Particularities in static and dynamic light scattering from branched polyelectrolytes in comparison to their linear analogues

The expansion behaviour of branched strong polyelectrolytes when lowering the ionic strength was compared with the corresponding linear chains. Two pairs of polymers were chosen (a) linear poly(allylamine hydrochloride) with the corresponding intramolecularly crosslinked microgel and (b) cationic amylose derivatives with the corresponding derivative of potato starch that consists mainly of the branched amylopectin. The cationic group in the polysaccharides was a quaternized amine. As expected the expansion behaviour of the branched materials is considerably less than of the linear chains. Surprisingly the ratio of the radius of gyration to the hydrodynamic radius rho = R(g)/R(h) showed opposite behaviour and increased for the linear chains (rod like behaviour) but decreased for the branched materials to sphere like behaviour when the ionic strength was lowered. A more detailed analysis of the structure was possible with the polysaccharides. Because of their large dimensions the angular dependence of the scattered light could be measured. The data were analysed from Kratky plots. The branched amylopectin showed no change in the self-similarity when the ionic strength was lowered and could be described by a fractal dimension of about d(f) = 2.2. Very different behaviour was found with the amylose derivative. A rod-like structure was obtained at high ionic strength that underwent a transition to a disordered conformation that however, is not that of a random coil. Various model calculation were made which give evidence for a folded double helix with loops in between. At low ionic strength the two double-helix section unfold and the loop strongly expands.