FLAT SHEET GEOMETRY

FIELD OF INVENTION

The present invention relates to a method of making macroporous or mesoporous films in flat sheet geometry with polymers .

BECKGROUND OF THE INVENTION

Macro or mesoporous polymeric films have recently gained in ^¬ terest due to their potential use in many fields. For example, these films may be used as supporting materials in tissue en ^¬ gineering, as inorganic growth templates, as optical materi ^¬ als, as antireflection coatings, in catalysis, as bio or gas sensors, as dielectric materials for electronic devices, as stamps for soft lithography, or as etching masks. Several methods are known for isoporous structure formation in macroporous or mesoporous films from both homopolymers and co ^¬ polymers. The methods include annealing, lithography, electron beam sculpting, track etching, self assembly followed by selective etching or non-solvent induced phase separation, or by the so-called "breath figure assembly" method (G. Widawski et al . "Self-organized honeycomb morphology of star-polymer poly ^¬ styrene films", Nature, 1994, 369, 387) . A carbon disulphide solution of star-shaped polystyrene was cast on solid sub ^¬ strates with a moist airflow across the polymer solution surface to create porous polymer films with monodisperse pore size. The polystyrene self-assembles into an ordered pattern with hexagonally arranged pores ranging from 0.2-10 pm in di ^¬ ameter .

A continuous process of producing a honeycomb structure in flat sheet geometry films is known from US 2006/0266463 Al . In many cases, only two-dimensional, i.e. single-layer, arrays of pores were obtained at the film surface. Three-dimensional structures, however, composed of several layers of air-bubble sheets, are also known (cf. M. Srinivasarao et al . , "Three- dimensionally ordered array of air bubbles in a polymer film", Science, 2001, 292, 79) . Air bubbles generally are of a larger diameter inside the film with the smaller diameter top open at the film surface. Different pore sizes from 0.2 to 20 pm in diameter have been reported in the literature. When referring to the reported values of pore sizes, two measurements have been used to represent the pore diameter. One measurement re ^¬ fers to the diameter of the top opening of the pores, and the other is the maximum pore diameter of the pores, which is be ^¬ low the film surfaces.

The main factors known to affect the formation and size of the pores are casting conditions, including solution concentra ^¬ tion, and the nature of the material systems. Casting condi ^¬ tions such as humidity and airflow speeds have effects on the size of the air pores and the morphology of the cast films. Generally, increased humidity and reduced airflow lead to large air bubbles. The pore size can be adjusted by changing the airflow or concentration of the prepared polymer solutions .

Thus far macro or mesoporous polymeric films have been pro ^¬ duced in flat sheet geometry. Techniques for producing hollow fiber membranes, where the pores are connected to extend throughout the whole width of the polymer layer, are, however, known .

Z.-H. Wang et al . „Diffuse-In/Condense-Out Behavior of Glycer ^¬ ol Induces Formation of Composite Membranes with Uniform Pores", Macromolecular Materials and Engineering 2016, pages 36-41 discloses a method of producing composite polymer films with uniform pores in their surface layers. Cellulose acetate (CA) solution was spin-casted on a porous substrate filled with glycerol, with the evaporation of the volatile solvent of CA, the nonvolatile glycerol in the substrate firstly diffuses up into the CA solution layer and then condenses out and ar ^¬ ranges into uniform droplets on the casted layer. The glycerol droplets act as template for the precipitation of the CA mole ^¬ cules and thus result in the formation of the uniform pores in the CA layer. A spin casting process, as described in Z.-H. Wang et al . is designed for non-continuous production of la ^¬ boratory-scale membranes in flat sheet geometry. Such spin casting process will not enable a larger-scale continuous pro ^¬ duction of hollow-fiber membranes.

Polymeric hollow fiber membranes are generally prepared by phase separation of polymer solutions using spinnerets. Phase separation can be induced by cooling or by the presence of precipitant. Precipitant induced phase separation (NIPS) is taught for example in U.S. Patents 3,615,024; 5,066,401; and 6,024,872. Thermally induced phase separation (TIPS) is taught for example in U.S. Patents 4,702,836 and 7,247,238. Spinner ^¬ ets are e.g. described in published European Patent Applica ^¬ tion 0 277 619 A2, in U.S. Patent 4,385,017, and in WO 2007/007051, which are all incorporated herein by reference.

It is an object of the present invention to produce macro- porous or mesoporous films from homopolymers or copolymers in flat sheet geometry having an ordered porous structure in a reliable manner. Preferably, the pores are isoporous. The method for producing the films should be applicable in a con ^¬ tinuous form. DEFINITIONS

In the context of the present invention the term "porous film" or "porous, polymeric film" is meant to designate polymeric films having an upper and lower surface, which films exhibit two-dimensional, i.e. single-layer arrays of pores at the film surface. Preferably, the pores have a larger diameter inside the film with the top open at the film surface. The pores are termed macroporous or microporous, depending on their size, i.e. diameter. The term "macroporous" is meant to designate pores having a mean pore size as determined by electron mi ^¬ croscopy in the range of from 50 nm to 10 pm, preferably from 1 pm to 2 pm. The term mesoporous is meant to designate pores having a mean pore size in the range of from 2 nm to less than 50 nm according to IUPAC (International Union of Pure and Ap ^¬ plied Chemistry), K.S.W. Sing et al . "Reporting physisorption data for gas/solid systems with special reference to the de ^¬ termination of surface area and porosity" , Pure Appl . Chem., 1985, 57, 603.

The term "isoporous" is meant to designate pores having pore size dispersity, i.e. ratio of the maximum pore diameter to the minimum pore diameter, of at most 3, preferably at most 2.

The pore sizes and pore size distribution can e.g. be deter ^¬ mined using microscopy such as electron microscopy. Scanning electron microscopy was used to take the image of the surface of the film and the size and the distribution of the pores on the surface of the film were determined by using the software IMS V15Q4 available from Imagic Bildverarbeitung AG, Glattbrugg, Switzerland.

The term "polymeric membrane", "porous membrane" or "membrane" as used herein is meant to designate porous films where the pores are connected to extend throughout the entire width of the polymer layer. Although the term porous polymeric films as used herein is not meant to exclude porous polymeric mem ^¬ branes, preferred porous polymeric films are those which do not exhibit pores which are connected to extend throughout the entire width of the polymer layer.

The term "optionally" is meant to indicate that separating the flat sheet polymer film from the adjacent carrier is not a necessary method step; and the flat-sheet polymer film and ad ^¬ jacent carrier may be kept together.

The term "room temperature" is meant to designate the range of air temperatures that people prefer for indoor settings, which feel comfortable when wearing typical indoor clothing. As a medical definition, the range generally considered to be suit ^¬ able for human occupancy is between 15°C and 25°C, such as 23°C.

The term "volatile" is meant to designate solvents which is able to evaporate (it has a measurable vapor pressure) at pro ^¬ cessing temperatures.

The term "carrier substrate" or "support substrate" is meant to designate a support which on which the membrane prepared according to the process of the invention is formed. It may be of flat sheet geometry. If desired, the carrier may be removed from the membrane.

SUMMARY OF THE INVENTION

According to an embodiment, the present invention relates to a method for preparing a polymer film in flat sheet geometry, comprising :

a. providing a polymer solution of at least one copolymer or at least one homopolymer in a volatile solvent, and providing a polyol which is liquid at room temperature; b. applying the liquid polyol onto a carrier substrate by means of a doctor blade to provide a coated carrier sub ^¬ strate;

c. applying the polymer solution of at the least one copoly ^¬ mer or at least one homopolymer onto the coated carrier substrate adjacent to the liquid polyol solution by means of a doctor blade to provide a cast polymer solution;

d. thereafter immersing the cast polymer solution into a coagulation bath to produce polymer film in flat sheet geometry; and

e. optionally, separating the flat sheet polymer film from the adjacent carrier substrate.

Without wishing to be bound to any theory, according to the present invention macroporous or mesoporous film occurs during evaporation at the film surface of the polymer solution of at least one copolymer or at least one homopolymer while the polyol diffuses into and subsequently condenses out of the polymer solution of at least one copolymer or at least one homopolymer. The diffusion of the polyol is at least in part directed towards the polymer solution of at least one copoly ^¬ mer or at least one homopolymer by the presence of a carrier substrate .

The method according to the present invention leads to macroporous film formation having a mean pore size as determined by electron microscopy in the range of from 50 nm to 2 pm, preferably from 50 nm to 200 nm, when a copolymer is used, and 1 pm to 2 pm, when a homopolymer is used, with uniform pore sizes, i.e. a ratio of the maximum pore diameter to the minimum pore diameter, of at most 3, preferably at most 2. The method according to the present invention may also lead to mesoporous film formation having a mean pore size in the range of from 2 nm to less than 50 nm. Scanning electron microscopy was used to take the image of the surface of the film and the size and the distribution of the pores were determined by us ^¬ ing software IMS V15Q4, available from Imagic Bildverarbeitung AG, Glattbrugg, Switzerland.

Preferably the polyol is selected from glycerol, ethylene gly ^¬ col and pentaerythritol , most preferably glycerol.

DETAILED DESCRIPTION OF THE INVENTION

The at least one copolymer or at least one homopolymer used in the polymer solution for producing films flat sheet geometry according to the present invention preferably comprises two or more different polymer blocks when block copolymer is used 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 or random copolymer or homopolymers . 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 ( -methylstyrene ) , poly (para-methylstyrene ) , poly(t-butyl styrene) , poly (trimethylsilylstyrene) , poly(4- vinylpyridine ) , poly ( 2-vinylpyridine ) , poly (vinyl cyclohex- ane) , 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), poly- acrylamide, poly-N-alkylacrylamide, polysulfone, polyaniline, polypyrrole, polytriazole, polyvinylimidazole, polytetrazole, polyethylene diamine, poly (vinyl alcohol), polyvinylpyrrolidone, polyoxadiazole, polyvinylsulfonic acid, polyvinyl phosphonic acid or polymers.

Preferred block copolymers for use in the present invention are selected from polystyrene-jb-poly ( 4-vinylpyridine ) copoly ^¬ mers, poly (α-methylstyrene ) -jb-poly ( 4-vinylpyridine ) copoly ^¬ mers, poly (para-methylstyrene ) -jb-poly ( 4-vinylpyridine ) copoly ^¬ mers, poly ( t-butylstyrene) -jb-poly ( 4-vinylpyridine) copolymers, poly (trimethylsilylstyrene) -jb-poly ( 4-vinylpyridine) copoly ^¬ mers, polystyrene-jb-poly ( 2-vinylpyridine ) copolymers, poly(a- methylstyrene ) -jb-poly ( 2-vinylpyridine ) copolymers, poly (para- methylstyrene ) -jb-poly ( 2-vinylpyridine ) copolymers, poly(t- butylstyrene ) -jb-poly ( 2-vinylpyridine ) copolymers, poly (tri ^¬ methylsilylstyrene ) -jb-poly ( 2-vinylpyridine ) copolymers, poly- styrene-jb-polybutadiene copolymers, poly (a-methylstyrene ) -b- polybutadiene copolymers, poly (para-methylstyrene ) -jb-polybuta- diene copolymers, poly ( t-butylstyrene) -jb-polybutadiene copoly ^¬ mers, poly (trimethylsilylstyrene) -jb-polybutadiene copolymers, polystyrene-jb-polyisoprene copolymers, poly (α-methylstyrene ) - jb-polyisoprene copolymers, poly (para-methylstyrene ) -jb-polyiso- prene copolymers, poly ( t-butylstyrene) -jb-polyisoprene copoly ^¬ mers, poly (trimethylsilyl-styrene) -jb-polyisoprene copolymers, polystyrene-jb-poly (ethylene-stat-butylene) copolymers, poly(a- methylstyrene) -jb-poly (ethylene-stat-butylene) copolymers, po ^¬ ly (para-methylstyrene) -jb-poly (ethylene-stat-butylene) copoly ^¬ mers, poly ( t-butylstyrene) -jb-poly (ethylene-stat-butylene) co ^¬ polymers, poly (trimethylsilylstyrene) -jb-poly (ethylene-stat- butylene) copolymers, polystyrene-jb- (ethylene-alt-propylene) copolymers, poly (α-methylstyrene) -b- (ethylene-alt-propylene) copolymers, poly (para-methylstyrene) -b- (ethylene-alt-propy ^¬ lene) copolymers, poly ( t-butylstyrene) -b- (ethylene-alt-propy ^¬ lene) copolymers, poly (trimethylsilylstyrene) -b- (ethylene-alt- propylene) copolymers, polystyrene-jb-polysiloxane copolymers, poly (α-methylstyrene ) -jb-polysiloxane copolymers, poly (para- methylstyrene ) -jb-polysiloxane copolymers, poly(t- butylstyrene ) -j -polysiloxane copolymers, poly ( trimethyl ^¬ silylstyrene ) -j -polysiloxane copolymers, polystyrene-jb- polyalkylene oxide copolymers, poly ( -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-jb-poly-ε- caprolactone copolymers, poly ( -methylstyrene ) -jb-poly-ε- caprolactone copolymers, poly (para-methylstyrene) -jb-poly-ε- caprolactone copolymers, poly ( t-butylstyrene) -jb-poly-ε- caprolactone copolymers, poly (trimethylsilylstyrene) -jb-poly-ε- caprolactone copolymers, polystyrene-jb-poly (methyl methacry- late) copolymers, poly (α-methylstyrene ) -jb-poly (methyl methac- rylate) copolymers, poly (para-methylstyrene ) -jb-poly (methyl methacrylate ) copolymers, poly ( t-butylstyrene) -jb-poly (methyl methacrylate ) copolymers, poly (trimethylsilylstyrene) -b- poly (methyl methacrylate) copolymers, polystyrene-jb- poly (methyl acrylate) copolymers, poly (a-methylstyrene ) -b- poly (methyl acrylate) copolymers, poly (para-methylstyrene ) -b- poly (methyl acrylate) copolymers, poly ( t-butylstyrene) -b- poly (methyl acrylate) copolymers, poly (trimethylsilylstyrene) - jb-poly (methyl acrylate), polystyrene-jb-poly (hydroxyethyl methacrylate) copolymers, poly (α-methylstyrene ) -jb-poly (hydroxyl- ethyl methacrylate) copolymers, poly (para-methylstyrene ) -b- poly (hydroxyethyl methacrylate) copolymers, poly(t- butylstyrene ) -jb-poly (hydroxyethyl methacrylate) copolymers, poly (trimethylsilylstyrene) -jb-poly (hydroxyethyl methacrylate) copolymers, polystyrene-jb-polyacrylamide copolymers, poly(a- methylstyrene ) -jb-polyacrylamide copolymers, poly (para- methylstyrene ) -jb-polyacrylamide copolymers, poly(t- butylstyrene ) -jb-polyacrylamide copolymers, poly ( trimethyl ^¬ silylstyrene ) -jb-polyacrylamide copolymers, polystyrene-jb- 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-j -poly- vinylpyrrolidone copolymers, poly ( -methylstyrene ) -j -poly- vinylpyrrolidone copolymers, poly (para-methylstyrene) -j -poly- vinylpyrrolidone copolymers, poly ( t-butylstyrene) -jb-poly- vinylpyrrolidone copolymers, poly (trimethylsilylstyrene) -b- poly-vinylpyrrolidone copolymers, polystyrene-j -poly-vinyl- cyclohexane copolymers, polystyrene-j -poly-vinylcyclohexane copolymers, polystyrene-j -poly (vinyl-cyclohexane ) copolymers, polystyrene-j -poly-vinylcyclohexane copolymers, poly (tri ^¬ methylsilylstyrene ) -j -poly (vinylcyclo-hexane ) copolymers .

The 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 copolymers preferably have a molecular weight between 25 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 par ^¬ ticular fine manner through selection of the molecular weight. The polymer preferably makes up a percentage by weight between 5 wt . % and 20 wt.%, and most preferably between 8 wt . % and 15 wt . % of the polymer solution.

Preferred homopolymers for use in the present invention are selected from cellulose acetate (CA) , cellulose acetate butyr- ate (CAB), polystyrene (PS), polyether sulfone (PES), polysulfone (PSf ) , polyphenylene sulfone (PPSU) , polyetherimide (PEI), polyacrylonitrile (PAN), polyvinyledenefluoride (PVDF) , matrimid.

The homopolymers preferably have a molecular weight between 40 kg/mol and 200 kg/mol, in particular between 50 kg/mol and 150 kg/mol. In this range, the pore size can be adjusted in a par- ticular fine manner through selection of the molecular weight. The polymer preferably makes up a percentage by weight between 5 wt . % and 20 wt.%, and most preferably between 8 wt . % and 15 wt . % of the polymer solution.

Several solvents are suitable for preparing the polymer solu ^¬ tions. Preferred solvents include diethyl ether, 1,4-dioxane, tetrahydrofuran (THF) , carbon disulfide, acetonitrile, ace ^¬ tone, and/or other low boiling solvent /solvents . Mixture of two or more volatile solvents can be used. Mixture of high boiling solvent/s and low boiling solvent/s can also be used in that case high boiling solvents include dimethyl formamide, dimethyl sulfoxide, dimethyl acetamide, N-methyl-2-pyrrolidone (NMP) , etc.

According to a further preferred embodiment of the present in ^¬ vention, the polymer solution comprises at least one metal compound. Preferably the compounds are selected from tetrae- thyl ortho silicate (TEOS), stannic chloride (SnCl ₄ ), chloroauric acid (HAuCl ₄ ) , titanium tetrachloride (TiCl ₄ ) . Preferably the metal is selected from main group or transition metals of the periodic system of elements, such as Ti, Sn or Si .

An illustrative setup of an assembly according to an embodi ^¬ ment of the present invention is schematically shown in Fig. 1. Fig. 1 shows a setup for casting a macroporous or mesoporous film in flat-sheet geometry wherein a nonwoven sheet is provided as a carrier substrate onto which glycerol as polyol is applied by casting blade 5. Subsequently, macroporous or mesoporous film forming layer is cast onto the glycerol layer by casting blade 6, the cast solution is di ^¬ rected through an air gap and then immersed into a coagulation bath . Fig. 2 depicts a macroporous film prepared with the method ac ^¬ cording to the present invention.

Accordingly, the present invention is directed towards a meth ^¬ od for the preparation of macroporous or mesoporous polymer films in flat sheet geometry. The method according to the pre ^¬ sent invention reliably produces macroporous or mesoporous polymer films in flat sheet geometry having an ordered pore structure and thickness of preferably about one micrometer. The method involves the purging or casting a polyol adjacent to a film forming polymer solution of at least one copolymer or at least one homopolymer in a suitable solvent before the solution is immersed into a coagulation bath. The methods also require the presence of a carrier.

The method makes macroporous or mesoporous film formation pos ^¬ sible with a single step processing method.