Abstract
Periodate oxidation introduces aldehyde functionality to cellulose. The use of dialdehyde cellulose has been demonstrated for crosslinking and as a chemical intermediate towards functionalized cellulose. Commercially available cellulose nanocrystals (CNCs) typically carry a surface sulfate half-ester functionality, which results from their manufacture via sulfuric acid hydrolysis and subsequent esterification. The sulfate half-ester group is a bulky group carrying a net negative charge above pH 2 that modifies the colloidal and electro-chemical properties of the CNCs. Periodate oxidation is regioselective to the bond between carbons in positions 2 and 3 in the anhydroglucose unit while the sulfate half-ester groups are mostly considered to be located in carbon in position 6. This regioselectivity could be the reason why the role played by the sulfate half-ester group on modification by periodate oxidation has not previously been elucidated. Here, the influence of the sulfate half-ester on the oxidation of CNCs, which is shown to steer the oxidation kinetics and the properties of the resulting materials, is studied. Conventional physicochemical analysis of the oxidant consumption is accompanied by elemental analysis, Fourier-transform infrared, X-ray photoelectron and solid-state nuclear magnetic resonance spectroscopy, and wide-angle x-ray scattering analyses; the zeta potential is used to characterize the colloidal properties of the suspensions and atomic force microscopy for determining particle dimensions. The presence of the sulfate half-ester group decreases the rate of oxidation. However, the content of the sulfate half-ester groups decreases when degree of oxidation reaches approx. 50%. We demonstrate that the CNC surfaces are affected by the oxidation beyond the C2–C3 bond cleavage: insight into the kinetics of the oxidation process is a prerequisite for optimizing CNC oxidation.
Highlights
It is only recently that native cellulose has been accessible to nanotechnology due to the heterogeneous morphology and the millimeter size of wood fibers
We demonstrate that the cellulose nanocrystals (CNCs) surfaces are affected by the oxidation beyond the C2–C3 bond cleavage: insight into the kinetics of the oxidation process is a prerequisite for optimizing CNC oxidation
The effect of sulfate half-ester groups on the periodate oxidation of CNCs are elucidated; we investigated how the initial content of sulfate half-ester groups affects the properties of the oxidized CNC product
Summary
It is only recently that native cellulose has been accessible to nanotechnology due to the heterogeneous morphology and the millimeter size of wood fibers. The development of the liberation of nanosized colloidal cellulose nanocrystals (CNCs) from cellulosic materials via hydrolysis with mineral acids, has transformed these entities into bio-based nanoparticles (Kontturi et al 2018). Sulfuric acid hydrolysis is employed to digest the most accessible, amorphous cellulose chain arrangements, leading to the liberation of the remaining, more ordered, segments into CNCs. Hydrolysis with sulfuric acid leads to the simultaneous esterification of the CNC surfaces with sulfate half-ester groups (Eyley and Thielemans 2014). CNCs have been demonstrated as being suitable for use in nanocomposites, biomedical devices, emulsions and foams, and as rheology modifiers (Sinha et al 2015; Vanderfleet and Cranston 2021)
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