New associative systems based on alkylated hyaluronic acid. Synthesis and aqueous solution properties

This paper concerns the preparation of new water-soluble alkylated derivatives of hyaluronan (HA); these derivatives were synthesized under mild aqueous and well controlled conditions. The first step was to prepare a HA derivative selectively modified with adipic dihydrazide (HA-ADH) with a substitution degree of 0.08; then, alkyl chains with different chain lengths were introduced with a substitution degree around 0.05. These polymers being still water-soluble exhibit associating properties in the presence of external salt. Formation of hydrophobic domains is demonstrated from fluorescence measurements in the presence of pyrene as a probe. Then, it is shown that the viscosity of polymer solutions in 0.1 M NaCl becomes non-Newtonian for HA grafted with C-10 and C-12 chains and increases rapidly with the polymer concentration over 1 g/L. When the polymer concentration increases, a gel-like behaviour was observed from rheological measurements with the G′ modulus larger than G″ in all the frequency range covered. The dependence of the shear stress on shear rate applied according to a continuous linear ramp provided evidence of the deformation or rupture of aggregates. The latter were shown to reform slowly. At the end, the influence of temperature was examined: first a slight increase in the modulus was observed followed by a decrease corresponding to reinforcement of the hydrophobic interactions.