Heterogeneous Dynamics and Microdomain Structure of High-Performance Chitosan Film as Revealed by Solid-State NMR
Fenfen Wang, Zhou Deng, Zhijun Yang, Pingchuan Sun
Abstract
Understanding the structure–property relationship of biopolymers is crucial for revealing the superior properties of advanced materials in nature, and thus facilitating material innovation. In this work, it was found that the chitosan film cast from acid solution represented a tremendous change of mechanical property, from brittle to tough when neutralized in a strong alkaline solution. The molecular mechanism of the enhancement in mechanical property was systematically investigated by multiscale solid-state NMR coupling with other techniques.1H and 13C NMR experiments revealed that the alkaline treatment leads to a distinct difference in structure and dynamics for the chitosan samples. X-ray experiments and polarized optical microscopic results demonstrated that the crystallinity of alkaline-treated chitosan film was remarkably enhanced due to the removal of acid counterions, resulting in the enhanced interchain hydrogen-bonding interaction. For chitosan films cast from acidic solution, the acid counterions act as a defect and impair the interchain hydrogen-bonding interaction as well as long-range organization, resulting in brittle films with amorphous structure. 1H double quantum filter spin diffusion experiments indicated the enhanced nanoscale phase separation and heterogeneous dynamics in the alkaline-treated chitosan films. Therefore, hydrogen-bonding plays a crucial role in affording high mechanical strength to the neutralized chitosan films with well-defined long-range structure. A deep understanding on the heterogeneous dynamics and nanostructure of the neutralized chitosan films with enhanced mechanical properties at the atomic level could guide the regeneration of high-performance natural polymer materials.