Receiving considerable interest in recent years, the reducing ends of cellulose nanocrystals (CNCs) with heterogeneous groups and different reactivity from surface chemistry contribute to the development of new strategies for regioselective modification and performance tailoring for this building block of natural biomass. However, the understanding of the phenomena involved at the reducing end is still at the research stage for cellulose chains, usually in dissolved form in a solvent, but lacks extensive investigation for nanocrystals, existing in the form of suspensions. Two issues are explored in this study. The first one concerns the comprehensive analysis of various conditions and parameters for the introduction of aldehyde groups at the reducing end of CNCs. The second investigates the influence of end-modified species on the crystalline reconfiguration of nanocrystals during mercerization. Quantitative measurements of the content of reducing end aldehyde groups are carried out for nine nanocellulose samples, offering scientific evidence to obtain more aldehyde groups available at the reducing end of CNCs for future studies. Furthermore, four species of CNCs with various chemical structures and macroscopic properties are modified at the reducing ends, and their reorganization as type-II nanocrystals during mercerization is investigated. The results indicate a critical effect of the steric hindrance of end species on the crystalline reconfiguration of nanocrystals, providing inspiration for a possible molecular structure and asymmetric modification for this ageless nanomaterial.
In recent years, click reactions with cellulose nanocrystals (CNC) participation have gradually become a research hotspot. Carboxylamine condensation is the most used method to introduce terminal alkyne groups at the reducing end of CNC as reaction sites for click reactions. However, hydroxyl groups on CNC surface would be slightly oxidized during the carboxyamine condensation process , inducing the potential positions of introduced alkynes would be not only at the reducing end but also on CNC surface. Here, aldimine condensation was proposed to introduce terminal alkyne groups just at the reducing end of CNC, and a systematic comparison analysis was conducted with carboxylamine condensation. Firstly, the selectivity and extent of alkynylation were characterized by XPS and EA. Secondly, the end aldehyde content in these CNC samples was measured by the BCA method, which quantitatively explained the grafting efficiency of aldimine condensation and further verified its feasibility. Thirdly, the clickability of the modified CNC samples was confirmed through XPS analysis of the products after a pre-designed click reaction. In sum, aldimine condensation was proven to be a simple and effective strategy for introducing terminal alkyne groups at the reducing end of CNC, which could be used as reaction sites for further click reactions.
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