RED MICROALGA EXOPOLYSACCHARIDES - 2 - STUDY OF THE RHEOLOGY, MORPHOLOGY AND THERMAL GELATION OF AQUEOUS PREPARATIONS

Porphyridium sp (P.sp) is a red marine microalga which is encapsulated within a charged polysaccharide envelope. The external part of this e nvelope dissolves in the growth medium. This mucilaginous polymeric ma terial harvested from the medium is an anionic heteropolysaccharide co ntaining both carboxyl and half-ester sulfate groups. Aqueous preparat ions of P.sp polysaccharide yield highly viscous samples with unique r heological properties which may be of commercial value. Rheological st udies of aqueous preparations have been combined with X-ray diffractio n studies of oriented fibers in order to investigate the conformation and interactions of the macromolecules. The X-ray fiber diffraction su ggests the presence of an oriented single two-fold helical structure o f pitch 1.6 nm, indicating that the P.sp. polysaccharide contains exte nded regions with a regular chemical repeat. Aqueous preparations exhi bit relatively high viscosity at low polymer concentrations. Within th e investigated concentration range the preparations show marked shear thinning behavior with no evidence for a Newtonian plateau. Small defo rmation mechanical studies of 1% show evidence for elastic behaviour i ndicative of a weak gel network. An unusual feature of these samples i s a reversible sharp transition upon heating from a weak to a strong e lastic network.