Project

Discipline

chirality; liquid crystals; cellulose nanocrystals; surface-initiated polymerization; self-assembly; polymer-tethered nanorods; cholesteric; hybrid materials

Lead
Eine der praktischsten Strategien um Materialien mit neuen Eigenschaften zu erzeugen, ist die Selbstassemblierung von chemischen Bausteinen in nanoskalige Strukturen. Die Bausteine selbstorganisierter Strukturen können aus kleinen Molekülen, Polymeren oder kolloidalen Partikeln bestehen. Besonders die Selbstassemblierung von Kolloiden bietet eine einfache Methode für die Herstellung komplexer, dreidimensionaler Materialstrukturen. Eine attraktive Klasse chiral-kolloidaler Bausteine sind Zellulose-Nanokristalle (CNCs), welche aus natürlichen, reichlich vorhandenen erneuerbaren Rohstoffen gewonnen werden. Eine im Vergleich zu anderen stabförmigen Nanopartikeln einzigartige Eigenschaft von CNCs ist die Bildung von Flüssigkristallphasen, deren Struktur bei der Herstellung von trockenen Filmen erhalten bleibt. Somit sind CNCs ein ideales Ausgangsmaterial für die Entwicklung neuartiger, poröser Materialien mit weiträumig chiraler Ordnung.
Lay summary
Inhalt und Ziel des Forschungsprojekts Das Ziel dieses Forschungsprojektes ist die systematische Untersuchung selektiver Endgruppenmodifikation der Zellulose-Nanokristalle mit dem Ziel, die Selbstassemblierung in Flüssigkristallphasen zu kontrollieren, sowohl in kolloidalen Lösungen, als auch in Filmen. Entsprechend werden die Enden von CNCs unterschiedlicher Kristallstruktur mit einer Reihe von verschiedenen Polymerketten modifiziert, sodass stabförmige Nanopartikel mit asymmetrischen und symmetrischen Formen entstehen. Schlussendlich werden die Flüssigkristalleigenschaften der modifizierten CNCs sowohl in unterschiedlichen Lösungsmitteln als auch in festem Zustand untersucht. Wissenschaftlicher und gesellschaftlicher Kontext des Forschungsprojekts Die Einführung von Chiralität, oder Händigkeit, in solch poröse Feststoffe könnte zu wünschenswerten Eigenschaften für die Anwendung ergeben, zum Beispiel für asymmetrische Katalysatoren, enantioselektive Sorptionsmittel, Biosensoren und Optiken. Diese neuen Materialien könnten einen wichtigen Beitrag zur Überwindung gesellschaftlicher Herausforderungen leisten, wie beispielsweise im Gesundheitswesen, in der Energiewende und dem Klimawandel.

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Last update: 13.02.2017

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Communication	Title	Media	Place	Year

Title	Year

One of the most practical strategies to produce nanoarchitectures in both two and three dimensions is through self-assembly. The building blocks of self-assembled structures span multiple length scales, from molecules and polymers to nanoparticles and macroscopic colloidal objects. Self-assembly of colloids, in particular, provides a simple means toward complex hierarchical structures, especially in three dimensions. Although still at its infancy, synthetic chiral colloids, such as twisted and helical nanoribbons/nanofibers, have started to emerge in nanoscience as a tool for synthetic materials with controlled handedness. One attractive class of chiral supracolloidal building blocks are cellulose nanocrystals (CNCs), which are readily extracted from naturally abundant renewable materials. Unique among rod-like chiral colloids, CNC chiral nematic (N*) liquid crystalline assemblies are preserved in dried films and have been utilized as templates for novel materials with mesoporous structures and long-range chiral order. Such chiral self-assembly concepts have contributed to recent developments in heterogeneous enantioselective catalysts, chiral plasmonics and stratified films with alternating chiral domains. Further development of chiral nanomaterials may lead to favorable properties for numerous applications including asymmetric catalysis, enantioselective separation/sorption media, chiral sensing, nanodevices/-machines and circular dichroism.Among the external factors that can be used to manipulate N* phases of CNCs, such as ionic strength, ultrasound, etc., surface chemical modification with tethered polymers remains less explored. This is probably because non-selective surface modification may have a negative impact on the formation of N* phases, as the packing density of nanorods would be disturbed by the presence of macromolecular layers. On the other hand, selective modification at the ends of nanorods with polymer tethers would still allow sufficient packing within N* phases. In fact, recent computer simulations have predicted the formation of N* phases of asymmetric polymer-tethered nanorods within a wide range of their phase diagram, even though the nanorods themselves were achiral. To date, there is no experimental platform available to support these theoretical descriptions of asymmetric nanorods with end-tethered polymer chains.The objective of the proposed research is to systematically investigate the selective-end group modification of colloidal nanorods as a means to manipulate their self-assembly into N* liquid crystalline phases, both in colloidal solution and in free-standing films. In this regard, CNCs represent an ideal platform, as the inherent directionality of individual cellulose chains, and therefore reducing end groups, can be controlled through chemical pre-treatments of raw cellulosic materials, resulting in different crystalline polymorphs, for example cellulose I and cellulose II. Through the acid hydrolysis of native cellulose I and cellulose II fibers, CNCs containing parallel and anti-parallel chains, respectively, will be produced as a new basis for symmetric and asymmetric chiral nanorods with end-tethered polymer chains.Important questions to be answered in this project to that objective are:•Do CNCs produced from cellulose II exhibit the same handedness (twist) as cellulose I?•Can the reducing end groups of CNCs be used as initiating sites for controlled radical polymerization toward colloidal analogues of rod-coil and coil-rod-coil block copolymers?•How do end-tethered polymer chains and symmetry affect the liquid crystalline behavior of CNCs?The development of symmetric and asymmetric CNCs with end-tethered polymer chains would not only provide an experimental platform to test recently developed theories of polymer-tethered nanorods, but also a fundamental understanding of the relationship between cellulose crystal structure, chirality and inversion over different length scales. As a unique nanomaterial with attractive thermomechanical and photonic properties, the development of CNC hybrids with end-tethered polymer chains will offer a new means to manipulate their liquid crystal ordering for chirality induction into porous solids. Finally, they could also provide new building blocks for novel supracolloidal concepts.