Structure Formation by Polysaccharides in Concentrated Solution

Molecular structure determination in concentrated solutions is generally considered unfeasible. This conclusion, drawn from experience with linear flexible chains, relates to the complete interpenetration of coils and their ensuing entanglement. Such deep interpenetration is prevented by the obstacles of branching units in branched macromolecules. The present study with branched (and one linear cellulose) polysaccharides demonstrates that different architectures can be distinguished via their different repulsive interactions. The procedure displayed in this article permits the estimation of true molar mass and true radius of gyration at finite concentration c. It furthermore allows, within upper and lower limits, an estimation of the growing size, when association takes place. Results from starches, glycogen, dextran, and cellulose, in aqueous media are discussed. The cellulose was dissolved in a recently developed aqueous cadmium complex. Association can lead to gel formation and/or to phase separation. The former is characterized by repulsive interactions which only slightly decrease when the gel point is approached. Phase separation, on the other hand, results from a decrease of the repulsive interaction induced, for instance, by another polymer. The effect is demonstrated with starches containing different amounts of amylose, where pure amylose forms unstable solutions and precipitates in time as a semicrystalline solid.