and u s pat no 4 333 805 the disclosure of the abovementioned references u s pat no 4 191 618 u s pat no 4 212 714 and u s pat no 4 333 805 in respect of the structure and production of membrane electrode units is hereby incorporated by reference into the present description the invention is illustrated below by means of examples and comparative examples without the invention being restricted to these examples a commercial polymer celazole pbi polymer in the form of a powder is separated into various fractions by means of a stack of sieves the results of the sieve analysis are shown in table 1 the sieve fractions obtained in this way are dried individually as soon as the water content of a sieve fraction is lt 0 1 a solution is prepared therefrom by mixing with dimethylacetamide using a method described in the prior art it is found that particles having a size of gt 1500 u03bcm cannot be brought completely into solution for this reason particles having a size of gt 1500 u03bcm should not be used for the preparation of solutions each solution prepared using a separate sieve fraction is then used to produce a film by conventional industrial casting processes or by a manual doctor blade technique the films produced in this way are doped by dipping into 85 h 3 po 4 at room temperature for 72 hours to determine the mechanical properties test specimens.