Molecular weight distribution and mass changes during polyamide hydrolysis

Samples of polyamide 6, 11 and 12 have been hydrolysed at various temperatures ranging from 80 to 120 degrees C in water baths of pH ranging from 0 to 4. The polymer degradation was monitored by steric exclusion chromatography (SEC), viscometry and gravimetry. SEC data indicate that the polydispersity of the polyamides tested does not increase during ageing. This can be considered as proof of the homogeneity of the reaction. Average molecular weight data from SEC or viscometry appear to be consistent with a random chain scission process, however gravimetry (at least in the case of relatively soft conditions, for instance pH greater than or equal to 4) indicates a high monomer yield. These apparently contradictory results are tentatively explained by the equilibrated character of hydrolysis. Acid-amine pairs resulting from hydrolysis events would recombine easily when both belong to macromolecular fragments. When one fragment is the monomer or a low oligomer, it can escape easily from the cage and the corresponding recombination yield is decreased. The monomer extraction by water complicates the gravimetric behaviour, especially in the most severe hydrolysis conditions. (C) 1997 Elsevier Science Ltd. All rights reserved.