PHASE-SEPARATION INVOLVING ORDERED BIOPOLYMER SYSTEMS

The formation of a gel phase is very common in the biopolymer field and is at the basis of many important biological and industrial processes. Among the several known gelling systems, those occurring by the local association of conformationally ordered chains are the most interesting for their peculiar reversibility. A structural and thermodynamic characterisation of these processes is presented, in particular, for three categories of biopolymers. A regular non-ionic microbial polysaccharide is shown to exhibit a very cooperative phase transition which is the prerequisite for a thermoreversible gelation process (even at very low polymer concentration, down to 0.1 g/L) and its behaviour is discussed in view of the supramolecular structures formed. The second example is given by the ionic and temperature dependence of the gelling properties of the sulphated algal polysaccharides, in which the interactions between ordered chains are dominated by the ionic interactions. Finally, the thermodynamic and kinetic characterisation of a new gel phase formed in a semi-concentrated solution of the microbial polyester, poly(D-(-)-beta-hydroxybutyrate, is reported.