WO1993025300A1

WO1993025300A1 - Ultrathin composite membranes

Info

Publication number: WO1993025300A1
Application number: PCT/US1993/005384
Authority: WO
Inventors: Charles R. Martin; Chao Liu
Original assignee: Research Corp Technologies Inc
Current assignee: Research Corp Technologies Inc
Priority date: 1992-06-05
Filing date: 1993-06-07
Publication date: 1993-12-23
Anticipated expiration: 1994-12-05

Abstract

The present invention relates to a method of forming defect-free ultrathin film composite membranes by the photochemical synthesis of an ultrathin polymer film on the surface of a microporous support membrane. In one particular embodiment, the method entails placing a support membrane on a filter paper which is saturated with a solution of the desired monomer(s). The solution then wicks to the top of the membrane, covering the surface with a thin solution film, where it is subjected to irradiation by ultraviolet (UV) light to initiate the polymerization of the ultrathin polymer film. The rate of polymerization can be enhanced by adding a photoinitiator to the gas phase above the membrane or into solution with the monomer or monomers to induce radical polymerization. The method results in a defect-free ultrathin composite membrane. In addition, to minimize photon penetration depth and confine the polymerization predominantly to the membrane surface, the incident at which the UV light strikes the membrane surface is kept at an acute angle. The ultrathin film composite membranes can be used in a vast number of chemical separation processes, bioreactors and sensors, energy conversion and drug delivery systems.

Description

ULTRATHIN COMPOSITE MEMBRANES

This invention was made with Government support under Grant Nos. 87-0173 and 90-0282 awarded by the Air Force Office of Scientific Research (AFOSR). The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

10 The present invention relates generally to defect- free ultrathin composite membranes and a method of forming the same by the photochemical synthesis of an ultrathin polymer film on the surface of a microporous support membrane. The ultrathin film composite membranes of the present invention ^■^ can be used in various applications of thin film technology, such as membrane separation processes, such as gas separation, reverse osmosis, pervaporation and liquid-liquid separation, bioreactors and sensors, energy conversions, drug delivery systems, and the like. 0

2. Description of the Prior Art

There has been a tremendous resurgence of interest in recent-...years in synthetic membranes and membrane-based processes due to their use in a wide diversity of potential

25 technological applications including numerous varieties of filtration and separation processes, bioreactors and sensors, and energy conversion and drug-delivery systems. These applications generally involve the permeation of gases or liquids through polymeric membranes. The transport of a 0 species through a membrane can occur by any of several different mechanisms, depending on the structure and nature of the membrane. In all cases, the driving force for the transport of a species through a membrane is a difference in free energy or chemical potential of that species across the 5 membrane. For example, the driving force may result from differences in pressure, concentration, electrical potential, or combinations of these factors.

The membrane prevents hydronamic flow and the species is transported by sorption and diffusion. Membranes are characteristically selective in what they transport and are said to be semipermeable if, under identical conditions, different molecular species are transported at different rates. The rate at which a species is transported through a membrane is a function of the membrane's permeability and is generally referred to as the flux. The satisfactory operation of membrane separation processes is dependent on such factors as the selectivity of the separation membrane, its flux and its resistance to chemical, biological and physical degradation.

Although an extremely thin membrane is preferred in order to obtain high fluxes, it is also desired to have membranes as free of flaws and imperfections as polymer chemistry and processing will allow. Unfortunately, however, these two desired characteristics do not go hand in hand. As the thickness of any polymeric film or membrane nears five micrometers (μM) and approaches molecular thickness on the order of a few nanometers, there is an increase in the probability of holes in the film structure or membrane. Obviously, even a minimal number of holes of larger-than- molecular size could result in less than sufficient operation. Accordingly, much of the effort in this art has been directed to the development of composite-type membranes.

Conventional composite membranes consist of a separating membrane and a porous supporting membrane to which the said separating membrane is applied. The supporting membrane and the separating membrane may be made of different materials. Typical composite-type membranes are disclosed in U.S. Patent Nos. 4,242,159 (Klimmek et al.), 4,277,344 (Cadotte) and 4,388,189 (Kawaguchi et al.). All of these patents relate to the development of composite-type membranes which include a microporous support and a thin layer of polymeric material coated thereon. These patents disclose composite membranes which are formed either by interfacial polymerization of the components forming the separating membrane on the supporting membrane or by applying a polymer solution to the supporting membrane followed by evaporation of the solvent. The result is a thin film of chemically selective material bonded to the surface of a microporous support membrane. Such composite membranes can provide good mechanical properties, high flux, and good chemical selectivity.

A variation of the above processes is plasma polymerization. This process of forming ultrathin membranes involves the formation of a thin polymer film on a porous substrate and exposing the same to a plasma of gas. By passing the monomer(s) over the substrate under conditions which generate a glow discharge, and which in turn creates the plasma, the monomeric surface layer is cross-linked by the resulting plasma. U.S. Patent Nos. 3,992,495 (Sano et al.), 4,533,369 (Okita) and 4,548,769 (Shimo ura et al.) disclose membranes formed by this technique.

Many of the thin film technological applications require the ability to prepare extremely thin, defect-free synthetic films, which are supported on microporous supports to form composite membranes. The U.S. Department of Energy, has recently published an extensive report on membrane separations, Membrane Separation Systems - A Research Needs Assessment, U.S. DOE, April 1990, which identifies a number of critical priorities in this area. One of the more significant goals is the development of methods for forming membranes with defect free "skins" (i.e. the films which accomplish the desired separation) which are less than 50nm thick. These ultrathin films must be completely defect-free because even minute numbers of defects will destroy chemical selectivity. The composite membranes known in the art, however, have not been completely satisfactory as they can or may exhibit a variety of defects resulting in the diminishing of the overall efficiency of the membrane processes. A more recent effort to produce composite membranes is disclosed in U.S. Patent No. 4,976,897 (Callahan, et al.) and relates to a method for curing a polymer deposited on top of a microporous membrane by ultraviolet irradiation. As disclosed therein, prior art membranes formed using ultraviolet irradiation have exhibited low flux values and/or little selectivity due to either the membranes being too thick or having too many defects. It is believed that the ultraviolet reactive mixtures, after coating, tend to "wick up" and fill the pores of the microporous support causing both insufficient flux and unsuitable effective thickness. Callahan, et al. avoids having the polymeric coating fill the pores of the microporous support by coating the support with a high viscosity UV curable polymer composition. In order to avoid defects, a double coating of the polymer composition was required. The photocuring of the large macromolecules of the polymeric material, however, still result in a film with a thickness much greater than that desired.

U.S. Patent No. 4,466,931 (Tanny) discloses a method for the manufacture of a microporous membrane involving the use of ultraviolet and/or electron beam radiation to induce rapid polymerization. The microporous membranes of Tanny have a thickness of less than 0.1 inch (0.254cm) with pore sizes of from 0.02 to 15 μm. Ultrathin films having a thickness of about 10 to about 5000nm are not disclosed. To date, defect-free ultrathin film composite membranes have not been commercially available.

It is therefore a primary object of the present invention to develop defect-free ultrathin film composite membranes, preferably having thicknesses of less than lOOnm. It is further object of the present invention to develop a method for synthesizing said defect-free ultrathin composite membranes using a radiation-induced polymerizable system. It is yet a further object of the present invention to develop processes of use for the defect-free ultrathin film composite membranes in a variety of thin film technological applications including chemical separation processes, bioreactors and sensors, energy conversion and drug delivery systems and the like.

The achievement of these and other objects will be apparent from the following description of the subject matter.

SUMMARY OF THE INVENTION The present invention is directed to a defect-free ultrathin film composite membrane comprised of an ultrathin polymer film and a microporous support membrane.

The present invention is further directed to a method of synthesizing an ultrathin film composite membrane comprising:

(a) forming a thin film of a radiation induced- polymerizable monomer or monomers on the surface of a microporous support membrane;

(b) polymerizing said monomer or monomers with a radiation source under conditions effective to form a polymer film on said membrane to provide an ultrathin composite membrane.

A photoinitiator may be used to enhance the rate of polymerization. The method of preparation of the present invention is based on the interfacial photochemical synthesis of an ultrathin high quality polymer film on the surface of a microporous support membrane and producing ultrathin films which are completely defect-free. This is extremely important because this means that the membranes of the present invention will achieve higher fluxes of permeate as compared to the prior art composite membranes. The method has proven to be extremely versatile as it has been used to synthesize a variety of functional ultrathin polymer films based on electroactive, photoactive and ion exchange polymers. The utility of the method of the present invention is demonstrated by the synthesis of a series of composite membranes exhibiting exceptional gas transport properties.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic illustration of one embodiment of the photopolymerization method of the present invention.

Figures 2a-2d are scanning electron micrographs of cross sections and surfaces of a microporous support membrane (an Anopore alumina filter) after 15 minutes (Fig. 2a), 20 minutes (Fig. 2b), 25 minutes (Fig. 2c) and 30 minutes (Fig. 2d) of photopolymerization in accordance with the embodiment of the method of the present invention shown in Figure 1.

DETAILED DESCRIPTION OF THE INVENTION

As indicated hereinabove, the ultrathin composite membranes of the present invention are defect-free and are comprised of an ultrathin polymer film formed on the surface of a microporous support membrane. As employed herein, "ultrathin film" means a film having a thickness of about lOnm to about 5000nm, preferably about 20 to about lOOnm. The ultrathin polymer film is formed by irradiating a thin film of monomer or monomers present on the surface of the support membrane. As employed herein, a "thin" film of monomer or monomers is one which when polymerized as described herein provides an ultrathin polymer film having a thickness in the range of about 10 to about 5000nm, perferably about 20 to about lOOnm. The thin film of radiation-poly erizable monomers(s) is irradiated, for example, by an ultraviolet light or electron beam source to polymerize the monomer(s) and results in the synthesis of an ultrathin composite membrane. The rate of polymerization of the monomer(s) can be enhanced by the addition of a photoinitiator. The photoinitiator may be added to the monomer solution or it can be present in the gaseous phase above the membrane. Some monomers, e.g. styrenics, can be photopolymerized without the use of a photoinitiator.

Photoinitiators, or photocatalysts, are in general, molecules which absorb a UV photon and then fragment into component parts which are free radicals. Examples of photoinitiators which may be used in the present invention may be any of those typically used in the art and include benzoin and its derivatives such as benzoin methyl ether, benzoin isobutyl ether, and benzoin ethyl ether, and benzoylperoxide, diethoxyacetophenone, benzophenone, 2-methoxy-l,2- diphenylethanone , trichloracetophenone , dimethyla inoethanolamine, and the like.

Those skilled in the art will appreciate that the particular choice of the photoinitiator will generally be a function of several formulation parameters including radiation polymerization response, cost effectiveness, toxicity and odor. The use of the photoinitiator in the gas phase helps to insure that photopolymerization occurs at the surface of the microporous support and not in bulk. When the photoinitiator is added to the monomer solution, there must be careful control of the polymerization time to insure photopolymerization only occurs near the surface.

With the option of using a photoinitiator either in solution with the monomer or monomers or in the gas phase over the membrane, the method of the present invention consequently is applicable to a vast number of monomers. It will be obvious to those skilled in the art that the choice of suitable radiation-reactive monomer(s) and/or oligσmer(s) and or combinations thereof will generally be a function of a variety of formulation and performance parameters such as film forming properties, acceptable viscosity, photoresponse, permeability and selectivity of the synthesized membrane, odor, toxicity and cost effectiveness. The one or more monomers or oligo ers or the combination thereof which are used in the present invention will have to be such that they will undergo a polymerization reaction in the presence of a radiation source, such as ultraviolet or electron beam irradiation, to form a solid polymer. Examples of such monomers which can be polymerized via the method of the present invention are those which contain a carbon-carbon double bond, i.e. a vinyl group, as part of the molecular structure. The method of the present invention may be practiced with the use of such suitable radiation- polymerizable monomers as styrenic monomers, methacryl monomers, and vinylene monomers such as vinyl chloride. Specific examples of monomers include acrylonitrile, methyl methacrylate and vinylidiene chloride and co-monomers of divinyl benzene and ethylene benzene in combination with sodium styrene sulfonate or vinylferrocene. The particular monomer(s) employed may be as a neat liquid or can be solubilized by any solvent which will not quench the free radicals of the monomers. Such solvents include nonpolar solvents such as benzene, toluene, polar solvents such as dimethylsulfoxide, acetonitrile and the like, and chlorinated solvents such as chloroform, methylene chloride and the like. The concentration of the monomer(s) used in the particular solvent will vary with the monomer/solvent system and with the desired film thickness. To illustrate the range of applicability of suitable monomers, ultrathin polymer films in accordance with the subject invention have been prepared from a large number of monomeric starting materials. A summary of these are presented in Table I. Table I . Representative polymer films prepared using the photopolymerization method of the present invention as shown in Figure I .

II . ^b CH₃

I

" Polymerized from neat acrylonitrile. Irradiation time = 60 min. Microporous alumina support. ^to Polymerized from neat methyl methacrylate. Irradiation time = 60 min. Microporous alumina support.

⁼ Polymerized from neat vinylidiene chloride. Irradiation time = 120 min. Microporous alumina support. Polymerized from a solution of 79% (w/w) sodium styrene sulfonate and 30% (w/w) technical grade divinylbenzene in dimethylsuIfoxide (62.1% (w/w)). Numerous films (e.g. Table II) were made on microporous alumina support, polycarbonate membrane support and a fluoropolyroer support.

* Polymerized from a 20% (w/w) vinylferrocene, 80% (w/w) technical grade divinylbenzene mixture. Irradiation time = 30 min. Microporous alumina support. As is evident from Table I, useful and functionalized co- and terpolymers can be prepared as well, including electroactive, photoactive and ion-exchange polymers. These polymers can be synthesized on a variety of microporous support membranes.

The microporous support membrane suitable for the present invention may be any of those typically used in the art. Those skilled in the art will understand that the material and specific porosity of a particular support will vary according to the ultimate desired end use. Generally, however, for the composite membranes of the present invention, the microporous membrane should have pores at the surface which are less than 0.1 μm in diameter, preferably less than 0.02 μm in diameter. Examples of suitable microporous support membranes include microporous alumina such as that sold under the trademark "Anopore", a fluoropoly er such as that sold under the trademark "Gore-Tex", polycarbonate membranes such as that sold under the trademark "Nucleopore", polyolifin membranes such as those sold under the trademark "Celgard", and the like.

The radiation-polymerizable monomer or mixture of monomers which will subsequently be polymerized, may be formed as a thin film on the surface of the microporous support by a variety of techniques known in the art, such as drop or dip coating. One technique which may be usefully employed is to saturate a porous material such as filter paper with the monomer or mixture of monomers in liquid form or dissolved in a solvent and then place the microporous support on the porous material whereby the liquid will wick up through the microporous support forming a thin liquid film on the upper surface of the support. This thin film is then subjected to photopolymerization to produce the required thin polymeric film. Where the monomer is used in a solvent, most of the solvent is evaporated during the polymerization treatment. Any solvent remaining is ultimately evaporated during the post-polymerization treatment.

It will be understood by those skilled in the art that irradiation may be effected using any means typically employed in the art for polymerizing and curing. Examples of suitable radiation sources include ultraviolet light, an electron beam source and the like. For ultraviolet light, a medium pressure mercury arc lamp may be used. Mercury vapor lamps are the most commonly available UV radiation sources though pulsed xenon and plasma arc UV generators are also known and used. The spectral range of interest for UV polymerizing and curing is in the 300 to 430nm region where most photoinitiators absorb.

The duration of the photopolymerization treatment depends on the nature of the polymeric system and, in particular, on the power of the photon source. For example, a Xe-arc lamp is a relatively low intensity source and would require photopolymerization times of 30 minutes or longer. If a higher intensity source were used, such as a laser, photopolymerization times could be decreased. As it is well understood, the shortening of the photopolymerization time is desirable, if such membranes are to be mass produced.

Following the initial polymerization process of the polymer film, the membrane may be subjected to a photocuring treatment to promote chain extension, cross-linking and to reduce the free volume of the film. As is the case for the photopolymerization treatment, the post-polymerization cure time is also dependent upon the intensity of the source being used. With the use of a Xe-arc source, cure times of about 1 to about 2 hours would be appropriate. If a laser source were available, the cure time, of course, could be substantially decreased.

In order to illustrate more specifically the method of preparing ultrathin composite membranes in accordance with the present invention, one particular embodiment thereof will be described with reference to the apparatus shown in Figure 1.

With respect to Figure 1, a microporous support membrane 2 is placed on a filter paper 4 which has been saturated with a solution of the desired monomer(s). The filter paper 4 is placed on a glass slide 6 and all are confined in a polymerization cell or chamber 8. The solution wicks into the pores 10, i.e. is drawn into the pores, rises to the upper surface of the membrane by capillary action and covers the surface with a thin film 12 of monomer solution. The surface is irradiated by an ultraviolet irradiation source 14, here being a 450 W xenon-arc lamp, which initiates the polymerization of the ultrathin polymer film 12. The rate of polymerization is enhanced by adding a photoinitiator 16 to the gas phase above the membrane via inlet 18 prior to irradiation. The photoinitiator 16 is added to the gas phase above the membrane by suitably entraining the photoinitiator in an inert gas. This is accomplished by first dissolving the photoinitiator 16 into a suitable solvent. Those skilled in the art will understand that it is desirable that the chosen solvent have a high boiling point so as to minimize the amount of solvent which will be introduced into the polymerization chamber 8. For example, for a benzoin derivative photoinitiator, a useful solvent is dimethylsulfoxide. After the photoinitiator is dissolved in a suitable solvent, an inert gas such as Ar or N₂ is then bubbled through the solution of the photoinitiator. As the photoinitiator has some volatility, photoinitiator vapor is entrained within the inert gas stream. This photoinitiator - containing gas stream is then admitted into the polymerization chamber 8 via inlet 18.

The thickness of the polymer film 12 is controlled by varying the irradiation time. The longer the irradiation time, the thicker the resulting polymer film will become as the monomer liquid will continue to wick up the pores and add to the polymer film forming on the surface.

The mechanism of film formation depends on the nature of the polymer. For the cross-linked polymers shown in Table I, the classical gel-point mechanism seemed to be operative. A series of electron micrographs taken during film formation are shown in Figures 2a through 2d. These figures show the surface of a microporous alumina support membrane taken at various times after the initiation of the photopolymerization treatment [Fig. 2a after 15 minutes of photopolymerization; Fig. 2b after 20 minutes; Fig. 2c after 25 minutes and Fig. 2d after 30 minutes]. The polymer used was the sulfonated styrenic (no. IV) shown in Table I, i.e. the terpolymer of divinyl benzene, ethylvinyl benzene and sodium styrene sulfonate. Although the total irradiation time was 30 minutes, no visible evidence for film formation can be seen until about 20 minutes (i.e. Fig. 2b). This long induction period is characteristic of the gel-point mechanism. This mechanism is also unique to cross linked polymers. Another factor to consider is that in the method of the present invention, the incident ultraviolet light 14 (or any other radiation source which is used) strikes the membrane surface 12 at an acute angle, as shown in Figure 1. This acute angle preferably is about 30° or less. This minimizes the penetration depth of the photons and consequently confines the polymerization process predominately to the membrane surface. The scanning electron micrographs of Figures 2a to 2d confirm that the polymerization is confined to the membrane surface when the polymerization is conducted in accordance with the subject invention. Nevertheless, at long polymerization times, the polymer does propagate into the pores, and polymeric fibers are obtained. This is, in general, undesirable since this would lead to large effective film thicknesses. As mentioned earlier, a post-polymerization photocure treatment may be typically employed in the method of the present invention. Again, the duration of the post- polymerization photocure treatment will depend mostly upon the power of the photon source being used. That is, the nature of the source will dictate the required length of photocure time. For example, if a Xe-arc lamp is employed, the post- polymerization photocuring treatment, will last for a period of about one to about two hours. The photocuring times could be decreased by using a higher intensity source such as a laser.

Figure 2d shows a scanning electron micrograph of a cross-section of a typical composite membrane made by the method of the present invention as illustrated in Figure 1. The film is so thin that it is close to the resolution limit for the microscope which was used (i.e., Phillips 505). The film thickness was determined by photographically enlarging such micrographs and making careful measurements of the film edge. A film thickness of 40nm was obtained. The gas- transport data, discussed below, provide independent corroboration for this measured film thickness.

The critical goal of the present invention is to produce defect-free and extraordinarily thin films. In order to address this factor, gas-transport measurements have been used. Permeability coefficients for 0₂ and N₂ in composite membranes of the present invention based on polymer IV (see Table I) and the method as illustrated in Figure 1 were measured using the conventional single gas permeation method defined by Koros, W.J. et al., J. Membr. Sci 2: 165-190 (1977) and Puleo, A.C. et al., J. Polyro. Sci Part B; Polym. Phys. 27: 2385-2406 (1989). The _,/_ ₂ selectivity coefficient value is the important measurement. As was shown by Hennis, J.M.S. et al., Science 220: 11-17 (1983), when even a minute number of defects are present in a thin polymer film, the selectivity coefficient will assume what has been termed the "Knudsen value", i.e. 0.93. In contrast, the thin polymer films of the prior art which are defect-free typically show selectivity coefficient values of about 4 to about 6.

Composite membranes of the present invention based on polymer IV of Table I which were as thin as 40nm exhibited selectivity coefficients of 8.0. See Table II.

Table II. Gas transport properties of composite membranes of the present invention based on polymer IV^*

Film Thickness ^to Permeability Coefficient Selectivity __ ~~1 0?=i_ Coefficient

2300 0.54 0.066 8.2

150 0.55 0.068 8.0

40 0.57 0.071 8.0

See Table 1. ^b Determined from electron micrographs. Irradiation times were 180 min. (2.3μm), 60 min. (150 n ) and 30 min. (40nm).

⁼ Calculated from gas flux data and film thickness. Units (in barrers) are 10^-αo cc(STP) cm/cm² Scm(Hg).

The data in Table II clearly show that these ultrathin films are essentially defect-free. In addition, because film thickness enters into the calculation of permeability coefficient, these data corroborate the film thickness determined by electron microscopy. More specifically, one skilled in the art to which the present invention pertains will appreciate that relatively thick films are essentially defect-free, the general proposition being that as the thickness of a film decreases, i.e. as the film becomes thinner, there is a greater likelihood that defects will be present therein. Furthermore, the scanning electron microscope measurement of thickness is very accurate for such thick films and in turn, the permeability coefficient for such thick films can be unequivocally calculated by using the known cell constant to convert the raw P/ t (change in pressure with time) data into permeance data. Permeance has units of cm³ (STP)/cm² S cm(Hg), i.e., volume of gas (cm³(STP)) per cm² of membrane area per second per unit of pressure (driving force across the membrane (cm(Hg) ) . To calculate the permeability coefficient from the permeance data, the permeance is simply multiplied by the film thickness. Thus, the permeability coefficient has units of cm³(STP) cm/cm² s cm(Hg). With respect to a thick film, an accurate permeability coefficient can be calculated from availably accurate permeance data and accurate thickness data obtained by an SEM measurement of the film.

A problem arises, however, in the SEM determination of film thickness for a thin or ultrathin film. The thickness of such a film is right at the resolution limit of the SEM instrument. In addition, one cannot know a priori that the permeance data are accurate because such a thin film might have defects. However, if the permeance data for a thin film, such as a 40nm thick film of the present invention, is multiplied by the SEM film thickness, i.e. 40nm, a permeability coefficient is obtained which is essentially identical to that of the thick film.

Consequently, one can conclude that the subject 40nm film is defect free. If it were not, the permeability coefficient would be much larger than the value for the thick film. Further, the SEM-determined 40nm measure must be accurate because if it were not, the permeability coefficient would not agree with the thick film value.

Due to the potential advantages in the areas of flexibility of chemical composition, durability, ease of construction and flux, the composite membranes of the present invention may be used in a variety of different membrane separation processes.

Both the permeability and selectivity coefficients obtained for polymer IV were quite high establishing that these materials could be useful in commercial gas-separation systems such as N₂/0₂ separations, reverse osmosis processes, pervaporation processes and liquid separation processes. One skilled in the art would also appreciate, based on the exceptional gas-transport data obtained, that the composite membranes of the present invention will also be beneficial in applications involving sensors and drug-delivery systems.

Specifically, the ultrathin composite membrane of the invention can be employed in gas seperations. Thus an 0₂/N₂ mixture could be separated with the ultrathin film of polymer IV (see Table I). This combination of gases with an ultrathin membrane of polymer IV in accordance with the subject invention can effectively be separated since it has an 0₂ permeability coefficient of 0.54 to 0.57 barrers and a N₂ permeability coefficient of 0.066 to 0.071. barrers, which provides a selectivity coefficient for 0₂ of 8.0 to 8.2 (see Table II).

Further, these ultrathin composite membranes have commercial potential in pervaporation. For example, a pervaporation process of separating methanol from methyl-t- butyl ether (MTBE) with an ultrathin membrane of polymer IV (see Table I) is promising since the selectivity coefficient for methanol relative to MTBE for the ultrathin membrane of this polymer is 4.0.

It is well known that nearly all chemical sensors use a thin polymer (or other material) film. It is further known that the response time of such a sensor is generally determined by the thickness of this film. It therefore follows that sensors based on the ultrathin film composite of the present invention should have very fast response times. The above preferred embodiments and examples are given to illustrate the scope and spirit of the present invention. The embodiments and examples described herein will make apparent, to those skilled in the art, other embodiments and examples. These other embodiments and examples are within the contemplation of the present invention. Therefore, the present invention should be limited only by the appended claims.

Claims

1 WHAT IS CLAIMED IS:

1. An ultrathin film composite membrane comprising an ultrathin polymer film formed on the surface of a microporous support membrane, said composite membrane having an 0₂/N₂ gas

5 transport selectivity coefficient greater than about 6.0 and an 0₂ premeability coefficient of greater than about 0.55 barrers.

2. The ultrathin film composite membrane according to Claim 1 wherein the ultrathin polymer film composite membrane

10 is about lOnm to about 5000nm in thickness.

3. The ultrathin film composite membrane according to Claim 2 wherein the ultrathin polymer film composite membrane is about 20nm to about lOOnm in thickness.

4. The ultrathin film composite membrane according to 15 Claim 3 wherein the ultrathin polymer film of said composite membrane is about 40nm in thickness.

5. The ultrathin film composite membrane according to Claim 1 wherein said composite membrane has an 0₂/N₂ gas transport selectivity coefficient of about 8.0 and an 0₂

20 permeability coefficient of about 0.55 barrers.

6. The ultrathin film composite membrane according to Claim 1 wherein said ultrathin polymer film comprises a styrenic monomer.

7. The ultrathin film composite membrane according to 25 Claim 1 wherein said ultrathin polymer film comprises a methacrylic monomer.

8. The ultrathin film composite membrane according to Claim 1 wherein said ultrathin polymer comprises a vinylene monomer.

30 9. The ultrathin composite membrane according to Claim

7 wherein the methacryl monomer is methyl methacrylate

10. The ultrathin composite membrane according to Claim

8 wherein the vinylene monomer is vinylidiene chloride.

11. The ultrathin composite membrane according to Claim oc 1 wherein the ultrathin polymer film is a terpolymer of 1 divinyl benzene, ethylvinyl benzene and sodium styrene sulfonate.

12. The ultrathin composite membrane according to Claim 1 wherein the ultrathin polymer film comprises a terpolymer of

5 divinyl benzene, ethylvinyl benzene and vinylferrocene.

13. The ultrathin composite membrane according to Claim 1 wherein the ultrathin polymer film is acrylonitrile

14. The method of synthesizing an ultrathin film composite membrane comprising:

10 (a) forming a thin film of a radiation induced- polymerizable monomer or monomers on the surface of a microporous support membrane;

(b) polymerizing said monomer or monomers with a radiation source under conditions effective to form a polymer 15 film on said membrane to provide an ultrathin composite membrane.

15. The method according to Claim 14 wherein the radiation source is ultraviolet light.

16. The method according to Claim 14 wherein the 20 radiation source is an electron beam source.

17. The method according to Claim 14 wherein the monomer or monomers is polymerized in the presence of a photoinitiator.

18. The method according to Claim 17 wherein the 25 photoinitiator is present in the thin film of monomer or monomers.

19. The method according to Claim 17 wherein the photoinitiator is present in the gaseous state above the thin film.

30 20. The method according to Claim 17 wherein the photoinitiator is benzoin or a derivative thereof.

21. The method according to Claim 20 wherein the photoinitiator is a benzoin Cι.-C₄ alkyl ether.

22. The method according to Claim 21 wherein the benzoin oc C_a-C₄ alkyl ether is benzoin methyl ether.

23. The method according to Claim 21 wherein the benzoin

Ci-C₄ alkyl-ether is benzoin isobutyl ether.

24. The method according to Claim 21 wherein the benzoin

C_!-C₄ alkyl ether is benzoin ethyl ether.

25. The method according to Claim 17 wherein the photoinitiator is 2-methoxy-l,2-diphenylethanone.

26. The method according to Claim 16 wherein said electron beam source is a laser.

27. The method according to Claim 15 wherein the ultraviolet light source is a xenon-arc lamp.

28. The method according to Claim 14 wherein the radiation source strikes the membrane surface at an acute angle.

29. The method according to Claim 28 wherein the acute angle at which the radiation source strikes the membrane surface is 30° or less.

30. The method according to Claim 27 wherein the thin film of monomer or monomers is irradiated for a time of about 20 to about 40 minutes.

31. The method according to Claim 30 wherein the thin film is irradiated for a time of about 30 minutes.

32. The method according to Claim 26 wherein the thin film of monomer or monomers is irradiated for a time of less than about 20 minutes.

33. The method according to Claim 14 wherein said microporous support membrane is microporous alumina.

34. The method according to Claim 14 wherein said microporous support membrane is a fluoropolymer.

35. The method according to Claim 14 wherein said microporous support membrane is a polycarbonate membrane.

36. The method according to Claim 14 wherein said microporous support is a polyolefin.

37. The method according to Claim 14 which further comprises following step (b), photocuring the ultrathin composite membrane for a sufficient time after the initial polymerization to promote chain extension, further cross- linking and to reduce the free volume of the film.

38. The method according to Claim 37 wherein the ultrathin composite membrane is photocured in the presence of a photoinitiator.

39. The method according to Claim 37 wherein the photocuring is performed by an ultraviolet light source.

40. The method according to Claim 39 wherein the ultraviolet light source is a Xe-arc lamp.

41. The method according to Claim 40 wherein the composite membrane is photocured for a period of about 1 to about 2 hours.

42. The method according to Claim 37 wherein the photocuring is performed by an electron beam source.

43. The method according to Claim 42 wherein the electron beam source is a laser.

44. The method according to Claim 43 wherein the composite membrane is photocured for a period of less than one hour.

45. The method according to Claim 14 wherein step (a) comprises forming the thin film of monomer or monomers on the microporous support membrane by saturating a porous material with said monomer or monomers in liquid form and placing the microporous support on the porous material whereby the liquid will wick up through the microporous support to form the thin film.

46. The method according to Claim 45 wherein the porous material is filter paper.

47. The ultrathin film composite membrane prepared by the method of Claim 14.

48. The ultrathin film composite membrane prepared by the method of Claim 15.

49. The ultrathin film composite membrane prepared by the method of Claim 16.

50. The ultrathin film composite membrane prepared by the method of Claim 17.

51. The ultrathin film composite membrane prepared by the method of Claim 28.

52. The ultrathin film composite membrane prepared by the method of Claim 37.

53. The ultrathin film composite membrane prepared by the method of Claim 45.

54. In the process of detecting changes in solute concentration in a solution, said process including the steps of passing a solution containing a solute in contact with a sensor containing a permeable membrane under conditions effective to cause at least some of the solute to pass through the permeable membrane and detecting the rate of passage of said solute through said membrane, the improvement which comprises employing as the permeable membrane the ultrathin composite membrane of Claim 1.

55. The process of Claim 54 wherein the ultrathin film composite membrane is about lOnm to about 5000nm in thickness.

56. The process of Claim 55 wherein the ultrathin film of said composite membrane is about 20nm to about lOOnm in thickness.

57. The process of Claim 54 wherein said composite membrane has a selectivity coefficient of about 8.0.

58. In a process of gas separation of the type employing a permeable membrane whereby a gas mixture is passed in contact with the upstream side of the permeable membrane and a sweep gas is passed in contact with the downstream side of the permeable membrane under conditions effective to provide a gas mixture at the downstream side wherein the more permeable gas is present is a higher concentration than is present in the gas mixture at the upstream side, the improvement which comprises employing as the permeable membrane the ultrathin composite membrane of Claim 1.

59. The process of Claim 58 wherein the ultrathin film composite membrane is about lOnm to about 5000nm in thickness.

60. The process of Claim 58 wherein the ultrathin film of said composite membrane is about 20nm to about lOOnm in thickness.

61. The process of Claim 58 wherein said composite membrane has a selectivity coefficient of about 8.0.

62. In a pervaporation process employing a permeable membrane wherein a mixture of liquids is passed in contact with the upstream side of the permeable membrane and a sweep gas is passed in contact with the downstream side of the permeable membrane under conditions effective to provide a mixture of vapors at the downstream side, said mixture of vapors composed of the components of the mixture of liquids but containing a higher concentration of the more permeable component than is present in the liquid mixture at the upstream side, the improvement which comprises e ployinq as the permeable membrane the ultrathin composite membrane of Claim 1.

63. The process of Claim 62 The process of Claim 54 wherein the ultrathin film composite membrane is about lOnm to about 5000nm in thickness.

64. The process of Claim 62 wherein the ultrathin film of said composite membrane is about 20nm to about lOOnm in thickness.

65. The process of Claim 62 wherein said composite membrane has a selectivity coefficient of about 8.0.