Starch fractions as examples for nonrandomly branched macromolecules .2. Behavior in the semidilute region

In a former paper, the dimensional properties of degraded starch fractions of different molar masses were reported. The behavior was found to be dominated by the highly branched amylopectin. The same samples are now investigated in semidilute solution by static-dynamic light scattering. The deviating values of the overlap concentrations calculated by four definitions ([eta], R(g), R(h), and A(2)) are discussed. The influence of polydispersity, excluded volume, and branching are checked. All starch fractions form one common master curve in the plot of the normalized osmotic modulus versus the reduced concentration (c/c*). The data are described in terms of the third and fourth virial coefficients A(3) and A(4). At concentrations higher than 9%, low-angle excess scattering starts to govern the behavior in static light scattering, and simultaneously slow motion becomes noticeable in the time correlations function (TCF) that grows as the concentration is increased. The TCFs are described by stretched exponentials (KWW functions). Three modes are observed: a fast one increasing with concentration and two other ones decreasing with concentration. The concentration dependencies of the first two motions are compared with the behavior of the cooperative and the reptation modes predicted by de Gennes for linear chains. The third (slow) motion is interpreted as being caused by associates.