Characterizing gelation kinetics of chitosan dissolved in an alkali/urea aqueous solution: Mechanisms accounting for the morphological development

Abstract

Dissolution in an alkali/urea aqueous solution offers a novel way to prepare chitosan membranes via immersion precipitation, whose underlying mechanisms are of fundamental interest to the improvement of filtration performance. This study employed optical coherence tomography (OCT) to in-situ characterize the gelation of chitosan dissolved in a LiOH/urea aqueous solution using pure water as the coagulation bath. The OCT datasets were numerically analyzed to resolve the conversion curves at various depths in the cast film, thereby establishing a method to estimate the phase-inversion rate in terms of the moving gelation front. The periodic competition between the solidification and diffusion of the polymer chains (i.e., the Liesegang phenomenon) was revealed and analyzed in terms of the assumption that the phase inversion should be dominated by the removal of the gelation inhibitors (i.e., the alkali and urea). It was further confirmed by a comparative study that the alkali could play a relatively important role and the porous morphology could be effectively tuned by varying the concentration of alkali in the coagulation bath. All the characterization results provided evidence correlating the reduction in the connectivity of the polymer network with the retarded gelation. This study would shed light on the development of novel methods for fabricating chitosan membranes with optimized geometrical and topological characteristics.