Morphological changes of sterically stabilized nanocrystalline cellulose after periodate oxidation

Abstract

Periodate oxidation breaks the C2–C3 bond in the glucose repeat units of cellulose, forming two vicinal aldehyde groups. When the cellulose is partially oxidized, three products were generated after periodate oxidation: fibrous cellulose, sterically stabilized nanocrystalline cellulose (SNCC) and dialdehyde modified cellulose. Thus, by periodate oxidation alone, we can produce nanocellulose. SNCCs were produced after 26, 42 and 84 h periodate oxidation. Their morphologies were examined by transmission electron microscopy, which show that the three SNCCs have similar diameters (5–10 nm). In contrast, the average length of SNCC decreases with aldehyde content: from approximately 590 nm after 26 h of oxidation to 100 nm for an oxidation period of 84 h. It indicates that the morphology of SNCC can be well controlled by the degree of periodate oxidation, which depends on the amount of periodate and the reaction time. Equivalent spherical diameters of SNCCs were also examined by dynamic light scattering, and the results correspond closely to the ones observed by TEM. The viscosities of SNCCs were measured by an Ubbelohde viscometer and compared with theory. Because the length of SNCC particles gradually reduces while their diameters remain almost the same, we propose that periodate reacts preferentially with the amorphous region of cellulose. After most of the amorphous regions have reacted, the reaction proceeds at the boundary of amorphous and crystalline regions, creating a reaction front that advances towards the crystalline regions, thus continually shortening them. Dynamic light scattering experiments on SNCC suspensions when adding cosolvents into them proved that SNCCs were sterically stabilized in water.