Nanocomposites of Polyurethane Featuring Cellulose Nanocrystals and Dynamic Boronic Ester Bonds

The nanocomposites of polyurethane (PU) with cellulose nanocrystals (CNCs) were fabricated and cross-linked with boronic ester bonds. First, a linear PU bearing numerous ketal moieties was synthesized, and then the ketal structural units were transformed into 1,3-diol moieties via reaction of hydrolysis. Thereafter, the linear PU was cross-linked with a diboronic acid with variable cross-linking densities. To achieve fine dispersion of CNCs in the cross-linked PUs, CNCs were simultaneously surface-treated with boronic acid. The morphological observation showed that fibrous cellulose nanocrystals were uniformly dispersed in the PU matrix, i.e., the nanocomposites of PU featuring CNCs and dynamic boronic ester bonds were successfully obtained. Owing to the incorporation of the fibrous nano-objects, the thermal and mechanical properties of PU networks were significantly improved. Thanks to the cross-linking with dynamic boronic bonds, the nanocomposites were recyclable (or reprocessable). Furthermore, the nanocomposites exhibited excellent shape memory properties, which featured the programmability of original shapes.