Browsing by Author "Alshammari, Ahlam Falah A"

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Non-symmetric liquid crystal dimers and twist-bend phases.
(Saudi Digital Library, 2023-10-10) Alshammari, Ahlam Falah A; Prof. C.T. Imrie; Prof. John M. D. Storey
The main aims of this Thesis are the synthesis and characterisation of a range of new liquid crystal dimers designed to exhibit new examples of the recently discovered twist-bend phases, and to establish and understand the relationships between molecular structure and the formation of these fascinating phases. In Chapter 3, the effects of increasing the length of both the spacer and the terminal alkyl chain in the CBnO.m series, the 4'-(ω-{4-[{[4-alkylphenyl]imino}methyl]phenoxy}alkyl)[1,1'-biphenyl]-4-carbonitriles. The CBnO.m series, on the NTB, and SmCTB phase behaviour are reported. In the acronym, n refers to the number of carbon atoms in the spacer, and m the in terminal chain. For the odd-membered CB8O.m and CB10O.m series, if m/n≲0.5 there is a tendency to form intercalated structures driven by the favorable interaction between the dissimilar mesogenic groups. As m increases such that m≳n, bilayer smectic phases are favoured and driven by the interaction between the polar and polarisable cyanobiphenyl units. For intermediate chain lengths, only nematic behaviour is observed. In the interdigitated, bilayer regime twist-bend smectic phases are observed whereas in the intercalated regime only conventional smectic phases are seen. As expected, the liner CB9O.m series does not exhibit twist-bend phases. The difference in behaviour between the CB8O.m and CB10O.m, and the CB9O.m series is accounted for in terms of how the parity of the spacer governs molecular shape and specifically, odd-membereds dimers are bent whereas even member are liner. This reinforced the view that a bent molecular shape is a prerequisite for the observation of twist-bend nematic and smectic phases. In Chapter 4, an investigation of the effects that a lateral methyl branch has on the liquid crystalline behaviour of non-symmetric dimers is reported. The synthesis and characterisation of the 4'-(ω-{4-[{[3-methyl-4-alkylphenyl]imino}methyl]phenoxy} alkyl)[1,1'-biphenyl]-4-carbonitriles, (CBnO2Me.m) have been reported and their properties compared to those of the corresponding CBnO.m series. For the odd-membered CB8O2Me.m series, nematic and twist-bend nematic phases were observed. The even-membered CB9O2Me.m series showed only a conventional nematic phase. The introduction of the methyl branch generally reduces the melting point. The values of TNI are lower for the methyl branched CB9O2Me.m and CB8O2Me.m series than the corresponding CBnO.m series by around 41°C and 31°C, respectively. This may be attributed to both the reduced structure anisotropy associated with the methyl group, and the reduced interaction strength between mesogenic units. The introduction of the methyl group extinguished smectic behaviour presumably by disrupting the specific interaction between the unlike mesogenic groups. In Chapter 5, the spacer is changed to include an extra ether link to give the, 4´-((ω-(3-methyl-4-alkylphenylimino)methylphenoxy)alkyl)oxy)-[1,1'-biphenyl]-4-carbonitriles, (CBOnO2Me.m). The CBO5O2Me.m, CBO6O2Me.m, CBO7O2Me.m, and CBO8O2Me.m series have been synthesised, and their properties characterised and compared with those of the corresponding CBOnO.m series. For the odd-membered dimers, CBO5O2Me.m and CBO7O2Me.m, the nematic, twist bend nematic, twist bend smectic and smectic A phases were observed. By comparison, the even-membered dimers, CBO6O2Me.m, and CBO8O2Me.m, showed only the conventional nematic, and smectic A phases. This further supports the view that molecular curvature is a prerequisite for the observation of twist-bend phase. In Chapter 6, we exchange the cyanobiphenyl unit for a cyanoterphenyl fragment to give the 4ʹʹ-(6-(3-methyl-4-((alkylphenylimino)methyl)phenoxy)hexyl)-[1,1ʹ:4,1ʹ-terphenyl]-4-carbonitriles, (CT6O2Me.m). The cyanoterphenyl units is known to promote smectic phase behaviour and also to significantly increase the transition temperatures, and in this respect the inclusion of the lateral methyl substituent is particularly important to offset the increase in the transition temperatures. The CT6O2Me.m series exhibits phase behaviour quite different to that of the CT6O.m series. For short and intermediate chain lengths, the intercalated smectic phases seen for the CT6O.m series are extinguished by the addition of the methyl substituent and this reveals NTB behaviour for the CT6O2Me.m series. In addition, a liquid like smectic phase is observed that exhibits spontaneous chirality. It suggested that this may be an example of an intercalated twist-bend smectic phase. For the longer homologues the CT6O2Me.m series shows twist-bend smectic phases. Chapter 7 provides both a summary of the key findings of this thesis and suggestions of future work.
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