Seminar über aktuelle Fragen zur Dynamik komplexer Fluide: Supermolecular Rings, Rods and Helices in Nanoporous Solids: From Quantized Self-Assembly to Opto-Electronic Functionality in Confined Liquid Crystals
May 25, 2018
10:15 - 11:15
Prof. Patrick Huber
Max-Planck-Institut für Dynamik und Selbstorganisation (MPIDS)
Motived by surprising findings in experiments on the imbibition dynamics of mesoporous solids with liquid crystals we explored the structure of disc-like and chiral rod-like liquid crystals (DLC, ChLC) in silica and anodic aluminum oxide (AAO) nanopores with diameters ranging from 5 nm to 200 nm.
Combining optical birefringence, measuring orientational order, and X-ray as well as neutron diffraction, measuring translational order, we found a pore size and anchoring condition dependent orientational transition from a circular (nanoring) to axial (nanowire/nanorod) orientation for a DLC (HAT6) in AAO. Additionally, and in combination with Monte Carlo simulations, we show that confining the same thermotropic DLC in cylindrical silica nanopores induces a quantized formation of annular layers consisting of concentric circular bent columns, unknown in the bulk state, see Fig. 1. This allows us to determine the otherwise hard to access bend elastic constant of the discotic system and to switch on and off reversibly nano-sized supermolecular rings by small temperature variations (1).
For a ferroelectric ChLC (2MBOMBC) the native oxidic pore walls of AAO do not provide a stable smectogen wall anchoring. By contrast, a polymeric wall grafting enforcing planar molecular anchoring results in a thermal-history independent formation of smectic C* helices and a reversible chevron-like layer buckling (2). An enhancement of the optical rotatory power by up to one order of magnitude of the confined compared to the bulk liquid crystal is traced to the pre-transitional formation of helical structures at the smectic-A*-to-smectic-C* transformation. A linear electro-optical birefringence effect evidences collective fluctuations in the molecular tilt vector direction along the confined helical superstructures, i.e. the Goldstone phason excitations typical of the para-to-ferroelectric transition. Their relaxation frequencies increase with the square of the inverse pore radii as characteristic of plane-wave excitations and are two orders of magnitude larger than in the bulk, evidencing an exceptionally fast electro-optical functionality of the liquid-crystalline-AAO nanohybrids.
(1) Quantized self-assembly of discotic rings in a liquid crystal confined in nanopores. K. Sentker, A.W. Zantop, M. Lippmann, T. Hofmann, O.H. Seeck, A.V. Kityk, A. Yildirim, A. Schoenhals, M.G. Mazza, and P. Huber, Physical Review Letters 120, 067801 (2018).
(2) A ferroelectric liquid crystal confined in cylindrical nanopores: Reversible smectic layer buckling, enhanced light rotation and extremely fast electro-optically active Goldstone excitations. M. Busch, A.V. Kityk, W. Piecek, T. Hofmann, D. Wallacher, S. Calus, P. Kula, M. Steinhart, M. Eich, and P. Huber, Nanoscale 9, 19086 (2017).