Partial periodate oxidation and thermal cross-linking for the processing of thermoset all-cellulose composites

Abstract With the goal of preparing a new type of all-cellulose composites (ACCs), partial periodate oxidation of microcrystalline cellulose (MCC) was performed leading to specimens oxidized at C2 and C3 of the anydroglucose moieties of cellulose. Samples having an overall degree of oxidation (DO) comprised between 0 and 1 were prepared. As confirmed by their absence in their 13 C solid-state NMR spectra, the created carbonyl moieties were not detected, due to their immediate recombination with the available OH groups from untouched cellulose to yield hemiacetal cross-linkings. The high reactivity of the neo-carbonyl and the lability of the hemiacetals linkages were successfully used to prepare novel cross-linked ACCs resulting from the hot-pressing of samples with DOs ranging from 0.2 to 0.85. In these composites, the oxidized component acted as an in-situ reactive matrix, gluing the non-oxidized MCC part playing the role of filler. Mechanical properties, measured with 3-point bending test, together with structural and thermal observations showed a direct correlation between the DO of the specimens, the cross-linking density, the macroscopic performances and the propensity to retain water when they were heated as shown by thermogravimetric (TGA) measurements. An optimum of the properties was reached for samples having a DO in the range of 0.4–0.65, where surface oxidized cellulose nanocrystals could be cross-linked with their oxidized or un-oxidized neighbors to yield nearly transparent samples of flexural modulus as high as 7 GPa and flexural strength of 177 MPa. Such values, obtained with samples resulting from the simple oxidation followed by thermosetting process presented here, match well the physical properties of ACCs resulting from more complex preparation scheme.