Producing homologous series of molar masses for establishing structure-property relationships with the aid of ultrasonic degradation

Abstract

Ultrasonic degradation was shown to be a very effective method for creating homologous series of molar masses in order to establish structure-property relationships such as the Mark-Houwink equation and the RG-M relation. It was tested mainly for cellulose derivatives. The detection of the molar masses, the radii of gyration and of their distributions was carried out by means of size-exclusion chromatography coupled with multi-angle laser light scattering and a refractometer (SEC/MALLS/DRI). The results were compared with existing data and checked according to their suitability. It was shown that the calculated data generally agree very well with the experimental results. Only the exponents of charged polymers were slightly lower than the calculated values. Thus the change in polydispersity has no effect on the establishment of the relationships. Furthermore the influence of the chemical structure on the degradation process of cellulose and starch derivatives was examined with special regard to the limiting molar mass and the degradation constant proposed by Ovenall et al. It was revealed that the conformation of the polymers in solution plays an important role on the degradation process. The change in the molar degree of substitution and the introduction of a second substituent of cellulose derivatives have no effect on the decomposition pattern because the slightly expanded conformation remains undisturbed. In contrast the degradation process of strongly coiled hydroxyethyl starch is slowed down and the limiting molar mass lies above that of cellulose derivatives. First investigations with the synthetic polyacrylamide (PAAm) which was grafted with starch showed that even small amounts of starch side groups lead to a retardation of degradation.