Highly crystalline chitosan produced by multi-steps acid hydrolysis in the solid-state

A multi-steps solid state hydrolysis of chitosan with 3 and 12 M HCl including typical intermediate washings was performed at room temperature. This original treatment induced the processing of highly crystalline materials with polymorphic changes. At each step, a slow decrease of the molecular weight and a plateau in the X-ray crystallinity index (CrI) variation was observed with time. CrI increased up to 88% with the chain re-crystallization, and the transformation of the hydrated to the anhydrous allomorph. Different factors were at the origin of the plateau of CrI observed: (i) the hydrolysis rate decreased after consumption of easily accessible amorphous parts, (ii) crystals of the anhydrous polymorph resulting from the reaction hindered the access of reagents to the remaining amorphous parts. Both mechanisms assumed a decreasing access to reagents: the former during the early stages of hydrolysis, the latter at longer times. The important role played by the multi-steps process with appropriate washings was interpreted and justified. A scheme is proposed to describe the mechanisms involved during hydrolyses.