,000). Hydrazine monohydrate (reagent grade, 98%, Sigma-Aldrich) (or more generally, NH₂-R-NH₂, where R = (CH₂)_n or aryl) was used for the cross-linking of the membranes. Ethanol was used as the inert medium to dilute hydrazine monohydrate and process thermal/chemical crosslinking (water, methanol, IPA, butanol can also be used as the inert medium). Polyethylene glycol and polyethylene oxide of various molecular weights (PEG, M_w = 400 g mol^-1, 1000 g mol^-1, 2000 g mol^-1, 4000 g mol^-1, 12,000 g mol^-1 and PEO, M_w = 100,000 g mol^-1, Sigma- Aldrich) were utilized to determine the molecular weight cut-off (MWCO), mean pore size and pore size distribution of the membranes in deionized (DI) water. Rose Bengal (M_w = 1017.64 g mol^-1), Brilliant Blue R (M_w = 825.97 g mol^-1) and Remazol Brilliant Blue R (M_w = 626.54 g mol^-1) were purchased from Sigma- Aldrich and used to determine the membrane rejections in the organic solvent ethanol (analytical reagent grade, Fisher Scientific). All chemicals were used as received unless otherwise stated.

Fabrication of PAN hollow fiber membranes

The PAN polymer was first dried in a vacuum oven overnight at 50°C to remove moisture prior to use. To prepare the dope solution, PAN and polyvinylpyrrolidone (PVP K30) were dissolved in DMSO and stirred overnight at 70°C until a clear solution was obtained. The dope solution was then allowed to stand still and degas for one day. Next, the solution was loaded into an ISCO syringe pump heated to 60°C with a heating jacket and further degassed overnight prior to spinning.

The hollow fibers were spun using a dry-jet wet-spinning technique where the dope solution was fed into the outer annulus of the spinneret while the bore fluid was fed into the inner annulus. The spinneret was wrapped in a heating jacket set at 55°C to maintain a more consistent temperature after the dope solution was extruded from the pump. Both extruded streams were allowed to pass through a 2.0 cm air gap before entering the coagulation bath of water with a temperature of 6-7°C. The hollow fibers were then collected on a take-up drum. After immersing the spun hollow fibers in water for 2 days to remove residual solvents, the membranes were post-treated in two different ways as follows. Firstly, to test the performance of the as-spun hollow fiber membranes, the unmodified fibers were immersed in a 50 wt% aqueous glycerol solution for 2 days and air dried under ambient conditions before being made into membrane modules. The humectant glycerol was meant to prevent the pores from collapsing during drying. Alternatively, some hollow fibers were freeze-dried for morphological characterizations.

To create a hollow fiber membrane with minimal macrovoids, the polymeric additive PVP K30 with an average molecular weight of 40,000 g mol^-1 was chosen. Simultaneously, the dope and bore fluid flow rates were adjusted, as illustrated in Table 1.

As can be seen from Figure 1(a), when the preliminary dope composition A was used for spinning, the hollow fiber membranes that were obtained under free-fall conditions gave a cross-section with a sponge-like morphology interspersed with numerous macrovoids. Both inward-pointed and outward-pointed macrovoids resulting from the intrusion of bore- fluid and external coagulant, respectively, co-existed. To increase the wall thickness and reduce the number of macrovoids, the dope viscosity was increased by increasing the PVP K30 amount while the bore fluid flow rate was reduced as shown by the conditions of dope B in Table 1. This successfully increased the wall thickness of the hollow fiber membranes and somewhat reduced the number of macrovoids as observed from Figure 1(b). To further reduce the number of macrovoids without subjecting the fiber to elongation yet, the viscosity was further increased in dope C by increasing the PAN concentration while the dope flow rate was slightly reduced to 4.0 mL/min to decrease the OD:ID ratio. As shown in Figure 1(c), with these changes, most inward-pointed macrovoids were eliminated and a mostly macrovoid-free morphology was obtained as desired. Thus, dope solution C and its respective flow rates were used for further modifications and studies.

FESEM images shown in Figure 2 illustrate hollow fiber membranes with different take-up speeds. It is important to note that dopes A and B with lower viscosities were also spun at higher take-up speeds to observe their morphologies. However, they were less capable of providing the mostly macrovoid-free morphology achieved by dope C. Hence, both dope composition and take-up speed are vital components in achieving the desired morphology. To estimate the performance of the hollow fiber membranes prior to making any modifications, the PWP, MWCO and pore size distribution of the membranes were determined in aqueous systems. As observed in Figure 3(a), both PWP and MWCO decline as the take-up speed increases. Concurrently, Figure 3(b) shows that the pore size distribution becomes noticeably narrower with a smaller median pore size as the take-up speed rises. Both of these observations are in line with the fact that a higher take-up speed results in better alignment of polymer chains. This is consistent with the observation from Figure 2 where smoother surfaces with less porous defects are observed for hollow fibers spun with a higher take-up speed than those obtained under free-fall conditions. More importantly, unmodified hollow fiber membranes with ultrafiltration properties were obtained.

Cross-linking of PAN hollow fiber membranes

The PAN hollow fibers were first placed in a solvent-exchange bath of ethanol to remove residual water. Subsequently, the fibers were immersed in a 25 v/v% solution of hydrazine monohydrate in ethanol and heated at 70°C for various lengths of time ranging from 8 to 48 h. The crosslinking is conducted not only on the surface of the membrane, but uniformly throughout the entire membrane matrix. The modified hollow fiber membranes were then washed and stored in DI water overnight to remove the excess cross-linker. Next, the cross-linked fibers were immersed in a 50 wt% aqueous glycerol solution for 2 days and air dried under ambient conditions before being made into membrane modules for performance testing. On the other hand, some cross-linked fibers were freeze-dried for morphological characterizations.

As with the unmodified membranes, the PWP, MWCO and pore size distributions were determined using aqueous systems. Firstly, comparing Figure 3a with Figure 4a, there is a noticeable decrease in PWP once the cross-linking reaction has been performed on hollow fibers spun from these three take-up speeds. In general, the use of a longer cross-linking period leads to a smaller PWP and this can be explained by the densification of the selective layer induced by the heating as well as pore shrinkage caused by cross-linkers pulling the polymer chains closer together. Hence, PAN hollow fiber membranes with ultrafiltration properties are first required such that cross-linking reactions involving heating may subsequently be performed without significantly reducing the flux. Secondly, as evident from Figure 4a, for each given cross-linking duration, there is a decrease in PWP between fibers spun from free-fall conditions and those from higher take-up speeds. This implies that the effect of elongation-induced polymer chain alignment, and consequently less porous selective layers, persists through the different cross-linking times used. However, when longer cross-linking times of 14 h and 18 h are utilized, differences in PWP between "50% stretch" and "100% stretch" hollow fibers become negligible. Hence, to decide on the optimal take-up speed and appropriate cross-linking period, the rejection tests and pore size distribution curves may also be considered. Firstly, from Figures 4(b) to (d), it is clear that hollow fibers obtained under free-fall conditions possess the broadest pore-size distribution regardless of the cross-linking period used. Since broad pore- size distributions are undesirable, hollow fibers obtained at higher take-up speeds are preferred. Secondly, as the period of cross-linking increases, the pore-size distributions become increasingly narrower. Eventually, only negligible differences exist between both "50% stretch" and "100% stretch" hollow fibers after 18 h of cross-linking. Thus, for the most consistent performance independent of the original state of the unmodified hollow fiber, 18 h of cross-linking is recommended. Lastly, since hollow fiber membranes obtained under elongation have less macrovoids, they may have less mechanical weak- spots in the membranes. As shown in Figure 5, the cross-linking reaction does not affect the macrovoids present within the hollow fiber membranes. "100% stretch" hollow fibers are chosen for further OSN tests. For clearer illustration, the morphologies of various parts of the "100% stretch" hollow fibers cross-linked for 18 h are shown in Figure 6. Characterizations

The morphologies of hollow fiber membranes were observed using a field emission scanning electron microscope (FESEM, JEOL JSM-6700F). The freeze-dried hollow fibers were immersed in liquid nitrogen and fractured before being coated with a layer of platinum using a JEOL JFC-1300 platinum coater. Surface chemical functionalities of freeze-dried samples were studied using X-ray photoelectron spectroscopy (XPS) on a Kratos AXIS UltraDLD spectrometer (Kratos Analytical Ltd.) equipped with a monochromatized Al Ka X-ray source (1486.71eV, 5mA, 15kV).

XPS Analyses - Control studies and fate of PVP K30

Since cross-linking is a vital aspect of determining the membrane performance in OSN, it is of interest and importance to understand the processes that take place during cross- linking. To gain insights into the possible changes or chemical reactions that might have occurred, XPS analyses were conducted on the hollow fiber samples. Firstly, a control sample was made by heating some unmodified hollow fiber membranes in pure ethanol at 70°C for 8 h. The procedure used was exactly the same as that described herein, except no hydrazine cross-linker was employed in this case. XPS analyses were then conducted on this sample and the results may then be compared accordingly. The results are summarized in Table 2.

The control sample provides a preliminary understanding of the fate of PVP K30 in the hollow fiber membrane. Given its relatively large molecular weight, it seemed unlikely for all PVP K30 to leach out of the membrane despite it being subjected to various processes prior to the XPS analysis. As shown in Table 2, this is indeed true as a non-zero atomic oxygen concentration was obtained for the control sample. Since the most likely source of oxygen is PVP K30 present in the matrix of the hollow fiber membrane, it may be said that PVP K30 persists in the hollow fibers even after heating in ethanol. Since little to no sulfur was detected via XPS, it may also be concluded that residual DMSO is not responsible for the presence of oxygen.

To further prove that little, if any, reactions occur from only simple heating in ethanol, deconvolution of the XPS peaks was conducted. As shown in Figure 7, under the C Is spectra for both the unmodified hollow fibers and those heated in ethanol, the peaks at the binding energies of 285.0, 286.0, 286.4, 286.9 and 288.3 eV may be assigned to the species C-C, C-C≡N, C-N, C≡N and N-C=0, respectively. A major difference between the C Is spectra of both samples is the lower intensities of the C-C, C-N and N-C=0 peaks relative to the C≡N peak in Figure 7(b) compared to Figure 7(a). Since these peaks, particularly the C-N and N-C=0 peaks, correspond to the chemical states found in PVP K30, it suggests that some PVP K30 may leach out during the heating process. This is supported by the N ls spectra in Figure 7 where the relative intensity of the N-C=0 peak to the C≡N peak is lower in the N Is spectrum of Figure 7(b) compared to Figure 7(a). However, the binding energies of the various peaks are generally similar and suggests no significant changes in chemical states under heating alone.

XPS Analysis - Reactions occurring during cross-linking

As seen from Table 2, a higher oxygen atomic concentration is observed for the hollow fibers that have been cross-linked for 8 h as compared to the control sample. Given the presence of water in the system derived from hydrazine monohydrate, alkaline hydrolysis of the nitrile groups to form carboxylic acids is believed to also occur in addition to nucleophilic addition of hydrazine. Furthermore, the oxygen concentration increases with the cross-linking duration, thus indicating that alkaline hydrolysis of the nitrile groups is indeed a plausible side reaction occurring across time. Further deconvolution of the high resolution XPS spectra of the hollow fiber membranes cross-linked for 8 h give additional clues to the possible reactions that may occur. As seen in Figure 8(a), the C Is spectrum of the cross-linked hollow fiber membrane appears different from that shown in Figure 7(b) by having greater intensities between the binding energies of 286 to 287 eV and a somewhat bimodal distribution. This may be attributed to the formation of the N-C=N moiety with a binding energy of approximately 286.3 eV and a fall in the C≡N peak intensity. Hence, this indicates that the cross-linking reaction between PAN and hydrazine monohydrate has taken place. The deconvolution of the O ls spectrum in Figure 8(b) also supports the possible alkaline hydrolysis of the nitrile groups in PAN to give carboxylic acids, as previously discussed. More importantly, a shift in the intensity of the overall peak towards lower binding energies is also seen in the N Is spectrum in Figure 8(c). This, too, may be attributed to the possible formation of the cross- linked moieties C-N-N-C and C=N-H at a binding energy of around 399.3 eV following a decrease in the intensity of the C≡N peak. From the high resolution XPS C Is spectra of the membranes cross-linked for even longer periods of 14 and 18 h in Figures 9(a) and 9(c), the overall peak intensity has grown to be even more focused in the region between 286 and 287 eV. This is indicative of further cross-linking of the nitrile groups to give rise to the N-C=N moiety. This observation is also substantiated by the N Is spectra where the relative intensity of the C≡N peak is seen to have fallen. Furthermore, Table 2 shows that the ratio of the atomic nitrogen to atomic carbon concentration ([N]/[C]) rises and eventually reaches a somewhat constant level with increasing cross-linking time. This may indicate the depletion of the cross-linker. Hence, more hydrazine monohydrate may be added if a more complete cross-linking reaction is desired. However, a complete reaction of all nitrile groups is not necessary for good solvent resistance and thus need not be the focus.

Additionally, a small peak corresponding to C=N-C at a binding energy of 398.3 eV can be seen in Figures 8(c), 9(b) and 9(d). In cross-linked PAN hollow fibers, a yellowish tint was also observed that may possibly be attributed to the formation of the conjugated cyclic structures.

Filtration Experiments

Performance parameters of hollow fiber membranes such as pure water permeance (PWP), MWCO and pore size distribution were determined using a laboratory- scale cross-flow setup. To determine PWP (L m- h- bar- ), DI water was pumped through the shell side of the hollow fibers at a pressure of 2.0 bar and calculated using the following equation:

2 where V (L) is the permeate volume collected in a given time period t (h), A (m ) is the effective membrane filtration area and ΔΡ (bar) is the applied transmembrane pressure. The pore size, pore size distribution and MWCO were determined according to procedures described elsewhere. Firstly, separate 200 ppm solutions of PEG and PEO in DI water were prepared as feeds. Each solution was pumped through the shell side of the hollow fibers at a pressure of 2.0 bar and a flow rate of 1.0 L/min. Concentrations of the feed and permeate solutions were then determined using a total organic carbon analyzer (TOC, ASI- 5000A, Shimadzu, Japan). The effective rejection R (%) for each solute was calculated using the equation:

where C_p is the solute concentration in the permeate and C/ is the solute concentration in the feed. When the solute rejection R is plotted against the solute diameter (d_s) on a log-normal probability graph, a linear relationship between R and d_s can be observed. Given that the solute molecular weight (M_w) is a function of d_s as follows:

R may thus be related to ds and Mw. The MWCO was obtained when R = 90% while the mean effective pore size (μ_ρ) was obtained by assuming it to be the same as d_s when R = 50%. Finally, the pore size distribution may be given by the probability density function:

where d_p is the effective pore diameter and σ_ρ is the geometric standard deviation. σ_ρ is defined as the ratio between the values of d_s at R = 84.13% and R = 50%.

The OSN performance of hollow fiber membranes were determined using a stainless steel cross-flow setup. Organic solvents were used as feeds and pumped through the shell side of the hollow fibers at a pressure of 2.8 bar and the pure solvent permeance was determined as per Eq. (1). Rejection performances of hollow fibers were obtained using various dyes dissolved in ethanol at a concentration of approximately 50 ppm. The pure solvent permeance was first determined before the rejection test was conducted. For the rejection tests, the dye solutions were pumped through the shell side of hollow fibers at a pressure of 1.0 bar and a flow rate of 140 mL/min. The rejection was determined using Eq. (2) where the dye concentration in a solution can be related to its absorbance via the Beer- Lambert Law. The absorbance spectra of the permeates and feeds were obtained using a UV-Vis spectrophotometer (Pharo 300, Merck). For the determination of permeance and rejection in either aqueous or organic systems, three consecutive permeates were obtained and measured to ensure that variations were minimal and the systems were at a sufficiently stable state. OSN performance

As a means of understanding the possible differences in performance between the hollow fiber membrane in aqueous and organic solvent systems, ethanol was chosen as a benchmark organic solvent. The results of the OSN experiments are summarized in Table 3. Firstly, the pure ethanol permeances were determined for the "100% stretch" hollow fibers that have been cross-linked for various time periods. As can be seen from Table 3, the pure ethanol permeances fell sharply when increasing the cross-linking time from 8 h to 14 h. On the other hand, there were little differences in ethanol permeance when increasing the cross-linking time to 18 h. This behavior is similar to that seen for the aqueous system tests discussed above.

More importantly, upon increasing the cross-linking time, the hollow fiber membranes are able to reject dyes of increasingly smaller molecular weights. In particular, the hollow fibers cross-linked for 18 h are able to give an excellent rejection of Remazol Brilliant Blue R with a molecular weight of 626.54 g mol^-1. Since ethanol has a significantly lower dielectric constant than water, the charges on Remazol Brilliant Blue R are well shielded in ethanol. This suggests that the rejection of Remazol Brilliant Blue R is mainly attributed to its steric effect rather than its charge. Although a very poor rejection for Methylene Blue was obtained using the same cross-linked hollow fibers, this may suggest a fairly sharp pore size distribution. Nonetheless, considering the balance between pure solvent permeance and rejection, a cross-linking period of 18 h remains fairly optimal. Notably, membranes cross-linked for 18 h also remained insoluble in aggressive solvents such as N- methylpyrrolidone (NMP) and dimethylformamide (DMF) during immersion tests lasting more than two months whereas unmodified membranes dissolved completely within minutes. In particular, unmodified membranes completely dissolve in NMP, DMF and DMSO within minutes. A performance comparison with some OSN membranes listed in Table 4 further highlights the performance achieved by PAN hollow fiber membranes. Without the use of complex or expensive polymers nor additional coatings, PAN hollow fiber membranes exhibited a good pure ethanol permeance and good rejection performance.

Given the pore size distribution shown in Figure 4(d) with a median pore diameter of approximately 1 nm for the "100% stretch" hollow fibers, the MWCO was determined to be approximately 1600 g mol^-1 using a series of PEG dissolved in DI water. Although this may seem to be in contrast with the ability of the membrane to reject Remazol Brilliant Blue R possessing a molecular weight of merely 626.54 g mol^-1, the contradiction may be resolved by several explanations as disclosed above. Nonetheless, the use of PEG in aqueous systems remains an important, simple and safer preliminary method to assess membrane performances and should not be lightly dismissed. Thus in conclusion, PAN membranes with minimal macrovoids and a sponge-like morphology was obtained by adjusting parameters such as the dope composition, take-up speed and bore fluid and dope flow rates. In particular, PAN hollow fiber membranes suitable for ultrafiltration can be obtained. For example, a sufficiently viscous dope, high take-up speed and balanced OD:ID ratio were needed to achieve the desired PAN hollow fiber membranes. PAN membrane was cross-linked using amine crosslinker (such as hydrazine monohydrate) at 70°C for improved solvent-resistance. In general, longer cross- linking times were associated with narrower pore size distributions and smaller pore sizes. Based on XPS analysis, it is believed that the cross-linking reaction occurred, and additionally the formation of conjugated cyclic structures that account for the yellowish tint observed on PAN membranes. Using dyes dissolved in ethanol as a model OSN system, PAN hollow fiber membranes were able to provide good rejection of Remazol Brilliant Blue R, indicating nanofiltration-level performance in alcohols. Combined with its simplicity of design, it is believed that PAN membranes can be applicable for use in OSN applications and for larger-scale operations.

Claims

THE CLAIMS DEFINING THE INVENTION

1. A polyacrylonitrile (PAN) membrane comprising:

wherein the PAN polymer is cross linked by an amine cross linker.

2. The PAN membrane according to claim 1, the PAN membrane is a PAN hollow fiber membrane.

3. The PAN membrane according to claim 1 or 2, wherein the PAN polymer is selected from a PAN homopolymer having a weight- average molecular weight of about 30,000 g mol^-1 to about 250,000 g mol^-1, copolymer PAN-methyl acrylate, PAN-methyl methacrylate and a combination thereof.

4. The PAN membrane according to any of claims 1 to 3, wherein the polymer additive has a weight-average molecular weight between about 10,000 g mol^-1 to about 1,300,000 g mol^-1.

5. The PAN membrane according to any of claim 1 to 4, wherein the polymer additive is polyvinylpyrrolidone.

6. The PAN membrane according to any of claim 1 to 5, wherein the amine cross linker is hydrazine monohydrate.

7. A polyacrylonitrile (PAN) hollow fiber membrane comprising:

wherein the PAN polymer is cross linked by hydrazine monohydrate.

8. The PAN hollow fiber membrane according to claim 7, wherein the PAN polymer is selected from PAN homopolymer, having a weight-average molecular weight of about 30,000 to about 250,000 g mol^-1, copolymer PAN-methyl acrylate or PAN-methyl methacrylate.

9. The PAN hollow fiber membrane according to claim 7 or 8, wherein the PAN polymer and polymer additive are subjected to a stretch in a direction of about 50% more to about 100% more than its original dimension in that direction so that less than about 8% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoid.

10. The PAN hollow fiber membrane according to claim 7 or 8, wherein the PAN polymer and polymer additive are subjected to a take-up speed of about 50% more to about 100% more than its free fall speed so that less than about 8% of the cross sectional area of the PAN hollow fiber membrane comprises macrovoid.

11. The PAN membrane or PAN hollow fiber membrane according to any of claim 1 to

10, the membrane having a pure water permeance of less than about 25 L m- h- bar- .

12. The PAN membrane or PAN hollow fiber membrane according to any of claim 1 to

11, the membrane having a pure ethanol permeance is less than about 8 L m- h- bar- .

13. The PAN membrane or PAN hollow fiber membrane according to any of claims 1 to 12, the membrane having a PEG molecular weight cut off of about 1,600 g mol^-1.

14. The PAN membrane or PAN hollow fiber membrane according to any of claims 1 to 13, the PAN membrane having a mean pore diameter of less than about 2 nm.

15. A method of forming a PAN membrane, comprising the steps of:

d) crosslinking the PAN polymer in the second polymer with an amine cross linker to form the PAN membrane;

wherein the PAN polymer is about 13 wt% to about 19 wt% of the dope composition; wherein the polymer additive is about 1 wt% to about 6 wt% of the dope composition; and wherein less than about 8% of the cross sectional area of the PAN membrane comprises macro void.

16. The method according to claim 15, the PAN membrane being formed as a hollow fiber, the method further comprises the step of providing a bore fluid substantially adjacent to the dope composition prior to step (b).

17. The method according to claim 15 or 16, wherein the dope composition further comprises a solvent.

18. The method according to any of claims 15 to 17, wherein the ratio of PAN polymer to polymer additive in the dope composition is about 2 to about 13.

19. A method of forming a polyacrylonitrile (PAN) hollow fiber membrane, comprising the steps of:

b) extruding the dope composition to form a first extruded polymer; c) stretching the first extruded polymer in a direction to a range of about 50% more to about 100% more than its original dimension in that direction to form a second polymer; and

20. The method according to claim 19, wherein the polyvinylpyrrolidone has a weight- average molecular weight of about 10,000 g mol ¹ to about 1,300,000 g mol ¹.

21. The method according to claim 19 or 20, wherein the PAN polymer is selected from PAN homopolymer, having a weight-average molecular weight of about 30,000 to about 250,000 g mol^-1, copolymer PAN-methyl acrylate or PAN-methyl methacrylate.

22. The method according to any of claims 19 to 21, wherein the step of stretching the first extruded polymer comprises taking-up the first extruded polymer on a take up drum at a speed in a range of about 50% more to about 100% more than its original free fall speed to form a second polymer.

23. A PAN membrane or PAN hollow fiber membrane formed by the method according to any of claims 15 to 22.