Facile nanofibrillation of chitin derivatives by gas bubbling and ultrasonic treatments in water

• Chitin derivatives were nanofibrillated by gas bubbling–ultrasonication in water.
• To provide re-nanofibrillation property, amidine groups were introduced in chitin.
• Conversion of amidine to amidinium by CO2 gas bubbling gave nanofibers.
• The amidinium chitin exhibited agglomeration–nanofibrillation behavior.

In this paper, we report that nanofiber network structures were constructed from chitin derivatives by gas bubbling and ultrasonic treatments in water. When chitin was first subjected to N2 gas bubbling with ultrasonication in water, the SEM images of the product showed nanofiber network morphology. However, nanofiber network was not re-constructed by the same N2 gas bubbling and ultrasonic treatments after agglomeration. We then have paid attention to an amidine group to provide the agglomeration–nanofibrillation behavior of chitin derivatives. An amidinated chitin was synthesized by the reaction of the amino groups in a partially deacetylated chitin with N,N-dimethylacetamide dimethyl acetal, which was subjected to CO2 gas bubbling and ultrasonic treatments in water to convert into an amidinium chitin by protonation. The SEM images of the product clearly showed nanofiber network morphology. We further examined re-nanofibrillation of the agglomerated material, which was obtained by mixing the nanofibrillated amidinium chitin with water, followed by drying under reduced pressure. Consequently, the material was re-nanofibrillated by N2 gas bubbling with ultrasonication in water owing to electrostatic repulsion between the amidinium groups. Furthermore, deprotonation of the amidinium chitin and re-protonation of the resulting amidinated chitin were conducted by alkaline treatment and CO2 gas bubbling–ultrasonic treatments, respectively. The material showed the agglomeration–nanofibrillation behavior during the processes.

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