Small Angle Neutron Scattering from Polyelectrolyte Solutions: From Disordered to Ordered Xanthan Chain Conformation

This work discusses the static properties of xanthan solutions in the semidilute concentration range as revealed by small angle neutron scattering (SANS). Samples having different molecular weights were used to investigate the scattering properties as a function of polymer concentration C,, added salt concentration C,, and temperature. The results show the existence of a broad peak in the investigated range of C, and momentum transfer q. The positions of these peaks scale roughly as C,1'2 in the high- concentration range. As the concentration decreases, the system shows a conformational transition characterized by a decrease in the exponent. This change in the exponent is interpreted as a progressive shift from ordered to disordered conformation of xanthan for which the peak position is expected to scale with the concentration as Cpm too. The equivalent Bragg interparticle distance has been found in good agreement with hexagonal packing, assuming a single stretched chain in the low-C, range (disordered xanthan conformation) and a double-stranded chain, at least locally, in the high C, range (ordered conformation). I. Introduction Solution properties of polyelectrolytes exhibit a be- havior which, in most cases, differs considerably from that of uncharged polymers. The origin of this feature comes obviously from the presence of charges along the chain and leads to intra- and intermacromolecular interactions. These interactions, ifthey are not screened out, by adding external salt for instance, have strong consequences on both static and dynamic properties of the system. These complex mechanisms involving interacting polyions, counterions, and co-ions have attracted many researchers, and over the past decade there has been considerable interest in the static and dynamic properties of such systems from both theo- reticall-ll and experimental points of vie^.'^-^^ Various experimental techniques have been used to understand the behavior of such charged complex systems: small angle X-ray (SAXS) and neutron scattering (SANS), elastic and quasi elastic light scattering, and viscosity. In this paper we shall present and discuss our experi- mental results obtained using SANS. The results concern xanthan which is a charged bacterial polysaccharide. Two conformations character- ize this polymer and were investigated previou~ly~~-~~ using different techniques. At low polymer concentra- tion, low salt concentration, and higher temperature, the xanthan is in its disordered conformation, i.e. a wormlike chain with a moderate persistence length (Lp - 50 A); when temperature is decreased or external salt concentration is added (above M at 25 "C), an ordered conformation corresponding to a helical struc- ture with a high intrinsic persistence length (Lp - 350 A, based on a monohelical structure assumption), is ~tabilized.~~ This conformation can be either a single helix or a double helix, as it is still under di~cussion.~~ Moveover, when the polyelectrolyte concentration in- creases and reaches a critical value, the system presents a cholesteric mesophase formation.33