Structure-Property Relationships in de Vries Smectic A Liquid Crystals

Moran, Mark James Conroy

Graduate Thesis Or Dissertation

Structure-Property Relationships in de Vries Smectic A Liquid Crystals Public Deposited

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Citeable URL: https://scholar.colorado.edu/concern/graduate_thesis_or_dissertations/rn3011585

Abstract

Liquid crystals (LCs) have become inextricably linked with the display industry in recent years. As screen sizes increase the response time of the LC must decrease to maintain picture quality. A rare subset of the chiral smectic A (SmA*) , known as the de Vries SmA* phase, shows fast switching times, large electroclinic tilts, native gray-scale and small layer shrinkage when driven by an electric field, making it an attractive alternative to SmC* materials for next-generation displays. The de Vries SmA* phase is typically formed by molecules with one or more tails that strongly suppress out-of-layer fluctuations.

The first part of this thesis describes a set of mesogens with four different liquid crystalline cores built on a scaffold possessing a phenyl and biphenyl moiety linked by an ester with two peripheral alkoxy side-chains (tails). One side-chain is the (S)-1-methylheptyloxy group ortho to a trifluoromethyl group. The second side chain is varied between an n-alkoxy-, n-alkenyloxy- and semifluorinated tails, all of the same length. The latter two tails have been shown to suppress out-of-layer fluctuations. Only the LCs built on the hydroquinone stereopolar unit (W652/W657/W530) show the desired liquid crystalline phases (SmA* and SmC*) over a convenient temperature range. Surprisingly, W652, which possesses no strong out-of-layer suppressing group, shows the de Vries SmA* phase, suggesting the core may be as important as the tails in the design of de Vries smecticss.

The second part of this thesis develops structure-property relationships of compounds built on the hydroquinone stereopolar unit, which is particularly good at promoting the de Vries SmA* phase. The set of lateral polar groups (H, Me, Cl, CF₃ or NO₂) and achiral tails were varied. The effect on the width of the SmA* phases, the clearing point, saturated electroclinic tilt, layer shrinkage from SmA* to SmC* and spontaneous polarization in the SmC* phase were examined. Materials with a large electronegative lateral polar group and strongly out-of-layer suppressing tails give the most desirable properties.

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