WO2007079004A2

WO2007079004A2 - Fluorinated polyarylenethioethersulfone polymers having sulfonate pendants and phenyl-endcapping groups for use as proton exchange membranes

Info

Publication number: WO2007079004A2
Application number: PCT/US2006/048854
Authority: WO
Inventors: Thuy D. Dang; Zongwu Bai
Original assignee: University Of Dayton
Priority date: 2005-12-28
Filing date: 2006-12-22
Publication date: 2007-07-12
Also published as: WO2007079004A3; WO2007079004B1

Abstract

The present invention provides a method for forming fluorinated polyarylenethioetheresulfone polymers having sulfonate pendants for use in proton exchange membrane materials. The polymers are formed by combining a sulfonated monomer, a non-sulfonated monomer, and a fluorinated monomer in a polar aprotic solvent and heating. The polymers contain up to two sulfonate groups per repeat unit to provide a wide range of proton conductivities in the resulting proton exchange membranes. The polymers are also preferably endcapped with phenyl-based endcapping groups to enhance the water resistivity and thermal stability. The membranes may also be 0 formed by dissolving the formed fluorinated sulfo-pendant polyarylenethioethersulfone polymers (in salt form) in a solvent such as N,N-dimethylacetamide or N,N-dimethylformamide, and casting the membrane from the solution. The resulting proton exchange membranes have proton conductivities ranging from about 120 to 360 mS/cm at 85°C and 85% relative humidity.

Description

FLUORINATED POLYARYLENETHIOETHERSULFONE POLYMERS HAVING

SULFONATE PENDANTS AND ENDCAPPING GROUPS FOR USE AS

PROTON EXCHANGE MEMBRANES

This invention was made with government support under Contract No.

F33615-OO-D-5OO8 awarded by the AFRL/ML. The government has certain rights in the invention.

The present invention relates to fluorinated sulfo-pendant polyarylenethioetheresulfone (6F-SPTES) polymers with endcapping groups, and more particularly, to a method of forming such polymers for use as proton exchange membrane materials.

Polymers which are designed for use as proton exchange membranes in fuel cell applications are currently based primarily on hydrocarbon structures for the polymer backbone. Membranes currently in use include fluorinated, partially fluorinated, and aliphatic polymer membranes, as well as composites derived from such polymers. One such commercially available perfluorinated polymer membrane is Nafion®, available from DuPont. The hydrophobic polytetrafiuoroethylene backbone of Nafion® provides thermal and chemical stability while the perfluorinated side chains with terminating hydrophilic sulfonate groups provide a mechanism for proton conduction. However, the use of Nafion® suffers from a number of drawbacks when used in fuel cell applications. For example, the proton conduction is extremely sensitive to hydration, limiting its performance at high temperatures (greater than 8O⁰C) and at low humidity levels. Additionally, the cost of Nafion® is high due to the complex and extensive chemistry required for synthesis.

Hydrocarbon polymers which contain polar groups are known in the art which can retain large amounts of water over a wide temperature range and are less expensive to synthesize than perfluorinated polymers such as Nafion®. For example, sulfonated high performance polymers based on aryleneether, sulfone, and thioether linkages in the backbone have been synthesized, and are potential candidates for fuel cell applications as they exhibit good thermal and chemical stability, high proton conductivity, and high temperature use. High molecular weight endcapped sulfonated polyarylenethioethersulfone (SPTES) polymers have also been developed. For example, an aromatic polymer backbone with a pendant acid functionality attached directly to the backbone is considered to be an economical alternative to the use of perfluorinated polymers as a proton exchange membrane material. However, SPTES polymers have a higher percentage of water- uptake, which, while beneficial from the standpoint of proton conduction, has the potential to be detrimental for long lifetime performance during thermal cycling.

Accordingly, there is still a need in the art for polymers which may be used as proton exchange membranes which provide enhanced water resistance, good proton conductivity, and which work well in high temperature applications.

The present invention meets that need by providing polymers formed by the addition of a -CF₃ group which is hydrophobic onto an aromatic polymer backbone having a high sulfonate content. The resulting fluorinated polyarylenethioethersulfone polymers containing sulfonate pendants exhibit good proton conductivity and may be used in high temperature (e.g., greater than 8O⁰C, and preferably, greater than 120⁰C) proton exchange membrane applications.

According to one aspect of the present invention, a fluorinated polyarylenethioethersulfone polymer containing sulfonate pendants and end-capped groups is provided for use as a proton exchange membrane, where the polymer has the formula

where

and Z=O, S, NH. In one embodiment of the invention, the polymer comprises a fluorinated sulfo- pendant polyarylenethioethersulfone copolymer (6F-SPTES-50) having the formula:

In another embodiment of the invention, the polymer comprises a fluorinated sulfo-pendant polyarylenethioethersulfone copolymer (6F-SPTES-70) having the formula:

In yet another embodiment, the polymer comprises a fluorinated sulfo-pendant polyarylenethioethersulfone homopolymer (6F-SPTES-100) having the formula:

In the method of making the fluorinated polyarylenethioethersulfone polymer having sulfonate pendant groups, the method includes providing a sulfonated monomer selected from the group consisting of 3,3'-disulfonate-4,4'-difluorodiphenyIsulfone, 3,3'- disulfonate-4,4'-dichlorodiphenylsulfone, providing a non-sulfonated monomer selected from the group consisting of bis(4-fluorophenyl)sulfone and bis(4-chlorophenyl)sulfone; providing a fluorinated monomer comprising 4,4'-(hexafluoroisopropylidene) diphenyldithiol; and dissolving the sulfonated monomer, non-sulfonated monomer, and fluorinated monomer in a polar aprotic solvent to form a solution. The solution is then heated. The solution is preferably heated at a temperature between about 120°C to 180⁰C.

Preferably, the polar aprotic solvent is selected from dimethylsulfoxide (DMSO), N, N-dimethylacetamide (DMAc), tetrahydrothiophene-1, 1 -dioxide (sulfolane), and N- methylpyrroHdone (NMP).

The method of forming the polymer also preferably includes endcapping the polymer with a phenyl-based endcapping agent. The endcapping agent preferably comprises a phenyl-based group selected from phenyl, biphenyl, benzophenone, phenylsulfone, benzothiazole, benzoimidazole, and benzoxazole. A preferred endcapping agent is 4-fluorobenzophenone. Preferably, about 1 mol% of the endcapping agent is used.

In the method of forming a fluorinated sulfo-pendant polyarylenethioethersulfone copolymer (6F-SPTES-50), the reaction scheme is as follows:

The reaction scheme for forming a fluorinated sulfo-pendant polyarylenethioethersulfone copolymer (6F-SPTES-70) is as follows:

The reaction scheme for forming a fluorinated sulfo-pendant polyarylenethioethersulfone homopolymer (6F-SPTES-100) is as follows:

The polymers formed from the reactions schemes described above may be used as proton exchange membrane materials.

In an alternative method of forming a proton exchange membrane, the fluorinated polyarylenethioetheresulfone polymer having sulfonate pendants and phenyl-based endcapping groups formed in the method of the present invention is dissolved in a solvent selected from N,N-dimethylacetamide (DMAc) or N,N-dimethyIformamide (DMF) to form a solution, and the membrane is cast from the solution. In this embodiment, the fluorinated sulfo-pendant polyarylenethioetheresulfone polymer is provided in salt form. The resulting proton exchange membrane preferably has a proton conductivity of from about 120 to about 360 mS/cm at 85°C and 85% relative humidity. The polymeric materials formed in the present invention may be used in a number of membrane applications including electro-dialysis, reverse osmosis, and fuel cell applications.

Accordingly, it is a feature of the invention to provide fluorinated sulfo-pendant polyarylenethioethersulfone polymer compositions containing sulfonate pendant groups and end-capping groups for use as proton exchange membrane materials. Other features and advantages will be apparent from the following description and the accompanying drawings.

Fig. 1 illustrates the thermogravimetric analysis (TGA) of fluorinated polyarylenethioethersulfone polymers with sulfonate pendants and endcaps (in salt form); and Fig. 2 illustrates the TGA of fluorinated polyarylenethioethersulfone polymers with sulfonate pendants and endcaps (in acid form).

We have found that the proton conductivities of polymeric membranes formed from the fluorinated sulfo-pendant polyarylenethioethersulfone polymers produced in accordance with the present invention increased by a factor of three to five over perfluorinated polymer membranes such as Nafion® under comparable conditions of measurement. The introduction of the fluorinated group onto the polymer backbone enhances the hydrophobicity of the polymer due to the high sulfonate content of the pendant groups. We have also found that the proton conductivity of the polymer membrane increases as the content of sulfonate pendants increases. Preferably, the polymeric backbone includes two sulfonate groups per polymer repeat unit to aid in maximizing the membrane water uptake and maximizing proton conductivity. When the polymers are endcapped with bulky aromatic end groups, the resulting polymers and membranes formed therefrom are suitable for high temperature use, which contributes to the lowering of water susceptibility without affecting the high proton conductivity due to the presence of the sulfonate groups. We have also found that the proton conductivities of the membranes may be varied by the type of aromatic dihalo functionality in the sulfonated monomer structure (difluoro or dichloro groups), the mole fraction of the sulfonated monomer as well as the introduction of the endcapping groups. In the method of synthesizing fluorinated polyarylenethioetheresulfone polymers containing sulfonate pendants and end-capping groups, the polymers are synthesized by dissolving difunctional sulfonated diphenylsulfone, difunctional diphenylsulfone, and difunctional hexafluoroisopropylidenebisbenzenethiol in polar aprotic solvents to form a solution. Suitable polar aprotic solvents for use in the present invention include dimethylsulfoxide (DMSO), N, N-dimethylacetamide (DMAc), tetrahydrothiophene-1, 1- dioxide (sulfolane), and N-methylpyrrolidone (NMP). The polymer concentration is preferably about 10-30 wt% under a dry nitrogen atmosphere. Preferably, a base reagent such as potassium carbonate is also included in the solution.

The solution is preferably heated for at least 20 hours at a temperature between about 120⁰C to 180°C to form white, Fibrous polymers.

The fluorinated sulfo-pendant polyarylenethioethersulfone polymer and copolymer are then endcapped with a phenyl-based monohalide endcapping agent. Suitable phenyl- based groups include phenyl, biphenyl, benzophenone, phenylsulfone, benzothiazole, benzoimidazole, and benzoxazole. Preferably, about 1 mol% of the endcapping agent is used.

We have found that the 6F-SPTES homopolymer compositions with the end- capping group are soluble in water, methanol, and polar aprotic organic solvents such as DMAc, DMF, and NMP at room temperature. The 6F-SPTES copolymers with the endcapping groups are insoluble in water, but soluble in methanol, DMAc, DMF, and NMP at room temperature. We have found that the solubility characteristics of the 6F- SPTES polymers with endcaps are dependent on the particular endcapping agent used. Flexible proton exchange membranes (100 to 150 μm) may also be formed from the fluorinated sulfo-pendant polyarylenethioethersulfone polymer with the end-capping group by dissolving the polymer in salt form in N,N-dimethylacetamide (DMAc) or N,N- dimethylformamide (DMF) solutions, followed by casting. The solution is preferably formed at about 100⁰C to obtain a clear solution. Polymer membranes may be directly cast from the solution at about 8O⁰C in vacuum for 24 hours. The clear films obtained may be converted to the corresponding sulfonic acids in the presence of dilute sulfuric acid (4M) at room temperature for 48 hours. The resulting membranes are then preferably dried at room temperature, followed by heating at about 100⁰C in vacuum for 24 hours. In order that the invention may be more readily understood, reference is made to the following examples which are intended to illustrate the invention, but not limit the scope thereof.

Example 1

Table 1 below summarizes the properties of various fluorinated sulfo-pendant polymers and copolymers prepared by the method of the present invention. Gel permeation chromatography (GPC) was used to determine molecular weights and molecular weight distributions (M_wM_n) of synthesized polymer samples with respect to polystyrene standards (Polysciences Corporation). Molecular weight measurement was performed on TriSEC Version 3.00 in 0.5% LiBr/N-methylpyrrolidone (NMP) at 70⁰C.

Intrinsic viscosity (IV) was measured at 3O⁰C in NMP using an Ubbelohde Viscometer. Thermogravimetry (TGA) was conducted using an Auto TGA 2950HR V5.4A instrument in air and helium at a heating rate of 10°C/min.

Table 1 - Properties and thermal characteristics of fluorinated polyarylenethioethersulfone polymers with sulfonate pendants

- GPC was performed in NMP, 0.5% LiBr at 70⁰C using sulfo-polystyrene as standard

- Intrinsic viscosity was measured at 30⁰C in NMP with Ubbelohde Viscometer ^Temperatures for 1^st stage and 2st stage weight loss

Example 2

Table 2 below illustrates the proton conductivity and solubility of membranes cast from DMAc and fluorinated polyarylenethioethersulfone polymers and copolymers with sulfonate pendants. The conductivity of the polymer membranes was measured using AC Impedance Spectroscopy and utilized a standard 4-electrode measurement setup to eliminate electrode and interfacial effects. A Teflon sample fixture was placed inside a temperature and humidity controlled oven and was fabricated so as to allow the sample to be exposed to humidified air within the chamber. The two outer electrodes were made of platinum foil and these acted to source the current in the sample. Two inner platinum wire electrodes (spaced 1 cm apart) were then used to measure the voltage drop across a known distance. By measuring the impedance of the material as a function of frequency at a set temperature and humidity, the conductivity of the film was obtained using the magnitude of the impedance in a region where the phase angle is relatively zero. Table 2 - Proton conductivity and solubility of fluorinated polyarylenethioethersulfone polymers with sulfonate pendants

Note: the proton conductivity was tested at 85⁰C, 85% relative humidity

The solubility of the samples was tested at room temperature (S=swollen, N=not soluble, Y=soluble)

Example 3 The fabrication of 6F-SPTES polymer membranes was achieved by converting the sulfonate salts of fluorinated polyarylenethioethersulfone polymers with the endcapping group to the corresponding sulfonic acids in the presence of dilute sulfuric acid. Figures 1 and 2 illustrate the TGA traces of the homopolymers and copolymers. The TGA of the polymers with the sulfonic acid pendant (Fig. 2) shows the typical two-stage decomposition profile, where the degradation at lower temperatures occurs due to the loss of the sulfonic acid group.

Example 4

A fluorinated polyarylenethioethersulfone polymer containing sulfonate pendants and an end-capping group was synthesized as follows. 3,3'-Disulfonate-4,4'- difluorodiphenylsulfone (6.8750 g.), 4,4'-(hexafluoroisopropylidene) diphenyldithiol (5.5252 g) (hexafluorobisphenylthiol A), and potassium carbonate (4.5609 g) were charged into a 250 ml round-bottom flask maintained under a back pressure of nitrogen and equipped with a magnetic stirrer and an oil bath on a hot plate. 100 ml of sulfolane was added to the flask, stirred for 45 minutes at room temperature, and at 100⁰C overnight; this was followed by the addition of 4-fluorobenzophenone (0.05 g) as an end-capping -U-

agent into the reaction solution. The reaction mixture was heated to 18O⁰C for 6 hours, then cooled down to room temperature, quenched with acetic acid, and precipitated in methanol. The polymer was collected by filtration, air-dried, dissolved in boiling water, and the solution was then reprecipitated in methanol. The polymer was filtered, soxhlet- extracted with methanol for 48 hours, and dried in vacuum at 80⁰C overnight to provide a yield of 89%.

Example 5

A fluorinated polyarylenethioethersulfone copolymer with sulfonate pendants and an endcapping group was synthesized as follows: 4, 4'-(hexafluoroisopropylidene) diphenyldithiol (3.6835 g) and potassium carbonate (3.6287 g) were charged into a 250 ml round-bottom flask maintained under a back pressure of nitrogen and equipped with a magnetic stirrer and an oil bath on a hot plate. 50 ml sulfolane was added into the flask, stirred for 45 minutes at room temperature and at 100⁰C for 4 hours. This was followed by the addition of 3, 3'-disulfonate-4, 4'-difluorophenylsulfone (3.2854 g) and 4,4'- difluorodiphenylsulfone (O.7688g) as well as 30 ml sulfolane into the reaction solution. The reaction mixture was stirred for 4 hours at 120⁰C, then the solution was heated up to to 18O⁰C, followed by the addition of 4-fluorobenzophenone (0.02 g), stirred for 2.5 hours, then cooled down to room temperature, quenched with acetic acid, and precipitated in methanol. The copolymer was collected by filtration, air-dried, dissolved in boiling water, and the solution was then reprecipitated in methanol. The polymer was filtered, soxhlet- extracted with methanol for 48 hours, and dried in vacuum at 8O⁰C overnight to provide a yield of 93.0%.

Example 6

A fluorinated polyarylenethioethersulfone copolymer with sulfonate pendants and an endcapping group was synthesized as follows: 4, 4'-(hexafluoroisopropylidene) diphenyldithiol (2.9468 g) and potassium carbonate (2.8748 g) were charged into a 250 ml round-bottom flask maintained under a back pressure of nitrogen and equipped with a magnetic stirrer and an oil bath on a hot plate. Sulfolane (40 ml) was added into the flask, stirred for 45 minutes at room temperature and at 100⁰C for 4 hours, followed by the addition of 3,3'-disulfonate-4,4'-difluorodiphenylsulfone (1.8333 g) and 4, 4'- difluorodiphenylsulfone (1.1484 g) as well as 30 ml sulfolane into the reaction solution. The reaction mixture was stirred overnight at 100⁰C. The solution was then heated to 120⁰C and stirred for 4 hours, then heated to 180⁰C, followed by the addition of 4- fluorobenzophenone (0.05 g) as an end-capping agent, stirred for 2.5 hours, then cooled down to room temperature, quenched with acetic acid, and precipitated in methanol. The copolymer was collected by filtration, air-dried, dissolved in boiling water, and the solution was then reprecipitated in methanol. The polymer was filtered, soxh let-extracted with methanol for 48 hours, and dried in vacuum at 80⁰C overnight to provide a yield of 91.0%.

Example 7

A fluorinated polyarylenethioethersulfone polymer with sulfonate pendants and an end- capping group was synthesized as follows: 4, 4'-(hexafiuoroisopropylidene) diphenyldithiol (5.5268 g) and potassium carbonate (5.9848 g) were charged into a 250 ml round-bottom flask maintained under a back pressure of nitrogen and equipped with a magnetic stirrer and an oil bath on a hot plate. 100 ml of sulfolane was added into the flask, stirred for 45 minutes at room temperature and at 100⁰C for 2 hours, followed by the addition of 3,3'-disulfonate-4,4'-difluorodiphenylsulfone (2.6248 g) and 4,4'- difluorodiphenylsulfone (2.5634 g) as well as 40 ml sulfolane into the reaction solution. The reaction mixture was heated up to 12O⁰C and stirred for 4-6 hours, then heated to 180⁰C, followed by the addition of 4-fluorobenzophenone (0.05 g) as an end-capping agent. The mixture was stirred for 2.5 hours, cooled down to room temperature, quenched vvith acetic acid, and precipitated in methanol. The copolymer was collected by filtration, air-dried, dissolved in boiling water, and the solution was then reprecipitated in methanol. The polymer was filtered, soxhlet-extracted with methanol for 48 hours, and dried in vacuum at 80⁰C overnight to provide a yield of 95.0%.

Example 8

A polymer membrane was formed from a fluorinated sulfo-pendant polyarylenethioethersulfone polymer by providing a fluorinated sulfo-pendant polyarylenethioethersulfone polymer (in salt form) with an end-capping group (as formed in the above Examples) and dissolving the polymer in N,N-dimethylacetamide (DMAc) or N,N-dimethyIformamide (DMF) at 100⁰C to obtain a clear solution which was filtered at room temperature. Polymer membranes were directly cast from a flat glass dish at 80⁰C in vacuum for 24 hours. The clear films were converted to the corresponding sulfonic acids in the presence of dilute sulfuric acid (4M) at room temperature for 48 hours. The films were dried at room temperature, then at 100⁰C in vacuum for 24 hours.

Having described the invention in detail and by reference to preferred embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention.

Claims

1. A fluorinated arylenethioethersulfone polymer containing sulfonate pendants and endcapped groups for use as a proton exchange membrane; said polymer having the formula:

and Z=O, S, NH. 5

2. The polymer of claim 1 comprising a fluorinated sulfo-pendant polyarylenethioethersulfone copolymer (6F-SPTES-50) having the formula:

3. The polymer of claim 1 comprising a fluorinated sulfo-pendant polyarylenethioethersulfone copolymer (6F-SPTES-70) having the formula: 5 _cp o -1 ^CflJ

^{U cp}5 HO₃S-^ ³ "

0

4. The polymer of claim 1 comprising a fluorinated sulfo-pendant polyarylenethioethersulfone homopolymer (6F-SPTES-100) having the formula:

5. A method of making a fluorinated polyarylenethioethersulfone polymer having sulfonate pendant groups comprising: providing a sulfonated monomer selected from the group consisting of 3,3'- disulfonate-4,4'-difiuorodiphenylsulfone and 3,3'-disulfonate-4,4'- dichlorodiphenylsulfone; providing a non-sulfonated monomer selected from the group consisting of bis(4- fluorophenyl)sulfone and bis(4-chlorophenyl)sulfone; providing a fluorinated monomer comprising 4,4'-(hexafluoroisopropylidene) diphenyldithiol; dissolving said sulfonated monomer, non-sulfonated monomer, and fluorinated monomer in a polar aprotic solvent to form a solution; and heating said solution.

6. The method of claim 5 wherein said polar aprotic solvent is selected from dimethylsulfoxide (DMSO), N, N-dimethylacetamide (DMAc), tetrahydrothiophene-1, 1- dioxide (sulfolane), and N-methylpyrrolidone (NMP).

7. The method of claim 5 wherein said solution is heated at a temperature of between about 120 to 18O⁰C.

8. The method of claim 5 including endcapping said polymer with a phenyl-based endcapping agent.

9. The method of claim 8 wherein said endcapping agent comprises a pheny-based group selected from phenyl, biphenyl, benzophenone, phenylsulfone, benzothiazole, benzoimidazole, and benzoxazole.

10. The method of claim 8 wherein said endcapping agent comprises 4- fluorobenzophenone.

1 1. The method of claim 5 having the reaction scheme

12. The method of claim 5 having the reaction scheme

13. The method of claim 5 having the reaction scheme

14. A method of forming a proton exchange membrane comprising providing a fluorinated polyarylenethioetheresulfone polymer having sulfonate pendants and phenyl-based endcapping groups; dissolving said polymer in a solvent selected from N,N-dimethylacetamide (DMAc) or NjN-dimethylformamide (DMF) to form a solution; and casting said membrane from said solution.

15. The method of claim 14 wherein said membrane has a proton conductivity of from about 120 to about 360 mS/cm at 85°C.