AbstractThe acid‐base titration curves of solid nylon 6 resins and nylon 66 fibers have been measured, and isoionic points and endgroups determined. Five nylon 6 polymers of different molecular weights and two commercial nylon 66 yarns have been studied. Titration curves of these materials with hydrochloric acid and sodium hydroxide in the absence of added salt, and of nylon 66 in the presence of added salt, at three different temperatures, and with benzenesulfonic, trichloroacetic, and naphthalene‐2‐sulfonic acid, allowed the effect of polymer molecular weight, salt, temperature, and anion affinity on the titration curves to be determined. All the results are consistent with the view that nylon possesses a zwitterion structure at neutral pH, carboxyl groups being protonated on acid titration. A proportion of the endgroups is not available to titration in the solid polymers. Existing theories of the acid–base combination of solid polymers fail to explain all the results. The titration curves and elastic properties of the following modified nylon 66 yarns have also been determined and interpreted: nylon deaminated by nitrous acid, dinitrophenylated, acetylated, carboxymethylated, hydrolyzed, and dihydrophenylated nylons, and nylon treated with alkaline hypochlorite, boiling water, and boiling benzene. In all cases the increase in the work to stretch the filament is independen
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