Synthesis and Characterisation of New Polymerisable Mesogens Containing Fluorene Moieties

Diplomarbeit von Philipp Gebhardt

Introduction:

Liquid crystals present an intermediate state of matter. In a liquid the molecules are in contact but are able to move past each other. In a crystal the molecules are not able to move past one another, they are incorporated in the cystalline lattice, giving the system a long-range order. In nematic liquid crystals, the molecules are arranged in such a way that their longitudinal axes are mutually parallel but they are easily able to move in the direction of their longitudinal axes. Thus, liquid properties like fluidity and viscosity as well as optic properties that are shown by crystals like the reflection of different colours depending on the viewing angle are observed simultaneously.

The incorporation of photopolymerisable groups provides monomers for temperature independent polymerisation. After polymerisation in the LC phase and subsequent cooling, the molecular orientation within the system can be frozen in, thus, materials with special qualities can be obtained. These materials have direction-depending optical and mechanical properties consequently they represent an area of scientific interest and technological potential.

In the present work three new mesogens, molecules with liquid crystalline behaviour in a determined temperature range were synthesized. They have the above illustrated structure. One of them is a direactive monomer for the creation of a threedimensional network. Due to their structure, the compounds show fluorescence and are suitable for new materials with application in electro-optical devices like LCDs.

The present thesis describes the synthesis of the new mesogens and their characterisation with FT-IR, 1H and 13C NMR. The influence of the molecular structure on the thermotropic properties is discussed and the liquid crystalline properties are examined by polarisation microscopy and DSC.

Moreover, ways for obtaining and characterising orientated thin films are bescribed.

Table of Contents:

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Chapter 1.4, Liquid Crystalline Polymers and their Application:

In recent years the photoinitiated polymerisation of LCs has been of high interest in the technological and scientific research area, because these monomers are suitable for the preparation of thin, orientated solid films for electronic components, especially for applications such as solid state polarisers, interference filters, etc. The advantage of photoinitiated polymerisation compared to thermal polymerisation is its independence of temperature which enables the selection of the mesophase formed to be frozen in.

Two classes of liquid crystalline polymers (LCPs) are distinguished. In main chain LCPs, the mesogens are part of the polymer main chain. A well known application for this class of compounds is „Kevlar”, the trade name for high-strength aramid fibers.

In side group LCPs, the mesogens are attached to the polymer backbone. If the mesogenic groups are covalently fixed side chains of a given main chain, their ability to move and orient is changed drastically. In the liquid state of the polymer, the tendency of statistical chain conformation hinders an orientation of the side chains. If the anisotropic interactions of side chains are strong enough to form the mesophase, a liquid crystalline structure can neverthless be formed, but only in accordance with the limited motions of the main chain. In the liquid state the motions of the polymer main chain have to be decoupled from those of the anisotropically oriented mesogenic side chains. The decoupling should be possible, if flexible spacer groups are inserted between the main chain and the rigid mesogenic side chains (Fig. 10: Polymer with mesogenic side groups).

In 1987, D. J. Broer and co-workers obtained a highly ordered polymer sample by „in-situ photopolymerisation of an oriented liquid-crystalline acrylate”. In the absence of solvents, the monomer is heated to the mesomorphic phase, macroscopically oriented and irradiated with ultraviolet (UV) light. The photoinitiator, which is dissolved in the mesomorphic monomer, is fragmented into free radicals which initiate the chain polymerisation. Homogenous orientation can be obtained by applying the monomer onto a substrate which has been coated with a thin polymer layer and rubbed unidirectionally with tissue. Subsequently, the monomer orients itself according to the rubbing direction.

The ordering of the mesogens is frozen-in, yielding uniaxially oriented networks by carrying out photo-initiated chain crosslinking of liquid crystalline diacrylates (Fig. 11: Photo-crosslinking of an oriented liquid crystalline diacrylate).

Liquid crystalline gels can be obtained by polymerisation of liquid crystal diacrylates in mixtures of nonreactive mesogens. Although these host molecules are not chemically incorporated in the crosslinked network, they support orientation due to anisotropic interactions. Some applications were found. The transparent material shows light scattering behavior as a response to an electric field and can be used for switchable polarisers and further display applications.

Liquid crystalline elastomers are materials with remarkable properties. They can be obtained by incorporating mesogenic monomers into poymer networks with a determined density of crosslinking. Nematic elastomers with a single, global director of orientation can be produced through the use of magnetic fields or mechanical treatment during the cross-linking procedure which results in a globally anisotropic chain trajectory. Such samples retain a memory of their chain shape and, through coupling, a memory of the single global director present at the time of network formation. Interesting applications are for example materials that change colour when stretched, and artificial muscles.