AbstractNylon 66 and nylon 6 yarns were exposed in air, nitrogen, and vacuum to temperatures form 136 to 215°C. for periods from 5 min. to 17.5 hr. The moisture absorption of both fibers is reduced by the exposure independently of the presence of oxygen or of antioxidant. This is ascribed to secondary crystallization. Simultaneously, the electrical resistivity is increased. The breaking strength is reduced by exposure in the presence of oxygen unless the polymer contains antioxidant. The extent of oxidative cleavage was measured by the viscosity‐average molecular weightM̄vof nylon 66. Simultaneously, a reduction of the amine endgroup content was observed. It occurs only in the presence of oxygen and approaches a limit indicating that some amine groups are not subject to elimination. If oxygen is absent or the polymer protected by antioxidant, theM̄vincreases. Sorption of hydrochloric acid is reduced by the exposure. Only 80% of the amine groups in the unexposed and 55% in the exposed nylon participate in acid binding at pH 2.5. The rate constants of cleavage as manifested byM̄vand of the process leading to disappearance of amine groups were determined for nylon 66. The activation energies were calculated 40 and 36 Kcal./mole, respect
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