Starch fractions as examples for nonrandomly branched macromolecules .4. Angular dependence in dynamic light scattering

The angular dependence of dynamic light scattering from five degraded potato starch samples was studied. The study allowed the determination of internal modes of motion in branched structures that resemble hyperbranched polymers. The properties of the investigated starches are dominated by the large size of the amylopectin. Measurements were made at infinite dilution (c = 0) and at concentrations c > c* where c* is the overlap concentration. The time correlation function (TCF) at low concentration was analyzed by its first cumulant at low values of qR(g) and in the asymptotic region of qR(g) > 2. The first cumulant Gamma describes the initial decay of the TCF, g(1)(q,t). A double logarithmic plot of Gamma/q(2) drop D-app(q) against qR(g) gave a curve that lies between those for hard spheres and flexible coils. In contrast to Linear chains, it showed an asymptotic slope of 0.80-0.85 instead of the theoretically expected slope of 1.00. The reduced first cumulant T*(q) drop (Gamma/q(3))(eta(o)/kT) did not approach a constant plateau at large q but decayed continuously below the experimentally observed plateau value for linear chains. The reason for this behavior is seen in the high branching density and a loss of internal flexibility. In a second part, the angular dependence of the first cumulant in the semidilute regime, up to c/c* = 5, was measured. The data from the different concentrations could be condensed to one common master curve when an empirical scaling parameter <(R-h(c))over bar> was used. A plot of the TCF's from the various concentrations as a function of Gamma(q,c,)t resulted in q-independent shape functions g(1)(Gamma t),), which gradually changed from Zimm to Rouse behavior when the concentration was increased beyond the overlap concentration c*. This change is interpreted as a result of hydrodynamic screening.