Macromolecules, Vol.30, No.7, 2008-2015, 1997
Static and Dynamic Light-Scattering by a Thermoreversible Gel from Rhizobium-Leguminosarum-8002 Exopolysaccharide
Thermally reversible gelation of an exocellular polysaccharide from the bacterium Rhizobium leguminosarum phaseoli strain 8002 was studied by static and dynamic light scattering (LS) and by small angle X-ray scattering (SAXS). The very stiff double helical polysaccharide spontaneously forms a gel when salt (0.1 M NaCl) is added to aqueous solutions of the polymer with concentrations larger than 1.5% (w/v). The combination of static LS and SAXS revealed a heterogeneous structure that could be described by three characteristic correlation lengths. Large globular particles with a radius of gyration of R(G) = 308 +/- 10 nm appear embedded in a disordered medium with a correlation length of xi(1) = 120 +/- 20 nm and a fractal dimension of d(f) = 1.6 +/- 0.2. The matrix consists of entangled or associated chain sections with a cross-sectional radius of r(c) = 0.7 +/- 0.1 nm. The large clusters have an apparent fractal dimension of d(f) = 3.2 +/- 0.3. A third correlation length of 2.1 +/- 0.2 nm found in the system may be assigned to the fairly long side chains which form a disordered layer around the double helix. The heterogeneous structure was confirmed by dynamic LS, which revealed a very slow translational mode of motion in addition to the common fast translational motion. The time domain between these two relaxation processes appeared to be filled with a spectrum of further relaxation times but could not unambiguously be analyzed by Laplace inversion of the time correlation function.
Keywords:ILL-POSED PROBLEMS;EXOCELLULAR POLYSACCHARIDE;REGULARIZATION METHOD;POLYMER-SOLUTIONS;GELATION;MECHANISM;NETWORKS;POINT