Self-Assembly of Surface-Acylated Cellulose Nanowhiskers
von Huan Liu
Datum der mündl. Prüfung:2020-09-28
Erschienen:2020-10-29
Betreuer:Prof. Dr. Kai Zhang
Gutachter:Prof. Dr. Kai Zhang
Gutachter:PD Dr. Markus Euring
Dateien
Name:Thesis-Huan Liu.pdf
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Zusammenfassung
Englisch
Self-assembly of nanoparticles into ordered materials is an attractive route for the synthesis of functional materials that are not feasible through conventional techniques. Recently, much attention has been paid to self-assembly of anisotropic cellulose nanowhiskers (CNWs) because of their unique properties, such as high surface to volume ratio, high mechanical performance, small thermal expansion coefficient, renewability, and sustainability. The objective of this research was to investigate the self-assembly of surface-acylated CNWs and to fabricate functional materials via this path. Helical fibers with the lengths of micrometers were fabricated by the evaporation-driven self-assembly of CNWs with surface-immobilized 10-undecenoyl groups (CNWs-SU). The self-assembly process and the final helical structures were affected by parameters including the wettability of substrates, dispersing solvents, the amount of 10-undecenoyl groups, the crystallinity, the dimension of CNWs, and the length of acyl chains. In particular, surface-acylated CNWs with a certain amount of 10-undecenoyl groups (ca. 3.52 mmol/g), an appropriate crystallinity (ca. 40%), a length of about 135 nm, and a diameter of around 4 nm, preferentially self-assembled into explicit left-handed helical fibers from their THF suspensions on wafers. The surface of CNWs-SU contains numerous olefin groups, allowing them to be further modified. After the photo-induced thiol-ene reaction on the terminal olefin groups with 1-octadecanethiol, CNWs-SU-C18 were obtained. Janus films of CNWs-SU-C18 with or without graphene oxide (GO) were fabricated via one-step evaporation-driven self-assembly process, 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 thermo-responsive and vanish after heating at 60 °C for few minutes, while they are reconstructed at room temperature with time-dependent evolving micro-/nanostructures in dry-state within two days. The thermo-responsive transition of patchy microparticles leads to accompanied switchable change between transparency and opacity of Janus 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 films. Different distributions of CNWs and GO in Janus films and the solvent-responsive self-assembled patchy microparticles of CNWs facilitate their reversible actuation by showing very fast curling in THF within 6 seconds and flattening in water for at least 25 cycles. The self-assembly of CNWs in confined geometries provides a powerful method to break the symmetry of an ordered structure, thus it can serve as an efficient route for the fabrication of novel morphologies. Three-dimensional (3D) ordered microporous CNWs-SU honeycomb films were prepared by breath figure (BF) method. The resulting films show high porous order over large regions and the iridescent color was only displayed by the rim of the obtained honeycomb film due to the confined 3D geometry drastically affects the self-assembly of CNWs, which is different with traditional dish-cast CNWs films showing the iridescent color over the whole area. In the confined 3D walls, CNWs self-assembly into cholesteric architectures which resulted in the iridescent color of the rims. Some critical influencing factors, such as the concentration of the CNWs solution and the relative humidity of the atmosphere were also investigated. Too low or too high concentrations of CNWs solutions/relative humidity are not preferable for the formation of uniform porous films and the self-assembly of CNWs in the confined geometry was also affected. This method provides a new idea to study the interactions of CNWs beyond what has been achieved in a planar geometry. This thesis is a cumulative work including 3 publications. All papers were submitted in peer-reviewed journals, one of them was already published and two of them were submitted. The background, the objective of the study, results and discussion of the three publications and the conclusion are presented in Section 1-4.
Keywords: Self-assembly; Cellulose nanowhiskers