Intramolecular and Intermolecular Association during Chemical Cross-Linking of Dilute Solutions of Different Polysaccharides under the Influence of Shear Flow

Abstract

In this work, intramolecular and intermolecular associations under the effect of shear flow of dilute aqueous alkaline solutions of dextan, hydroxyethylcellulose (HEC), and the hydrophobically modified analogue (HM-2-HEC) in the presence of a chemical cross-linking agent were characterized with the aid of viscometry and rheo-small angle light scattering (rheo-SALS) methods. The picture that emerges at short times during the cross-linking reaction at a constant shear rate is that HEC coils contract because of intramolecular cross-linking; whereas the HM-2-HEC species show an incipient association and the dextran molecules are unaffected due to their compact structures. At longer times, interchain cross-linking of the polymer promotes the growth of large flocs, which are disrupted by shear forces when they are sufficiently large. The delicate interplay between intramolecular and intermolecular association is governed by factors such as the magnitude of the shear rate, the cross-linking agent concentration, the pH value and the reaction temperature. Two-dimensional SALS intensity patterns at different stages during the cross-linking reaction were acquired by the rheo-SALS method. From the analysis of these pictures we gain the q dependence of the scattered intensity, and the time evolution of the scattered intensity at a fixed low q value. The results from the rheo-SALS measurements are in good agreement with the viscometry data. The cross-linking behaviour of HM-HEC (HM-1-HEC or HM-2-HEC) under the influence of shear stress in the presence of both a chemical cross-linking agent and a cosolute (β-CD or HP- β-CD) were also explored. The effect on the cross-linking reaction on the addition of β-CD depends on how the samples are prepared. By one preparation method, a dextran-like behaviour is observed for the HM-HEC-cross-linker-cosolute system, while the other preparation procedure shows no big difference from what is observed in the absence of the cosolute.