STARCH FRACTIONS AS EXAMPLES FOR NONRANDOMLY BRANCHED MACROMOLECULES .1. DIMENSIONAL PROPERTIES

Starch is well-known as a blend consisting of highly branched amylopectin and much smaller linear amylose. At a low amylose content the light scattering behavior is dominated by the amylopectin part. Applying a recent description by Fox and Robyt, a series of different molecular weights were prepared by acid degradation of potato starch. The obtained samples were characterized by static-dynamic light scattering and viscometry in a solution of 0.5 N NaOH. The results are compared with data from literature for amylose. The applicability of scaling relationships between exponents is checked. Determination of contraction factors allows evaluation of the branching density. These results are in good agreement with the theoretical model of ABC polycondensation. A comparison between the branching density obtained by LS measurements (0.016) and the branching density determined by NMR and enzymatical techniques (0.04) is made. The observed discrepancy is explained by a modified model of heterogeneously branched amylopectin. The approximate nature of the ABC model is also displayed by deviations of the experimental rho (=R(g)/R(h)) parameters from those predicted by the model.