Gel-Sol Transition and Thermotropic Behavior of a Chitosan Derivative in Lyotropic Solution

A chitosan derivative which has a structure that approximates N-phthaloyl-3,6-di-Oacetylchitosan (AcPhCh) was used to investigate the thermal behavior of lyotropic gel liquid crystalline phases in dimethylsulfoxide (DMSO). A distinct gel-sol transition in addition to a liquid crystal-liquid (or chiral nematic-isotropic) transition was observed in the AcPhCh-DMSO system by differential scanning calorimetry (DSC). The detected gel-sol transition was reversible with a considerable amount of temporal hysteresis. Mesophase ordering occurred prior to gel formation for AcPhCh-DMSO samples while cooling from above the order-disorder “clearing” transition temperature. Similarly, a transition to a liquid crystal phase takes place prior to gelation while increasing the polymer concentration in the isotropic solution at ambient temperature. Circular dichroism (CD) spectra in addition to polarized light microscopy (PLM) observations as a function of temperature supported the existence of liquid crystal-liquid transitions at the temperatures recorded by DSC. A ball-drop experiment carried out on the lyotropic gel confirmed that the DSC step transition was indeed a gel-sol transition and provided additional mechanistic information. The above studies demonstrate, contrary to previous reports, that development of a lyotropic gel by rodlike polymers may occur first by forming an anisotropic liquid followed by gelation.