WO2006130117A1

WO2006130117A1 - Novel polymer blends and applications for their use in hollow polymer fibers

Info

Publication number: WO2006130117A1
Application number: PCT/SG2006/000139
Authority: WO
Inventors: Renbi Bai; Chunxiu Liu
Original assignee: National University Of Singapore
Priority date: 2005-06-03
Filing date: 2006-06-02
Publication date: 2006-12-07

Abstract

There is provided a blended polymer comprising cellulose ester and amino-substituted polysaccharide. There is also disclosed a novel method for making the hollow fibers. Fig. 2

Description

NOVEL POLYMER BLENDS AND APPLICATIONS FOR THEIR USE IN HOLLOW POLYMER FIBERS

Technical Field

The present invention relates to novel polymer blends and to hollow fibers fibers made from said polymer blends and to a method for fabricating the hollow fibers and to their associated uses.

Background

Chitosan is a deacetylated form of chitin, a naturally abundant biopolymer widely available from seafood processing waste. Chitosan may be considered as a cellulose-derived compound in which one of the hydroxyl groups in the cellulose ring is substituted by an amino group. Normally, the nitrogen atom accounts for up to about 7% (by weight) of the chitosan polymer.

Chitosan is non-toxic to humans, is biocompatible and has good haemostatic and antimicrobial ability.

Chitosan has been processed into a variety of forms or shapes, such as beads; gels, fibers, flat membranes, scaffolds. The preparation of these chitosan products normally involves steps of dissolving chitosan in dilute acid solutions and subsequently solidifying the polymer solutions into desired shapes with alkali solutions. Since chitosan is readily soluble in organic and inorganic acid solutions (normally at pH < 4) by forming polycations, the preparation of chitosan products is seldom difficult. However, due to poor mechanical strength of chitosan, it is difficult to process chitosan into hollow fibers and hence the fibers will not support themselves. The poor mechanical strength of chitosan hollow fibers has limited their useful applications.

In one method to solve the problems associated with mechanical strength of chitosan, chitosan was dissolved in acid solution and then coated onto various supports to make composite membranes or hollow fibers.. Examples can be found in the following patents and articles: U.S. Pat No. 5,259,950 to Shirό et al., Chinese Pat. No. CN1119553 to Gong et al., Japanese Pat. No. 2001038173 to Saito Tomonari et al. and article "Immobilization of polyphenol oxidase on chitosan-coated polysulphone capillary membranes for improved phenolic effluent bioremediation" (Edwards et al., Enzyme and Microbial Technology, 25, 769-773, 1999). The supports reported in these patents and articles include polyvinylidene fluoride (PVDF), polysulfone (PSU), ceramic and alumina. The former two materials are hydrophobic polymers while the later two materials are hydrophilic inorganic compounds. This method produces hollow fibers with heterogeneous structures with one thin chitosan layer supported on another porous supporting material. The thin chitosan layer, as an active layer, can improve the property of the hollow fibers and make the composite membranes effective in separation of polar compounds from non-polar substances. However, a drawback of the coating method includes incomplete coverage of the support materials, irreproducible surfaces of the composite hollow fibers, and possible detachment of the coated chitosan layer. Accordingly, these composite hollow fibers fail to perform well in fields where the adsorbing capability of chitosan is used for applications such as removal or recovery of metal ions and affinity separation of biomacromolecules. This is mainly due to the fact that the coated chitosan layer on the hollow fibers is very thin or the amount of chitosan coated on the surfaces has been very small.

Another known method to solve the problems associated with mechanical strength of chitosan, involves through chemical modification of the support hollow fibers with chitosan. Chinese Pat. No. CN1413761 to Shi et al. discloses a process for preparing asymmetric ultra filter affinity polysulfone-chitosan membrane, including such steps as hydrolyzing the chloromethylated polysulfone in alkaline solution at 50- 90⁰C to obtain hydroxymethyl polysulfone membrane, followed by direct or indirect reaction of the membrane with the acidic chitosan solution for coupling of chitosan on the surface of the membrane. One drawback of this method is the requirement for treatment of the support material under harsh physical and chemical conditions, which may damage to the support matrix and cause irreproducible and uncontrollable membrane structures. In addition, the weakly bound chitosan molecules on the surface are vulnerable to detachment or release over time.

The known approaches are also somewhat complicated structures because a support has to be prepared first, followed by a second process for the coating or grafting of chitosan.

There is a need to provide hollow fibers and processes for making the same that overcome, or at least ameliorate, one or more of the disadvantages described above. Summary

According to a first aspect, there is provided a blended polymer comprising cellulose ester and amino-substituted polysaccharide.

In one embodiment, there is provided a blended polymer comprising cellulose acetate and chitosan.

In one embodiment, there is provided a hollow fiber comprising the blended polymer of the first aspect.

According to a second aspect, there is provided a method of making a blended polymer comprising the steps of:

(a) providing a polymer blend solution of amino-substituted polysaccharide and cellulose ester; and

(b) introducing a non-solvent into the polymer blend solution to precipitate the blended polymer.

In one embodiment of the method, the amino-substituted polysaccharide comprises chitosan and the cellulose ester comprises cellulose acetate.

According to a third aspect, there is provided a hollow fiber comprising a cellulose acetate and chitosan polymers.

According to a fourth aspect, there is provided a process for making a hollow fiber comprising the steps of:

(a) providing a spinning solution comprising a polymer dope of cellulose acetate and chitosan polymer blends and a solvent; and;

(b) extruding the spinning solution through an annular slit;

(c) extruding, during step (b), a quench liquid from an orifice encircled by said annular slit, said quench liquid being substantially miscible with the solvent of said spinning solution but substantially immiscible with the polymer dope of said spinning solution to develop a phase separation between the polymers dope and the solvent of said spinning solution; and

(d) introducing the extruded spinning solution and extruded quench liquid into a quench bath comprising quench liquid to precipitate said polymers from said solution and thereby form said hollow fiber.

In one aspect, there is provided the use of blended polymer of the first aspect or a hollow fiber of the third aspect, for adsorbing metal ions. In one aspect, there is provided the use of blended polymer of the first aspect or a hollow fiber of the third aspect, for adsorbing proteins.

In one aspect, there is provided the use of blended polymer of the first aspect or a hollow fiber of the third aspect, for affinity separation of biomolecules.

In one aspect, there is provided the use of blended polymer of the first aspect or a hollow fiber of the third aspect, for adsorbing toxins from a mammals blood.

In one aspect, there is provided the use of blended polymer of the first aspect or a hollow fiber of the third aspect, for removing impurities from water.

In one aspect, there is provided the use of blended polymer of the first aspect or a hollow fiber of the third aspect, for desalinating saline water.

In one aspect, there is provided the a process for making a hollow fiber comprising the steps of:

(a) preparing a chitosan and cellulose acetate blend solution;

(b) filtering the blend solution to substantially remove insoluble or undissolved particles;

(c) degassing the blend solution to free any air bubbles entrapped therein;

(d) extruding the blend solution into a quenching bath (or coagulation solution) while a core liquid is delivered through the lumen of the fibers, by a wet phase inversion method;

(e) collecting by a PVC drum the blend hollow fibers drawn out of the liquid bath and rinsing them with clean water;

(f) cutting the hollow fiber from the drum and immersing them in 10% m/v acetate buffers to speed up the leaching of the solvent;

(g) treating the chitosan/cellulose acetate blend hollow fiber with a plasticizer which acts as a softening agent; and

(a) drying the hollow fibers. Definitions

The following words and terms used herein shall have the meaning indicated:

The term "amino-substituted polysaccharide" refers to a polysaccharide having one or more amino substituents groups.

The term 'chitosan polymer" as used herein includes chitosans, modified chitosans, crosslinked chitosans and chitosan salts. The chitosans may be isolated from natural sources, synthetic chitosan and commercially-available chitosan, such as from various suppliers such as Aldrich of Milwaukee, Wisconsin United States of America. Methods for the manufacture of pure chitosan are well known. Generally, chitin is milled into a powder and demineralized with an organic acid such as acetic acid. Proteins and lipids are then removed by treatment with a base, such as sodium hydroxide, followed by chitin deacetylation by treatment with concentrated base, such as 40 percent sodium hydroxide. The chitosan formed may be washed with water to obtain a desired pH. Chitosan is not a single, definite chemical entity but varies in composition depending on the conditions of manufacture. It may be equally defined as chitin sufficiently deacetylated to form soluble amine salts, and various grades of chitosan having different average molecular weight and different extents of deacetylation are encompassed within this term. Accordingly, the terms "chitosan" and "chitosan polymer" include various derivatives of chitosan having the necessary solubility in a reaction solution and having at least a portion of the amino functional groups available for reaction.

The term "cellulose ester" is to be interpreted broadly to refer to include derivatives of cellulose in which the free hydroxyl groups attached to the cellulose chain are replaced, wholly or in part, by substituents ester groups. The term includes polymers of cellulose ester, cellulose monoester, cellulose diester, cellulose triester, cellulose ether, cellulose ester ether; mono-, di- and tri-cellulose alkanylate; mono-, di- and tri alkenylate; and/or mono-, di- and tri-aroylate.

The term 'acetylation degree' refers to the amount of bonded acetic acid (in percentage by weight) that is bonded to the cellulose in a sample of polymer, and it is intended to mean the average acetylation degree.

Unless specified otherwise, the terms "comprising" and "comprise", and grammatical variants thereof, are intended to represent "open" or "inclusive" language such that they include recited elements but also permit inclusion of additional, unrecited elements. As used herein, the term "about", in the context of concentrations of components of the formulations, typically means +/- 5% of the stated value, more typically +/- 4% of the stated value, more typically +/- 3% of the stated value,. more typically, +/- 2% of the stated value, even more typically +/- 1 % of the stated value, and even more typically +/- 0.5% of the stated value.

Throughout this disclosure, certain embodiments may be disclosed in a range format. It should be understood that the description in range format is merely for convenience and brevity and should not be construed as an inflexible limitation on the scope of the disclosed ranges. Accordingly, the description of a range should be considered to have specifically disclosed all the possible sub-ranges as well as individual numerical values within that range. For example, description of a range such as from 1 to 6 should be considered to have specifieally^disclosed sub-ranges such as from 1 to 3, from 1 to 4, from 1 to 5, from 2 to 4, from 2 to 6, from 3 to 6 etc., as well as individual numbers within that range, for example, 1 , 2, 3, 4, 5, and 6. This applies regardless of the breadth of the range.

Disclosure of Optional Embodiments

Exemplary, non-limiting embodiments of a novel polymer blend, a hollow fiber comprising the polymer blend and a process for making the same will now be disclosed.

Polymer blend

One embodiment provides a polymer blend comprising a cellulose ester and chitosan polymer.

In one embodiment, the weight percentage of chitosan polymer in the polymer blend is selected from the group consisting of about 1% to about 99%, about 10% to about 99%, about 20% to 99%, about 30% to about 99%, about 40% to about 99%, about 50% to about 99%, about 60% to about 99%, about 70% to about 99%, about 80% to 99%, about 90% to about 99%, about 1% to about 90%, about 1% to about 80%, about 1% to about 70%, about 1% to about 60%, about 1% to about 50%, about 1% to about 40%, about 1% to about 30%, about 1% to about 20%, about 1% to about 10%. In one embodiment weight percentage of chitosan polymer in the polymer blend is about 5% to about 25% or about 10% to about 25%, and wherein the remainder is cellulose ester polymer and any incidental impurities. Chitosan may be prepared by deacetylation of chitin (poly-beta (1,4)-N-acetyl- D-glucosamϊne). Chitin occurs widely in nature, for example, the waste from shrimp, lobster, and crab typically contains about 10% to about 15 % by weight chitin.

Chitosan is the beta-(1-4) polysaccharide of D-glucosamine, and is structurally similar to cellulose, except that the C-2 hydroxy! group in cellulose is substituted with a primary amine group in chitosan. The large number of free amine groups makes chitosan. a polymeric weak base.

Chitosan is generally insoluble in water, in alkaline solutions at pH levels above about 6.5, or in organic solvents. However, it generally dissolves readily in dilute solutions of organic acids such as formic, acetic, tartaric, glycolic, lactic and citric acids, and also in dilute mineral acids, except, for example, sulfuric acid.

The amount of acid required to dissolve chitosan is approximately stoichiometric with the amino groups. Since the pKa for the amino groups present in chitosan material is between 6.0 and 7.0, they can be protonated in very dilute acids to render a cationic nature with a high charge density. Advantageously, the cationic nature of chitosan allows it to interact with negatively charged surfaces (ie like proteins) to function as an anionic absorbent gelling material

The chitosan materials for use herein may have an average degree of deacetylation (D.A.) of more than 75%, preferably from 80% to about 100%, even more preferably from 90% to 100% and most preferably from 95% to about 100%. The degree of deacetylation refers to the percentage of the amine groups that are deacetylated from chitin.

The average molecular weight of the chitosan polymer may be selected from the group consisting of a viscosity-averaged molecular weight of about 25,000 to about 2,000,000, about 25,000 to about 1,000,000, about 25,000 to about 500,000, about 25,000 to about 400,000, about 25,000 to about 350,000, about 30,000 to about 500,000, about 35,000 to about 500,000, about 40,000 to about 500,000, about 50,000 to about 500,000, about 60,000 to about 500,000, and about 75,000 to about 320,000 g/mole.

The cellulose ester may be selected from the group consisting of cellulose acetate, cellulose acylate, cellulose diacylate, cellulose triacylate, cellulose acetate, cellulose diacetate, cellulose triacetate, cellulose nitrate, cellulose propionate, cellulose butyrate and mixtures thereof. One suitable cellulose ester includes cellulose acetate polymers consisting of cellulose monomers wherein the hydroxyl radicals of the cellulose monomers have been acetylated to some degree. The cellulose acetate polymers may comprise monomers of cellulose acetate, cellulose diacetate and/or cellulose triacetate, wherein the monomers have an acetyl content of at least 40%. In one embodiment, the acetyl content of said cellulose acetate is selected from the group consisting of at least about 50%, at least about 60%, at least about 70%, at least about 80%, at least about 90%, at least about 95%, and at least about 99% or about 100%.

Exemplary cellulose acetates include cellulose acetate propionate cellulose acetate butyrate, cellulose triacylates, such as cellulose trivalerate, cellulose trilaurate, cellulose tripalrήitate, cellulose trioctanoate, and cellulose tripropionate; cellulose diesters, such as cellulose disuccinate, cellulose dipalmitate, cellulose dioctanoate, and cellulose dicaprylate; cellulose propionate morpholinbutyrate; cellulose acetate butyrate; cellulose acetate phthalate; mixed cellulose esters, such as cellulose acetate valerate, cellulose acetate succinate, cellulose propionate succinate, cellulose acetate octanoate, cellulose valerate palmitate, cellulose acetate heptonate, and the like. Other exemplary cellulose ester polymers include cellulose acetaldehyde, dimethyl cellulose acetate; cellulose acetate ethylcarbamate; cellulose acetate methylcarbamate; cellulose dimethylaminoacetate; a cellulose composition comprising cellulose acetate and hydroxypropylmethylcellulose; a composition comprising cellulose acetate and cellulose acetate butyrate; a cellulose composition comprising cellulose acetate butyrate and hydroxypropylmethylcellulose, and mixtures thereof. Cellulose esters useful herein are commercially available from Eastman Chemical Products, Inc., Kingsport, Tenn., U.S.A. or can be made by techniques known in the art, for example, as taught in Kirk-Othmer, Encyclopedia of Chemical Technology, 3rd Edition, Vol. 5, Wiley-lnterscience, New York (1979), pp. 120-126. Examples of cellulose esters which are commercially available from Eastman Chemical Products, Inc. include, but are not limited to, the following: CAB-553, CAB-551, CAB-381, CAP- 504, CAP-482, CA-320S, and CA-398.

The number-averaged molecular weight of the cellulose acetate polymer may be selected from the group consisting of about 10,000 to about 500,000, about 10,000 to about 300,000, about 10,000 to about 250,000, about 10,000 to about 200,000, about 15,000 to about 200,000, about 20,000 to about 200,000, about 25,000 to about 200,000, and about 30,000 to about 200,000. Formation of blended polymer

The blended polymer may be prepared by the steps of:

(a) providing a polymer blend solution of chitosan and cellulose ester; and

(b) introducing a non-_^solvent into the polymer blend solution to precipitate the blended polymer.

The polymer blend solution may comprise: about 2% to about 4% by weight chitosan, more preferably about 3% by weight chitosan; about 10% to about 30% by weight cellulose ester, such as cellulose acetate, more preferably about 12% to about 26% by weight cellulose acetate; and the remainder comprising an acidic organic solvent. The method may comprise the step of:

(c) introducing a cross-linking agent into the polymer blend solution.

Suitable crosslinking agents for use herein are organic compounds having at least two functional groups or functionalities capable of reacting with active groups located on the chitosan. Exemplary active groups include, but are not limited to, carboxylic acid (-COOH), aldehyde, chlorine, epoxy groups. Examples of such suitable crosslinking agents include, but are not limited to, dicarboxylic acids, polycarboxylic acids, glutaraldehyde (GA), epichlorohydrin (ECH), ethylene glycol diglycidyl ether (EGDE) and the like. One way to introduce a crosslinking agent with the polymer blend solution is to mix the crosslinking agent with chitosan during preparation of the solution. Another suitable crosslinking agent comprises a metal ion with more than two positive charges, such as Ca²⁺, Al³⁺, Fe³⁺, Ce³⁺, Ce⁴⁺, Ti⁴⁺, Zr⁴⁺, and Cr³⁺.

In embodiments where crosslinking agents are used, a suitable amount of crosslinking agent may be from about 0.001% to about 30% by weight based on the total dry weight of chitosan used to prepare the blended polymer, more specifically from about 0.02% to about 20% weight percent, more specifically from about 0.05% to about 10% by weight and from about 0.1% to about 5% by weight.

The blended polymer may be used to form polymer sheets, hollow polymer fibers, and beads. The hollow fibers

One embodiment provides a hollow fiber comprising a cellulose acetate and chitosan polymer blend. In one embodiment, the weight percentage of chitosaή polymer in the hollow fiber is selected from the group consisting of about 10% to about 25%, about 15% to about 25%, about 20% to 25%, about 10% to about 20%, about 15% to about 20%, about 10% to about 15%, and wherein the remainder is cellulose acetate polymer and any incidental impurities.

In one embodiment, the hollow fibers have an external diameter selected from the group consisting of about 0.25 mm to about 3 mm, about 0.25 mm to about 2 mm, about 0.3 mm to about 2 mm, about 0.5 mm to about 2 mm, about 0.5 mm to about 1.5 mm, about 0.5 mm to about 1 mm, about 1 mm to about 2 mm, about 1.5 mm to about 2 mm, about 1 mm to about 1.5mm. In one embodiment, the hollow fibers have an internal diameter selected from the group consisting of about 0.16 mm to about 2 mm, about 0.16 mm to about 1.3 mm, about 0.1 mm to about 1 mm, about 0.2 mm to about 1 mm, about 0.5 mm to about 1 mm, about 0.1 mm to about 0.8 mm, about 0.1 mm to about 0.5 mm. .

In one embodiment, the inner diameter of the hollow fibers is about 2/3 of the outer diameter.

Advantageously, the hollow fibers are characterized in that they are capable of exhibiting enhanced adsorption properties for metal ions or proteins relative to hollow fibers consisting of cellulose acetate only. The disclosed hollow fibers are also characterized in that they are capable of being easily modified for specific affinity or selectivity in separation.

The Process for making the hollow fibers

One embodiment discloses a process for making a hollow fiber comprising the steps of:

(b) extruding the spinning solution through an annular slit;

(c) extruding, during step (b), a quench liquid from an orifice encircled by said annular slit, said quench liquid being substantially miscible with the solvent of said spinning solution but substantially immiscible with the polymer of said spinning solution to develop a phase separation between the polymers and the solvent of said spinning solution; and

In one embodiment, the spinning solution comprises: about 2% to about 4% by weight chitosan, more preferably about 3% by weight chitosan; about 10% to about 30% by weight cellulose acetate, more preferably about 12% to about 26% by weight cellulose acetate; and the remainder comprising an organic solvent.

In one embodiment, the organic solvent is an acidic organic solvent.

In one embodiment, the organic solvent comprises at least one of a protic solvent and an aprotic solvent.

In one embodiment, the volume percentage of protic solvent in said organic solvent is selected from the group consisting of at least 60%, at least 70%, at least 80%, at least 90%, at least 98%, and about 100%. The protic solvent may comprise carboxylic acids. The carboxylic acid may be selected from the group consisting of formic (methanoic acid), ethanoic acid, propanoic acid and mixtures thereof.

In one embodiment, the organic solvent is an aprotic solvent. In one embodiment, the volume percentage of aprotic solvent in said organic solvent is selected from the group consisting of at least 60%, at least 70%, at least 80%, at least 90%, at least 98%, and about 100%. The aprotic solvent may be selected from the group consisting of water, ketones such as acetone, alkyl formamides such as dimethylformamide (DMF), Tetrahydrofuran (THF), alkyl sulfoxides such as Dimethyl sulfoxide (DMSO).

In one embodiment, the providing step comprises the step of:

(a1) stirring said cellulose acetate polymer, said chitosan polymer with an acidic solvent to form the spinning solution having a substantially homogenous composition.

The stirring step (a1 ) may be undertaken for at least one hour. In one embodiment, the providing step comprises the step of: (a2) filtering said spinning solution.

In one embodiment, the providing step comprises the step of:

(a3) degassing said spinning solution to substantially remove any gas entrained therein.

In one embodiment, the providing step may comprise, during step (a3), the step of:

(a4) applying a vacuum to the spinning solution to assist in the removal of said entrained gas.

In one embodiment, the distance between the orifice surrounded by the annular slit from which said quench liquid and said spinning solution are respectively extruded, and the surface of said quench liquid contained in said quench bath, is in the range of 0- 50cm.

In one embodiment, the composition of the quench liquid being extruded from said orifice is substantially the same or different composition as the quench liquid in said quench bath.

In one embodiment, the quench liquid comprises a solvent that is substantially immiscible with said polymers of said spinning solution. The quench liquid may be selected from the group consisting of an alkaline solution, an aqueous salt solution, water and acidic salt solutions containing di-valent or multi-valent ions or cations, such as SO₄ ^2", PO₄ ^3" ions and Ca²⁺ cations.

In one embodiment, the alkaline solution may be sodium hydroxide, potassium hydroxide, lithium hydroxide, ammonia solution, sodium acetate, and mixtures thereof.

In one embodiment, the aqueous salt solution may be an inorganic salt selected from the group consisting of phosphate, sulphate, calcium salts, and mixtures thereof. A surfactant may also be added to the salt solution.

In one embodiment, the aqueous salt solution may be an organic salt selected from the group consisting of organic carboxylic acid salts, organic sulfonic acid salts and mixtures thereof.

In one embodiment, the method may comprise the step of:

(e) removing said formed hollow fiber from said quench bath. In one embodiment, the method may comprise the step of:

(f) washing solvent from removed hollow fiber. In one embodiment, the method may comprise the step of:

(g) leaching the solvent from said hollow fiber.

In one embodiment, the method may comprise the step of:

(h) treating said hollow fiber with a softening agent to reduce the hardness of said hollow fiber.

The softening agent may be.a glycerol or an alkylene glycol.

In one embodiment, the method step (h) may comprise the step of:

(hi) treating said hollow fiber with an aqueous glycerol or an aqueous alkylene glycol solution for at least 48 hours, more preferably at least 60 hours.

In one embodiment, the method step may comprise the step of: (i) drying hollow fiber to substantially remove all solvent therefrom.

Brief Description Of Drawings

The accompanying drawings illustrate disclosed embodiments and serve to explain the principles of the disclosed embodiments. It is to be understood, however, that the drawings are designed for purposes of illustration only, and not as a definition of the limits of the invention.

Fig. 1 is a schematic diagram of a process for spinning hollow fibers.

FIG. 2 consists of scanning electron microscopic photographs of a chitosan/cellulose acetate blend hollow fiber produced by the method of Example 1 described further below: FIGS. 2a shows an overview of the hollow fiber at *100 magnification; FIG. 2b shows a cross-section of the hollow fiber at *30,000 magnification; FIG. 2c shows the outer surface of the hollow fiber at * 50,000 magnification; and FIG. 2d shows the inner surface of the hollow fiber at *50,000 magnification.

FIG. 3 consists of scanning electron microscopic photographs of a chitosan/cellulose acetate blend hollow fiber produced by the method of Example 3 described further below: FIG. 3a shows the outer surface of the hollow fiber at *10,000 magnification; and FIG. 3b shows the inner surface of the hollow fiber at ^χ10,000 magnification. FIG. 4 consists of scanning electron microscopic photographs of a chitosan/cellulose acetate blend hollow fiber produced by the method of Example 4 described further below: FIG. 4a shows the outer surface of the hollow fiber at xi 0,000 magnification; and FIG. 4b shows the inner surface of the hollow fiber at *10,000 magnification.

Fig. 5. is an SEM images showing the overall (a and c) and cross-sectional (b and d) structures of a CS/CA blend hollow fibers made in the disclosed Experiments.

Fig. 6 shows graphs showing the effect of CA concentration in the spinning dope in the disclosed experiments: Fig. 6(a) shows specific surface areas and porosities and Fig. 6(b) shows averaged pore size of the outer and inner surfaces (water was used as both the external and internal coagulant).

Fig. 7 & Fig. 8 show SEM images of the outer and inner surfaces of hollow fiber membranes fabricated at different cellulose acetate concentrations.

Fig. 9 shows SEM images of the outer surfaces of the hollow fibers prepared at various chitosan concentrations.

Fig. 10 shows the effect of CS concentrations on the surface pore sizes, porosities and specific surface areas of the hollow fibers.

Fig. 11 shows a picture of a polymer blend hollow fibers prepared with 3.0 wt% NaOH solution as the external coagulant.

Fig. 12 shows SEM showing the effects of different external coagulants on the outer surfaces of hollow fibers.

Fig. 13 are pictures showing the effect of internal coagulant (bore fluid) composition on the cross-sectional structures of the polymer blend hollow fibers, wherein NaOH concentration was (a) 2 wt% and (b) 3 wt%.

Fig. 14 are pictures showing the effects of internal coagulant (bore fluid) composition on the inner edges of the CS/CA blend hollow fibers.

Fig. 15 are SEM images showing the effect of internal coagulant and chitosan concentrations on the inner surfaces of the hollow fibers.

Fig. 16 shows the morphologies of four different polymer blend hollow fibers;

Fig. 17 shows the equilibrium copper ion adsorption of the four polymer blend hollow fibers of Fig. 16 versus (a) the initial copper ion concentration; and (b) the equilibrium copper ion concentration. Fig. 18 shows graphs of the adsorption kinetics of copper ions on the hollow fibers of Fig. 16 wherein: (a) are experimental results showing the change of copper ion concentration (C, mg/L) in the bulk solution with adsorption time (t, min); (b) the fitting of diffusion- or transport-controlled kinetic model; (c) the fitting of the pseudo first-order kinetic model; and (d) the fitting of the pseudo second-order kinetic model to the experimental results.

Fig. 19 shows copper ion removal at low concentrations (C₀ = 0.28-6.5 mg/L) with the hollow fibers, showing the equilibrium copper ion concentrations (C_e) versus the initial copper ion concentrations (C₀) in the bulk solution.

Fig. 20 shows N 1s XPS spectra of the fibers before (a) and after (b) copper ion adsorption.

Fig. 21 is a graph showing equilibrium adsorption of BSA with the CS/CA and CS/CA-Cu blend hollow fiber membranes.

Fig. 22 shows the correlation of experimental data in Fig. 21 with the Langmuir model (a) and the Freundlich model (b) for CS/CA-Cu hollow fibers.

Fig. 23 shows the effect of solution pH (a) and ionic strength (b) on BSA binding capacity on the hollow fibers. NaCI concentration in (a) was 12OmM and solution pH in (b) was 7.4..

Fig. 24 shows the BSA adsorption rates on CS/CA-Cu hollow fibers.

Fig. 25 shows fitting of BSA adsorption kinetics data in Fig. 24 with the pseudo- first (a) and the pseudo-second order (b) kinetic models.

Fig. 26 shows copper ion leakages from CS/CA-Cu hollow fibers during BSA adsorption, (a) Cu leakage with equilibrium BSA concentrations, and (b) modeling of Cu leakage with the Freundlich isotherm model.

Detailed Description Of Embodiments

Non-limiting embodiments of a hollow fiber comprising a chitosan and cellulose acetate blend polymer blend will be further described in greater detail, including reference to specific Examples. The following disclosure should not be construed as in any way limiting the scope of the invention.

Preparation of Polymer Blend Solution

The disclosed hollow fiber is comprised of a cellulose acetate and chitosan polymer blend. In the polymer blend, chitosan acts as a functional polymer and cellulose acetate acts as a support matrix.

To prepare the blend hollow fibers, chitosan is blended with cellulose acetate by dissolving the polymers in an appropriate solvent to obtain a homogeneous and viscous polymer blend solution. The polymer blend solution is then spun through a spinneret into a quench bath in the form of a "quench bath", which comprises a quench liquid which is capable of precipitating the polymer blend from the polymer blend solution, to thereby form solid hollow fibers comprising the cellulose acetate and chitosan polymer blend.

Commercially available chitosan and cellulose acetate (including cellulose di- and tri-acetate) is used. In the disclosed embodiment, the chitosan was obtained from Aldrich of Milwaukee, Wisconsin United States of America (Low molecule weight, Brookfield viscosity 20-200cps) and from Sigma-Aldrich of St. Louis, Missouri, United States of America(practical grade from crab shells, Brookfield viscosity > 200,000 cps) and the cellulose acetate obtained from Fluka and Riedel-de Haen of Switzerland with molecular weight of 37,000 and acetyl content of 40% .

The solvent employed to dissolve both the chitosan and the cellulose acetate was a protic solvent, particularly formic acid (98-100%) obtained from Merck of New Jersey, United States of America.

In the disclosed embodiments, the chitosan and cellulose acetate blend solution was prepared by dissolving an appropriate amount of chitosan and a sufficient amount of cellulose acetate in the formic acid to yield a blend solution containing up to about 4.0% by weight of the chitosan and 12-26% by weight of cellulose acetate in the total weight of the blend solution. It is important to note that the chitosan concentration in the polymer blend solution should not be arbitrarily selected because the chitosan forms polyelectrolyte (or polycations) in formic acid.

The final determination of the concentration of chitosan in the blend solution is dependent on the molecular weight of chitosan and the desired characteristics of the blend hollow fibers to be prepared. Normally, the larger the chitosan molecular weight, the more viscous of the blend solution and thus the less content of chitosan should be used in the polymer blend solution, and vise visa. A too high concentration of chitosan can result in the polymer blend solution becoming too viscous and hence unspinnable.

As a guide, if the molecular weight of the chitosan is 75,000 g/mole, then the concentration in the polymer solution should be up to 4 wt%.

With regard to the cellulose acetate, it is preferred that the cellulose acetate concentration in the blend solution be high enough to produce hollow fibers that have sufficient mechanical strength. It has been found by the inventors that the blend solution with cellulose acetate concentration in the range of 12.0-26.0 wt% can be used to fabricate hollow fibers having sufficient mechanical strength for practical use in such applications as membrane separation systems.

The relative concentration between the chitosan and cellulose acetate is also important in the preparation of the blend hollow fibers. It is preferred that the total concentration of the two polymers in the blend solution is such that the viscosity of the solution is acceptable for subsequent processing and spinning. As a guide, if the molecular weight of the chitosan is 75,000 g/mole, and the molecular weight of the cellulose acetate is 37,000 g/mole, then the concentration of the two polymers in solution should be within the range 14 wt% to 30 wt%.

In the disclosed embodiment, the chitosan/cellulose acetate polymer blend solution is obtained by mechanically stirring the polymers in formic acid. The vessel for the mixture and the mixer shaft and blades should be made of material that is acid resistant and is not reactive with the polymer blend solution, such as glassware, stainless steel or Teflon™ from E. I. du Pont de Nemours and Company in Delaware, United States of America.

During the stirring step, the vessel is air-tightly sealed to prevent the evaporation of the solvent. The dissolution of both polymers in formic acid is very fast, and hence it normally takes less than one hour to obtain a homogeneous and clear polymer blend solution. However, to ensure formation of a homogenous solution, the polymer blend solution may be stirred for more than one hour.

It should be appreciated that the two polymers can be dissolved simultaneously in the solvent, or separately dissolved in the solvent and then desired portions of the resulting solutions can be mixed together to form a blend solution of the two polymers.

Before the fabrication of the hollow fibers, the solution is pre-treated. Firstly, the polymer blend solution is placed in an air-tight stainless steel dope tank before being forced, under compressed nitrogen gas, through a 15 μm stainless steel filter. The purpose of the filtration step is to remove any insoluble or undissolved particles that may be present in the solution.

The filtered blend solution is then degassed by leaving it in the dope tank in order to free any air bubbles entrapped therein. The time needed for degassing depends on the dope viscosity but generally, the higher the viscosity, the longer time needed for degassing. For example, it takes about several hours and about 2 days respectively, for the blend solutions prepared with 12.0% by weight of cellulose acetate but 2.0% or 3.0% by weight of chitosan (Aldrich) to be completely degassed.

In order to fasten the degassing step, the dope can be degassed under vacuum conditions. These two steps of pretreatment of the blend solution are important to ensure the fabrication of defect-free and smooth hollow fibers.

Once the polymer blend solution has been filtered and degassed, it is ready to be extruded into hollow fibers.

Formation of hollow fibers from polymer blend solution

Any commercially available wet spinning apparatus can be used for spinning the hollow fibers of the disclosed embodiment. The spinning apparatus comprises a spinneret for extruding the polymer blend and a core quench liquid into a quench bath. It should be noted that the tubing connecting the major parts of the wet spinning device, (i.e. such as the dope tank, spinneret and quench bath), should be made from an acid resistant material such as stainless steel.

The spinneret comprises an annular ring and the outer and inner diameters of the ring are dependent on the size of the hollow fibers to be prepared. As an illustrative embodiment, the spinneret with an OD/ID of 1.3/0.5 mm is used in this disclosed embodiment. The hollow fibers are prepared by a wet phase inversion method by extruding the blend solution into a quenching bath comprising a quench liquid (or coagulation solution) while a core quench liquid is delivered through the lumen of the fibers.

The "air gap" (i.e. the distance from the outlet of spinneret to the surface of the quench liquid) can be varied from 0 to 50 cm, depending on the characteristics of the hollow fibers to be prepared.

The flow rate of the polymer blend solution can be controlled by compressed nitrogen gas and the flow rate of the core quench liquid can be controlled by a syringe pump for feeding to the spinneret.

The quench liquid in the quench bath and the core quench liquid are also called as the "external coagulant" and "internal coagulant", respectively. The external coagulant and the internal coagulant are composed of "nonsolvent" which is miscible with the solvent of the polymer blend solution but which the polymer dope will precipitate to form a solid.

Exemplary nonsolvents include, but are not limited to, NaOH solution, NaAc solution, ammonia solution, tripolyphoaphate solution, alginate solution, water and mixtures thereof.

Nonsolvents may be categorized as alkali and water. The choice of external coagulant and internal coagulant depends on the amount of chitosan to be used and the structure of the hollow fibers to be prepared. Under alkali coagulation conditions for example, the precipitation rate of the polymer dope to form the hollow fibers is much faster than that in water, thereby producing hollow fibers with relatively denser outer surfaces in alkali conditions relative to water conditions Generally, the higher the alkalinity of the nonsolvent solution, the faster the hollow fibers solidify, thereby producing a denser surface.

Under basic coagulation conditions (relatively high pH), the amount of chitosan on the produced hollow fiber is higher than that with water as the quench liquid. When water is used as the coagulant, the hollow fibers undergo a slower precipitation rate, producing hollow fibers with highly porous structures. The possible reasons for the formation of highly porous hollow fibers when water is used as the coagulant may be attributed to (1) the slow phase separation of dope solution in water and (2) the partial dissolution of chitosan in the quench bath because it is thought that the out-diffusion of formic acid from the nascent hollow fibers results in the water having a pH lower than 4. The loss of chitosan from the blend hollow fibers was verified by the adsorption experiments described below, wherein the hollow fibers which coagulated in water showed a lower adsorption capacity toward copper ions than those coagulated in alkali solutions.

Post-treatment of Hollow Fibers

After formation, the hollow fibers are drawn out of the quench bath and collected on a PVC drum while being rinsed with water.

The collection rate of the hollow fibers is accurately controlled by a motor to avoid a dragging force being imposed on the hollow fibers.

The hollow fibers are cut from the drum and then immersed in 10% m/v acetate buffers, which assists in speeding-up leaching of the solvent.

The hollow fibers may, before drying, be treated with a plasticizer softening agent. Such plasticizers include, but are not limited to, glycerol, ethylene glycol, propylene glycol, diethylene glycol, Methylene glycol and trimethylene glycol. Glycerol is a particularly suitable plasticizer. The treatment of hollow fibers with plasticizer is carried out by immersing the hollow fibers in a 10% by weight of glycerol aqueous solution for three days.

To obtain dry blend hollow fibers, the plasticized hollow fibers can be dried directly in air while under tension.

In order to obtain a small amount of dry sample for analysis, a multi-step solvent exchange method which is rapid can be used. [Refer to:- (1): A. Z. Gollan, U.S.Patent 4,681,605 (1987); (2): K. Vasarhelyi, J. A. Ronner, M. H. V. Mulder, C. A. Smolders, Desalination 61 (1987) 211-235.] Such methods involve immersion of the hollow fibers in 1-propanol or ethanol for 2 hours and subsequently in 1-heptane or 1- hexaηe for another two hours. The replacement of water in the hollow fibers with organic solvents prevents the pores of the hollow fibers from collapsing.

Referring to Fig. 1, there is shown wet spinning system for spinning hollow fibers, which was used in this experiment. The system comprises Gas cylinder (1), pressure meter(2), dope tank (3), fιlter(4), syringe pump(5), spinneret(6), coagulation bath (7) and rinsing tank(8). Details of operation of the system of Fig. 1 are disclosed in CX. Liu, R.B. Bat, Preparation of chitosan/cellulose acetate blend hollow fibers for adsorptive performance, J. Membr. ScL 267 (2005) 68-77, which is incorporated in its entirety herein by way of reference.

The present invention is not limited to blend hollow fibers, but can be used in other forms including, but not limited to, blend flat membranes, fibers and beads.

Examples

To obtain a better understanding of the present invention, some examples are described below. In Example 1, a relatively large molecular weight chitosan supplied by Sigma was used while in Examples 2 to 4, a low molecular weight chitosan supplied by Aldrich was used.

Example 1

A polymer blend solution was prepared from, by weight:

1.0% chitosan (Sigma), 319,000 g/mole, practical grade (The N-deacetylation degree (DDA) of chitosan was measured with the titration method and was found to be

26.0% cellulose acetate with an acetyl content of 40% was purchased from Fluka and

73.0% formic acid (98-100% solution) as the solvent. The reagents were mixed in a dope tank for 12 hours.

The reagents were filtered through a 15 μm filter before being subjected to degasification by leaving the reagent solution in the dope tank for about 3 days.

The wet spinning system of Fig. 1 was used to undertake the spinning step. The spinneret had an OD/ID of 1.3/0.5 mm.

In the spinning step, NaOH solution (3% m/v) was used as both the external coagulant and internal coagulant.

The nitrogen gas pressure for extruding the dope was adjusted to 483 Kpa (70.0 psi) and the core quench liquid flow rate was set as 0.133 ml/min. The collection rate of the hollow fibers was 36.4 cm/min. The produced hollow fibers showed high mechanical strength. The tensile stress, elongation ratio and Young's modulus at breakage of the wet blend hollow fibers were 22MPa, 24% and 90MPa respectively.

The tensile stress value of the resultant hollow fibers appeared to be slightly greater than that of known hollow fibers such as cellulose acetate fibers while the values of elongation ratio and Young's modulus were generally comparable with those of other known hollow fibers.

The water contact angle of the hollow fibers made in this example was 47.5°. By varying the relative concentration between the chitosan and cellulose acetate in the hollow fibers, the hydrophilicity, indicated by the water contact angle, can be effectively varied in the range from 36.5° to 63.0°.

The morphologies and structures of the resultant hollow fibers are shown in FIG. 2. The hollow fibers show a spongy-like and porous cross-section (FIG. 2a and FIG. 2b), a porous outer surface (FIG. 2c) and a porous inner surface (FIG. 2d). The outer and inner diameter of the blend hollow fibers was 1.31 mm and 0.76 mm, respectively.

The adsorption capacities of the blend hollow fibers for copper ions and bovine serum albumin (BSA) from batch experiments were 4.1 mg and 13.8 mg per gram of dry hollow fibers respectively. In contrast, the hollow fibers prepared from the dope containing 27.0% of cellulose acetate and 73.0% of formic acid (i.e. no chitosan) showed negligible adsorption capacity towards these two substances.

The adsorption capacity was calculated from the concentration difference between initial and adsorption equilibrium concentrations in the solution. The reaction time was long enough for the adsorption to reach equilibrium and was 2 hours for copper ion adsorption, and 10 hours for BSA adsorption. The solution pH for adsorption was 6 and 6.3 for copper ions and BSA respectively. Accordingly, the addition of chitosan in the hollow fibers significantly improved the reactivity and adsorptive properties of the hollow fibers.

Example 2

A polymer blend solution was prepared from, by. weight:

3.0% of chitosan (Aldrich, low molecular weight). The deacetylation degree and the molecular weight of chitosan were 73.5% and 75,000 g/nrtol respectively. 12.0% cellulose acetate of the type used in Example 1 and 85.0% formic acid (98-100% solution) as the solvent.

After being filtered through a 15 μm filter, the blend solution was degassed by leaving it in the dope tank for about 2 days.

The same spinning equipment was used as for Example 1. In the spinning process, NaAc solution (10% m/v) was used as both the external and internal coagulants. The nitrogen gas pressure for extruding the dope was adjusted to 483 Kpa (70.0 psi) and the core quench liquid flow rate was set at 0.6 ml/min. The collection rate of the hollow fibers was 185 cm/min.

The hollow fibers prepared in this example contained a higher weight percentage of chitosan and they showed excellent chelating capability towards copper ion adsorption. The adsorption capacity for copper ions in the batch adsorption experiment was found to be 58.4 mg per gram of dry hollow fibers (The adsorption capacity protocol was as follows: 110 mg dry fibers were placed in 5 ml of 1.5 g/L copper ion solution at pH 6 for 2 hours, and the final concentration was found to be 0.2152 g/L). Therefore, the hollow fibers made in this example are particularly suitable for the recovery or removal of heavy metal ions from water, wastewater and industrial effluents.

The blend hollow fibers made in this example have similar surface morphology and membrane structure to that of the hollow fibers made in Example 1.

Example 3

A blend solution was prepared from, by weight: 3.0% of chitosan as used in Example 2; 12.0% cellulose acetate as used in Example 1; and 85.0% formic acid (98-100% solution) as the solvent.

Water was utilized as both the external and internal coagulants and all other spinning conditions were the same as those in Example 2.

The surface morphologies of the resultant blend hollow fibers are shown in FIG. 3. The hollow fibers show a porous outer surface (FIG. 3a) with a larger pore size and a highly porous inner surface (FIG. 3b). The mean pore size of the outer surface was 0.22 μm. The adsorption capacity of the hollow fibers for copper ions in the batch adsorption experiment was 6.5 mg per gram of dry hollow fibers (The adsorption capacity protocol was as follows: 110 mg dry fibers were placed in 5 ml of 1.5 g/L copper ion solutions at pH 6 for 2 hours, and the final concentration was found to be 1.357 g/L), much less than that in Example 2, attributed to the loss of chitosan in the spinning process due to the weaker coagulant used.

The resultant hollow fibers made in this example are suitable as support matrix for coupling of ligaηds such as dyes for affinity separation of biomolecules because of the large surface pore size of the blend hollow fibers.

Example 4

A blend solution was prepared from, by weight: 2.0% of chitosan as used in Example 2; 12.0% cellulose acetate as used in Example 1; and 86.0% formic acid (~100% solution) as the solvent.

After being filtered through a 15 μm filter, the blend solution was degassed by leaving it in the dope tank for about 2 hours. In the spinning process, water was used as both the external and internal coagulants. The nitrogen gas pressure for extruding the dope was adjusted to 30.3 psi and the core fluid flow rate was set as 0.2 ml/min. The collection rate of the fibers was 163 cm/min.

The surface morphologies of the resultant blend hollow fibers are shown in FIG. 4. The hollow fibers show a highly porous outer surface with uniform surface pores (FIG. 4a) and a highly porous inner surface (FIG. 4b). The mean pore size of the outer surface is 0.54 μm.

The adsorption capacity of the blend hollow fibers for copper ions in the batch experiment was 4.8 mg per gram of dry hollow fibers (The adsorption capacity protocol was as follows: 110 mg dry fibers were placed in 5 ml of 1.5 g/L copper ion solutions at pH 6 for 2 hours, and the final concentration was found to be 1.3944 g/L)

The resultant hollow fibers made in this example are also suitable as support matrix for coupling of ligands such as dyes for affinity separation of biomolecules. Example 5

Chitosan (CS) was purchased from Aldrich (labeled as low molecular weight) and used as received. The degree of deacetylation and the molecular weight of the CS were determined to be 73.5% and 75,000 g/mol, respectively. The reason in choosing CS with low molecule weight was to allow a greater amount of chitosan could be added into the CS/CA (Cellulose Acetate) blend dope. CA was supplied by Fluka and the acetyl content and molecular weight of the CA was 40% and 37,000 g/mol, respectively. Formic acid ((FA)₁ 98-100%) from Fluka was used as the co-solvent for both CS and CA.

The blend hollow fiber membranes were fabricated using the wet spinning system of Fig. 1 and according to the protocol described in CX. Liu, R.B. Bai, Preparation of chitosan/cellulose acetate blend hollow fibers for adsorptive performance, J. Membr. Sci. 267 (2005) 68-77.

The blend spinning dope solution was prepared by mechanically stirring cellulose acetate (CA) and CS together in the co-solvent at 3.33 Hz(200 rpm) overnight. The resultant blend spinning dope solution was then degassed and finally filtered through a 15 μm stainless filter to remove any insoluble particles under the force of high pressure N2 gas. The clear and homogeneous blend dope solution was then forced through a stainless steel spinneret comprising an annular ring (with o.d. and i.d. of 1.3 and 0.5 mm, respectively) and extruded into an external coagulation bath. A bore liquid coagulant was simultaneously delivered through the inner core of the spinneret by a high pressure syringe pump (ISCO 100DX). The hollow fiber membranes were collected by a drum from the external coagulation tank and were then rinsed with 10 wt% NaAc solution to leach out excess solvent, followed by the rinse with tap water. Finally, the hollow fibers were stored in Dl water for further use. To get dried samples for analyses, the hollow fibers were subjected to treatment of multi-step solvent exchange (refer to :- (1): A. Z. Gollan, U.S.Patent 4,681,605 (1987); (2): K. Vasarhelyi, J. A. Ronner, M. H. V. Mulder, C. A. Smojders, Desalination 61 (1987) 211-235.) with 1-propanol and 1 -heptane to retain the original porous structures.

A number of experimental runs were conducted with different dope compositions and external and internal coagulants and the conditions are summarized in Table 1 below. Table 1

Fabrication conditions of the CS/CA blend hollow fibers . Experimental series Dope composition Composition of Bore fluid by weight external coagulant composition

(CS/CA/FA)

Effect of CA 2.0/12.0/86.0 Tap water Dl water concentrations 2.0/14.0/84.0 2.0/16.0/82.0 2.0/18.0/80.0

Effect of CS 2.0/12.0/86.0 Tap water Dl water concentrations 3.0/12.0/85.0 4.0/12.0/84.0

Effect of external 2.0/12.0/86.0 3 wt% NaOH Dl water coagulants 10 wt% NaAc

3.6 X iO -⁴ Wt⁰ZoFA

Effect of internal 2.0/12.0/86.0 Tap water 2 wt% NaOH; coagulants 3 wt% NaOH 3.0/12.0/85.0 Tap water 2 wt% NaOH;

3 wt% NaOH

The total weight of the three components in each dope, i.e., CS, CA and FA, was set at 100 g while the CA concentrations in the dopes varied from 12 to 18 wt% and the CS concentrations varied from 2 to 4 wt%. Water was first examined as both the external and internal coagulants. Then, the composition of the external coagulant was adjusted by adding an adequate amount of NaOH (3 wt%, 6.2 kg) or sodium acetate (10 wt%, 22 kg) or a small amount of FA (3.6*10^"4 wt%, 60 mL) in the coagulation bath of 200 L water (200 kg) and that of the internal coagulant was adjusted by the addition of a different amount of NaOH in Dl water.

The cloud points of the dope solutions were measured by titration method. CA solution, CS solution and CS/CA blend solutions were prepared by dissolving them in formic acid, respectively, with mechanical stirring. Then, nonsolvent or coagulant (i.e., Dl water or NaOH solution in this case) was added slowly into each of the solutions through a syringe pump. The cloud point was observed visually from the sudden occurrence of the turbidity of the solutions (indicating the production of polymer solid particles due to phase separation/inversion).

The structures and morphologies of the blend hollow fiber membranes were investigated through SEM (JEOL JSM-5600 SEM) and FESEM (JEOL JSM-6700F FESEM) analysis. The dried hollowfibers were snapped in liquid nitrogen to give a generally clean break of the cross-section. As the polymers were non-conductive, the hollow fibers were coated with platinum powder on the surface for 40 s at 4*10^"3 Pa (40 mbar) vacuum.

The average surface pore sizes of the hollow fibers were measured with the software supplied by the manufacturers of the SEM/FESEM. The specific surface areas of the blend hollow fibers were measured with the BET method using a Quantachrome Nova 3000 Multi-point Gas Adsorption Analyzer at the liquid nitrogen temperature. Before the analysis, samples were fully degassed overnight with pure nitrogen gas. Specific surface areas were calculated from five-point adsorption data in the relative pressure range of p/p₀ = 0.05-0.30.

The porosities of the blend hollow fibers were measured by the dry-wet weighing method. The dried hollow fibers were equilibrated with Dl water for 24 h. The porosity was then determined by dividing the amount of water adsorbed (mL) with the amount of the wet hollow fibers (mL). The experiment was done for five samples and the average porosity was used for each type of the blend hollow fibers.

The mechanical property of the wet hollow fibers was evaluated through the measurement of the tensile strength and strain at break. Tests were conducted with lnstron 3345 Material Tester at a temperature of 25 ⁰G and a relative humidity of 60%. The initial gauge length was set to be 25mm and the draw speed was set at 10 mm/min. In each measurement, sample of each fiber was cut into 5 cm length, and attached onto the two clamps of the machine. For reliability, five readings were taken for each type of the sample, and the average value was used in this paper.

Results and discussion

5.1 Cloud point data

The cloud point data provide useful thermodynamic information about the phase separation/inversion process of the polymer solutions. Table 2 below shows the experimental results of the cloud points for a few types of spinning dope solutions containing CA/FA, CS/CA/FA and CS/FA, respectively.

TABLE 2 Cloud point data of different dope solution compositions (at 25°C)

CA (wt%) CS (wt%) FA (wt%) Nonsolvent concentration at cloud point (wt%)

Water as NaOH (3 wt%) nonsolvent solution as nonsolvent

12 0 88 39.3 34.5

12 2 86 32.7 30.2

12 3 85 29.6 27.3

0 2 98 NO^a 95.6 ^a Not observed.

The results of Table 2 indicate that both the ternary CS/CA/FA blend solutions and the binary CA/FA solution had high tolerance with the addition of the nonsolvent (or coagulant), i.e., water or NaOH solutions in this case, because the nonsolvent added to obtain the cloud point was as high as 40-27% by weight. This suggests that FA was indeed a solvent with high solubility for both CS and CA polymers. The cloud point for the CS/FA binary solution was however not observed with the addition of water up to 100% by weight, and was only observed when the addition of NaOH solution (3 wt%) reached about 95% by weight. Since CS can dissolve in solutions of pH < 4, the cloud point (or phase separation/inversion) of the CS/FA solution would only occur when the addition of water or NaOH solution raised the solution pH to a value of 4 or above. The results in Table 2 also suggest that, though both the tap water and the NaOH solution may be used as the coagulants for the fabrication of the CS/CA blend hollow fibers, higher pH coagulant (e.g., NaOH solution) would serve as a stronger coagulant since the amount of the NaOH solution to be added was less than that of tap water to obtain the cloud point. With the increase of the polymer concentrations (consequently a reduction of FA), the phase separation/inversion also appeared to be easier as the cloud point occurred at a less addition of the water or NaOH solution. The CS concentration seemed to have a significant impact on the cloud point since a slight increase of the CS concentration caused a large reduction in the need of adding water or NaOH solution to obtain the cloud point (e.g., the dope solution of 12% CA, 2% CS and 86% FA needed 39.3% water, and that of 12% CA, 3% CS and 85% FA needed only 29.6% water, see Table 2). This trend of change indicates that thermodynamically CS worked favorably in enhancing the demixing of the polymers with FA in the CS/CA/FA spinning dope solutions.

5.2 Effect of cellulose acetate concentrations

Spinning dope solutions with a constant CS (2 wt%) and different CA concentrations (i.e., 12, 14, 16 and 18 wt%) were used to spin the hollow fiber membranes and water was used as both the external and internal coagulants (nonsolvent). Some typical results showing the overall and the cross-sectional structures of the hollow fiber membranes are given in Fig. 5, which shows SEM images showing the overall (a and c) and cross-sectional (b and d) structures of a CS/CA blend hollow fibers prepared from spinning solutions containing 12 wt% CA and 2 wt% CS (a and b) and 18 wt% CA and 2 wt% CS (c and d) (water was used as both the external and internal coagulant).

Referring to Fig. 6, there is shown graphs showing the effect of CA concentration in the spinning dope (with 2 wt% CS) on the structural characteristics of the CS/CA blend hollow fibers: Fig. 6(a) specific surface areas and porosities and Fig. 6(b) averaged pore size of the outer and inner surfaces (water was used as both the external and internal coagulant).

It has been found that all the hollow fiber membranes had spongy-like and macrovoids-free structures, with the pores across the cross-section being highly interconnected and displaying open porous networks, which is the desirable structure for adsorptive membranes to achieve large specific surface areas and uniform fluid flow. As indicated in Fig. 6, the porosities and surface pore sizes of the hollow fiber membranes decreased but the specific surface areas of the hollow fiber membranes increased with the increase of the CA concentrations in the dope solutions. For example, for CA from 12 to 18 wt%, the porosity reduced from 80.6 to 70.4%, and the outer surface pore size from 0.54 to 0.09_m, but the specific surface area increased from 10.4 to 14.5m²/g. Since CA acted as the matrix polymer, it is easy to understand that more CA in the spinning dope solution resulted in the formation of denser matrix networks, hence lower porosity, smaller pore sizes but greater specific surface areas. However, there was a limitation in the polymer concentrations that can be used to prepare the spinning dope solutions as the high viscosity can eventually cause the dope solution to be non-spinnable.

To further illustrate the effect of the CA concentrations on the morphologies of the hollow fiber membranes, Figs. 7 and 8: Fig. 7 shows SEM images showing the outer surfaces of the CS/CA blend hollow fibers prepared at CS concentration of 2 wt% but CA concentration of 12 wt% (a), 14wt% (b), 16wt% (c) and 18wt% (d) (water was used as both the external and internal coagulant); and Fig. 8 shows SEM images showing the inner surfaces of the CS/CA blend hollow fibers prepared at CS concentration of 2 wt% but CA concentration of 12 wt% (a), 14wt% (b), 16 wt% (c) and 18 wt% (d) (water was used as both the external and internal coagulant).

Extremely open porous outer surfaces with a latex structure were obtained in the CA concentrations ranging from 12 to 16 wt% (see Fig. 7a-c) while a much less porous surface was obtained at the CA concentration of 18 wt% (see Fig. 7d). Although water was used as both the external and internal coagulants, the inner surfaces appeared to be even more porous with much larger pore sizes than the corresponding outer surfaces, with beak-like structure at the CA concentration of 12 wt% (see Fig. 8a) or latex structure at the CA concentrations of 14-18 wt% (see Fig. 8b-d). The larger inner surface pore sizes are also clearly shown in Fig. 6b. As seen in both Figs. 8 and 6b, the average pore sizes of the inner surfaces also decreased with the increase of the CA concentrations. Although the external and internal coagulation processes occurred simultaneously in the wet spinning system, the coagulation behavior at the lumen side can be different from that at the shell side. The internal coagulant or bore fluid (i.e., water here) was small in the amount and can soon become a mixture of water and the polymer solvent (i.e., FA), and, as a consequence, the coagulation rate of the polymers at the lumen side can be much slower due to the lowered pH than that at the shell side with a large quantity of external coagulant (i.e., water). Hence, more porous inner surfaces were formed and the delayed phase separation (and possibly the erosion of the un-solidified polymers) at the lumen side resulted in the formation of the larger pores.

5.3. Effect of chitosan concentrations

CS was added as a functional polymer to provide the CS/CA blend hollow fiber membranes with excellent adsorptive performance. Although it was desirable to fabricate the CS/CA blend hollow fiber membranes with greater amounts of CS, but the addition of CS significantly increased the viscosity of the spinning dope solution and the spinning process became increasingly more difficult. It was found that when CS concentration exceeded 4 wt%, a non-flow behavior of the spinning dope solution occurred. Therefore, the effect of CS concentration on the morphologies and structures of the blend hollow fiber membranes was examined at the CS concentrations from 2 to 4 wt% with the CA concentration being set constant at 12 wt% in this case. Again, water was used as both the external and internal coagulants in the study.

In general, all the hollow fiber membranes showed the spongy-like and macrovoids-free porous structures, similar to those discussed above in section 5.2. However, the most significant differences in this case are that a small increase in the CS concentration would largely decrease the pore sizes of the hollow fiber membranes, as shown by the typical SEM images in Fig. 9, which shows SEM images showing the outer surfaces of the hollow fibers prepared at CA concentration of 12 wt% but CS concentration of 2.0 wt% (a), 3.0 wt% (b) and 4.0 wt% (c). Water was used as both the external and internal coagulants.

The effect of CS concentrations (with 12 wt% CA) on the surface pore sizes, porosities and specific surface areas of the hollow fibers are shown in Fig. 10: (a) averaged pore size of outer surfaces and (b) specific surface areas and porosities (water was used as both the external and internal coagulant). The average outer surface pore sizes were found to decrease from 0.54 to 0.22 and to 0.063 _μm when CS concentration was increased from 2 to 3 and 4 wt%, respectively (see Fig. 10a). The surface pore size of the hollow fiber membrane at the CS concentration of 4 wt% was only about one-ninth of that of the hollow fiber membrane at the CS concentration of 2 wt%. This phenomenon may be attributed to the much higher viscosity of the dope solution at higher CS concentrations. As expected, the porosities of the hollow fibers were not significantly changed due to the addition of a small amount of CS in the blend (see Fig. 10b). The results support the advantages of entrapping CS in the CA matrix to increase the reactivity of the blend hollow fiber membranes for improved or enhanced adsorptive performance. Also, the addition of CS may be used as an effective way to change the pore sizes of the blend hollowfiber membranes when hollowfibers of different pore sizes are desirable for applications involving the separation of substances with different sizes. 5.4. Effect of coagulant composition

The hollow fiber membranes mentioned in sections 5.2 and 5.3 were fabricated by using water as both the external and internal coagulants. Generally, most polymeric hollow fiber membranes are prepared through the wet or dry-jet wet spinning process of a polymer dope solution, with water being frequently used as the coagulant, largely due to the fact that water is a good nonsolvent for many polymers, has high mutual affinity with many polymer solvents, and is inexpensive as a large quantity of the external coagulant is usually needed. For the CS/CA blend spinning dope solutions, the results in Sections 5.2 and 5.3 also clearly illustrate that water can be used as the coagulant to make highly porous and macrovoids-free hollow fiber membranes. The relatively uniform porous structures of the hollow fibers suggest that delayed demixing of the hollow fibers during phase separation/inversion took place and water was a relatively weak coagulant for the CS/CA/FA ternary spinning dope solutions.

The now well established wet phase inversion method for fabrication of hollow fiber membranes usually uses aprotic polymer solvents and water as the coagulant, resulting in the production of asymmetric membranes, typically with a thin dense top layer (or skin) supported on a porous layer with macrovoids. The skin is formed by the instantaneous demixing of the membranes at the surface with the strong polymer nonsolvent (which also has high mutual affinity with the polymer solvent). One of the major difference in this work from many other methods lies in that a protic polymer solvent, instead of aprotic polymer solvent, was used.

5.4.1. Effect of external coagulant compositions

Instead of using water as the external coagulant, NaOH solution (3 wt%), NaAc solution (10 wt%) and FA solution (3.6x10-4 wt%) were examined as the external coagulant to spin CS/CA blend hollow fibers with the spinning dope solutions containing 2 wt% CS and 12 wt% CA (see Table 1).

In the case of using the NaOH solution, the hollow fibers were observed to show a faster phase separation/inversion than using water in the coagulation bath, supporting that the NaOH solution is a relatively stronger coagulant than water, and agreeing with the results from the cloud point experiments described above. The resultant hollow fibers were observed to have some sparkly distributed "tear-drop" shaped macrovoids which appeared near the outer surface of the fiber wall, which as can be seen in Fig. 11 , which shows a picture of a CS/CA blend hollow fibers prepared with 3.0 wt% NaOH solution as the external coagulant (CS/CA in the dope solution was 2.0/12.0 (g/g), and water was used as the internal coagulant).

With the use of the NaAc and FA solutions, the resultant hollow fibers did not show any such macrovoids across the cross-sections of the hollow fibers (results not shown), indicating that the NaAc and FA solutions were relatively weaker coagulants than the NaOH solution for the CS/CA blend hollow fibers.

Fig. 12 shows the effect of external coagulant composition on the outer surface morphology: (a) 3 wt% NaOH solution, (b) 10 wt% NaAc solution and (c) FA solution (3.6*10^"4 wt%) with pH of 3.21 (CS/CA) in the dope was 2.0/12.0 (g/g), and water was used as the internal coagulant). The corresponding surface pore sizes, specific surface areas and porosities are also given in Table 3 below:

Table 3

Effect of external coagulant composition on the structural characteristics of the CS/CA blend hollow fibers (CS/CA in the dope was 2.0/12.0 (g/g), and water was used as the internal coagulant)

External coagulant Average pore size Specific surface Porosity (%) of outer area (m²/g) surface (μm)

3 wt% NaOH 0.049 11.7 79.0

10 wt% NaAc 0.087 11.2 79.2

3.6 x 10^"4 wt% FA 0.47 10.4 80.4

Water 0.54 10.4 80.6

It can be observed from the above data that is clear that the hollow fibers generally had smaller surface pore sizes (surface pores from 0.47 to 0.087 to 0.049μm) and consequently greater specific surface areas (from 10.4 to 11.2 to 11.7m²/g) when the external coagulant was changed from the FA to NaAc to NaOH solutions (i.e., the solution pH increased). This result may be attributed to the relatively more rapid coagulation rate of the hollow fibers in a more basic coagulation solution. With basic (or alkali) coagulants, the porosity of the membranes also appeared to be slightly reduced (see Table 3).

5.4.2. Effect of internal coagulant (bore fluid) compositions

Only NaOH solutions (i.e., strong nonsolvent) were examined. The spinning dope solutions were prepared with 12 wt% CA, plus 2 or 3 wt% CS. Water was used as the external coagulant, but NaOH solutions at 1-3 wt% were used as the internal coagulant, respectively.

It was found that for the hollow fibers prepared with 2 wt% CS, large macrovoids were formed near the lumen side in the hollow fibers and more and larger macrovoids appeared for the NaOH solution of a higher concentration (see Fig. 13 which shows the effect of internal coagulant (bore fluid) composition on the cross- sectional structures of the CS/CA blend hollow fibers, wherein NaOH concentration was (a) 2 wt% and (b) 3 wt% (CS/CA in the dope was 2.0/12.0 (g/g), and water was used as the external coagulant)).

In contrast, the hollow fibers prepared with 3 wt% CS did not show apparent macrovoids in the cross-sections (results not shown) for the NaOH solutions studied, possibly due to the high viscosity of the spinning dope solution. It is interesting to note that the shape of these macrovoids was different from the tear-drop shape shown in Fig. 11 and also different from the typical finger-like shape usually reported in literature (the macrovoids in this case were very wide). The special shape of the macrovoids may be attributed to the different phase separation behaviors of the CS/CA blend from known polymers utilizing a single polymer in the spinning fope solution.

It was observed that the dope solution at 3 wt% CS showed much higher viscosity than that at 2 wt% CS. At such a high viscosity, the influx of the FA from the surrounding polymer solutions into the nonsolvent droplet was very slow. Under this condition, due to the high concentration nonsolvent in contacting with the surrounding polymers the surrounding polymers solidified rapidly before a large droplet of solvent and nonsolvent mixture formed, leading to the formation of microporous structures rather than macrovoids structures.

Although hollow fibers prepared at 3 wt% CS did not show macrovoids in the cross-sections, a tendency of phase separation into polymer rich and polymer lean phases was also observed when NaOH solution was used as internal coagulant. The inner edges of the hollow fibers are shown in Fig. 14, which shows the effects of internal coagulant (bore fluid) composition on the inner edges of the CS/CA blend hollow fibers. NaOH concentration was (a) 2 wt% and (b) 3 wt% (CS/CA in the dope was 3.0/12.0 (g/g), and water was used as the external coagulant).

It can be seen from Fig. 14 that a more porous region existed below the relatively denser inner surfaces. With increasing the NaOH concentration in the bore fluid, the pore size and porosity of this subsurface region obviously increased. Moreover, the starting point of this region was much closer to the inner surfaces with increasing NaOH concentration in the bore fluid. It has been speculated that this might be caused by the different phase separation behaviors of CS and CA.

SEM images showing the inner surfaces of these hollow fibers are given in Fig. 15: Effect of internal coagulant (bore fluid) composition on the inner surfaces of the CS/CA blend hollow fibers, (a) CS/CA = 2.0/12.0 (g/g), 2 wt% NaOH; (b) CS/CA = 2.0/12.0 (g/g), 3 wt% NaOH; (c) CS/CA = 3.0/12.0 (g/g), 2 wt% NaOH; (d) CS/CA = 3.0/12.0 (g/g), 3 wt% NaOH. Water was used as the external coagulant.

It can be observed that all these membranes had open porous inner surfaces, but the pore sizes were much smaller than those prepared with water as the internal coagulant (bore fluid) shown in Fig. 8. Also, a slight reduction in the surface pore sizes was observed with an increase of the NaOH concentrations in the bore fluid and with the increase of CS concentrations in the spinning dope solutions. The corresponding specific surface areas and porosities of these hollow fiber membranes are given in Table 4 below.

Table 4

Effect of bore fluid composition and CS concentration on the structural characteristics of the CS/CA blend hollow fibers (water, was used as the external coagulant)

Dope composition Bore fluid Specific surface Porosity (%) by weight composition area (m²/g)

(CS/CA/FA)

2.0/12.0/86.0 Dl water 10.4 80.6

2 wt% NaOH 10.9 82.3

3 wt% NaOH 11.3 86.7

3.0/12.0/85.0 Dl water 12.2 79.7

2 wt% NaOH 13.0 79.0

3 wt% NaOH 13.4 79.2

At CS concentrations of 2 wt%, the porosity increased at a high NaOH solution, which may be attributed to the presence of more and larger macrovoids, though the changes in the specific surface areas were not significant in this case. At CS concentration of 3 wt%, however, the changes of both the specific surface areas and the porosities did not appear to be significant.

5.5. Tensile sfress and strain of the blend hollow fiber

Although for adsorptive membranes the mechanical strength is not as critical as the filtration membranes (because the process pressures do not have to be high), the tensile stress and strain of the porous hollow fiber membranes was examined and some of the typical results are given in Table 5 below.

Table 5 Typical results of tensile stress and stain of some of the CS/CA blend hollow fibers

Dope Composition of Bore fluid Tensile stress Strain (%) composition by external composition (MPa) weight coagulant

(CS/CA/FA)

2.0/12.0/86.0 Tap water Dl water 7.8 22.1

2.0/18.0/80.0 Tap water Dl water 18.2 27.6

3.0/12.0/85.0 Tap water DI water 7.6 23.7

3.0/12.0/85.0 Tap water 3 wt% NaOH 8.3 25.3

It has been found that the blend hollow fibers generally had sufficiently high tensile stress (7.8-8.3MPa) and break elongations (22.1-25.3%) even for the highly porous membranes prepared at low polymer concentrations (CS/CA = 2/12 or 3/12). With the increase of the CA concentration up to 18 wt%, the tensile stress was increased significantly from 7.8 to 18.2MPa while the strain also increased moderately. These changes can be attributed to the increased cohesions among the CA molecules which formed the membrane matrix. The mechanical strength of the hollow fiber membranes also appears to be strengthened when NaOH solution instead of water was used as the internal coagulant.

5.6 Conclusions

Highly porous CS/CA blend hollow fibers for adsorptive membranes were successfully fabricated through a wet spinning process with CA in the concentration range of 12-18 wt% and CS concentration at up to 4 wt% in the spinning dope solutions. Depending on the coagulant compositions, the outer surface pore sizes, the specific surface areas and the porosities of the blend hollow fibers can change from 0.54 to 0.049μm, 10.4 to 14.5m²/g and 80.6 to 70.4%, respectively, with the increase of CA or CS amount in the spinning dope solutions for the polymer concentrations studied. Water can be used as both the external and internal coagulants in the fabrication process and the resultant hollow fibers showed spongy-like, macrovoids- free and relatively uniform porous structures which are desirable for adsorptive membranes, attributed to water being a weaker coagulant for CS and CA. The composition of the coagulants, especially the internal coagulant, also greatly affected the blend hollow fibers' structures. By increasing the alkalinity of the coagulants, the coagulation rate of the blend hollow fibers was increased, resulting in the formation of relatively denser surface layers and smaller surfaces pore sizes. In particular, when NaOH solutions (1-3 wt%) were used as the internal coagulant, more and larger macrovoids were formed in the blend hollow fibers at the near lumen side, when the concentration of NaOH solution was increased (>1 wt%) and the CS concentration in the spinning dope solutions was low (<3 wt%). The results demonstrate that the CS/CA blend hollow fibers can be made into highly porous adsorptive membranes with large specific surface areas and various desirable pore sizes, by properly controlling the CS and CA concentrations in the spinning dope solutions and by choosing the compositions of the external and internal coagulants.

Example 6

Blended hollow fibre membranes of Chitosan (CS) and Cellulose Acetate (CA) were fabricated according to the protocol described in Example 5 above. In this example, four polymer blend types were studied in relation to their adsorption of copper ions. The particulars of the polymer blend hollow fibers is given in Table 6 below:

Table 6- Information on the CS/CA blend hollow fiber adsorptive membranes prepared in this work

Hollow fiber ID 3-12-w 3-12-OH 2-18-w 2-18-OH

Blend dope composition: 3/12/85 3/12/85 2/18/80 2/18/80 CS/CA/FA (g/g/g)

Weight ratio of CS/(CS+CA) 20% 20% 10% 10% in the spinning blend dope

External coagulant Tap water 3 wt% Tap water 3 wt%

NaOH NaOH

Bore fluid Dl water 3 wt% Dl water 3 wt%

(internal coagulants) NaOH NaOH

Specific surface area (m²/g) 12.2 14.1 14.5 15.2

Porosity (%) 79.7 79.1 70.4 70.4

Outer surface pore size (μm) 0.22 0.07 0.09 0.05

CS content in the dry hollow 3.7% 12% 8.5% 9.3% fibers

6.1. Adsorption of copper ions

The hollow fibers were cut into pieces of about 0.5 cm length, treated by solvent exchange with 1-propanol and 1 -heptane and then dried in the air before adsorption study. To examine the adsorption capacities of the hollow fibers, a 1.1 g amount of the dried hollow fiber piece from a particular type of the hollow fibers were added, respectively, into a number of flasks, each of which contained 50 mL of a copper ion solution with an initial concentration varying in the range of 10-150 mg/L The initial pH of the copper ion solutions in the flasks were adjusted to 5 with 0.01 M HCi and NaOH solutions. The mixture in the flasks were stirred in a water bath shaker at 150 rpm and at 25 ⁰C for 2 h, which was more than the adsorption equilibrium time. The final copper ion concentrations in the solutions were then analyzed.

The amounts of copper ions adsorbed on the hollow fibers were then calculated from the concentration difference before and after the adsorption. Adsorption kinetic studies were conducted for two types of the hollow fibers. Again, a 1.1 g amount of the dried hollow fiber pieces from a particular type was added into 50 mL of a copper ion solution in a flask, with an initial solution pH of 5 and an initial copper ion concentration of 50 mg/L. The mixture was stirred in a water bath shaker at 150 rpm and at 25⁰C and samples were taken from the solution at desired time intervals for the analysis of copper ion concentrations in the solution.

One type of the hollow fibers was also examined for their removal of copper ions at very low copper ion concentrations. In this case, the initial concentrations of copper ions in the solutions were changed in the range of 0.28-6.5 mg/L and other experimental conditions were the same as those in the adsorption capacity study. Copper ion concentrations in all the samples in this study were determined with an inductively coupled plasma mass spectrometer (ICP-MS, Perkin- Elmer Elan 6100).

6.2. Desorption of copper ions and reuse of the hollow fibers

Desorption of copper ions from the hollow fibers was examined in a batch mode with EDTA (Ethylenedinitrilo tetraacetic acid disodium salt) or HCI solution in the concentration range from 0.01 to 50 mM. The hollow fibers were first equilibrated with copper ions in a solution with an initial concentration of 150 mg/L at pH 5 (1.1 g hollow fibers were added into 50 mL copper solution). Then, the hollow fibers were separated by filtration and added into 150 mL of the desorption solution. The mixture was stirred in a water bath shaker at 150 rpm and at 25 ⁰C, and samples were taken from the solution to monitor the amount of copper ions desorbed into the solution. After the desorption test, the hollow fibers were separated and washed in a 50 mM NaOH solution followed with Dl water, and reused in the next cycle of adsorption experiment. The adsorption- desorption experiments were conducted for four cycles.

6.3. Other analyses

To examine the mechanism of copper ion adsorption on the hollow fiber membranes, X-ray photoelectron spectroscopies (XPS) of the CS/CA blend hollow fibers before and after copper ion adsorption were obtained with a VGESCALAB MKII spectrometer using an Al Ka X-ray source (1486.6 eV of photons). The elements of C, O and N on each sample were scanned and the XPSpeak 4.1 software was used to fit the XPS spectra peaks. To eliminate the effect of surface charging on the analysis results, all XPS spectra were referred to the C 1s peak of the aliphatic carbons at 284.6 eV. 6.4 Results and discussion

6.4.1. Characteristics of the CS/CA blend hollow fiber membranes

Fig. 16 shows the surface morphologies and cross-sectional structures of the hollow fiber membranes obtained from the SEM analysis of the four types of CS/CA blend hollow fiber membranes of Table 6 (Magnification for all the images are *10, 000 and the bar in each image represents 1 μm).

In general, the hollow fibers with water as the coagulant (i.e., 3-12-w and 2-18-w) had larger pore sizes than their corresponding ones with the NaOH solution as the coagulant (i.e., 3-12-OH and 2-18-OH). Consequently, the hollow fibers prepared with water as the coagulant showed slightly lower specific surface areas than the hollow fibers coagulated with the NaOH solution (see Table 6). The two different coagulants however did not seem to significantly affect the porosity of the hollow fiber membranes. With the increasing of the CA contents (or the decrease in the CS/CA ratio) in the spinning blend dope solutions, the surface pore sizes and porosities of the hollow fibers decreased but the specific surface areas increased (see Table 6). However, all the four types of hollow fibers possessed spongy-like and open porous structures across the cross-sections, which is desirable and beneficial for adsorptive membranes to have high surface areas and hence provide high binding capacities.

As can be found in Table 6, the specific surface areas of the hollow fibers reached as high as 12.2-15.2 m²/g and the hollow fibers were highly porous with porosities in the range of 70.4-79.7%. The pore sizes of the hollow fibers were in the range of 0.05- 0.22 μm which is sufficiently large enough to allow free passage of any heavy metal ions into the internal adsorptive sites of the membranes.

It has been found that the coagulant type and the CA content in the spinning blend dope solutions significantly affected the CS contents in the resultant hollow fibers. The CS contents or the weight ratios of CS/(CS+CA) oh the blend hollow fibers were always lower than their corresponding ones in the spinning blend dope solutions (see Table 6). This may be caused by the partial dissolution of the CS polymers during the spinning and coagulation process. When the CS/CA blend dope solution was spun into the coagulant, the formic acid (solvent) was extracted into the coagulant (water or NaOH solution), which lowered the pH value of the coagulant. This was especially the case in the lumen side of the hollow fibers where the amount of coagulant (or bore fluid) was very small. Hence, before the CS polymers were solidified, the CS polymers on the fiber surfaces can partially dissolve into the coagulant solution at the fiber/coagulant interfaces if the coagulant solution pH was brought down to lower than 4. From the results given in Table 6, it is clear that the dissolution of CS from the hollow fibers can be effectively controlled by using an alkali solution as the coagulant and/or by spinning the CS/CA blend dope solution at a higher CA content. This is due to the fact that the CS polymers can solidify more rapidly in alkali solutions and the presence of more CA molecules in the blend formed much denser matrix webs that can hinder the diffusion of the CS polymers into the coagulant during the spinning and coagulation process.

6.4.2 Copper adsorption capacities

Figure 17 shows graphs of the equilibrium copper ion adsorption amount (q_e, mg/g) on the four types of CS/CA blend hollow fibers versus (a) the initial copper ion concentration (C₀, mg/L) ranging from 10 to 150 mg/L and (b) the equilibrium copper ion concentration C_e in the bulk solution.

In general, the equilibrium adsorption amounts, q_e, increased with an increase of the initial copper ion concentrations (Figure 17a) or increased with the equilibrium copper ion concentrations in the bulk solution (Figure 17b). The hollow fibers 3-12-OH always had the highest adsorption amounts, and, in all cases, the amounts of copper ion uptake by the hollow fibers followed the order of 3-12-OH>2-18-OH«2-18-w>3-12-w. This order is closely related to the order of CS contents in the four different types of the hollow fibers (see Table 6).

The results suggest that CS was the main reactive polymer that provided adsorptive sites on the blend hollow fibers for copper ion adsorption. In terms of the amounts of CS in the hollow fibers, the amounts of copper ions adsorbed can be calculated to be 48.2 mgCu²⁺/g chitosan for 3-12-OH, 44.9 mg for 2-18-OH, 35.3 mg for 3-12-w, and 43.8 mg for 2-18-w, at an initial copper ion concentration of 150 mg/L. Therefore, increasing the contents of CS in the blend hollow fiber membrane significantly enhanced the adsorptive performance of the hollow fiber membranes for copper ion removal.

6.4.3. Adsorption kinetics

A kinetic adsorption study was undertaken for the the hollow fibers 3-12-w and 3-12- OH and the results are shown in Fig. 18. Very high adsorption rates were observed for copper ions (i.e., rapid change of concentrations with time) on the hollow fibers in the initial stage of the adsorption process and the adsorption process finally reached the adsorption equilibrium in about 20 and 70 min respectively for the hollow fibers 3-12-w and 3-12-OH. These kinetic adsorption equilibrium times were much shorter than those with chitosan hydrogei beads reported to be at least several hours in literature {Dambie et a\, 2000; Lee et at, 2001; Oshita et at, 2003; Merrrfield et a\, 2004; Li and Bai, 2005). The fast adsorption kinetics may be attributed to the high porosities and large pore sizes of the hollow fibers, which facilitated the transport of copper ions to the adsorption or binding sites.

The kinetic adsorption results can be analyzed with various adsorption kinetic models to reveal the control factors in the adsorption process. For diffusion or transport controlled adsorption kinetics, the adsorption amount q_t at time t versus f⁵ would satisfy the following equation (Zhang and Bai, 2003):

q_t=kf-⁵ Eq. (1)

where q_t is the amount of copper ions adsorbed on per unit weight of the hollow fibers (mg/g) at time t (min), and k_d (mg g^"1 min^"05) depicts the intrinsic kinetic rate constant for diffusion-controlled adsorption and is related to the initial concentration of the copper ions in the bulk solution, the specific surface area of the hollow fibers and the diffusion coefficient of the copper ions in this case.

The fitting of Eq.(1) to the experimental results in Fig. 18(a) is shown by the linear lines in Fig. 18(b). The linear relationship of q_t against f³ is indeed observed for both types of the hollow fibers in the initial stage of the adsorption process. This means that the transport of copper ions from the solution to the adsorption sites on the external and internal surfaces of the hollow fibers was in fact the rate-controlling step in the initial stage. This effect however would be reduced in actual applications when the hollow fiber membranes are operated in a filtration mode as the copper ions to be removed can be bought to the binding sites by the convective flow. From the slopes of the straight lines in Figure 18(b), the transport-controlled rate constants /c_d (mg g^'1 min^"05) can be calculated to be 0.49 and 0.069 for the hollow fibers 3-12-OH and 3-12-w, respectively. The higher transport-controlled rate constant for the hollow fibers 3-12-OH may be attributed to the much higher CS content or adsorption capacity of the hollow fibers, under which the more rapid adsorption facilitated the transport of copper ions to the adsorption sites on the hollow fibers 3-12-OH than on 3-12-w.

The pseudo first-order and pseudo second-order kinetic models have often been used to fit the experimental adsorption kinetic results to determine whether the adsoφtion attachment is dominated by a physical or chemical mechanism. The linearized forms of the pseudo first-order and pseudo second-order model can be given respectively in Eq. (2) and Eq. (3) (Tien, 1994):

kgtø. -?.)

E^q. ⁽2)

q_t K₂q_e q_e where qr_e is the amount of copper ions adsorbed at adsorption equilibrium (mg/g), q_t (mg/g) is the amount of copper ions adsorbed at time t (min), and Ki (mirf ¹)and K₂ (g mg^"1 min^"1)) are the rate constants of the pseudo first-order and pseudo second-order adsoφtion models respectively and the constants are related to the reaction temperatures for a given adsorbent .

Based on Eq.(2) and Eq.(3), the plots of log (q_θ-qi) vs. t and t/q_t vs. t for the two types of hollow fibers for the experimental results in Figure 18(a) are shown in Figure 18(c) and Figure 18(d), respectively. From the linear relationships shown in Figure 18(d), it is clear that the adsoφtion of copper ions on both types of the hollow fibers well followed the pseudo second-order kinetic model (i.e., t/q_t vs. t gives a straight line), with the correlation coefficients of R²=0.999 for the hollow fibers 3-12-OH and R²=0.996 for the hollow fibers 3-12-w. The results suggest that the chemical interaction between the copper ions to be adsorbed in the solution and the functional groups on the hollow fiber surfaces dominated the adsoφtion attachment. The pseudo second-order rate constants (K₂) are found to be 0.29 and 0.14 g ^• mg^{"1 ■} min^"1 for the hollow fibers 3-12-w and 3-12- OH respectively. The higher attachment-controlled rate constant for the hollow fibers 3- 12-w may be attributed to the larger pore sizes of the hollow fibers that exposed the adsorption sites to a greater extent for copper ion attachment.

6.4.4 Copper adsorption at low copper ion concentrations

The adsoφtion of copper ions at very low initial concentration ranges of 0.28-6.5 mg/L was investigated with the hollow fibers 3-12-OH because many adsorbents do not perform well at low metal ion concentrations.

Figure 19 shows the final concentration (C₉) versus the initial concentration (Co ~ 0.28-6.5 mg/L) of copper ions in the solution. The results indicate that the CS/CA hollow fibers can effectively reduce copper ion concentrations down to 0.1-0.6 mg/L, a level well below the USEPA maximum contaminant level for copper ion at 1.3 mg/L for drinking water supply. The effectiveness of the CS/CA hollow fibers for heavy metal removal at low concentrations has great significance since traditional methods that are used for metal ion removal, such as chemical coagulation, electrodialysis, and adsorption with activated carbon, are usually ineffective or inefficient to remove metal ions to such a low level. If an adsorption process for metal ion removal is dominated by a physical process, the removal is usually reversible and dependent on the equilibrium between the metal concentration in the solution and the metal content on the surface of the adsorbent, and thus the process performs poorly at low concentrations in the solution. Hence, the results in Fig. 19 also support conclusion of the above kinetic study that the adsorption of copper ions on the hollow fibers was primarily a chemical adsorption process.

It is interesting to note that there are two plateaus of C_θ observed in Fig. 19 at the initial concentrations of 0.28-0.84 mg/L and 0.84-3.13 mg/L, respectively. At each plateau, the C_β remained unchanged even though C₀ increased. The presence of the plateaus in Fig. 19 is again an indication of the irreversible chemical adsorption phenomenon in the process because the C_β in the solution will increase with C₀ if a reversible physical adsorption process existed as the main adsorption mechanisms. The presence of the two different plateaus may suggest that two different functional groups were possibly involved in the chemical adsorption of copper ions and the first type of functional groups may have a slightly higher adsorption energy than the second type of functional groups. From the chemical structures of chitosan and cellulose acetate, it may be possible that the amine groups in CS and the hydroxyl groups in CA and CS may contribute to these functional groups. It is also observed in Figure 19 that, with the further increase of C₀ at above 3.13 mg/L, C_θ increased as well, suggesting that physical adsoφtion phenomenon still probably played some roles in copper ion removal in the process.

6.4.5 Adsorption mechanisms

To further elucidate the mechanism of copper ion adsorption on the CS/CA blend hollow fibers, XPS analysis was conducted on the hollow fibers before and after copper ion adsorption. From the XPS analysis, the 1s binding energies for the elements of C, O and N at different oxidization states before and after copper ion adsoφtion are summarized in Table 7 below. Table 7 XPS C Is, O Is and N Is binding energies of the CS/CA blend hollow fibers 3-12-OH before and after copper ion adsorption

Binding Energy (BE), eV

Element Before adsorption After adsorption Shift of BE (eV)

C Is 284.6 284.6 0.00

286.0 286.3 +0.3

287.5 287.8 +0.3

O ls 531.1 531.1 0.00

532.3 532.4 +0.1

N Is 399.0 399.7 +0.7

400.7 401.7 +1.00

It is clear that all the C 1s and O 1s binding energies did not show significant changes before and after copper ion adsorption (less than 0.5 eV was considered to be insignificant). Without being bound by these observations, the results at least indicate that there is no clear evidence to prove that the hydroxyl groups were involved in the chemical adsorption of copper ions on the hollow fibers (unless a noticeable increase in the O 1s binding energy was observed after copper ion adsorption). In contrast, the N 1s binding energy of the CS/CA blend hollow fibers showed significant changes before and after copper ion adsorption. Figure 20 also shows the N 1s XPS spectra of the hollow fibers before (a) and after (b) copper ion adsorption. Before copper ion adsorption, there are two peaks at the binding energies of 399.0 and 400.7 eV. The peak at the binding energy of 399.0 eV is attributed to the nitrogen atoms in the -NH₂ and/or the -NH- groups of CS and the peak at the binding energy of 400.7 eV is attributed to the protonated nitrogen atoms in the -NH₃ ⁺ groups of CS on the surfaces of the hollow fibers (Dambies et al 2000; Zhang and Bai, 2003). After copper ion adsorption, significant shifts or increases of the binding energies at both peaks are observed (399.0 to 399.7 eV and 400.7 to 401.7 eV, respectively). The increase of the binding energies provide evidence that the N atoms in the -NH₂ or -NH- and -NH₃ ⁺ were all involved in the adsorption of the copper ions, possibly through forming a surface complex in which a pair of lone electrons from the N atoms were shared with the copper ions, which increased the oxidation states and thus binding energies of the nitrogen atoms. It is also noted that the area under the peak at 400.7 eV before copper ion adsorption increased after copper ion adsorption (see the area under the peak at 401.7 eV). The adsorption process seemed to result in some nitrogen atoms in the -NH₂ or -NH- groups to form Cu-- NH₃ ⁺- complexes when the Cu-- NH₂- complexes were formed, which increased the area under the peak at 401.7 eV after copper ion adsorption.

6.4.6 Desorotion and reuse

The desorbing the copper ions adsorbed on the hollow fibers was observed to determine if the hollow fibers could be 're-used' after an adsorption step. Both EDTA and HCI solutions were examined in the study for hollow fibers 3-12-OH. Table 8 below shows the results obtained at different concentrations of EDTA and HCI solutions and the corresponding desorption efficiencies.

Table 8 - Desorption of copper ions from CS/CA blend hollow fibers 3-12-OH with

EDTA and HCl solutions

EDTA solution HCl solution

Concentration (mM) pH Desorption PH Desorption percentage (%) percentage (%)

0.01 5.40 2.6 6.09 1.7

0.1 5.38 3.8 4.98 2.2

1 5.34 16.7 3.83 10.9

10 5.21 99.3 2.80 90.2

50 5.13 98.8 2.11 90.5

As can be found from table 8, the desorption efficiencies increased generally with the concentrations of the EDTA or HCI solutions. Although both EDTA and HCI solutions appeared to be effective for copper ion desorption, the EDTA solution performed much better than the HCI solution and the maximum desorption efficiency reached about 99% with the EDTA solution but only 90% with the HCI solution.

The desorbed hollow fibers were reused in the next adsorption experiments to examine the effect of the desorption process. Table 9 below shows the results of adsorption from four adsorption-desorption cycles. Table 9 - Adsorption amounts of copper ions on CS/CA blend hollow fibers 3-12-OH from four adsorption-desorption cycles

Amounts of copper ions adsorbed

Cycle No. (mg/g)

Desorbedwith Desorbed with

1O mM EDTA 1O mM HCl

1 8.74 8.74

2 ^• 8.12 4.60

3 8.09 3.57

4 7.96 2.88

It is clear from Table 9 that the adsorptive hollow fibers can be effectively regenerated with EDTA and reused almost without any significant loss in their adsorption capacity for copper ions. For desorption conducted with the HCI solution, the adsorption capacity reduced significantly after each cycle. This may be attributed to the dissolution of CS in the hollow fibers in the HCI solution (with pH<4) during the desorption tests.

6.5 Conclusions

Adsoφtive porous hollow fiber membranes for copper ion removal can be directly prepared from chitosan (CS) and cellulose acetate (CA) blend solutions with water or NaOH solution as the coagulant. Higher CS contents provided better adsorption performances for the hollow fibers to remove copper ions. NaOH solution served as a stronger coagulant and prevented the dissolusion of CS during the hollow fiber spinning/coagulation process and hence retained higher CS contents for the hollow fibers. The hollow fibers were highly porous and had large specific surface areas, with pore sizes sufficiently large to allow full access of copper ions to be removed from solutions to the external or internal surfaces or binding sites of the hollow fibers. The batch adsorption study showed that the CS/CA blend hollow fiber membranes had high adsorption capacity, fast adsorption rate and short adsorption equilibrium time for copper ions. The CS/CA adsoφtive hollow fiber membranes can also work effectively at low copper ion concentrations and were able to reduce the concentration of copper ions in the solution down to a level of 0.1-0.6 mg/L. Copper ion adsoφtion on the hollow fibers followed the pseudo second-order kinetic model, indicating the importance of chemical adsoφtion of copper ions on the hollow fibers in the adsorption process. Mechanisms study through the XPS spectroscopy suggested that copper ion adsoφtion involved forming surface complexes with the nitrogen atoms of CS in the hollow fibers. The copper ions adsorbed on the hollow fibers can be effectively desorbed in an EDTA solution and the hollow fibers can be reused almost without loss of the adsorption capacity. An implication of the study is that the CS/CA adsorptive membranes have great potentials to be used in the MF or UF process to expand the function of these membrane processes from removing particles only to removing particles plus heavy metal ions together in water or wastewater treatment.

Example 7

The hollow fibers of Example 6 were studied to determine their binding capacities with a model protein, Bovine serum albumin (BSA). The binding capacities of the hollow fibers of Example 6 that were free of Cu²⁺ ions (CS/CA) and hollow fibers that had complexed with Cu²⁺ ions (CS/CA-Cu) were used in this example.

The BSA binding capacities with the CS/CA and CS/CA-Cu hollow fibers are shown in Fig. 21. The hollow fiber weight was 1.1g, BSA solution volume was 50ml, NaCI concentration was 12OmM, and solution pH was 7.4.

The nonspecific adsorption capacity by the CS/CA hollow fibers was less than 9 mg/g, which may be attributed to the electrostatic and hydrophobic interactions of BSA with the CS/CA hollow fibers. After coupling with copper ions, the binding capacity of the CS/CA-Cu hollow fibers increased significantly to about 60 mg/g.

The adsorption isotherms models (Langmuir and Freundlich) were used to correlate the experimental data in Fig. 21, the results of which are shown in Fig. 22: (a) Langmuir and (b) Freundlich. The Langmuir model fitted the experimental results much better than the Freundlich model (see R² = 0.98 in Fig. 22a, and R² = 0.88 in Fig. 22b). This suggests that the adsorption of BSA on the CS/CA-Cu fibers displayed a monolayer adsorption behavior at each adsorption site. The adsorption equilibrium constants (b) and adsorption capacity (q_m) were calculated to be 4.4*10^"3 L/mg and 69 mg/g hollow fibers, respectively.

The utilization of the immobilized copper ions was calculated. The utilization of metal ions is defined as the protein binding capacity on per unit mass of the metal ion ligands coupled [refer to Chυn-Yi Wu, Shing-Yi Suen, Shiow-Ching Chen and Jau~ Hwan Tzeng, 2003, Analysis of protein adsorption on regenerated cellulose-based immobilized copper ion affinity membranes, J. Chromatogr. A, 996: 53-70]. In this example, the copper ion utilization was 18 mg BSA/mg Cu²⁺, which is much higher than those for most of other reported IMAMs. Referring to Fig. 23, there is shown a graph showing the effects of solution pH (a) and ionic strength (b) on BSA binding capacity on the hollow fibers. The weight of hollow fibers was 1.1g, solution volume was 50ml, and BSA initial concentration was 2.5mg/ml. NaCI concentration in (a) was 12OmM and solution pH in (b) was 7.4.

It can be observed from Fig. 23 that the binding capacity increased remarkably from 35mg/g to 58mg/g when pH increased from 4.9 to 6.6 and then reached a plateau at pH greater than 6.6. Therefore, the binding was favored under neutral or weak basic solution conditions. As indicated in Fig. 23, the increase of ionic strength in the solution would result in significant reduction of the binding capacity for BSA on both the CS/CA and CS/CA-Cu fibers.

Referring to Fig. 24, there is shown a graph showing the BSA adsorption rates on CS/CA-Cu hollow fibers. Weight of hollow fibers was 1.1g, solution volume was 5OmI₁ BSA initial concentration was 2.5mg/ml, NaCI concentration was 12OmM and pH was 7.4. It can be observed from Fig. 24 that the adsorption equilibrium was achieved in about 10 hours.

Referring to Figure 25, there is shown fitting of BSA adsorption kinetics data with the pseudo-first (a) and the pseudo-second order (b) kinetic models. The pseudo first-order and second-order kinetic models were used to analyze the adsorption kinetic data. The linear regression coefficients for these two models were 0.90 and 0.97, respectively. So, the adsorption kinetics on the CS/CA-Cu hollow fibers can be better fitted with the pseudo second-order model than the first-order model. This suggests that chemical adsorption between BSA and membrane surface dominated the adsorption kinetics. The adsorption rate constant, K₂, of the hollow fibers was 3.8^χ10^'3 g/mg.hr.

The leakage of copper ions from CS/CA-Cu hollow fibers with BSA concentrations is shown in Figure 26, wherein (a) Cu leakage with equilibrium BSA concentrations, and (b) modeling of Cu leakage with the Freundlich isotherm model, Weight of hollow fibers was 1.1g, solution volume was 50ml, pH was 7.4, and NaCI concentration was 12OmM. The amounts of released copper ion were small, only 1- 17% of the total immobilized copper ions in the BSA initial concentration range of 0.2-5 mg/ml studied. The leakage amounts usually increased with the BSA equilibrium concentrations in the solution. More interestingly, the copper ion leakage with the equilibrium BSA concentrations can be well correlated by one of the adsorption isotherm models, i.e., the Freundlich model (see Fig. 26b). It will be appreciated that the data of this example indicates that the CS/CA-Cu hollow fiber membranes can be used as immobilized affinity membranes (IMAMs) for protein separations. The BSA binding capacity was as high as 69 mgBSA/g hollow fibers at pH 7.4 and a NaCI concentration of 12OmM. The data shows that the developed IMAMs in the study possessed higher binding capacity in pH 6.6-8 and at low ionic strengths. It was found that the release of Cu²⁺, although small, increased with the BSA concentrations, and the amounts can be well correlated with the equilibrium BSA concentrations in the bulk solution by the Freundlich adsorption isotherm model. The data shows that the metal ion immobilized CS/CA blend hollow fiber membranes have great potential to be used economically in large scale separations of proteins.

Applications

The disclosed hollow fibers have sufficient mechanical strength such that they are able to be used in numerous applications.

Advantageously, the disclosed hollow fibers have exhibit good absorption to certain substances and are particularly useful in separation applications. For example, the disclosed hollow fibers are particularly useful for the recovery or removal of various valuable and toxic transitional metal ions from water, wastewater or industrial effluents. Metal ions which can be effectively recovered or concentrated using the disclosed hollow fibers include, but are not limited to, copper, lead, gold, silver, mercury, zinc, nickel and chromium.

Advantageously, the disclosed hollow fibers have a microporous structure and a relatively high amount of chitosan (i.e. up to 25% by weight), which assists the absorption properties of the hollow fibers.

The hollow fibers of the present invention can be used in the pharmaceutical industry as an affinity membrane chromatography matrix to selectively separate proteins and enzymes. For example, ligands such as metal ions and dyes can be easily and rapidly immobilized onto the disclosed hollow fibers.

The disclosed hollow fibers can be applied potentially in medicine as blood or kidney dialysis membranes. The presence of chitosan in the hollow fibers can adsorb and hence remove some known toxic macromolecules which are metabolized by human bodies and cannot be removed by conventional dialysis devices. The disclosed hollow fibers can be applied potentially in the field of seawater desalination as the presence of chitosan can enhance the rejection rate toward sodium ions and potassium ions.

Advantageously, the disclosed method for making the disclosed hollow fibers teaches the preparation of a chitosan and cellulose acetate polymer blend solution using a suitable solvent. Hollow fibers can easily be fabricated from these polymer blends.

The disclosed process can produce hollow fibers having desired surface pore sizes and membrane structures.

One advantage of the disclosed process here is that the hollow fibers can be produced from a relatively simple and reproducible process, as compared to known methods. Hence, the hollow fibers are relatively easy to produce on an industrial scale.

Another advantage of the disclosed hollow fibers is that the chitosan is fixed and distributed homogeneously in the hollow fiber matrix, and therefore the hollow fibers do not have the detachment or dissolution problems as with other chitosan composite membranes prepared by known method of the prior art.

Another advantage of the hollow fibers disclosed here is that such hollow fibers can contain much higher amounts of chitosan than other chitosan composite membranes prepared by the existing methods. It has surprisingly been discovered that the amount of chitosan in the blend hollow fibers can reach as high as up to about 25% by weight.

Another advantage of the hollow fibers disclosed here is that the morphologies and structures of the hollow fibers can be easily controlled and varied in the fabrication process. It has been discovered that the disclosed hollow fibers can be made to show microfiltration, or ultrafiltration characteristics, or porous adsorbent properties.

Another advantage of the hollow fibers disclosed here is that such hollow fibers can be easily tailored in their hydrophilicity or hydrophobicity for specific separation demands.

Another advantage of the hollow fibers disclosed here is that such hollow fibers have high biocompatibility and can be used in the biomedical field.

It will be apparent that various other modifications and adaptations of the invention will be apparent to the person skilled in the art after reading the foregoing disclosure without departing from the spirit and scope of the invention and it is intended that all such modifications and adaptations come within the scope of the appended claims.

Claims

1. A blended polymer comprising cellulose ester and amino-sυbstituted polysaccharide.

2. A blended polymer as claimed in claϊm 1, wherein said amino-substituted polysaccharide is a chitosan polymer.

3. A blended polymer as claimed in claim 2, wherein the weight percentage of said chitosan polymer in the blended polymer is about 5% to about 25% and wherein the remainder is cellulose ester and any incidental impurities.

4. A blended polymer as claimed in claim 2, wherein the chitosan polymer has an average degree of deacetylation (D.A.) selected from the group consisting of more than 75%, more than 80%, more than 90%, from 90% to 100% and from 95% to 100%.

5. A blended polymer as claimed in claim 2, wherein the average molecular weight of the chitosan polymer is 25,000 to 2,000,000 or 30,000 to 200,000 g/mole.

6. A blended polymer as claimed in claim 1, wherein the cellulose ester is selected from the group consisting of cellulose acetate, cellulose acylate, cellulose diacylate, cellulose triacylate, cellulose diacetate, cellulose triacetate, cellulose nitrate, cellulose propionate, cellulose butyrate and mixtures thereof.

7. A blended polymer as claimed in claim 1, wherein the cellulose ester is cellulose acetate and wherein the acetylation degree is at least about 40% or at least about 95% to about 100%.

8. A blended polymer as claimed in claim 7, wherein the number-averaged molecular weight of the cellulose acetate is 10,000 to 500,000 or 30,000 to 200,000.

9. A hollow fiber comprising of the blended polymer of claim 1.

10. A method of making a blended polymer comprising the steps of:

11. A method as claimed in claim 10, wherein the amino-substituted polysaccharide comprises chitosan polymer and the cellulose ester comprises cellulose acetate.

12. A method as claimed in claim 11, wherein the polymer blend solution comprises:

2% to 4% by weight chitosan polymer;

10% to 30% by weight cellulose ester; and the remainder comprising an acidic organic solvent.

13. A method as claimed in claim 10 comprising the step of:

(c) introducing a cross-linking agent into the polymer blend solution.

14. A hollow fiber comprising a cellulose acetate and chitosan polymer.

15. A hollow fiber as claimed in claim 14, wherein the weight percentage of chitosan polymer in the hollow fiber is about 1% to about 25%.

16. A hollow fiber as claimed in claim 14, wherein the hollow fiber has an external diameter of about 0.25 mm to about 3 mm.

17. A hollow fiber as claimed in claim 14, wherein the hollow fiber has an internal diameter of about 0.16 mm to about 2 mm.

18. A process for making a hollow fiber comprising the steps of:

(a) providing a spinning solution comprising a polymer dope of cellulose acetate and chitosan polymer blends and a solvent;

(b) extruding the spinning solution through an annular slit;

(c) extruding, during step (b), a quench liquid from an orifice encircled by said annular slit, said quench liquid being substantially miscible with the solvent of said spinning solution but substantially immiscible with the polymer dope of said spinning solution to develop a phase separation between the polymer dope and the solvent of said spinning solution; and

(d) introducing the extruded spinning solution and extruded quench liquid into a quench bath comprising quench liquid to precipitate said blended polymer from said solution and thereby form said hollow fiber.

19. A process as claimed in claim 18, wherein the spinning solution comprises: up to about 4% by weight chitosan polymer;

10% to 30% by weight cellulose acetate; and the remainder comprising an organic solvent.

20. A process as claimed in claim 19, wherein the spinning solution comprises 12% to 26% by weight cellulose acetate.

21. A process as claimed in claim 19, wherein the organic solvent comprises at least one of a protic solvent and an aprotic solvent.

22. A process as claimed in claim 21 , wherein the volume percentage of said at least one protic solvent and said aprotic solvent in said organic solvent is selected from the group consisting of at least 60%, at least 70%, at least 80%, at least 90%, at least 98%, and about 100%.

23. A process as claimed in claim 21, wherein the protic solvent comprises carboxylic acids selected from the group consisting of formic (methanoic acid), ethanoic acid, propanoic acid and mixtures thereof.

24. A process as claimed in claim 21 , wherein the aprotic solvent comprises is selected from the group consisting of water, ketones, alkyl formamides, Tetrahydrofuran (THF), alkyl sulfoxides and mixtures thereof.

25. A process as claimed in claim 19, comprising the step of:

(a1) stirring said cellulose acetate polymer and said chitosan polymer with an acidic solvent to form the spinning solution having a substantially homogenous composition.

26. A process as claimed in claim 25, wherein the stirring step (a1) is undertaken for at least one hour.

27. A process as claimed in claim 19, comprising the step of: (a2) filtering said spinning solution.

28. A process as claimed in claim 19, comprising the step of:

29. A process as claimed in claim 28, wherein, during step (a3), the method comprises the step of: (a4) applying a vacuum to the spinning solution to assist in the removal of said entrained gas.

30. A process as claimed in claim 18, wherein the distance between the orifice surrounded by the annular slit from which said quench liquid and said spinning solution are respectively extruded, and the surface of said quench liquid contained in said quenching bath, is between 0 to about 50 cm.

31. A process as claimed in claim 18, wherein the quench liquid being extruded from said orifice has substantially the same composition or different composition as the quench liquid in said quench bath.

32. A process as claimed in claim 18, wherein the quench liquid is selected from the group consisting of an alkaline solution, an aqueous salt solution, water and an acidic salt solutions containing di-valent or multi-valent ions.

33. A process as claimed in claim 18, comprising the step of:

(f) washing solvent from said hollow fiber.

34. A process as claimed in claim 18, comprising the step of:

(g) leaching the solvent from said hollow fiber.

35. A process as claimed in claim 18, comprising the step of:

36. A process as claimed in claim 35, wherein the softening agent is a glycerol or an alkylene glycol.

37. Use of blended polymer of claim 1 or a hollow fiber of claim 14, for adsorbing metal ions.

38. Use of blended polymer of claim 1 or a hollow fiber of claim 14, for adsorbing proteins.

39. Use of blended polymer of claim 1 or a hollow fiber of claim 14, for affinity separation of biomolecules.

40. Use of blended polymer of claim 1 or a hollow fiber of claim 14, for adsorbing toxins from a mammal's blood.

41. Use of blended polymer of claim 1 or a hollow fiber of claim 14, for removing impurities from water.

42. Use of blended polymer of claim 1 or a hollow fiber of claim 14, for desalinating saline water.

43. A blended polymer as claimed in claim 1, further comprising a metal ion complexed with at least one of said polymers.

44. A blended polymer as claimed in claim 43, wherein the metal of the metal ion is a transition metal.

45. A hollow fiber as claimed in claim 14, further comprising a metal ion complexed with at least one of said polymers.

46. A hollow fiber as claimed in claim 45, wherein the metal of the metal ion is a transition metal. There is provided a blended polymer comprising cellulose ester and amino-substituted polysaccharide. There is also disclosed a novel method for making the hollow fibers.

Fig. 2