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Structure/Property Relationships in Homologous Libraries of Bent Core Mesogens Forming Orthogonal, Polar, and Achiral Smectic Liquid Crystal Phases
| Content Provider | Semantic Scholar |
|---|---|
| Author | Kolber, Maria |
| Copyright Year | 2014 |
| Abstract | According to a new design strategy for “de Vries-like” liquid crystal materials, which are characterized by a maximum layer contraction of e1% upon transition from the SmA phase to the SmC phase, we report the synthesis and characterization of two homologous series of organosiloxane mesogens. The design of these new materials is based on a frustration between one structural element that promotes the formation of a SmC phase (a trisiloxane-terminated side-chain) and one that promotes the formation of a SmA phase (either a chloro-terminated side-chain or a 5-phenylpyrimidine core). Measurements of smectic layer spacing d as a function of temperature by small-angle X-ray scattering (SAXS) combined with optical tilt angle measurements revealed that the mesogens 5-(4-(11-(1,1,1,3,3,5,5-heptamethyltrisiloxanyl)-undecyloxy)phenyl)-2-(1-alkyloxy)pyrimidine (3(n)) undergo SmA-SmC phas transitions with maximum layer contractions ranging from 0.5% to 1.4%. A comparison of reduction factors R and f suggests that this behavior is due in part to a pronounced negative thermal expansion in the SmC phase that counterbalances the layer contraction caused by increasing tilt. SAXS measurements also revealed that compounds 3(n) are characterized by low orientational and high translational order, which is consistent with theoretical predictions that such materials should exhibit de Vries-like properties. The R values for series 3(n) are comparable to, and even lower than, those reported for established de Vries-like materials such as the perfluorinated 2-phenylpyrimidine material 3M 8422. Introduction The self-organization of amphiphilic molecules forming liquid crystal phases is generally described as the nanosegregation of two or more incompatible segments into distinct domains with topographies dictated by molecular shape and amphiphilic interfacial curvature. The formation of lamellar smectic phases by calamitic (rod-shaped) mesogens is driven by the nanosegregation of rigid aromatic cores from flexible aliphatic sidechains and may be further promoted by the introduction of incompatible segments such as perfluorinated side-chains or oligomeric siloxane end-groups. The fluid smectic A (SmA) and smectic C (SmC) phases have diffuse lamellar structures described by a density wave with a period d corresponding to the smectic layer spacing. In the uniaxial SmA phase, the axes of orientational and translational order are coincident, with the director n parallel to the smectic layer normal z; in the biaxial SmC phase, n is tilted relative to z by a tilt angle θ that varies with temperature. According to the classic rigid-rod model shown in Figure 1a, the SmA-SmC phase transition is accompanied by a contraction of the layer spacing d that scales with the cosine of the tilt angle θ and is typically on the order of 7-10% in conventional calamitic m terials. This layer contraction has been a problem in the formulation of chiral SmC* mixtures for ferroelectric liquid crystal (FLC) display devices, which may be solved by a design strategy focused on balancing nanosegregation and low orientational order (vide infra). † Queen’s University. ‡ Universität Stuttgart. (1) (a) Tschierske, C. J. Mater. Chem. 1998, 8, 1485–1508. (b) Goodby, J. W.; Mehl, G. H.; Saez, I. M.; Tuffin, R. P.; Mackenzie, G.; AuzélyVelty, R.; Benvegnu, T.; Plusquellec, D. Chem. Commun. 1998, 2057– 2070. (c) Tschierske, C. J. Mater. Chem. 2001, 11, 2647–2671. (2) (a) McMillan, W. L. Phys. ReV. A 1971, 4, 1238–1246. (b) McMillan, W. L. Phys. ReV. A 1972, 6, 936–947. (3) Lagerwall, J. P. F.; Giesselmann, F. ChemPhysChem 2006, 7, 20–45. Figure 1. Schematic representations of the SmA-SmC phase transition according to (a) a classic rigid-rod model and (b) the diffuse cone model proposed by de Vries. Published on Web 12/08/2009 10.1021/ja9087727 © 2010 American Chemical Society 364 9 J. AM. CHEM. SOC. 2010, 132, 364–370 |
| File Format | PDF HTM / HTML |
| Alternate Webpage(s) | https://scholar.colorado.edu/cgi/viewcontent.cgi?article=1207&context=chem_gradetds |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
