Thermo-thickening behavior and its mechanism in a chitosan-graft-polyacrylamide aqueous solution.

A novel thermo-thickening behavior of a chitosan-g-polyacrylamide (CS-g-PAM, GPAM) aqueous solution is reported for the first time in this work. The viscosity of GPAM aqueous solutions significantly increases above a critical temperature upon heating, as observed in dynamic and steady rheological experiments. Differing from the widely reported hydrophobic modified CS, GPAM was prepared by grafting hydrophilic polyacrylamide side chains onto the CS backbone, therefore the thermo-thickening behavior of the GPAM aqueous solution could not be explained by the usual thermo-thickening mechanism induced by the additional hydrophobic moiety or LCST segment. The origin of the thermo-thickening in GPAM solutions was explored using transmission electron microscopy (TEM), dynamic light scattering (DLS), and nuclear magnetic resonance (NMR) tests of the GPAM solution. A transformation from a hydrogen bonding (H-bonding) aggregate to a hydrophobic aggregate upon heating was confirmed to be responsible for the thermo-thickening. The heating initiates a transformation of large loose H-bonding aggregates into abundant small compact ones formed by self-assembled hydrophobic chitosan backbones, resulting in aggregate associations and thus flocculated aggregate networks. Some factors of the thermo-thickening were investigated and discussed in detail, including the heating history, concentration, grafting ratio, and length of the PAM side chain. Besides the influence caused by the heating history, this thermo-thickening process is influenced by kinetic factors, including the mobility of the macromolecule chains and the formation of new aggregate networks that are dependent on the number of hydrophobic clusters.