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-p, added salt concentration C-s, and temperature. The results show the existence of a broad peak in the investigated range of C-p and momentum transfer q. The positions of these peaks scale roughly as C-p(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 C-p(1/2) too. The equivalent Bragg interparticle distance has been found in good agreement with hexagonal packing, assuming a single stretched chain in the low-C-p range (disordered xanthan conformation) and a double-stranded chain, at least locally, in the high C-p range (ordered conformation).