ULTRATHIN-FILM COMPOSITE MEMBRANE BASED ON THERMALLY REARRANGED POLY(BENZOXAZOLE-IMIDE) COPOLYMER, AND PRODUCTION METHOD THEREFOR

Abstract

The present invention relates to an ultrathin-film composite membrane based on a thermally rearranged poly(benzoxazole-imide) copolymer and a production method therefor and to a technique for forming a porous support by means of a thermally rearranged poly(benzoxazole-imide)copolymer and then producing, on the porous support, an ultrathin-film composite membrane comprising a thin-film active layer. The ultrathin-film composite membrane produced according to the present invention has excellent thermal/chemical stability and mechanical physical properties, thus is not only capable of withstanding high operating pressure, but also capable of minimizing internal concentration polarization and thereby obtaining high water permeance and, as a result, high power density, and thus can be applied to a pressure-retarded osmosis or forward osmosis process. Further, said ultrathin-film composite membrane has excellent chemical/thermal stability against organic solvents, has superior organic solvent nano-filtration performance, particularly maintains nano-filtration performance stably even under a high-temperature organic solvent condition, and thus can be applied as an organic solvent nano-filtration membrane.

Claims

1. An ultrathin-film composite membrane comprising: a porous thermally rearranged poly(benzoxazole-imide) copolymer support having a repeating unit represented by the following Chemical Formula 1; and a thin-film active layer formed on the support. ##STR00014## wherein Ar.sub.1 is an aromatic cyclic group selected from a substituted or non-substituted tetravalent C6-C24 arylene group and a substituted or non-substituted tetravalent C4-C24 heterocyclic group, wherein the aromatic cyclic group is present alone; two or more aromatic cyclic groups may form a condensed ring; or two or more aromatic cyclic groups may be linked by means of a single bond, O, S, CO, SO.sub.2, Si(CH.sub.3).sub.2, (CH.sub.2) (1≤P≤10), (CF.sub.2).sub.q (1≤q≤10), C(CH.sub.3).sub.2, C(CF.sub.3).sub.2 or CO—NH; Ar.sub.2 is an aromatic cyclic group selected from a substituted or non-substituted divalent C6-C24 arylene group and a substituted or non-substituted divalent C4-C24 heterocyclic group, wherein the aromatic cyclic group is present alone; two or more aromatic cyclic groups may form a condensed ring; or two or more aromatic cyclic groups may be linked by means of a single bond, O, S, CO, SO.sub.2, Si(CH.sub.3).sub.2, (CH.sub.2).sub.p (1≤P≤10), (CF.sub.2).sub.q (1≤q≤10), C(CH.sub.3).sub.2, C(CF.sub.3).sub.2 or CO—NH; Q is a single bond, O, S, CO, SO.sub.2, Si(CH.sub.3).sub.2, (CH.sub.2).sub.p (1≤P≤10), (CF.sub.2).sub.q (1≤q≤10), C(CH.sub.3).sub.2, C(CF.sub.3).sub.2, CO—NH, C(CH.sub.3)(CF.sub.3), or substituted or non-substituted phenylene group; and each of x and y represents a molar fraction in the repeating unit, wherein 0.1≤x≤0.9, 0.1≤y≤0.9, and x+y=1.

2. The ultrathin-film composite membrane according to claim 1, wherein the porous thermally rearranged poly(benzoxazole-imide) copolymer support is an electrospun membrane or hollow fiber membrane.

3. The ultrathin-film composite membrane according to claim 2, wherein the electrospun membrane has a thickness of 10-80 μm and a porosity of 60-80%.

4. The ultrathin-film composite membrane according to claim 1, wherein the active layer of the thin-film is an aromatic polyamide having a repeating unit represented by the following Chemical Formula 2: ##STR00015##

5. The ultrathin-film composite membrane according to claim 4, wherein the active layer of the thin-film has a thickness of 50-300 nm.

6. The ultrathin-film composite membrane according to claim 1, which is for use in a pressure retarded osmosis process.

7. The ultrathin-film composite membrane according to claim 1, which is for use in a forward osmosis process.

8. The ultrathin-film composite membrane according to claim 1, which is for use in nano-filtration of organic solvents.

9. A method for producing an ultrathin-film composite membrane, comprising the steps of: I) carrying out reaction of acid dianhydride, ortho-hydroxydiamine and aromatic diamine to obtain polyamic acid solution and forming a hydroxyl group-containing polyimide-polyimide copolymer through an azeotropic thermal imidization process; II) forming a membrane from a polymer solution containing the hydroxyl group-containing polyimide-polyimide copolymer of step I) dissolved in an organic solvent through an electrospinning process or non-solvent induced phase separation process; III) carrying out thermal rearrangement of the membrane obtained from step II) to obtain a porous thermally rearranged poly(benzoxazole-imide) copolymer support having a repeating unit represented by the above Chemical Formula 1; and IV) forming an active layer on the support by using a crosslinked aromatic polyamide thin film having a repeating unit represented by the above Chemical Formula 2.

10. The method for producing an ultrathin-film composite membrane according to claim 9, wherein the acid dianhydride in step I) is represented by the following Chemical Formula 3: ##STR00016## wherein Ar.sub.1 is the same as defined in the above Chemical Formula 1.

11. The method for producing an ultrathin-film composite membrane according to claim 9, wherein the ortho-hydroxydiamine in step I) is represented by the following Chemical Formula 4: ##STR00017## wherein Q is the same as defined in the above Chemical Formula 1.

12. The method for producing an ultrathin-film composite membrane according to claim 9, wherein the aromatic diamine in step I) is represented by the following Chemical Formula 5:
H.sub.2N—Ar.sub.2—NH.sub.2  [Chemical Formula 5] wherein Ar.sub.2 is the same as defined in the above Chemical Formula 1.

13. The method for producing an ultrathin-film composite membrane according to claim 9, wherein the thermal rearrangement in step III) is carried out by increasing the temperature to 300-400° C. at a warming rate of 1-20° C./min and maintaining the isothermal state for 1-2 hours under a high purity inert gas atmosphere.

14. The method for producing an ultrathin-film composite membrane according to claim 9, which further comprises a step of carrying out hydrophilization treatment of the support obtained from step III) before carrying out step Iv).

15. The method for producing an ultrathin-film composite membrane according to claim 9, which further comprises a step of carrying out post-treatment of the ultrathin-film composite membrane obtained from step IV) with aqueous sodium hypochlorite.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0041] FIG. 1 illustrates the process for producing the porous thermally rearranged poly(benzoxazole-imide) copolymer supports (electrospun membrane) according to Examples 1-9 and scanning electron microscopic (SEM) images thereof.

[0042] FIG. 2 illustrates the attenuated total reflectance-infrared ray (ATR-IR) spectrum of the porous thermally rearranged poly(benzoxazole-imide) copolymer support according to each of Examples 1-9.

[0043] FIG. 3 illustrates the ATR-IR spectrum of each of the porous thermally rearranged poly(benzoxazole-imide) copolymer support (a) according to Example 1 and the ultrathin-film composite membrane (b) according to Example 11.

[0044] FIG. 4 is a thermogravimetric analysis (TGA) graph illustrating the weight reduction characteristics of the porous thermally rearranged poly(benzoxazole-imide) copolymer support according to Example 1 depending on thermal rearrangement conditions.

[0045] FIG. 5 is a photographic image illustrating the results of observation of the stability of the porous thermally rearranged poly(benzoxazole-imide) copolymer support according to Example 1 against an organic solvent.

[0046] FIG. 6 illustrates the SEM images of the surface, active layer and the total membrane of each of the commercially available polysulfone-based composite membrane (a) for reverse osmosis, cellulose-based ultrathin-film composite membrane (b) for forward osmosis and the ultrathin-film composite membrane (c) according to Example 11.

[0047] FIG. 7 is a graph illustrating the water permeance and salt rejection ratio of the ultrathin-film composite membrane according to Example 11 before and after the post-treatment (500 ppm NaOCl, 1000 ppm NaOCl) [charge: 2000 ppm NaCl (20° C.)].

[0048] FIG. 8 is a graph illustrating the water permeation amount and power density of the ultrathin-film composite membrane according to an embodiment of the present disclosure [inducing solution: 1M NaCl (20° C.), charge: deionized water (20° C.)].

[0049] FIG. 9 is a graph illustrating the pure solvent permeance test results of the porous thermally rearranged poly(benzoxazole-imide) copolymer support according to Example 1.

[0050] FIG. 10 illustrates the results of the observing a change in shape and structure of the porous thermally rearranged poly(benzoxazole-imide) copolymer support according to Example 1 in high-temperature DMF [(a) graph of dimensional change, (b) photograph taken by the naked eyes, (c) scanning electron microscopic (SEM) image].

[0051] FIG. 11 is a graph illustrating the THF permeance (a) and rejection ratio (b) of the ultrathin-film composite membrane according to Example 11.

[0052] FIG. 12 is a graph illustrating the DMF permeance (a) and rejection ratio (b) of the ultrathin-film composite membrane according to Example 11.

[0053] FIG. 13 is a graph illustrating the high-temperature DMF permeance (a) and rejection ratio (b) of the ultrathin-film composite membrane according to Example 11.

[0054] FIG. 14 is a scanning electron microscopic (SEM) image of the morphology of the ultrathin-film composite membrane according to Example 11, taken before and after using the membrane as an organic solvent nano-filtration membrane.

BEST MODE

[0055] In one aspect, there is provided an ultrathin-film composite membrane including: a porous thermally rearranged poly(benzoxazole-imide) copolymer support having a repeating unit represented by the following Chemical Formula 1; and a thin-film active layer formed on the support.

##STR00005##

[0056] wherein Ar.sub.1 is an aromatic cyclic group selected from a substituted or non-substituted tetravalent C6-C24 arylene group and a substituted or non-substituted tetravalent C4-C24 heterocyclic group, wherein the aromatic cyclic group is present alone; two or more aromatic cyclic groups may form a condensed ring; or two or more aromatic cyclic groups may be linked by means of a single bond, O, S, CO, SO.sub.2, Si(CH.sub.3).sub.2, (CH.sub.2).sub.p (1≤P≤10), (CF.sub.2).sub.q (1≤q≤10), C(CH.sub.3).sub.2, C(CF.sub.3).sub.2 or CO—NH;

[0057] Ar.sub.2 is an aromatic cyclic group selected from a substituted or non-substituted divalent C6-C24 arylene group and a substituted or non-substituted divalent C4-C24 heterocyclic group, wherein the aromatic cyclic group is present alone; two or more aromatic cyclic groups may form a condensed ring; or two or more aromatic cyclic groups may be linked by means of a single bond, O, S, CO, SO.sub.2, Si(CH.sub.3).sub.2, (CH.sub.2).sub.p (1≤P≤10), (CF.sub.2).sub.q (1≤q≤10), C(CH.sub.3).sub.2, C(CF.sub.3).sub.2 or CO—NH;

[0058] Q is a single bond, O, S, CO, SO.sub.2, Si(CH.sub.3).sub.2, (CH.sub.2).sub.p (1≤P≤10), (CF.sub.2).sub.q (1≤q≤10), C(CH.sub.3).sub.2, C(CF.sub.3).sub.2, CO—NH, C(CH.sub.3)(CF.sub.3), or substituted or non-substituted phenylene group; and

[0059] each of x and y represents a molar fraction in the repeating unit, wherein 0.1≤x≤0.9, 0.1≤y≤0.9, and x+y=1.

[0060] It can be seen that the porous thermally rearranged poly(benzoxazole-imide) copolymer support has excellent chemical/thermal stability by virtue of the structure of the repeating unit as defined in the above Chemical Formula 1.

[0061] In addition, preferably, the porous thermally rearranged poly(benzoxazole-imide) copolymer support is an electrospun membrane or hollow fiber membrane. In general, the electrospun membrane can be formed into a porous support having high porosity with a small thickness and interconnected pore structure by stacking fibers having a size of several hundreds of nanometers in a bottom-up mode through an electrospinning process. Therefore, according to the present disclosure, when the porous thermally rearranged poly(benzoxazole-imide) copolymer support is an electrospun membrane, it may have a thickness of 10-80 μm and a porosity of 60-80% preferably.

[0062] Since the polysulfone-based or polyethylene terephthalate-based porous support of the ultrathin-film composite membrane used conventionally as a separation membrane for water treatment has a large thickness of 100-200 μm, internal concentration polarization occurs inside of such a thick porous support, when it is used as a separation membrane for a pressure retarded osmosis process for generating energy or a forward osmosis process for producing water, resulting in a decrease in concentration gradient, which is driving force of water permeation. As a result, there have been problems of degradation of water permeance and a decrease in power density according thereto.

[0063] Therefore, when using the porous support obtained as an electrospun membrane and having a small thickness of 10-80 μm and a significantly high porosity of 60-80% according to the present disclosure, it is possible to minimize internal concentration polarization and to obtain high water permeance and high power density according thereto. Thus, it is possible to apply the membrane to a pressure retarded osmosis or forward osmosis process and to minimize mass transport resistance. As a result, the membrane not only has excellent chemical/thermal stability but also may be applied as an organic solvent nano-filtration membrane.

[0064] Herein, when the porous support obtained as an electrospun membrane has a thickness less than 10 μm, such an excessively small thickness may cause degradation of mechanical properties. When the porous support has a thickness larger than 80 μm, concentration polarization may occur in the support or mass transport resistance may be increased undesirably. In addition, when the porous support has a porosity less than 60%, water permeance or organic solvent separation performance may be degraded. When the porosity is larger than 80%, it is difficult to form a membrane.

[0065] The active layer of the thin-film formed on the porous support may be a crosslinked aromatic polyamide having a repeating unit represented by the following Chemical Formula 2.

##STR00006##

[0066] Preferably, the active layer of the thin-film has a thickness of 50-300 nm. When the active layer has a thickness less than 50 nm, it is difficult for the membrane to resist high operating pressure when it is applied to a pressure retarded osmosis process. When the active layer has a thickness larger than 300 nm, water permeance or mass transport resistance may be degraded.

[0067] In addition, the structure of the poly(benzoxazole-imide) copolymer is based on the synthesis of polyimide prepared by imidizing polyamic acid obtained from the reaction of acid dianhydride with diamine. Further, the thermally rearranged polybenzoxazole is obtained by allowing the functional group, such as hydroxyl group, present at the ortho-position of the aromatic imide connection ring to attack the carbonyl group of the imide ring to form a carboxy-benzoxazole intermediate, and then carrying out decarboxylation through heat treatment. Thus, the present disclosure provides a method for producing an ultrathin-film composite membrane including the following steps.

[0068] In another aspect, there is provided a method for producing an ultrathin-film composite membrane, including the steps of:

[0069] I) carrying out reaction of acid dianhydride, ortho-hydroxydiamine and aromatic diamine to obtain polyamic acid solution and forming a hydroxyl group-containing polyimide-polyimide copolymer through an azeotropic thermal imidization process;

[0070] II) forming a membrane from a polymer solution containing the hydroxyl group-containing polyimide-polyimide copolymer of step I) dissolved in an organic solvent through an electrospinning process or non-solvent induced phase separation process;

[0071] III) carrying out thermal rearrangement of the membrane obtained from step II) to obtain a porous thermally rearranged poly(benzoxazole-imide) copolymer support having a repeating unit represented by the above Chemical Formula 1; and

[0072] IV) forming an active layer on the support by using a crosslinked aromatic polyamide thin film having a repeating unit represented by the above Chemical Formula 2.

[0073] In general, acid dianhydride is allowed to react with diamine to obtain polyimide. Thus, according to the present disclosure, the compound represented by the following Chemical Formula 3 is used as acid dianhydride.

##STR00007##

[0074] wherein Ar.sub.1 is the same as defined in the above Chemical Formula 1.

[0075] Any acid dianhydride represented by Chemical Formula 3 may be used as a monomer for preparing polyimide with no particular limitation. However, in view of improvement of the thermal/chemical properties of the resultant polyimide, it is preferred to use 4,4′-hexafluoroisopropylidene phthalic dianhydride (6FDA) or 4,4′-oxydiphthalic dianhydride (ODPA) having a fluoro group.

[0076] In addition, according to the present disclosure, the copolymer ultimately has a poly(benzoxazole-imide) copolymer structure. Thus, considering that a polybenzoxazole unit can be introduced by thermal rearrangement of ortho-hydroxypolyimide, the compound represented by the following Chemical Formula 4 is used as an ortho-hydroxydiamine in order to obtain ortho-hydroxypolyimide.

##STR00008##

[0077] wherein Q is the same as defined in the above Chemical Formula 1. Any ortho-hydroxydiaime represented by Chemical Formula 4 may be used with no particular limitation. However, in view of improvement of the thermal/chemical properties of the resultant polyimide, it is preferred to use 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane (APAF) or 3,3′-diamino-4,4′-dihydroxybiphenyl (HAB) having a fluoro group.

[0078] Further, according to the present disclosure, the aromatic diamine represented by the following Chemical Formula 5 may be used as a to comonomer, which is allowed to react with the acid dianhydride represented by Chemical Formula 3 and ortho-hydroxydiamine represented by Chemical Formula 4 to obtain the hydroxyl group-containing polyimide-polyimide copolymer.

H.sub.2N—Ar.sub.2—NH.sub.2  [Chemical Formula 5]

[0079] wherein Ar.sub.2 is the same as defined in the above Chemical Formula 1.

[0080] Any aromatic diamine represented by Chemical Formula 5 may be used with no particular limitation. However, it is preferred to use 4,4′-oxydianiline (ODA) or 2,4,6-trimethylphenylene diamine (DAM).

[0081] In other words, in step I), the acid dianhydride of Chemical Formula 3, ortho-hydroxydiamine of Chemical Formula 4 and aromatic diamine of Chemical Formula 5 are dissolved and agitated in an organic solvent such as N-methyl pyrrolidone (NMP) to obtain polyamic acid solution, which, in turn, is subjected to azeotropic thermal imidization to provide a hydroxyl group-containing polyimide-polyimide copolymer represented by the following General Formula 1.

##STR00009##

[0082] wherein Ar.sub.1, Ar.sub.2, Q, x and y are the same as defined in Chemical Formula 1.

[0083] Herein, the azeotropic thermal imidization method is carried out by adding toluene or xylene to the polyamic acid solution, agitating the mixture and performing imidization at 160-200° C. for 6-24 hours. During this, water released while an imide ring is formed is separated as an azeotropic mixture of toluene or xylene.

[0084] Then, the hydroxyl group-containing polyimide-polyimide copolymer of step I) represented by General Formula 1 is dissolved in an organic solvent such as N-methyl pyrrolidone (NMP) to provide a polymer solution, which, in turn, is formed into a film through a conventional electrospinning or non-solvent induced phase separation process to obtain an electrospun membrane or hollow fiber membrane as a support.

[0085] Then, the hydroxyl group-containing polyimide-polyimide copolymer electrospun membrane or hollow fiber membrane is thermally rearranged to obtain a porous thermally rearranged poly(benzoxazole-imide) copolymer support having a repeating unit represented by Chemical Formula 1.

[0086] Herein, the thermal rearrangement is carried out by increasing the temperature to 300-400° C. at a warming rate of 1-20° C./min and maintaining the isothermal state for 1-2 hours under a high purity inert gas atmosphere.

[0087] Finally, an active layer of the crosslinked aromatic polyamide thin-film having a repeating unit represented by Chemical Formula 2 is formed on the porous thermally rearranged poly(benzoxazole-imide) copolymer support having a repeating unit represented by Chemical Formula 1 to obtain the target ultrathin-film composite membrane according to the present disclosure.

[0088] Herein, the active layer of the crosslinked aromatic polyamide having a repeating unit represented by Chemical Formula 2 is preferably formed by interfacial polymerization of meta-phenylene diamine (MPD) with trimesoyl chloride (TMC) according to the following Reaction Scheme 1.

##STR00010##

[0089] Meanwhile, according to an embodiment, before forming the active layer of the crosslinked aromatic polyamide thin-film on the porous thermally rearranged poly(benzoxazole-imide) copolymer support, the support may be hydrophilized to facilitate formation of the thin-film active layer. For the hydrophilization treatment of the support, various methods, such as known polydopamine (PDA) coating, polyvinyl alcohol (PVA) coating or plasma coating, may be used. Particularly, it is preferred to carry out hydrophilization by coating the support with polydopamine.

[0090] Actually, after carrying out hydrophilization by coating the porous thermally rearranged poly(benzoxazole-imide) copolymer support with polydopamine according to an embodiment of the present disclosure, the contact angle is decreased by about two times from 114° before coating to 58° after coating. This demonstrates that hydrophilization treatment is made clearly. Also, it can be seen that the porous thermally rearranged poly(benzoxazole-imide) copolymer support is coated with polydopamine by observing hydroxyl groups and acetal groups through attenuated total reflectance-infrared ray (ATR-IR) analysis.

[0091] In addition, the above-described method for producing an ultrathin-film composite membrane may further include a step of carrying out post-treatment of the ultrathin-film composite membrane obtained from step IV) with aqueous sodium hypochlorite. Through the post-treatment step, the crosslinked polyamide thin-film on the porous support undergoes decomposition of polyamide as shown in the following Reaction Scheme 2.

##STR00011##

[0092] Exemplary embodiments now will be described more fully hereinafter with reference to the accompanying drawings.

[Preparation Example 1] Preparation of Hydroxyl Group-Containing Polyimide-Polyimide Copolymer

[0093] First, 5.0 mmol of 3,3′-diamino-4,4′-dihydroxybiphenyl (HAB) and 5.0 mmol of 4,4′-oxydianiline (ODA) were dissolved in 10 mL of dry NMP, the resultant mixture was cooled to 0° C., and then 10.0 mmol of 4,4′-oxydiphthalic dianhydride (ODPA) dissolved in 10 mL of dry NMP was added thereto. The reaction mixture was agitated at 0° C. for 15 minutes, warmed to room temperature and allowed to stand overnight to obtain viscous polyamic acid solution. Then, 20 mL of ortho-xylene was added to the polyamic acid solution and the resultant mixture was agitated vigorously and heated to carry out imidization at 180° C. for 6 hours. During this, water released by the formation of an imide ring was separated as an azeotropic mixture with xylene. The resultant brown-colored solution was subjected to a series of processes including cooling to room temperature, precipitation in distilled water, washing several times with hot water and drying in a convection oven at 120° C. for 12 hours to obtain a hydroxyl group-containing polyimide-polyimide copolymer represented by the following Chemical Formula 6, designated as ODPA-HAB.sub.5-ODA.sub.5.

##STR00012##

[0094] Synthesis of the hydroxyl group-containing polyimide-polyimide copolymer represented by Chemical Formula 6 according to Preparation Example 1 was demonstrated by .sup.1H-NMR and FT-IR data as follows. .sup.1H-NMR (300 MHz, DMSO-d.sub.6, ppm): 10.41 (s, —OH), 8.10 (d, H.sub.ar, J=8.0 Hz), 7.92 (d, H.sub.ar, J=8.0 Hz), 7.85 (s, H.sub.ar), 7.80 (5, H.sub.ar), 7.71 (S, H.sub.ar), 7.47 (S, H.sub.ar), 7.20 (d, H.sub.ar, J=8.3 Hz), 7.04 (d, H.sub.ar, J=8.3 Hz). FT-IR (film): v(O—H) at 3400 cm.sup.−1, v(C═O) at 1786 and 1705 cm.sup.−1, Ar (C—C) at 1619, 1519 cm.sup.−1, imide v(C—N) at 1377 cm.sup.−1, imide (C—N—C) at 1102 and 720 cm.sup.−1.

[Preparation Examples 2-9] Preparation of Hydroxyl Group-Containing Polyimide-Polyimide Copolymers

[0095] Preparation Example 1 was repeated to obtain hydroxyl group-containing polyimide-polyimide copolymers, except that various acid dianhydrides, ortho-hydroxyldiamines and aromatic diamines as shown in the following Table 1 were used. Each of the resultant samples was designated in the same manner as described in Preparation Example 1.

TABLE-US-00001 TABLE 1 Preparation Example Sample Name Molar Fraction Prep. Ex. 2 ODPA-HAB.sub.8-ODA.sub.2 X = 0.8, y = 0.2 Prep. Ex. 3 6FDA-APAF.sub.8-ODA.sub.2 X = 0.8, y = 0.2 Prep. Ex. 4 6FDA-APAF.sub.5-DAM.sub.5 X = 0.5, y = 0.5 Prep. Ex. 5 6FDA-HAB.sub.5-ODA.sub.5 X = 0.5, y = 0.5 Prep. Ex. 6 6FDA-HAB.sub.8-ODA.sub.2 X = 0.8, y = 0.2 Prep. Ex. 7 6FDA-HAB.sub.5-DAM.sub.5 X = 0.5, y = 0.5 Prep. Ex. 8 6FDA-APAF.sub.2-ODA.sub.8 X = 0.2, y = 0.8 Prep. Ex. 9 6FDA-APAF.sub.5-ODA.sub.5 X = 0.5, y = 0.5 6FDA (4,4′-hexafluoroisopropylidene phthalic dianhydride) APAF (2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane DAM (2,4,6-trimethylphenylene diamine)

[Example 1] Preparation of Thermally Rearranged Poly(Benzoxazole-Imide) Copolymer Support (Electrospun Membrane)

[0096] ODPA-HAB.sub.5-ODA.sub.5 obtained from Preparation Example 1 was dissolved in dimethyl acetamide (DMAc) to prepare 10 wt % solution. Next, 6 mL of the polymer solution was charged to a 10 mL syringe equipped with a 23G needle and the syringe was mounted to the syringe pump of an electrospinning system (ES-robot, NanoNC, Korea). Then, spinning was carried out under the conventional electrospinning conditions to obtain an electrospun membrane (HPI).

[0097] The resultant electrospun membrane was inserted between an alumina sheet and carbon cloth, the temperature was increased to 400° C. at a rate of 3° C./min under high-purity argon gas atmosphere, and then the isothermal state was maintained at 400° C. for 2 hours to carry out thermal rearrangement, thereby providing a thermally rearranged poly(benzoxazole-imide) copolymer electrospun membrane (PBO) represented by the following Chemical Formula 7.

##STR00013##

[Examples 2-9] Preparation of Thermally Rearranged Poly(Benzoxazole-Imide) Copolymer Support (Electrospun Membrane)

[0098] Each of the samples obtained from Preparation Examples 2-9 was used to obtain each of the thermally rearranged poly(benzoxazole-imide) copolymer electrospun membranes as shown in FIG. 1 in the same manner as Example 1. It can be seen from FIG. 1, which illustrates the process for producing the porous thermally rearranged poly(benzoxazole-imide) copolymer supports (electrospun membrane) according to Examples 1-9 and scanning electron microscopic (SEM) images thereof, that nanofibrous porous electrospun membranes were formed.

[Example 10] Preparation of Thermally Rearranged Poly(Benzoxazole-Imide) Copolymer Support (Hollow Fiber Membrane)

[0099] A doping solution for forming hollow fibers was prepared from the ODPA-HAB.sub.5-ODA.sub.5 obtained from Preparation Example 1 [composition of doping solution: ODPA-HAB.sub.5-ODA.sub.5 25 wt %, mixture of N-methyl pyrrolidone (NMP) with propionic acid (PA) (NMP:PA=50:50 mol %) 65 wt %, ethylene glycol 10 wt %]. Then, the doping solution was supplied and ejected (air gap: 5 cm) together with Bohr solution (water) through a double spinning nozzle to obtain a hollow fiber membrane according to the conventional non-solvent induced phase separation method (NIPS). The resultant hollow fiber membrane was warmed to 400° C. at a rate of 10° C./min, and the isothermal state was maintained at 400° C. for 2 hours to obtain a thermally rearranged (benzoxazole-imide) copolymer.

[Example 11] Preparation of Ultrathin-Film Composite Membrane Including Thermally Rearranged Poly(Benzoxazole-Imide) Copolymer Support

[0100] The thermally rearranged poly(benzoxazole-imide) copolymer electrospun membrane obtained from Example 1 was coated with polydopamine (PDA) to carry out hydrophilization and then dipped into aqueous meta-phenylene diamine (MPD) solution. After removing an excessive amount of solution, 0.15% trimesoyl chloride hexane solution was poured to the surface of the support to carry out interfacial polymerization. Then, hexane was washed and the resultant product was allowed to stand in air and cured in an oven at 90° C. to obtain an ultrathin-film composite membrane having a crosslinked polyamide thin-film active layer formed on the thermally rearranged poly(benzoxazole-imide) copolymer support (electrospun membrane).

[Example 12] Preparation of Ultrathin-Film Composite Membrane Including Thermally Rearranged Poly(Benzoxazole-Imide) Copolymer Support

[0101] The thermally rearranged poly(benzoxazole-imide) copolymer hollow fiber membrane obtained from Example 10 was used as a support and 3.5 wt % aqueous meta-phenylene diamine (MPD) solution was allowed to flow into the hollow fibers. After removing an excessive amount of solution, 0.15% trimesoyl chloride hexane solution was allowed to flow into the hollow fibers to carry out interfacial polymerization. Then, an excessive amount of solution was removed again and the resultant product was allowed to stand in air and dried to obtain an ultrathin-film composite membrane having a crosslinked polyamide thin-film active layer formed on the thermally rearranged poly(benzoxazole-imide) copolymer support (hollow fiber membrane).

[0102] FIG. 2 illustrates the attenuated total reflectance-infrared ray (ATR-IR) spectrum of the porous thermally rearranged poly(benzoxazole-imide) copolymer support according to each of Examples 1-9. It can be seen that O—H stretching peaks appearing at around 1480 cm.sup.−1 and 1054 cm.sup.−1 have disappeared and two clear peaks derived from a typical benzoxazole ring have appeared. This suggests that a benzoxazole ring was formed during the heat treatment process. In addition, absorption bands unique to an imide group are observed at around 1784 cm.sup.−1 and 1717 cm.sup.−1. This demonstrates excellent thermal stability of an aromatic imide connection ring even under a high thermal rearrangement temperature of 400° C.

[0103] The following Table 2 shows the mechanical properties, average pore diameter, porosity and water permeance of the thermally rearranged poly(benzoxazole-imide) copolymer support (electrospun membrane) according to Example 1 as a function of thickness.

TABLE-US-00002 TABLE 2 Mechanical properties (MD/TD) Average Tensile pore Water Thickness strength Elongation diameter permeance (μm) (Mpa) (%) (μm) Porosity (%) (LMH) 20 35/51 11/28 0.22 75 8541 40 23/29  6/13 0.20 64 3304 60 23/34  5/12 0.12 61 2334 MD: machine direction, TD: transverse direction

[0104] It can be seen from Table 2 that the thermally rearranged poly(benzoxazole-imide) copolymer support according to the present disclosure has excellent mechanical properties, even though it has a significantly smaller thickness than the thickness (100-200 μm) of the conventional porous support applied as a membrane for water treatment, and has significantly high porosity, and thus provides significantly improved water permeance.

[0105] In addition, FIG. 3 illustrates the ATR-IR spectrum of each of the porous thermally rearranged poly(benzoxazole-imide) copolymer support (a) according to Example 1 and the ultrathin-film composite membrane (b) according to Example 11. As shown in FIG. 3, unlike the porous thermally rearranged poly(benzoxazole-imide) copolymer support (a) according to Example 1, the ultrathin-film composite membrane (b) according to Example 11 shows N—H stretching vibration at around 3444 cm.sup.−1 and 3310 cm.sup.−1, and C═O stretching and N—H plane bending at around 1667 cm.sup.−1 and 1542 cm.sup.−1, respectively.

[0106] FIG. 4 is a thermogravimetric analysis (TGA) graph illustrating the weight reduction characteristics of the porous thermally rearranged poly(benzoxazole-imide) copolymer support according to Example 1 depending on various thermal rearrangement conditions (0.5 hours at 375° C., 1 hours at 375° C., 2 hours at 375° C., 2 hours at 400° C.). The thermogravimetric analysis was carried out by heating a sample to 400° C. at a rate of 10° C./min, maintaining the sample at 400° C. for 2 hours and heating the sample to 800° C. In general, weight reduction caused by thermal rearrangement is about 9% when thermal rearrangement is completed to 100% theoretically. As can be seen from FIG. 4, the weight reduction of pristine (support before thermal rearrangement) is 10% between 40 min and 160 min. This suggests that thermal rearrangement was performed smoothly. In addition, it is possible to calculate the thermal rearrangement degree of each treated support reversely from the quantitative weight reduction data thereof under each thermal rearrangement condition.

[0107] In addition, FIG. 5 is a photographic image illustrating the results of observation of the stability of the porous thermally rearranged poly(benzoxazole-imide) copolymer support according to Example 1 against an organic solvent. The organic solvent, dimethyl acetamide (DMAc), used for forming a membrane was used to carry out a chemical stability test. It can be seen from the test that the support (HPI) before thermal rearrangement was dissolved in the organic solvent, while the support (PBO) after thermal rearrangement was not dissolved in the organic solvent but retains its shape.

[0108] FIG. 6 illustrates the SEM images of the surface, active layer and the total membrane of each of the commercially available polysulfone-based composite membrane (a) for reverse osmosis, cellulose-based ultrathin-film composite membrane (b) for forward osmosis and the ultrathin-film composite membrane (c) according to Example 11. It can be seen that an ultrathin-film composite membrane having a polyamide thin-film layer formed thereon was prepared according to Example 11 and the polyamide thin-film layer has a thickness of 60 nm, which is about 3 times smaller than the thickness (209 nm) of the conventional polysulfone-based composite membrane for reverse osmosis. It can be also seen that the total thickness of the membrane is 16 μm, which is at least 12 times smaller than the total thickness (204 μm) of the conventional polysulfone-based composite membrane for reverse osmosis. In other words, it can be seen from FIG. 6 that the ultrathin-film composite membrane according to Example 11 has a significantly smaller thickness as compared to the conventional polysulfone-based composite membrane for reverse osmosis and cellulose-based ultrathin-film composite membrane, has a porous structure, and the active layer thereof is significantly thin, thereby minimizing concentration polarization occurring in the composite membrane and mass transport resistance. Therefore, it can be expected that the ultrathin-film composite membrane according to Example 11 shows excellent performance as a separation membrane and can be applied to a pressure retarded osmosis or forward osmosis process and used as an organic solvent nano-filtration membrane by virtue of its excellent heat resistance and chemical resistance of the support.

[0109] In addition, FIG. 7 is a graph illustrating the water permeance and salt rejection ratio of the ultrathin-film composite membrane according to Example 11 before and after the post-treatment (500 ppm NaOCl, 1000 ppm NaOCl) [charge: 2000 ppm NaCl (20° C.)]. After carrying out treatment with NaOCl, it is possible to improve water permeance by about at least two times or more, while not adversely affecting the salt rejection ratio. Thus, it can be seen that the ultrathin-film composite membrane according to the present disclosure is suitable for a forward osmosis process.

[0110] Further, FIG. 8 is a graph illustrating the water flux and power density of the ultrathin-film composite membrane according to an embodiment of the present disclosure [inducing solution: 1M NaCl (20° C.), charge: deionized water (20° C.), conventional polysulfone-based ultrathin-film composite membrane (HTI) available from Hydration Technology Innovations, ultrathin-film composite membranes according to the present disclosure TR 40 (thickness 40 μm), TR 60 (thickness 60 μm), TR40.sub.NaOCl (thickness 40 μm, treated with 1000 ppm of NaOCl for 10 minutes]. As shown in FIG. 8, while the conventional HTI shows a low power density of 5 W/m.sup.2, the ultrathin-film composite membrane (TR40.sub.NaOCl) according to the present disclosure provides a high power density up to 21 W/m.sup.2. In addition, after comparing TR40 with TR 60 in order to determine the resistance of the support depending on thickness, it can be seen that TR40 reduces mass transport resistance and shows high power density.

[0111] FIG. 9 is a graph illustrating the pure solvent permeance test results of the porous thermally rearranged poly(benzoxazole-imide) copolymer support according to Example 1. As shown in FIG. 9, while the permeance test is carried out by using various organic solvents, such as isopropyl alcohol (IPA), distilled water, chloroform, dimethyl formamide (DMF), tetrahydrofuran (THF), toluene, acetonitrile and heptane, the support shows high chemical resistance and high pure solvent permeance derived from high porosity. Thus, it can be seen that the support can be used not only as a support for organic solvent nano-filtration but also as an organic solvent nano-filtration membrane by virtue of its chemical resistance and heat resistance.

[0112] In addition, FIG. 10 illustrates the results of the observing a change in shape and structure of the porous thermally rearranged poly(benzoxazole-imide) copolymer support according to Example 1 in high-temperature DMF [(a) graph of dimensional change, (b) photograph taken by the naked eyes, (c) scanning electron microscopic (SEM) image] to determine the heat resistance and chemical resistance. Even under more severe conditions including high temperature (30° C., 60° C., 90° C., 120° C.) and DMF as a solvent, the support causes no significant change in terms of dimension, observation by naked eyes and scanning electron microscopic (SEM) images.

[0113] Further, FIG. 11 is a graph illustrating the THF permeance (a) and rejection ratio (b) of the ultrathin-film composite membrane according to Example 11. The test was carried out by using a volumetric cylinder in a polystyrene/THF solution at 30° C. under 30 bars with a flow rate of 50 L/hr. The permeate and charge were collected in the same manner to determine the rejection ratio by using HPLC-UV/Vis. As can be seen from FIG. 11, the ultrathin-film composite membrane shows a high permeance of 5 LMH/bar and a rejection ratio of at least 99% vs. polystyrene having a molecular weight of 236-1600 g/mol.

[0114] In addition, FIG. 12 is a graph illustrating the DMF permeance (a) and rejection ratio (b) of the ultrathin-film composite membrane according to Example 11. The test was carried out by using a volumetric cylinder in a 2 g/L polystyrene/DMF solution and 1 g/L dye solution at 30° C. under 30 bar with a flow rate of 50 L/hr. The dyes used for the test were Chrysoidine G (− charge, 249 g/mol), Methylene Orange (+ charge, 327 g/mol) and Brilliant Blue (+ charge, 826 g/mol). When carrying out the test, the volume of the permeate for a predetermined time was measured in the same manner as described above to calculate the permeance. The dye rejection ratio was determined by observing a difference in wavelength through UV spectroscopy. As can be seen from FIG. 12, the ultrathin-film composite membrane shows a high permeance of about 8 LMH/bar. It can be also seen that the rejection ratio profile depends on solute size regardless of the type of a charge.

[0115] In addition, FIG. 13 is a graph illustrating the high-temperature DMF permeance (a) and rejection ratio (b) of the ultrathin-film composite membrane according to Example 11. The ultrathin-film composite membrane according to Example 11 shows stable and excellent performance even under more severe conditions including high temperature (30° C., 60° C., 90° C.) and DMF as a solvent. In other words, since the solvent viscosity is decreased as the system temperature is increased, the permeance is increased while causing little change in rejection ratio. It is thought that since the active layer and support have excellent chemical stability even at high temperature, only the permeance is increased while the rejection ratio is maintained. This suggests that the ultrathin-film composite membrane can be used as an organic solvent nano-filtration membrane even under severe conditions.

[0116] Further, FIG. 14 is a scanning electron microscopic (SEM) image of the morphology of the ultrathin-film composite membrane according to Example 11, taken before and after using the membrane as an organic solvent nano-filtration membrane. There is no significant change in SEM images before and after using the membrane as an organic solvent nano-filtration membrane. Thus, it can be seen that the ultrathin-film composite membrane according to the present disclosure has excellent stability.

INDUSTRIAL APPLICABILITY

[0117] As described above, the ultrathin-film composite membrane including a thin-film active layer formed on a thermally rearranged poly(benzoxazole-imide) copolymer support has excellent thermal/chemical stability and mechanical properties so that it can resist even under high operating pressure, minimizes internal concentration polarization to provide high water permeance and high power density according thereto so that it may be applied to a pressure retarded osmosis or forward osmosis process. In addition, the ultrathin-film composite membrane shows excellent chemical/thermal stability against organic solvents, and particularly maintains nano-filtration performance even under the condition of a high-temperature organic solvent so that it may be applied to an organic solvent nano-filtration process.