Thermo-rheological characterization of konjac glucomannan aqueous solutions

Konjac glucomannan (KGM) is a high molecular weight, nonionic polysaccharide, which has attracted considerable interest in drug delivery, tissue engineering, biomedicine, and food processing. KGM is known to form thermally irreversible gels only upon alkaline driven deacetylation and thermally reversible gels when compounded with other polysaccharides. When KGM alone is dissolved in neutral water, however, it is not known to form a gel and, despite its widespread use, a thorough rheological characterization is still lacking. In this paper, we explore the rheology of KGM aqueous solutions under rotational shear flow in both linear and nonlinear regimes, within a wide range of concentrations and temperatures. The results show that the rheology of KGM is complex, due to a relatively sharp conformation transition of the polymer chain as a function of temperature. Based on previous observations reported in the literature, we show through rheology that the transition separates a random coil conformation at higher temperatures from a more rigid structure, where helical chain segments are present, at lower temperatures. The conformational changes reflect also a qualitative change of the rheological response in the concentrated regime, where the solution becomes viscoplastic at lower temperatures and thermorheologically not simple, suggesting the formation of a weak gel.