Preparation of Nanochitin Films with Oligochitin Graft Chains

Even nowadays, chitin is mostly unutilized as a biomass resource, although it is abundantly present in nature. To develop an efficient method to use chitin as the component in new functional bio-based materials, in this study, we investigated the preparation of a flexible nanochitin (chitin nanofiber, ChNF) film with oligochitin dihexanoate graft chains. The parent ChNF film was prepared by regeneration of a chitin ion gel with an ionic liquid, 1-allyl-3-methylimidazolium bromide (AMIMBr), using methanol and subsequent filtration. However, the obtained film showed a quite brittle nature, probably because of the high crystallinity of the chitin chains. To reduce the crystallinity, oligochitin dihexanoate, which was provided by partial depolymerization of the parent chitin dihexanoate under acidic conditions, was modified on the partially deacetylated ChNF film by reductive amination. The introduction of the oligochitin dihexanoate graft chains was supported by 1H NMR and IR measurements. The powder X-ray diffraction (XRD) profile of a film, which was obtained from an aqueous acetic acid suspension of the grafted product, indicated a reduction in chitin crystallinity, which contributes to the disappearance of nanofiber morphology and enhancement of flexibility. The removal of hexanoyl groups from the film was performed by treatment with aqueous NaOH. The IR and XRD measurements of the obtained film suggested the compete dehexanoylation and the reformation of the chitin crystalline structure, respectively. This study provides a method to fabricate new bio-based graft and soft materials entirely comprising chitin moieties.