Syntheses and kinetics of piperazine-modified poly(vinyl chloride)s for use as fixed-site proton carrier membranes


Analytical-grade piperazine was dissolved in methanol and reacted with commercial-grade poly(vinyl chloride) powders over a range of temperatures from 25 to 64°C and times from 0.17 to 49 days. The products were aminated polymers that also contained conjugated double bonds. Chemical analysis of the resulting powders showed that up to 1.6 wt% N was possible in this solid-liquid heterogeneous reaction, although insolubility of the aminated polymer in tetrahydrofuran was always realized by 0.8 wt% N content. In those polymer formulations that were soluble, membranes were cast from solution using o-nitrophenyloctyl ether and potassium tetrakis(p-chlorophenyl)borate as the plasticizer and negatively charged sites, respectively. After conditioning, many of these fixed-site proton carrier membranes showed Nernstian pH sensitivity in potentiometric cells, independent of synthesis parameters. The nitrogen content of the aminated PVC and the pH sensitivity of the membrane correlated below a limiting nitrogen content, where Nernstian behaviour was generally observed over a range from 0.4 to 0.8 wt% N content. A three-dimensional plot over the soluble region of the polymer predicted the overall dependence of the reaction parameters, from which an activation energy (17 kcal mol-1) was determined for the substitution process. A time-temperature correspondence was demonstrated that allowed the construction of a master curve of amination in which the shift factor was of the same form as either the Williams-Landel-Ferry equation or a rearranged Doolittle equation. This format was extended to three dimensions, where the pH sensitivity is shown as a function of both the weight percentage N content and the shift factor. © 1994.

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