Cross-linked sulphonated polymers and method for preparing same

Abstract

The present invention is concerned with cross-linked sulfonated polymers, eventually perfluorinated, and their preparation process. When molded in the form of membranes, the polymers are useful in electrochemical cells, in a chlorine-sodium electrolysis process, as separator in an electrochemical preparation or organic and inorganic compounds, as separators between an aqueous phase and an organic phase, or as catalyst for Diels-Alder additions, Friedel-Craft reactions, aldol concentrations, cationic polymerisation, esterification, and acetal formation.

Description

[0001] The present invention is concerned with cationic ion-exchange resins, particularly in the form of membranes, preferably partially or completely fluorinated, their applications, in particular in electrochemical applications such as fuel cells, alkali-chloride processes, electrodialysis, ozone production, as well as any other application related to the dissociation of anionic centers linked to the membrane, such as heterogeneous catalysis in organic chemistry.BACKGROUND OF THE INTENTION[0002] Because of their chemical inertia, ion-exchange membranes partially or completely fluorinated are usually chosen for alkali-chloride processes or fuel cells consuming hydrogen or methanol. Such membranes are commercially available under trade names like Nafion.TM., Flemion.TM., Dow.TM.. Other similar membranes are proposed by Ballard Inc. in application WO 97 / 25369 that describes copolymers of tetrafluoroethylene and perfluorovinylethers or trifluorovinylstyrene. The active monomers from w...

AI Technical Summary

Benefits of technology

[0041] An advantage of the present invention is that the cross-linking agents provide negatively charged species that are bound to the sulfonyl groups of the polymers, and used as bridges between adjacent polymeric chains. It is well known that sulfonylimide groups and di- or trisulfonylmethane groups are strong electrolytes in most media, and therefore, the cross-linking reaction, in addition to improving the mechanical properties, does not have any detrimental effect on the conductivity. In fact, the latter is often increased.

[0042] The following compounds are preferred cross-linking ionogenes agents, i.e., ionic groups generators, when L is on the polymeric chain: Li.sub.3N; C.sub.3Al.sub.4; [(CH.sub.3).sub.3Si].sub.2NLi (or Na or K); NH.sub.3+3 DABCO; CF.sub.3SO.sub.2C[(CH.sub.3).sub.3Si][Li(TMEDA)].sub.2; (CH.sub.3).sub.3CNH.sub.2+3 TEA; NH.sub.2SO.sub.2NH.sub.2+4 TEA; [[(CH.sub.3).sub.3Si](Li)N].sub.2SO.sub.2; [(TMEDA)(Mg)N].sub.2SO.sub.2; CH.sub.3Li; (CH.sub.3).sub.3Al; NH.sub.2Li (or Na or K); [[Si(CH.sub.3).sub.3](Li)NSO.sub.2].sub.2CF.sub.2; [Li[Si(CH.sub.3).sub.3]NSO.sub.2CF.sub.2].sub.2CF.sub.2; [(Li)Si(CH.sub.3).sub.3NSO.sub.2CF.sub.2]; and [Li[Si(CH.sub.3).sub.3]NSO-.sub.2CF.sub.2CF.sub.2].sub.2O, wherein TEA=triethylamine; TMEDA=N,N,N'N' tetramethylethylene diamine and DABCO=1,4-diazabicyclo-[2,2,2,]-octane.

[0043] Alternately, the cross-linking reaction can take place when the Y group is already on the precursor of the polymer, for instance in the case of a substituted amide. In such a case, the general scheme is as follows: 5

[0044] The following compounds are examples of preferred ionogene cross-linking agents when L is on the reagent: SO.sub.2Cl.sub.2+3 DABCO; SO.sub.2(imidazole).sub.2; [FSO.sub.2CF.sub.2].sub.2+3 TEA; (CISO.sub.2CF.sub.2)CF.sub.2+3 DABCO and (FSO.sub.2CF.sub.2CF.sub.1).sub.-2O+3 DABCO.

[0045] The cross-linking reaction may imply all the sulfonyl groups, or only a fraction thereof. These cross-linking reagents can be added or used according to various techniques well known to those skilled in the art. Advantageously, the polymer is molded in the desired form prior to the cross-linking, for example in the form of a membrane or a hollow fiber, and the material is immerged or covered with a solution of the cross-linking agent in one or more solvent favoring the coupling reaction. Preferred solvents are polyhalocarbons, tetrahydrofuran (THF), glymes, tertiary alkylamides such as dimethylformamide, N-methylpyrrolidone, tetramethylurea and its cyclic analogues, N-alkylimidazoles, and tetraalkylsulfamides. The desired cross-linking degree can be controlled through various factors, such as the time of immersion in the solvent containing the cross-linking agent, the temperature of the solvent, the concentration of the cross-linking agent in the solvent, or a combination thereof. Preferably, these parameters are adjusted to produce the desired properties in a relatively short period of time varying between a few seconds to about ten hours, and the temperatures are chosen to be compatible with the usual solvents, from -10.degree. C. to 250.degree. C. For comparison purposes, hydrolysis of a Nafion.RTM. membrane takes more than 24 hours for usual thicknesses.

[0046] Alternately, a latex of the polymer to be molded is mixed preferably in the presence of fluids that are not solvents, such as ordinary or fluorinated hydrocarbons, with the solid cross-linking agent, and the mixture is heat pressed or calendered. This technique can be applied advantageously to thin membranes, and provides high productivity eventhough it is possible that the membrane be less homogeneous. Reinforcing agents such as fillers, organic or inorganic, like powders, fibers or strands woven or not, can be added to the polymers before the cross-linking reaction to reinforce the structure. Also, agents creating of porosity can be incorporated if necessary to increase the exchange surfaces with external fluids (catalytic purposes).

Problems solved by technology

A) Although the copolymers forming the membrane are insoluble in their ionic form, the membrane does not have a good dimensional stability and swells significantly in water or polar solvents.

However, this film regenerated in the solid form does not have good mechanical properties.

B) Tetrafluoroethylene (TFE) is a hazardous product to handle, because its polymerisation is performed under pressure and can cause uncontrolled reactions, particularly in the presence of oxygen.

Because of the difference of boiling points between the two monomers forming the copolymer, as well as their polarity difference, it is difficult to obtain a statistical copolymer corresponding to the addition rate of each monomer.

C) The ionic groups in high concentration on the chain have a tendency to cause solubilisation of the copolymer.

D) The penetration of methanol and oxygen through the membrane is high, because the perfluorocarbonated portion of the polymer allows an easy diffusion of the molecular species, which will chemically react at the opposite electrode and cause a loss of faradic efficiency, mainly in methanol fuel cells.

Non-fluorinated systems like sulfonated polyimides or sulfonated polyether sulfones have the same drawbacks because one must compromise between the charged density, and thus the conductivity, and the solubility or excessive swelling.

It is well known that perfluorinated polymers cannot usually be cross-linked by conventional techniques used for non-fluorinated polymers because of the easy elimination of the fluoride ion and the steric hindrance of the perfluorinated chains.

Images