Surface Functionalisation Effects in Liquid Crystal Nanocomposites

Doping of liquid crystal (LC) materials with nanomaterials is of great interest for tuning the optical, dielectric and elastic properties of the LC nanocomposites which are of particular importance for device applications. Additionally, the self-organising nature of LCs can be used to self-assemble nanomaterials into mesoscale structures. This thesis aims to report on the effect that functionalised nanoparticles (NPs) have on the properties of the LC (nanocomposites) and understand how this influences the process of NP self-assembly in the LC host. Sensitive measurements of the LC nanocomposite properties are therefore necessary, particularly at low NP concentrations. Thus, the work presented in this thesis is comprised of two parts. The first describes an investigation of the methodology used to perform accurate dielectric spectroscopy measurements of liquid crystal (LC) materials and subsequent data analysis procedure. The effects of the LC test cell on dielectric spectroscopy measurements of LC materials is found to have a considerable impact on the measured dielectric properties. In particular, for high dielectric anisotropy materials, the thin-film polymeric layers, required to provide homogeneous alignment of the LC are shown to significantly alter the measured permittivities at certain frequencies. An equivalent circuit approach is therefore proposed and used in order to model not only the dielectric spectra obtain using LC test cells, but also correct for the effects of the test cells in order to more accurately determine the properties of the LC. The second part of the work performed in this thesis is concerned with the characterisation of LC nanocomposites. In particular, quantum dots (QDs) are used as the nano dopant due to their photoluminescence properties. The effect of using different QD surface functionalisation is studied and the influence on the phase behaviour, order and physical properties of the QD-LC nanocomposites directly compared. The phase transition driven QD self-assembly of QD micro-structures is also studied and the process of which, directly related to the surface functionalisation effects on the order and phase behaviour of the nanocomposites. Two different types of surface functionalisation are used in this work. The first is a side-on mesogenic surface functionalisation, where the mesogenic group is attached with the long molecular axis connected tangentially to the surface of the QD by a flexible alkyl linker. This type of surface functionalisation is thought to allow the mesogenic group to freely align with the director of the host LC, therefore reducing the distortion of the director. The second is the alkyl chain, octadecylamine, which is thought to homeotropically anchor the LC and therefore create significant distortion of the director as a result. Through a combination of studying the phase behaviour and measuring the order parameter, permittivities and elastic constants of the nanocomposites, it is deduced that the mesogenic QD surface functionalisation behaves as expected and the mesogenic group freely aligns with the director of the host LC. The alkyl chain functionalisation, unlike the mesogenic functionalisation, was found to significantly reduce the order parameter and thus also reduce the nematic-isotropic phase transition temperature (TNI) of the nanocomposites compared to the pure LC host. Measurement of the permittivities and elastic constants also confirmed the order preserving nature of the mesogen functionalised QD and the disordering nature of the alkyl chain functionalised QD. The disordering alkyl chain surface functionalisation and the order preserving mesogenic surface functionalisation had a profound effect on the QD self-assembly through the isotropic to nematic phase transition. Using the side-on mesogenic surface functionalisation, 'tubular worm-like' QD networks could be formed, whereas the alkyl chain functionalisation produced 'solid' QD micro-structures. These results confirm the use of side-on mesogen functionalisation of nanomaterials is capable of preserving the order and physical properties of the host LC when forming LC nanocomposites. On the other hand, the alkyl chain functionalisation is actively disordering, thus the phase behaviour and physical properties of the nanocomposites are significantly altered compared to the pure LC. Further investigation into the dielectric and elastic properties of the QD-LC nanocomposites with consideration of the alignment effects of the different QDs were also performed. The measurement and data analysis methods developed and outlined in the first part of the results in this thesis were used to do so. In agreement with the phase behaviour and order of the mesogen functionalised QD nanocomposites, the elastic constants and permittivities were found to be unchanged compared to the pure LC host. An increase in pretilt with increasing QD concentration in planar aligned LC test cells was found, however. Permittivity measurements of the alkyl chain functionalised QD nanocomposites also agreed with the reduction in order and $T_{NI}$ by showing a decrease in the permittivity parallel to the director and the dielectric anisotropy. Interestingly, however, interpretation of the permittivity-voltage curves, even when accounting for the effect of pretilt did not reveal a reduction in the elastic constants as would be expected due to the reduced order.