METHODS FOR MONITORING SULFONYL HALIDE FUNCTIONAL GROUP CONTENT OF MATERIALS

Abstract: A method for monitoring the sulfonyl fluoride functional group content of a material in order to determine the degree of conversion of said sulfonyl fluoride functional groups into other functional groups; said method comprising measuring the absorbance of electromagnetic radiation transmitted through the material, said electromagnetic radiation being of an intensity and frequency such that it is at least partially, but not entirely, absorbed by the sulfonyl fluoride functional groups within the material.

Description: Methods for Monitoring Sulfonyl Halide Functional Group Content of Materials The present invention is concerned with methods for monitoring the sulfonyl halide functional group content of materials. In particular, it is concerned with methods for monitoring the conversion of sulfonyl halide functional groups into other functional groups and, especially, the conversion of sulfonyl fluoride functional groups (-S0 2 F) into sulfonic acid functional groups (-S0 2 OH) . Although the present invention may be applicable to a variety of materials, it is particularly concerned with the technology of cation selective ion exchange membranes. Cation selective ion exchange membranes are commonly used within many types of electrochemical apparatus such as electrolysers, fuel cells and secondary batteries . Cation selective ion exchange membranes are commonly manufactured from copolymers which comprise a fluorinated carbon polymer backbone with a plurality of pendant side chains. Said side chains may commonly comprise one or more fluorinated carbon chains and essentially comprise one or more cation selective functional groups. Such a co-polymer may be, for example, a graft copolymer comprising a fluorinated carbon polymer backbone, such as polytetrafluoroethylene or polyhexafluoropropylene, with additional monomer units grafted thereon so as to provide the pendant side chains. Said side chains may comprise fluorinated carbon chains and essentially comprise cation selective functional groups. Such a co-polymer may also be, for example, a statistical, random, alternating or block co-polymer, comprising as monomer units one or more fluorinated alkenes such as tetrafluoroethylene or hexafluoropropylene and one or more fluorinated alkenes which are substituted with pendant side chains . Said side chains may comprise fluorinated carbon chains and essentially comprise cation selective functional groups. Such cation selective ion exchange membranes commonly utilise sulfonic acid (-S0 2 OH) functional groups as the cation selective functional groups. Such membranes are well known in the art. Some examples of commercially available cation selective ion exchange membranes of this type include the Nafion™ range of materials (produced by DuPont) , the Plemion™ range of materials (produced by Asahi Glass) and the Aciplex™ range of materials (produced by Asahi Chemical) . Unfortunately, materials which incorporate sulfonic acid groups as the cation selective functional groups are not melt-processable and consequently are not readily formed into membranes. When forming membranes of such materials it is common to manufacture the films from a precursor material in which the sulfonic acid groups are in the form of a sulfonyl fluoride. Such precursor materials are melt processable and can be formed into thin films. They may then be converted into cation selective ion exchange membranes by hydrolysis of the sulfonyl fluoride functional groups to provide sulfonic acid functional groups. These types of cation selective ion exchange membrane precursors are also well known in the art . The sulfonyl fluoride groups may preferably be converted into sulfonic acid groups by hydrolysis, preferably by reaction with an alkali metal hydroxide in a mixture of dimethyl sulfoxide and water. An example of a co-polymer which may be used in the manufacture of cation selective ion exchange membranes is that sold under the trade name Nafion 115™. The structure for this material is shown below: Such a polymer may be formed from a precursor wherein the -SO 3 H groups are -S0 2 F groups. As mentioned above, the polymer material is melt processable in the sulfonyl fluoride form and can be formed into a film suitable for use as a membrane . Once formed into a film the sulfonyl fluoride groups may then be converted into sulfonic acid groups by hydrolysis, preferably by reaction with an alkali metal hydroxide in a mixture of dimethyl sulfoxide and water. A reaction scheme for the conversion process is shown below: Precursor O-CF 2 CF 2 SO 2 F Hydrolysis Material The concentration of sulfonic acid groups within the membrane material is an important factor in determining the ion exchange properties of the resultant film. It is therefore important, when using a sulfonyl fluoride precursor material, to be able to evaluate the degree of conversion of the sulfonyl fluoride groups into sulfonic acid groups. In order to obtain the best membrane performance properties it is preferred that 100% conversion should occur. Thus, the present invention is particularly concerned with methods for evaluating the degree of conversion of sulfonyl fluoride groups to sulfonic acid groups within materials, in particular polymeric materials and more particularly in cation selective ion exchange membranes such as those described above. Methods for monitoring the degree of conversion may involve either monitoring the amount of sulfonyl fluoride functionality remaining in the material or monitoring the amount of sulfonic acid functionality which has been generated. A prior solution to the problem of ascertaining the degree of conversion involves measuring the sulfonic acid group content. This is achieved by determination of its ion exchange capacity (IEC) . This method involves the steps of : (i) Ensuring all sulfonate groups are in the acid form. This may be achieved by immersing the film in dilute acid for approximately 48 hours, blotting the film dry with filter paper then immersing in dilute alkali for approximately 48 hours. This process is repeated twice then the film is finally placed back into dilute acid. (ii) Adding a standard solution of NaOH, sufficient to fully exchange all the acid functional groups, stirring for approximately 2 hours and allowing to stand for 48 hours. (iii) Titrating a portion of the resultant NaOH solution with a standard acid solution to determine the number of moles of hydroxide ions reacted from which can be calculated the number of moles of acid functional groups. (iv) Evaluating the dry weight of the membrane by weighing it after heating under a vacuum. (v) Calculating IEC in mol/g (or milliequivalents per gram, meq) by dividing the number of moles of acid functional groups by the mass of the sample. This process suffers from the disadvantages that it is a destructive process and it is also a very time-consuming technique. Accordingly, the present invention provides a method for monitoring the sulfonyl fluoride functional group content of a material in order to determine the degree of conversion of said sulfonyl fluoride functional groups into other functional groups, said sulfonyl fluoride functional groups being defined by the structure : O II —S— F II O said method comprising measuring the absorbance of electromagnetic radiation transmitted through the material, said electromagnetic radiation being of an intensity and frequency such that it is at least partially, but not entirely, absorbed by the sulfonyl fluoride functional groups within the material. It will be appreciated by a person skilled in the art that the frequency of electromagnetic radiation used will vary depending upon the nature of the material under investigation. Preferably, the frequency of electromagnetic radiation used lies in the mid-infra- red (MIR) region of the spectrum, i.e. from 500 to 4000 cm "1 . More preferably, the frequency of the electromagnetic radiation is in the range of from 1000 to 1700cm "1 . Still more preferably, the frequency of the electromagnetic radiation is in the range of from 1400 to 1500 cm "1 . Even more preferably, the frequency of the electromagnetic radiation is in the range of from 1440 to 1455 cm "1 , most preferably approximately 1450 cm "1 . In general, the frequency of electromagnetic radiation which is preferred is that which approximately corresponds to a shoulder found adjacent to the anti-symmetric S0 2 stretch which is characteristic of the sulfonyl fluoride functional group. It will also be appreciated by a person skilled in the art that the intensity of electromagnetic radiation used will vary depending, primarily, upon the strength of absorbance of the material at the chosen frequency. This will depend upon the nature of the material under investigation. Since the method involves transmission through the material under investigation it is important that the electromagnetic radiation should not be totally absorbed by the material. Thus, if the absorbance at the chosen frequency is strong the intensity of electromagnetic radiation will need to be higher. However, the intensity which may be used is limited by the ability of the material to withstand irradiation at high intensity. It will be appreciated by a person skilled in the art that the method of the present invention may be applied to a wide range of materials which contain sulfonyl fluoride functional groups. However, preferably, the material under investigation is a precursor to a cation selective ion exchange membrane. Said precursor may be formed from a copolymer comprising a fluorinated carbon polymer backbone with a plurality of pendant side chains. Said pendant side chains may also comprise one or more fluorinated carbon chains and essentially comprise at least one sulfonyl fluoride functional group. The method may be applied to said material at any point during a process of conversion of the sulfonyl fluoride functional groups into other functional groups. Preferably, the process of conversion comprises hydrolysis of the sulfonyl fluoride functional groups in which case they are converted into sulfonic acid groups. Even more preferably, the process of conversion comprises hydrolysis using a solution of an alkali metal hydroxide in water and dimethyl sulfoxide. The advantages of the present invention are that it is non-destructive of the material under investigation and it is much quicker to carry out than methods presently known in the art. It could also be applied as an on-line process, continuously monitoring material as it is produced. The present invention also includes within its scope a method of manufacturing a cation selective ion exchange membrane which comprises the steps of : (i) providing a precursor polymer comprising a fluorinated polymer backbone with a plurality of pendant side chains, said side chains each comprising at least one sulfonyl fluoride functional group, (ii) forming said precursor polymer into a film, (iii) converting the sulfonyl fluoride functional groups of said precursor polymer film into sulfonic acid groups by hydrolysis and, (iv) monitoring the sulfonyl fluoride functional group content of said precursor polymer film by a method as hereinbefore described. This method of manufacture is particularly advantageous because it enables determination of the degree of sulfonyl fluoride conversion by a rapid non- destructive technique. This provides improvements in the speed of production of the final product and decreases waste thus reducing costs. The method of determination of the degree of conversion can be implemented as an on-line process which involves minimum interference of the production line. The present invention also includes within its scope a method for quantifying the sulfonyl fluoride content of a polymer material, said method comprising the steps of : (1) measuring the absorbance of electromagnetic radiation transmitted through the material, said electromagnetic radiation being of an intensity and frequency such that it is at least partially, but not entirely, absorbed by the sulfonyl fluoride functional groups within the material, (n) measuring the thickness of the material, (in) comparing the results with polymer materials of known sulfonyl fluoride content and known thickness. The present invention will now be described by way of the following example which is intended to illustrate, but not limit, the present invention. The example makes reference to the drawings m which: Figure 1 is an IR ATR spectrum of unconverted and partially converted precusor polymer. Figure 2 shows transmission IR spectra for a precursor polymer at various stages of conversion. Figure 3 shows peak area analysis for a selected absorption peak at various stages of conversion. Example Specimens of a precursor to a cation selective ion exchange membrane with a range of degrees of conversion of sulfonyl fluoride functional groups into sulfonic acid groups were prepared by the following method from the precursor membrane material NX115f which is the precursor for the commercially available product Nafion™ 115. Potassium hydroxide (105g) , dimethylsulfoxide (DMSO, 100ml) and distilled water (500ml) were mixed to provide a solution of 3.75M KOH in distilled water with 16.6 volume % DMSO. The solution was placed in a beaker, covered with a watch glass and heated to 75°C in an oven. A piece of NX115f (100mm x 100mm x approximately 120μm) was placed in the solution at 75°C and small specimens were cut from the membrane sample at 15 minute intervals up to 120 minutes. The freshly cut pieces of membrane were immediately placed in specimen bottles containing 50ml of 2M H 2 S0 4 where they remained for 24 hours. Samples were washed with distilled water then left to soak in distilled water for 2 hours. Prior to spectroscopic measurements, these samples were washed with distilled water and placed on tissue paper to remove any surface water. A reaction scheme for the process is shown below: Precursor O-CF 2 CF 2 SO 2 F KOH/DMSO/H 2 O 75 degC brane Material In the present example, transmission IR spectroscopy experiments were carried out using a Nicolet Magna FTIR 760 instrument using the following scanning conditions: IR source, KBr beamsplitter, MCT liquid nitrogen cooled detector, wavelength range = 4000-400cm "1 , spectrum resolution = 2cm "1 , 32 scans. In the present example, attenuated total -internal reflection (ATR) spectroscopy was carried out using a Nicolet 80SX FTIR spectrometer using a KRS5 horizontal ATR unit and the following scan conditions: wavelength range 4000-400cm "1 , spectrum resolution = 4cm "1 , 32 scans . IR ATR spectra were recorded for the unconverted precursor NX115f and the precursor after 15 minutes hydrolysis. The spectra are shown in Figure 1. Analysis of the spectrum for the unconverted precursor showed a small shoulder peak at 1449cm "1 adjacent to the large peak at 1468cm "1 . This peak was selected for transmission spectroscopy analysis since it is very weak. Transmission spectroscopy was carried out for each of the membrane films converted for 0 to 120 minutes. The results in the region of 1449cm "1 are presented in Figure 2. Figure 3 shows the peak area analysis for the peak area at 1449cm "1 over the range 1454-1441cm "1 . Figures 2 and 3 illustrate that monitoring the absorbance of the membrane at approximately 1449cm " can provide an indication of the degree of conversion of S0 2 F into S0 2 OH .

Claims: Claims 1. A method for monitoring the sulfonyl fluoride functional group content of a material in order to determine the degree of conversion of said sulfonyl fluoride functional groups into other functional groups, said sulfonyl fluoride functional groups being defined by the structure: O II —S— F II O said method comprising measuring the absorbance of electromagnetic radiation transmitted through the material, said electromagnetic radiation being of an intensity and frequency such that it is at least partially, but not entirely, absorbed by the sulfonyl fluoride functional groups within the material . 2. A method according to claim 1 wherein the frequency of the electromagnetic radiation is in the range of from 500 to 4000cm "1 . 3. A method according to claim 1 or claim 2 wherein the frequency of the electromagnetic radiation is in the range of from 1000 to 1700cm "1 . 4. A method according to any one of the preceding claims wherein the frequency of the electromagnetic radiation is in the range of from 1400 to 1500cm "1 . 5. A method according to any one of the preceding claims wherein the frequency of the electromagnetic radiation is in the range of from 1440 to 1455cm "1 . 6. A method according to any one of the preceding claims wherein the frequency of the electromagnetic radiation is approximately 1450cm "1 . 7. A method according to claim 1 wherein the frequency of electromagnetic radiation is that which approximately corresponds to a shoulder found adjacent to the anti-symmetric S0 2 stretch of the sulfonyl fluoride group. 8. A method according to any one of the preceding claims wherein the material comprises a precursor to a cation selective ion exchange membrane, said precursor being formed from a copolymer which comprises a fluorinated carbon polymer backbone with a plurality of pendant side chains, said pendant side chains comprising at . least one sulfonyl fluoride functional group. 9. A method of manufacturing a cation selective ion exchange membrane comprising the steps of: (i) providing a precursor polymer comprising a fluorinated polymer backbone with a plurality of pendant side chains, said side chains each comprising at least one sulfonyl fluoride functional group, (ii) forming said precursor polymer into a film, (iii) converting the sulfonyl fluoride functional groups of said precursor polymer film into sulfonic acid groups by hydrolysis, and (iv) monitoring the sulfonyl fluoride functional group content of said precursor polymer film by a method as claimed in any one of claims 1 to 8. 10. A method for quantifying the sulfonyl fluoride content of a polymer material, said method comprising the steps of: (i) measuring the absorbance of electromagnetic radiation transmitted through the material, said electromagnetic radiation being of an intensity and frequency such that it is at least partially, but not entirely, absorbed by the sulfonyl fluoride functional groups within the material, (ii) measuring the thickness of the material, (iii) comparing the results with polymer materials of known sulfonyl fluoride content and known thickness.

Inventors: CROSBY PETER, MORRISSEY JOHN PATRICK, CROSBY, PETER, MORRISSEY, JOHN, PATRICK
Applicants: CROSBY PETER, MORRISSEY JOHN PATRICK, NAT POWERS PLC, CROSBY, PETER, MORRISSEY, JOHN, PATRICK, NATIONAL POWERS PLC
Application year: 2000
Main classification: C08F8
IPC codes: C08J5, G01N21