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 scat tering (SANS). Samples having different molecular weights were used to investigate the scattering properties as a function of polymer concen tration C-p, added salt concentration C-s, and temperature. The result s 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 decrea ses, the system shows a conformational transition characterized by a d ecrease in the exponent. This change in the exponent is interpreted as a progressive shift from ordered to disordered conformation of xantha n for which the peak position is expected to scale with the concentrat ion as C-p(1/2) too. The equivalent Bragg interparticle distance has b een 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).