The high level of hysteresis observed in the current PAN gel fibers is due to the presence of both highly basic pyridine and carboxylic acid groups. There is a large pH range for which the pyridine groups are protonated but the carboxylic acid moieties remain anionic. There are a few approaches to this problem, each of which involves either deactivation of the basic pyridine group and/or conversion of the carboxylic acid group to a more basic functionality.
1) Conversion of carboxylic acid groups to amino groups: Two methods are:
a) Amidization of carboxylic acid with dimethylamino ethylamine to obtain a dimethylamino group as the electrolytic group. The pKa of such a functionality would be about 10 or 11. In the presence of the pyridine groups, the pH range should be considerably narrower, and is expected to fall between approximately 9 and 11.
b) Conversion of carboxylic acid to primary amine groups using standard reagents such as i) NaN3, H2SO4, followed by NaOH (Curtius rearrangement); ii) aminoalcohol, followed by thionyl chloride and heat (Lossen reaction), or other common methods. The primary amine is also expected to be a strong base, and to respond in a range close to that of the pyridine groups.
2) Convert the basic pyridine groups to charged species: This approach eliminates the effects of modulation of the gel behavior between two different electrolytic groups. (In this case, the polymer gel fiber may exhibit an amphoteric response moderated or affected in part by the presence of permanently charged species.) Two methods are:
a) Alkylation of the pyridine groups with an alkyl bromide, thus forming a positively charged pyridinium bromide group. This approach would work best in conjunction with conversion to the carboxylic acid groups to amino groups, as the charges incorporated would be the same.
b) Substitution reaction of pyridine with sultone to produce pyridinium alkyl sulfonate groups in which the positively charged pyridinium group is covalently attached to a negatively charged sulfonate group; the net charge of the moiety would be effectively neutral. This would work in conjunction with either carboxylic acid groups or amino groups.
Each of the above methods could be used alone, or in conjunction with each other, to determine the amphoteric response of the gel. Other gel systems, including systems based on changes in ionic strength rather than pH, are possibilities for further polymer design.