Starch fractions as examples for nonrandomly branched macromolecules .3. Angular dependence in static light scattering

The angular dependences of seven degraded potato starch samples were measured by static light scattering in the dilute and semidilute regimes and analyzed on the basis of a previously treated model that resembles hyperbranched structures. These nonrandomly branched samples show only very limited intermediate range power law behavior and are not fractals. Furthermore the various samples of different molar masses are not self-similar to each other, but each sample exhibits its own exponent in the asymptotic region. These findings are in principle agreement with the predictions for the above-mentioned model that, however, neglects excluded volume effects. This fact causes a change in the meaning of the structure-determining parameter C of the model. The scattering curves from the semidilute regime could be condensed to one master curve when plotted against qR(app)(c), where R-app(c) is the apparent radius of gyration and q = (4 pi/lambda) sin(theta/2) is the magnitude of the scattering vector with theta being the scattering angle. This master curve coincides with the particle scattering factor at zero concentration. No change in the shape of the macromolecules as a result of the osmotic force is deduced from this behavior. However association becomes effective when the overlap concentration is exceeded, which is recognized by a strong increase of R-g,R-app(c) and pronounced deviations from the master curve of the angular dependence. A decrease of R-g,R-app(c) as c(-1) was predicted from scaling arguments, and up to c = 5c* this prediction was fulfilled.