Effect of urea on solution behavior and heat-induced gelationof chitosan-β-glycerophosphate

In this study, we have investigated the effect of urea on the physicochemical (pH and conductivity) and rheological properties of the chitosan-β-GP system in order to assess the main polymer–polymer interactions at low and high temperature. The pH of the solutions was slightly increased due to the consumption of H+ in solution through the hydrolysis of urea. Furthermore, the addition of urea considerably decreased the conductivity, and therefore the ionic strength of the solutions, and this effect was more important at high temperature. It indicated that urea strongly affects polymer–polymer interactions by weakening hydrogen bonds at low temperature, but in addition can hinder hydrophobic effect at high temperature since the reduction of ionic strength results in less screening of electrostatic repulsion between protonated glucosamine groups. At 15 °C, the addition of urea to chitosan-β-GP solutions decreased their elasticity, shortened relaxation times and simplified the relaxation process due to the disruption of hydrogen bonds. Heat-induced gelation of the chitosan system in the presence of urea showed higher gelation temperature (Tgel) in non-isothermal tests and longer gelation time (tgel) in isothermal conditions. The activation energy for gelation also increased with increasing urea concentration. We concluded that the detrimental effect of urea on the gelation process was mainly related to a decrease in polymer-polymer hydrophobic effect, as shown by the decrease in conductivity.