Nanocomposites of\nPolyurethane Featuring Cellulose\nNanocrystals and Dynamic Boronic Ester Bonds

The nanocomposites of polyurethane (PU) with cellulose\nnanocrystals\n(CNCs) were fabricated and cross-linked with boronic ester bonds.\nFirst, a linear PU bearing numerous ketal moieties was synthesized,\nand then the ketal structural units were transformed into 1,3-diol\nmoieties via reaction of hydrolysis. Thereafter, the linear PU was\ncross-linked with a diboronic acid with variable cross-linking densities.\nTo achieve fine dispersion of CNCs in the cross-linked PUs, CNCs were\nsimultaneously surface-treated with boronic acid. The morphological\nobservation showed that fibrous cellulose nanocrystals were uniformly\ndispersed in the PU matrix, i.e., the nanocomposites of PU featuring\nCNCs and dynamic boronic ester bonds were successfully obtained. Owing\nto the incorporation of the fibrous nano-objects, the thermal and\nmechanical properties of PU networks were significantly improved.\nThanks to the cross-linking with dynamic boronic bonds, the nanocomposites\nwere recyclable (or reprocessable). Furthermore, the nanocomposites\nexhibited excellent shape memory properties, which featured the programmability\nof original shapes.