Preparation of thin cation-exchange membranes using glow discharge plasma polymerization and its reactions

Thin cation-exchange films with Fixed sulfonic acid groups were prepared by plasma polymerization followed by hydrolysis of sulfonyl halide groups using benzenesulfonyl fluoride and benzenesulfonyl chloride as starting materials. For benzenesulfonyl fluoride, sulfonyl fluoride groups were introduced into the plasma-formed polymers, whereas benzenesulfonyl chloride tended to decompose during plasma polymerization. The difference between the reaction in the glow discharge plasma of benzenesulfonyl fluoride and that of benzenesulfonyl chloride is discussed based on results obtained by in situ mass spectrometry and molecular orbital calculations. For benzenesulfonyl fluoride, the parent ion ([C6H5SO2F](+.)) is the major species which introduces the sulfonic fluoride groups into the plasma polymer because S-F bond cleavage is difficult. However, the parent ion of benzenesulfonyl chloride is unstable in the glow discharge plasma, and cleavage of the S-C1 bond is facile and produces Ci radicals. The plasma polymer formed at 3.5 W using benzenesulfonyl fluoride exhibits a cation-exchange capacity of 1.3 mequiv g(-1) after hydrolysis, which is comparable to the capacities of commercially available cation-exchange membranes. (C) 2000 The Electrochemical Society. S0013-4651 (99)05-046-6. All rights reserved.