Dynamics of xanthan biopolymer in semi-dilute aqueous solution studied by photon correlation spectroscopy. Comparison with solution viscosities

Abstract

Recent theoretical treatments of molecular motion in entangled solutions of rod-like polymers lead to results which describe the rheological behaviour in terms of the concentration-dependent rotational diffusion coefficient, D_r. Thus a comparison between experimental values for D_r and rheological data is possible and provides an exacting test of such theory. Here dynamic light scattering (DLS) studies of semi-dilute aqueous solutions of xanthan gum are presented and compared with independent rheological data. Despite extensive precautions, unambiguous interpretation of these data is precluded by the known tendency of xanthan for self-association. Such problems appear to be common when dealing with rod-like polymers and indeed many features of the observed DLS and viscometric data for xanthan in water are strikingly similar to published results on other rod-like polymers. In particular, strong correspondences are found when comparing numerical estimates of D_r determined by DLS and shear-rate-dependent viscosities, η(), with a similar analysis of D_r and η() for semi-dilute methanesulphonic acid solutions of poly(p-phenylene-2,6-benzobisthiazole), a rod-like polymer with a reportedly weak potential for self-association. For each system the D_r and η() data are qualitatively consistent with recent modified versions of the Doi–Edwards dynamical theory for entangled solutions of rod-like polymers. However, large (× 100) quantitative discrepancies are found when comparing η(0) and D_rvia this theory. A survey of rod-like polymer solvent systems for which D_r and η() are available indicates comparable discrepancies and none produces a completely self-consistent interpretation in terms of the above theory.