Shear-induced structure in enzymatically-synthesized dextran solutions

Abstract

A number of dextran derivatives and low molecular weight dextrans have found outlets in different fields such as fine chemistry and pharmaceutical industry, whereas only a few native (high molecular weight) dextrans have found applications. Thus, theoretical studies have scarcely concerned the rheological properties of native dextran solutions. Among the different synthesis processes, only the enzymatic in vitro synthesis produces a pure dextran, thereby allowing the rheological behavior of dextran solutions to be interpreted in terms of molecular considerations.The shear viscosity curve reveals an irreversible threshold-type shear thickening at a specific critical shear rate ~*. The value of ?* depends on the temperature of experiments and on the dextran concentration. Transient experiments give a description of the shear-thickening as a function of time and allows us to study the effect of several parameters (temperature, solvents, etc.). The shear stress growth, during the strueture formation, is made of two different kinetics: the first one depends strongly on the molecular conformation; the second one is the result of intermolecular interactions of the hydrogen-type bonds.

Threshold-type shear-thickening in polymeric solutions

“…The thresholdtype shear-thickening divides the viscosity curve in two regions: -for low values of shear rate (~ < :)*), the curve exhibits a Newtonian plateau followed by a smooth transition from Newtonian behaviour to shear-thinning behaviour (this is not unusual for natural polymers), this part of the curve is denoted by BSF (Before Structure Formation); -for values of shear rate higher than 9", the shear-thinning is more pronounced and the curve is denoted by ASF (After Structure Formation). Below this ASF curve, the extrapolation of the BSF curve (dashed lines) corresponds to the locus of minima (t/0 = r0/90, with 9o > 9*) exhibited by the shear stress growth curve just after the sharp overshoot shown in figure 3 of [39]. This is exactly the same phenomenon as that observed with Pusher 700.…”

“…This is the case however only when the shear rate does not exceed a critical value, ~*. For shear rates higher than ~*, the stress growth data exhibited a typical pattern of a shear-induced structure formation as illustrated in figure 3 of [39]. For the shear stress growth curve, all the characteristics observed for the Pusher 700 solutions are exhibited: a sharp overshoot, then a minimum v0 followed by a monotonically increase of shear stress with time.…”

“…It is therefore not so easy to obtain the viscosity and primary normal-stress difference curves which are represented in figure 4 of [39]. At the critical shear rate ~*, both curves show a discontinuity.…”

“…All the data represented by open symbols on figure 4 of [39] and corresponding to values of shear rate lower than 9" can be obtained with an increasing or a decreasing rate sweep. These data correspond to the BSF curves.…”

“…This is why we used an enzymic "in vitro" synthesis technique allowing us to obtain a pure dextran. Therefore, molecular considerations can be made in order to interpret the unusual rheological properties of dextran solutions [39,40].…”