Enabling mechanically adaptive 4D printing with cellulose nanocrystals

Additive manufacturing of stimulus-responsive materials is an area of four-dimensional (4D) printing that is continuing to gain interest. Cellulose nanocrystal (CNC) thermoplastic nanocomposites have been demonstrated as a water-responsive, mechanically adaptive material that shows promise in generating 4D-printed structures. In this study, a 10 wt% CNC thermoplastic polyurethane (TPU) nanocomposite was produced through a masterbatching process and printed using fused filament fabrication. A design of experiments was implemented to establish a processing window to highlight the effects of thermal energy input on the mechanical adaptivity of the printed parts. The combination of high temperatures and low speeds resulted in thermal energies that induced degradation of the CNC/TPU network and reduced the absolute values of storage moduli, but the mechanical adaptation persisted for all the printed samples. However, for slower speeds and increasing temperatures, the nanocomposites experienced a 15% decrease in adaptability. Further, a folded box structure was printed to establish the reversibility of the mechanical response and corresponding ability to generate a structure that can serve as a deployable shape-memory material based on response to water. The printed structure demonstrated fixity and recovery values of 76 and 42%, respectively. These results show significant promise for CNC/TPU nanocomposites in 4D-printed adaptable structures for academic and industrial applications.