Sustainable-solvent-induced polymorphism in chitin films

We report a simple route to produce chitin films with different crystalline structures. The use of a green deep-eutectic-solvent (DES) choline chloride: urea (CCU) or hexafluoroisopropanol (HFIP) allows fine-tuning of the crystallization process to produce films with beta-dihydrated- or gamma-chitin structures. With the advent of a Grazing-Incidence X-Ray Diffraction (GIXD) technique, we propose a new and fully indexed structure of gamma-chitin. This new gamma-structure is modeled as a super-cell, in contrast to the idea of a simple physical mixture of alpha and beta phases. The influence of purification, dissolution or dispersion media on chitin's crystallinity was investigated by FTIR, solid-state NMR and XRD. Results presented here may trigger applications in which a well-defined crystalline structure is required.