Sol and Gel Based on Polysaccharide: Characterization and Structure-properties Relationship

The objective of this chapter is to describe the main techniques adapted to characterize ionic or neutral polysaccharides, especially when they are water soluble. The steric exclusion chromatography (SEC), rheology and NMR spectroscopy are, in our opinion, the most important techniques to study the main characteristics of polysaccharides. The same techniques and approaches may also be valid for other water soluble biopolymers such as proteins and nucleic acids. The main characteristics of polyelectrolytes are recalled and it is shown that the ionic behavior of the polymers can be used to establish the nature of the polymer conformation (coil, helix). In this case, it is necessary to combine thermodynamic characteristics with optical rotary power (or differential scanning calorimetry DSC and circular dichroism CD) and molar mass determinationss. The semi-rigid characterization of the polysaccharide chains depending on their conformation is introduced. The persistence length Lp which characterizes the local stiffness of the chain determined by steric exclusion chromatography in application of the worm-like chain model and validated by molecular modelling. Rheology of polysaccharides is of great interest due to fundamental and applied point of views (especially in food, cosmetic or biomedical applications). The solutions even at low polymer concentration are often non Newtonian due to the stiffness of the polysaccharides which also depresses the critical overlap concentration. The main relationships relating viscosity to molar mass and/or polymer concentration are given; the influence of the shear rate for measurement is pointed out; and, the flow and dynamic measurements are described for sol systems. Originality of polysaccharides is that they may associate to form 3-D network, or physical network. This gel is controlled by the thermodynamic conditions and environmental conditions (ionic concentration, nature of ions, temperature, pH… ). The main mechanisms of physical gelation are recalled and characterization of the sol-gel transition is introduced. The techniques to follow this transition as well as those for gel rheology are also introduced.

Keywords: Biopolymer, Characterization, Ionic selectivity, Polyelectrolyte, Polysaccharide, Rheology, Sol-gel transition, Steric exclusion chromatography, Viscosity, Water-soluble systems.