Self-Assembly of Surface-Acylated Cellulose Nanowhiskers and Graphene Oxide for Multiresponsive Janus-Like Films with Time-Dependent Dry-State Structures.

For the first time Janus-like films of surface-acylated cellulose nanowhiskers (CNWs) with or without graphene oxide (GO) via one-step evaporation-driven self-assembly process are reported, which have reconstructible time-dependent micro-/nanostructures and asymmetric wettability. The heterogeneous aggregation of CNWs on rough Teflon substrates favors the formation of uniform films, leading to hydrophobic smooth bottom surface. The homogeneous nucleation of residual CNWs in bulk suspensions promotes the growth of patchy microspheres with an average diameter of 22.7 ± 2.1 µm, which precipitate on the top surface leading to enhanced hydrophobicity. These patchy microspheres are thermoresponsive and vanish after heating at 60 °C within 1 min, while they are reconstructed at room temperature with time-dependent evolving micro-/nanostructures in dry state within 2 d. The thermoresponsive transition of patchy microparticles leads to accompanied switchable change between transparency and opacity of Janus-like films. Furthermore, the incorporation of GO generates more patchy microspheres with an average diameter of 13.5 ± 1.3 µm on the top surface of hybrid Janus-like films. Different distributions of CNWs and GO in Janus-like films and the solvent-responsive self-assembled patchy microparticles of CNWs facilitate their reversible actuation by showing fast curling in THF within 6 s and flattening in water for at least 25 cycles.