INTEGRALLY ASYMMETRICAL, ISOPOROUS BLOCK COPOLYMER MEMBRANES IN FLAT SHEET GEOMETRY

Abstract

The present invention relates to a method of producing block copolymer membranes in fat sheet geometry having a surface morphology comprising ordered, isoporous nanopores. The method comprises providing a polymer solution of at least one amphiphilic block copolymer in a solvent; applying the polymer solution onto a substrate to provide a cast polymer solution; applying an electrical field to the cast polymer solution in a direction substantially perpendicular to the cast polymer solution; and thereafter immersing the cast polymer solution into a coagulation bath thereby inducing phase inversion to produce an integrally asymmetrical block copolymer membrane in flat sheet geometry.

Claims

1. A method for producing a block copolymer membrane in flat sheet geometry, the method comprising: providing a polymer solution of at least one amphiphilic block copolymer in a solvent; applying the polymer solution onto a substrate to provide a cast polymer solution; applying an electrical field having a field strength from 0.5 kV/cm to 10 kV/cm for a time period between 1 second and 120 seconds to the cast polymer solution in a direction substantially perpendicular to the cast polymer solution; and thereafter leaving the cast polymer solution to rest for a period of time of 0 seconds to 120 seconds and immersing the cast polymer solution into a coagulation bath thereby inducing phase inversion to produce block copolymer membrane in flat sheet geometry.

2. The method of claim 1, wherein the electrical field applied to the cast polymer solution is a direct current electrical field.

3. The method of claim 1, wherein the electrical field applied to the cast polymer solution is created by two flat electrodes.

4. The method of claim 3, wherein a gap between the electrodes is between 4 cm and 10 cm.

5. The method of claim 1 wherein the electrical field is applied to the cast polymer solution for a time period between 5 seconds and 1 minute.

6. The method of claim 1, wherein the amphiphilic block copolymer is selected from polystyrene-b-poly(4-vinylpyridine) copolymers, poly(α-methylstyrene)-b-poly(4-vinylpyridine) copolymers, poly(para-methylstyrene)-b-poly(4-vinylpyridine) copolymers, poly(t-butylstyrene)-b-poly(4-vinylpyridine) copolymers, poly(trimethylsilylstyrene)-b-poly(4-vinylpyridine) copolymers, polystyrene-b-poly(2-vinylpyridine) copolymers, poly(α-methylstyrene)-b-poly(2-vinylpyridine) copolymers, poly(para-methylstyrene)-b-poly(2-vinylpyridine) copolymers, poly(t-butylstyrene)-b-poly(2-vinylpyridine) copolymers, poly(trimethylsilylstyrene)-b-poly(2-vinylpyridine) copolymers, polystyrene-b-polybutadiene copolymers, poly(α-methylstyrene)-b-polybutadiene copolymers, poly-(para-methylstyrene)-b-polybutadiene copolymers, poly(t-butylstyrene)-b-polybutadiene copolymers, poly(trimethyl-silylstyrene)-b-polybutadiene copolymers, polystyrene-b-polyisoprene copolymers, poly(α-methylstyrene)-b-polyiso-prene copolymers, poly(para-methylstyrene)-b-polyisoprene copolymers, poly(t-butylstyrene)-polyiso-prene copolymers, poly(trimethylsilyl-styrene)-b-polyisoprene copolymers, polystyrene-b-poly(ethylene-stat-butylene) copolymers, poly(α-methylstyrene)-b-poly(ethylene-stat-butylene) copolymers, poly(para-methylstyrene)-b-poly(ethylene-stat-butylene) copolymers, poly(t-butylstyrene)-b-poly(ethylene-stat-butylene) copolymers, poly(trimethyl-silylstyrene)-b-poly(ethylene-stat-butylene) copolymers, polystyrene-b-(ethylene-alt-propylene) copolymers, poly(α-methylstyrene)-b-(ethylene-alt-propylene) copolymers, poly(para-methylstyrene)-b-(ethylene-alt-propylene) copolymers, poly(t-butylstyrene)-b-(ethylene-alt-propylene) copolymers, poly(trimethylsilylstyrene)-b-(ethylene-alt-propylene) copolymers, polystyrene-b-polysiloxane copolymers, poly(α-methylstyrene)-b-polysiloxane copolymers, poly(para-methylstyrene)-b-polysiloxane copolymers, poly(t-butylstyrene)-b-polysiloxane copolymers, poly(trimethylsilylstyrene)-b-polysiloxane copolymers, polystyrene-b-polyalkylene oxide copolymers, poly(α-methylstyrene)-b-polyalkylene oxide copolymers, poly(para-methylstyrene)-b-polyalkylene oxide copolymers, poly(t-butylstyrene)-b-polyalkylene oxide copolymers, poly(trimethyl-silylstyrene)-b-polyalkylene oxide copolymers, polystyrene-b-poly-ε-caprolactone copolymers, poly(α-methylstyrene)-b-poly-ε-caprolactone copolymers, poly(para-methylstyrene)-b-poly-ε-caprolactone copolymers, poly(t-butylstyrene)-b-poly-ε-caprolactone copolymers, poly(trimethylsilylstyrene)-b-poly-ε-caprolactone copolymers, polystyrene-b-poly(methyl methacrylate) copolymers, poly(α-methylstyrene)-b-poly(methyl methacrylate) copolymers, poly(para-methylstyrene)-b-poly(methyl methacrylate) copolymers, poly(t-butylstyrene)-b-poly(methyl methacrylate) copolymers, poly(trimethylsilylstyrene)-b-poly(methyl methacrylate) copolymers, polystyrene-b-poly(methyl acrylate) copolymers, poly(α-methylstyrene)-b-poly(methyl acrylate) copolymers, poly(para-methylstyrene)-b-poly(methyl acrylate) copolymers, poly(t-butylstyrene)-b-poly(methyl acrylate) copolymers, poly(trimethylsilylstyrene)-b-poly(methyl acrylate), polystyrene-b-poly(hydroxyethyl methacrylate) copolymers, poly(α-methylstyrene)-b-poly(hydroxyethyl methacrylate) copolymers, poly(para-methylstyrene)-b-poly(hydroxyethyl methacrylate) copolymers, poly(t-butylstyrene)-b-poly(hydroxyethyl methacrylate) copolymers, poly(trimethylsilylstyrene)-b-poly(hydroxyethyl methacrylate) copolymers, polystyrene-b-polyacrylamide copolymers, poly(α-methylstyrene)-b-polyacrylamide copolymers, poly(para-methylstyrene)-b-polyacrylamide copolymers, poly(t-butylstyrene)-b-polyacrylamide copolymers, poly(trimethyl-silylstyrene)-b-polyacrylamide copolymers, polystyrene-b-poly(vinyl alcohol) copolymers, poly(α-methylstyrene)-b-poly(vinyl alcohol) copolymers, poly(para-methylstyrene)-b-poly(vinyl alcohol) copolymers, poly(t-butylstyrene)-b-poly(vinyl alcohol) copolymers, poly(trimethylsilylstyrene)-b-poly(vinyl alcohol) copolymers, polystyrene-b-polyvinylpyrrolidone copolymers, poly(α-methylstyrene)-b-polyvinylpyrrolidone copolymers, poly(para-methylstyrene)-b-polyvinylpyrrolidone copolymers, poly(t-butylstyrene)-b-polyvinylpyrrolidone copolymers, poly(trimethylsilylstyrene)-b-polyvinyl-pyrrolidone copolymers, polystyrene-b-poly-vinylcyclohexane copolymers, polystyrene-b-poly-vinylcyclohexane copolymers, poly(trimethylsilylstyrene)-b-polyvinyl-cyclohexane copolymers and mixtures thereof.

7. The method according to claim 1, wherein the amphiphilic block copolymer has a polydispersity of less than 2.5

8. The method according to claim 1, wherein the solvent for the polymer is selected from diethyl ether, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, dimethyl-sulfoxide, acetonitrile, dioxane, acetone, tetrahydrofuran, and mixtures thereof.

9. The method according to claim 1, wherein the polymer solution further comprises an organic metal salt, wherein the metal is an element of the second main group of the periodic system.

10. The method according to claim 1, wherein the polymer solution further comprises a carbohydrate selected from a multifunctional phenol, a multifunctional organic acid, saccharose, D(+)-glucose, D(−)-fructose, cyclodextrin, and mixtures thereof.

11. The method according to claim 1, wherein the polymer solution further comprises cyclodextrin.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0024] The at least one amphiphilic block copolymer used in the polymer solution for producing the flat sheet membranes according to the present invention preferably comprises two or more different polymer blocks such as blocks A, B; or A, B, C; or A, B, C, D forming block copolymers of the configuration A-B, A-B-A, A-B-C, A-B-C-B-A, A-B-C-D, A-B-C-D-C-B-A or multiblock copolymers based on the aforementioned configurations. Multiblock copolymers comprise structures of the base configurations that repeat multiple times. The polymer blocks are preferably selected from the group consisting of polystyrene, poly(α-methylstyrene), poly(para-methylstyrene), poly(t-butyl styrene), poly(trimethylsilylstyrene), poly(4-vinylpyridine), poly(2-vinylpyridine), poly(vinyl cyclohexane), polybutadiene, polyisoprene, poly(ethylene-stat-butylene), poly(ethylene-alt-propylene), polysiloxane, poly(alkylene oxide) such as poly(ethylene oxide), poly-ε-caprolactone, polylactic acid, poly(alkyl methacrylate) such as poly(methyl methacrylate), polymeth-acrylic acid, poly(alkyl acrylate) such as poly(methyl acrylate), poly(acrylic acid), poly(hydroxyethyl methacrylate), polyacrylamide, poly-N-alkylacrylamide, polysulfone, polyaniline, polypyrrole, polytriazole, polyvinylimidazole, polytetrazole, polyethylene diamine, poly(vinyl alcohol), polyvinylpyrrolidone, polyoxadiazole, polyvinylsulfonic acid, polyvinyl phosphonic acid or polymers.

[0025] Preferred amphiphilic block copolymers for use in the present invention are selected from polystyrene-b-poly(4-vinylpyridine) copolymers, poly(α-methylstyrene)-b-poly(4-vinylpyridine) copolymers, poly(para-methylstyrene)-b-poly(4-vinylpyridine) copolymers, poly(t-butylstyrene)-b-poly(4-vinylpyridine) copolymers, poly(trimethylsilylstyrene)-b-poly(4-vinylpyridine) copolymers, polystyrene-b-poly(2-vinylpyridine) copolymers, poly(α-methylstyrene)-b-poly(2-vinylpyridine) copolymers, poly(para-methylstyrene)-b-poly(2-vinylpyridine) copolymers, poly(t-butylstyrene)-b-poly(2-vinylpyridine) copolymers, poly(trimethylsilylstyrene)-b-poly(2-vinylpyridine) copolymers, polystyrene-b-polybutadiene copolymers, poly(α-methylstyrene)-b-polybutadiene copolymers, poly(para-methylstyrene)-b-polybutadiene copolymers, poly(t-butylstyrene)-b-polybutadiene copolymers, poly(trimethylsilylstyrene)-b-polybutadiene copolymers, polystyrene-b-polyiso-prene copolymers, poly(α-methylstyrene)-b-polyiso-prene copolymers, poly(para-methylstyrene)-b-polyisoprene copolymers, poly(t-butylstyrene)-b-polyisoprene copolymers, poly(trimethylsilyl-styrene)-b-polyisoprene copolymers, polystyrene-b-poly(ethylene-stat-butylene) copolymers, poly(α-methylstyrene)-b-poly(ethylene-stat-butylene) copolymers, poly(para-methylstyrene)-b-poly(ethylene-stat-butylene) copolymers, poly(t-butylstyrene)-b-poly(ethylene-stat-butylene) copolymers, poly(trimethylsilylstyrene)-b-poly(ethylene-stat-butylene) copolymers, polystyrene-b-(ethylene-alt-propylene) copolymers, poly(α-methylstyrene)-b-(ethylene-alt-propylene) copolymers, poly(para-methylstyrene)-b-(ethylene-alt-propylene) copolymers, poly(t-butylstyrene)-b-(ethylene-alt-propylene) copolymers, poly(trimethylsilylstyrene)-b-(ethylene-alt-propylene) copolymers, polystyrene-b-polysiloxane copolymers, poly(α-methylstyrene)-b-polysiloxane copolymers, poly(para-methylstyrene)-b-polysiloxane copolymers, poly(t-butylstyrene)-b-polysiloxane copolymers, poly(trimethylsilylstyrene)-b-polysiloxane copolymers, polystyrene-b-polyalkylene oxide copolymers, poly(α-methylstyrene)-b-polyalkylene oxide copolymers, poly(para-methylstyrene)-b-polyalkylene oxide copolymers, poly(t-butylstyrene)-b-polyalkylene oxide copolymers, poly(trimethylsilylstyrene)-b-polyalkylene oxide copolymers, polystyrene-b-poly-ε-caprolactone copolymers, poly(α-methylstyrene)-b-poly-ε-caprolactone copolymers, poly(para-methylstyrene)-b-poly-ε-caprolactone copolymers, poly(t-butylstyrene)-b-poly-ε-caprolactone copolymers, poly(trimethylsilylstyrene)-b-poly-ε-caprolactone copolymers, polystyrene-b-poly(methyl methacrylate) copolymers, poly(α-methylstyrene)-b-poly(methyl methacrylate) copolymers, poly(para-methylstyrene)-b-poly(methyl methacrylate) copolymers, poly(t-butylstyrene)-b-poly(methyl methacrylate) copolymers, poly(trimethylsilylstyrene)-b-poly(methyl methacrylate) copolymers, polystyrene-b-poly(methyl acrylate) copolymers, poly(α-methylstyrene)-b-poly(methyl acrylate) copolymers, poly(para-methylstyrene)-b-poly(methyl acrylate) copolymers, poly(t-butylstyrene)-b-poly(methyl acrylate) copolymers, poly(trimethylsilylstyrene)-b-poly(methyl acrylate), polystyrene-b-poly(hydroxyethyl methacrylate) copolymers, poly(α-methylstyrene)-b-poly(hydroxyethyl methacrylate) copolymers, poly(para-methylstyrene)-b-poly(hydroxyethyl methacrylate) copolymers, poly(t-butylstyrene)-b-poly(hydroxyethyl methacrylate) copolymers, poly(trimethylsilylstyrene)-b-poly(hydroxyethyl methacrylate) copolymers, polystyrene-b-polyacrylamide copolymers, poly(α-methylstyrene)-b-polyacrylamide copolymers, poly(para-methylstyrene)-b-polyacrylamide copolymers, poly(t-butylstyrene)-b-polyacrylamide copolymers, poly(trimethylsilylstyrene)-b-polyacrylamide copolymers, polystyrene-b-poly(vinyl alcohol) copolymers, poly(α-methylstyrene)-b-poly(vinyl alcohol) copolymers, poly(para-methylstyrene)-b-poly(vinyl alcohol) copolymers, poly(t-butylstyrene)-b-poly(vinyl alcohol) copolymers, poly(trimethylsilylstyrene)-b-poly(vinyl alcohol) copolymers, polystyrene-b-polyvinyl-pyrrolidone copolymers, poly(α-methylstyrene)-b-polyvinyl-pyrrolidone copolymers, poly(para-methylstyrene)-b-polyvinyl-pyrrolidone copolymers, poly(t-butylstyrene)-b-polyvinyl-pyrrolidone copolymers, poly(trimethylsilylstyrene)-b-polyvinylpyrrolidone copolymers, polystyrene-b-poly-vinylcyclohexane copolymers, polystyrene-b-poly(vinylcyclohexane) copolymers, polystyrene-b-poly-vinylcyclohexane copolymers, poly(trimethylsilylstyrene)-b-poly(vinylcyclohexane) copolymers and the like.

[0026] The block copolymers and the polymer blocks used according to the present invention preferably have a polydispersity of less than 2.5, more preferably of less than 2.2, more preferably of less than 2.0. The polymer lengths of the at least two polymer blocks of the amphiphilic block copolymers are preferably selected with respect to each other so that a self-organization in the solvent leads to the formation of a spherical, cylindrical or co-continuous, in particular gyroidal, micelle structures or microphase structures in the solvent, in particular a length ratio between approximately 2:1 and approximately 10:1, in particular between approx. 3:1 and 6:1. These length ratios of the majority components to the minority components of the block copolymers lead to the desired micelle structure, i.e. to the inclusion of individual spherical micelles of the minority components in the bulk of the majority components or to cylindrical or continuous, for example gyroidal, micelle structures, in which the minority components form the cylinders or respectively gyroidal filaments or respectively branchings in the bulk of the majority components.

[0027] The block copolymers preferably have a molecular weight between 50 kg/mol and 200 kg/mol, in particular between 75 kg/mol and 150 kg/mol. In this range, the pore size can be adjusted in a particular fine manner through selection of the molecular weight. The polymer preferably makes up a percentage by weight between 10 wt. % and 50 wt. %, and most preferably between 15 wt. % and 35 wt. % of the polymer solution.

[0028] Several solvents are suitable for preparing the polymer solution. Preferred solvents include diethyl ether, dimethylformamide, dimethylacetamide, N-methylpyrrolidone, dimethylsulfoxide, acetonitrile, dioxane, acetone, and/or tetrahydrofurane. According to an embodiment a pure solvent or a solvent mixture is applied which is preferably selected such that different polymer blocks of the amphiphilic block copolymers are soluble up to different degrees and such that solvents are volatile to different degrees. The use of a solvent mixture supports the solidification of the self-organization and microphase formation on the surface of the membrane before immersion in the precipitation bath.

[0029] According to a further preferred embodiment of the present invention, the polymer solution comprises at least one metal salt. Preferably the metal is selected from an element of the second main group of the periodic system of elements, such as Mg, Ca or Sr or from non-toxic transition metals such as Fe. More preferably, the salt is an organic salt of Mg, Ca or Sr, most preferably magnesium acetate. The metals of the second main group of the periodic system are biocompatible making them preferred for membranes with biological or medical applications. The supporting effect of the salt in the phase separation can probably be explained in that the metal salt leads to the formation of partially charged polyelectrolytic micelle cores, which positively impact the precipitant-induced phase separation.

[0030] According to a still further preferred embodiment, the polymer solution comprises at least one carbohydrate, multifunctional alcohol, multifunctional phenol and/or multifunctional organic acid. Preferred carbohydrates include saccharose, D(+)glucose, D(−)-fructose and/or cyclodextrin, in particular α-cyclodextrin. Carbohydrates as used in the present invention lead to a stabilization of the isoporous separation-active surface during the phase inversion. The supporting effect of the at least one carbohydrate in phase separation can probably be explained in that the carbohydrates form hydrogen bonds with the hydrophilic block of the block copolymers.

[0031] A variety of materials can be selected as a substrate material, provided that it does not react with the polymer solution of at least one amphiphilic block copolymer in a solvent. Preferably the substrate material is a glass plate. According to another embodiment, the polymer solution is cast onto a glass plate using a doctor blade. Alternatively, the polymer solution is applied onto the substrate by spraying or dipping. Also in these embodiments the substrate material is preferably a glass plate.

[0032] The method of the present invention applies a phase separation process where the cast polymer solution is transferred into a coagulation bath. The cast polymer solution can be leave to rest for another period of time, preferably 0 seconds to 120 seconds, most preferably 0 to 60 seconds, before immersing into a coagulation bath. However, it should not dry out, as otherwise phase separation cannot occur in the coagulation bath.

[0033] The membrane precipitates in the coagulation bath by phase separation to form an integrally asymmetric polymer membrane.

[0034] The liquid in the coagulation bath preferably comprises water, methanol, ethanol or a mixture of two or more thereof.

[0035] According to a preferred embodiment of the present invention, the electrical field is generated by placing the cast polymer solution between two flat electrodes. Preferably the gap between the two electrodes is set as between 4 and 10 cm. Different voltages of direct current can be applied, such as between 5 kV and 50 kV, preferably for periods of time between 1 second and 5 minutes, more preferably between 5 seconds and 60 seconds, most preferably for periods of time between 10 seconds and 30 seconds.

[0036] The electric field strength is given by the high voltage and gap distance between the two electrodes. The electric field strength is calculated by the model of parallel plate capacitor, namely, the ratio between the high voltage and gap distance. The electric field is from 0.5 kV/cm to 10 kV/cm, preferably 0.9 kV/cm to 6.9 kV/cm. In this situation, the gap distance between two electrodes could e.g. be set as between 3 cm and 1 cm, such as 5.8 cm. Different voltages of direct current can be applied, such as between 5 kV and 40 kV.

[0037] An illustrative setup of the electrical field assembly according to the present invention is schematically shown in FIG. 1. FIG. 1 shows a cast polymer solution on a substrate to which, during initial evaporation of solvent, a direct current electrical field is applied in a direction substantially perpendicular to the cast polymer solution.

[0038] The invention is further described by the appending examples, which are of illustrative purposes only, and which shall not limit the present invention.

Example 1 and Comparative Example 1

[0039] A block copolymer of polystyrene-b-poly-4-vinylpyridine (PS-b-P4VP) with a molecular weight of 100 kg/mol and 25 wt % of P4VP was dissolved in a mixture of dimethylformamide (DMF) and tatrahydrofurane (THF) to produce a solution of at least one amphiphilic block copolymer in a solvent. The concentration of PS-b-P4VP in the solution was 25 wt %. DMF and THF each were present in a weight concentration of 37.5 wt %.

[0040] The polymer solution was applied onto a glass plate using a doctor blade to produce a cast polymer solution on the glass plate with 200 μm in thickness. Thereafter, the glass plate with the cast polymer solution was then placed between two flat electrodes having a gap of 5.8 cm between the two electrodes, and then a direct current electrical field of 30 kV for 10 seconds.

[0041] Thereafter, the glass plate with the cast polymer solution was thereafter—after a rest period of 5 seconds—immersed into a water bath thereby inducing phase inversion to produce block copolymer membrane in flat sheet geometry. After sufficient immersion time, the block copolymer membrane in flat sheet geometry was removed and dried in vacuum.

[0042] For comparison purposes, a comparative block copolymer membrane in flat sheet geometry membrane was produced in the same way as above with the exception of the electric field treatment. Instead, the glass plate with the cast polymer solution was left to rest for 15 seconds before immersing the glass plate with the cast polymer solution into a water bath.

[0043] The surface morphologies of both the block copolymer membrane in flat sheet geometry according to the present invention and the comparative block copolymer membrane in flat sheet geometry membrane were measured using scanning electron microscopy (SEM), as shown in FIGS. 2a and 2b.

[0044] FIG. 2 shows the surface morphology of membranes fabricated (a) by casting (5 sec) followed by an electric field with a voltage of 30 kV for 10 sec; (b) by casting (5 sec) followed by another 10 sec in air atmosphere. The concentration of PS (75 k)-b-P4VP (25 k) was 25 wt %.

[0045] The comparative PS-b-P4VP block copolymer membrane in flat sheet geometry membrane (FIG. 2b) shows parallel lamellae, whereas the PS-b-P4VP block copolymer membrane in flat sheet geometry membrane produced according to the present invention shows a morphology with ordered, isoporous nanopores (FIG. 2a).

Example 2 and Comparative Example 2

[0046] Example 1 was repeated with the exception that the concentration of PS-b-P4VP was 20 wt % and that an electrical field of 10 kV was applied for 10 seconds before immersing the glass plate with the cast polymer solution into a water bath. Again, for comparison, a comparative membrane was produced according to the method of Example 2 without electric field treatment. Instead, the glass plate with the cast polymer solution was left to rest for 5 seconds before immersing the glass plate with the cast polymer solution into a water bath.

[0047] The surface morphologies of both the block copolymer membrane in flat sheet geometry according to the present invention and the comparative block copolymer membrane in flat sheet geometry membrane were measured using scanning electron microscopy (SEM), as shown in FIGS. 3a and 3b.

[0048] FIG. 3 shows the surface morphology of membranes fabricated (a) by casting (5 sec) followed by an electric field with a voltage of 10 kV for 5 s; (b) by casting (5 sec) in an air atmosphere. The concentration of PS (75 k)-b-P4VP (25 k) was 20 wt %.

[0049] The comparative PS-b-P4VP block copolymer membrane in flat sheet geometry membrane (FIG. 3b) shows spares and disordered nanopores, whereas the PS-b-P4VP block copolymer membrane in flat sheet geometry membrane produced according to the present invention shows a morphology with ordered, isoporous nanopores (FIG. 3a).