Michel Milas

Water soluble derivatives obtained by controlled chemical modifications of chitosan

Abstract Chemical modifications of polysaccharides are increasingly studied as they have the potential of providing new applications for such abundant polymers. Water soluble chitosan derivatives (quaternized and N-carboxylated chitosans) are prepared, using techniques which allow homogeneous modifications without any sharp decrease in molecular weight, and characterized using NMR spectroscopy and viscometry. In addition a new method for nearly complete deacetylation of chitin is proposed and the role of the acid used for solubilizing chitosan derivatives is mentioned.

Conformational investigation on the bacterial polysaccharide xanthan.

The conformation of xanthan has been investigated as a function of temperature, ionic strength, and polymer concentration. A reversible transition induced by temperature is demonstrated; the melting temperature (TM) is directly correlated to the total ionic-strength and is independent of the polymer concentration. Measurements of circular dichroism show that the polysaccharide exists in a combination of only two characteristic conformations (random and ordered), regardless of the temperature and the concentrations of salt and polymer. Hydrodynamic measurements show that the hydrodynamic volume of both conformations is almost constant over the range of temperature investigated. The mechanism proposed by Morris for melting is confirmed, and a multichain process is excluded. The birefringence stability of the concentrated solutions is discussed.

Properties of xanthan gum in aqueous solutions: Role of the conformational transition

Abstract Xanthan is shown to exist in native (I) and denatured (II) forms, both of which are ordered by the criterion of optical activity, and both of which undergo a temperature-driven, conformational transition to a common, disordered form (III). Changes in optical activity and conductivity are identical for the transformations I → III and II → III. The denatured, ordered form II displays a considerably larger viscosity increment than the native, ordered form I under given conditions of added salt at temperatures below the transition temperature (Tm). Light-scattering measurements yield the same molecular weight for forms I and II; consequently, the observed difference in viscosity increment appears to reflect inherent differences in the chain extension and conformation of forms I and II. Measured persistence lengths for forms I and II are consistent with earlier reports of the persistence length of single-stranded, ordered xanthan in solution. In the present viscosity studies, the absence of any dependence of measured properties of the native form I and the denatured form II on the thermal and concentration history strongly suggests that forms I and II are not aggregated species. Similar comparisons of the chain extension of disordered form III with the ordered forms I and II are more difficult, owing to the absence of conditions under which ordered and disordered forms are simultaneously stable. It is possible, nevertheless, to conclude that the viscosity increment of form II exceeds that of form III, whereas those of forms I and III are similar. Measurements of the molecular weight of form III, needed to convert this observation reliably into information about chain extension, are lacking. These observations are interpreted in terms of a model for xanthan in which the native, single-stranded, ordered form I is stabilized by side chain-backbone interactions that are shown, by experiments reported here, to be strong. These strong interactions are disrupted upon temperature-induced conversion into the disordered form III above the Tm. On subsequent cooling, there is established an alternative pattern of side chain-backbone interactions, presumably dictated by kinetic rather than thermodynamic factors, which stabilizes a more-extended backbone conformation in the denatured, ordered form II. The assumption is made that these strong, side chain-backbone interactions can bridge some backbone breaks which can appear during partial degradation of forms I and II.

The viscosity dependence on concentration, molecular weight and shear rate of xanthan solutions

SummaryThis paper concerns the viscosity dependence of Xanthan as a function of polymer concentration, shear rate and molecular weight in the ordered conformation. The different samples with various molecular weights are obtained by ultrasonication. A unique curve is obtained for the reduced specific viscosity (

Role of conformation and acetylation of xanthan on xanthan-guar interaction

\eta _{sp/} \eta _{sp_0 }

Polyelectrolytes from polysaccharides: selective oxidation of guar gum- a revisited reaction

) as a function of γ · γr−1 for the different molecular weight samples and polymer concentrations below an overlap concentration C [η]0⩽ 1.5. The master curve giving the relation

Polyelectrolyte and rheological studies on the polysaccharide welan.

\eta _{sp_0 }