Thermorheological complex behaviour of maltosyl-chitosan derivatives in aqueous solution

Abstract

N-maltosyl-chitosans with different substitution degrees were prepared by reductive N-alkylation. Dynamic shear experiments were used to study the influence of degree of substitution (DS), polymer concentration and temperature on the viscoelastic properties of these semi-synthetic polysaccharides. The attachment of the disaccharide as side chains drastically changed solubility and rheological behaviour of the ‘native’ biopolymer. At lower DS extensive interchain associations may develop, yet allowing for water solubility, but originating temporary gel-like networks. At higher DS, the bulky of the disaccharide groups leads to less extensive hydrogen bonding or hydrophobic interactions and the viscoelastic profile resembles more to an entangled high-molecular weight polymer. The combined effect of different types of interactions among the polysaccharide chains, including topological entanglements, but also more specific hydrogen bonding and hydrophobic interactions, is responsible for several peculiar rheological characteristics, such as strain-induced structuring, complex relaxation processes, elastic plateaus at low frequencies, anomalous scaling behaviour on concentration regarding the relaxation times and modulus, and complex temperature effects, including departures to the time–temperature superposition principle, which are strongly dependent on the polymer concentration, DS and temperature. Therefore, the explored approach demonstrated that branched chitosan derivatives could be produced with varied and tailorable rheological properties in aqueous media with expected enhanced applications.