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 ([η], 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.

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 (= Rg/Rh) parameters from those predicted by the model.

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 = (4π/λ) sin(θ/2) is the magnitude of the scattering vector with θ 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,app (c) and pronounced deviations from the master curve of the angular dependence. A decrease of R g,app (c) as c -1 was predicted from scaling arguments, and up to c = 5c* this prediction was fulfilled.

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 Γ describes the initial decay of the TCF, g 1 (q,t). A double logarithmic plot of Γ/q 2 ≡ 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 Γ*(q) ≡ (Γ/q 3 )(η0/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 was used. A plot of the TCF's from the various concentrations as a function of Γ(q,c,)t resulted in q-independent shape functions g 1 (Γt), 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.

Because of a well defined supramolecular architecture of the native starch granules the preparation of molecularly dispersed starch solutions is achieved only after autoclaving at temperatures of 135 to 160 C. A detailed analysis of static light scattering data allowed the determination of the molecular parameters of both the amylopectin and amylose. The results were confirmed by (1) measurements in the iron sodium tartrate complex FeTNa, (2) by extrapolation of the data obtained with degraded starches to no degradation and (3) by sedimentation field‐flow‐fractionation sFFF. Above the overlap concentration strong aggregation due to H‐bonding commenced and eventually led to gelation. The process is promoted by the amylose content and could be followed by static and dynamic light scattering. Cationic starches and cationic amyloses display remarkably different behavior. The branched amylopectin expanded uniformly when the ionic strength was lowered but the corresponding amylose exhibited an unusual helix‐disorder transition.