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Fundamentals and applications of ferro- and antiferroelectric liquid crystals - the physics and chemistry of "de Vries"-type materials

Subject Area Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
Experimental and Theoretical Physics of Polymers
Term from 2010 to 2015
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 173853318
 
Final Report Year 2016

Final Report Abstract

This project was part of an interdisciplinary and multi-national NSF/DFG Materials World Network (MWN) between the liquid crystal materials research groups at the University of Colorado at Boulder (USA), Chalmers University of Technology (Sweden), University of Strathclyde (UK), Queen’s University (Canada) and the University of Stuttgart (Germany) to advance the physics and chemistry of polar liquid crystals, in particular of the so-called “de Vries-type” smectics. Polar liquid crystals, such as ferro- and antiferroelectric smectic liquid crystals (FLCs and AFLCs), are the prime example of macroscopically polar fluids and, as such, considered to be key components in the development of future electro-optic devices such as high-speed high-resolution displays. Since many years however, the roadblock for the broad commercialization of FLCs and AFLCs is related to the temperature-dependent contraction of the smectic layer thickness in the ferroelectric and antiferroelectric phases. The only clear-cut solution to this problem is to find FLC/AFLC materials that do not change their layer thickness, despite the temperaturedependent average tilt of the molecules inside the smectic layers of FLC and AFLC phases. Indeed, until 2006 very few exotic examples of such materials, named “de Vries smectics”, were known but the mechanism behind the practical absence of smectic layer contraction in these materials remained mysterious. It was thus the main challenge of our project (i) to elucidate the mechanism of how the tilt-induced smectic layer contraction is compensated in de Vries-type smectics and (ii) to develop a rational molecular design strategy leading to new de Vries-type liquid crystal materials. In the course of the project detailed studies of orientational order turned out to be the key for the understanding of de Vries-type smectics. Extensive X-ray diffraction and polarized Raman spectroscopy studies of smectic monodomains revealed that the layer contraction in de Vries-type smectics is counteracted by a combination of two mechanisms: a substantial improvement of molecular orientational order, i.e., an increase of the orientational order parameter S2, which effectively increases the smectic layer spacing, and – in the case of partial bi-layer structures – a reduction of the degree of molecular interdigitation, which increases the effective molecular length. Surprisingly, the role of molecular orientational order in this mechanism is rather general and does not require the ad-hoc assumption of certain orientational distribution functions (hollow-cone or diffuse-cone distributions), as often proposed since the original work of de Vries. These results were further confirmed by MD simulations (developed together with our physics partner group at CU Boulder) and emanated into phenomenological and molecular theories of de Vries-type smectics, developed together with our theory partner group at the University of Strathclyde. The recognition of the molecular mechanisms behind the compensation of smectic layer contraction led us to a rational molecular design strategy. This concept is essentially based on terminal siloxane- or carbosilane groups, the nanosegregating effect of which leads to smectic bi-layer structures with exceptionally low orientational order in the high-temperature paraelectric smectic phase. This allows the substantial increase of the molecular orientational order which is needed to compensate the layer contraction in the lowtemperature FLC phase. Thanks to the synthetic efforts of our organic chemistry partners at Queen’s we identified a library of now more than 40 new de Vries-type materials, among them the best de Vries-type materials reported heretofore with a maximum layer contraction of less than 0.5%. The chemical stability of these materials makes them perfect components to formulate FLC mixtures tailored for future electro-optic applications. We wish to express our opinion that the NSF/DFG Materials World Network program created an excellent framework for our international research collaboration. We thus regret even more that the program was not continued.

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