Gelation of Liquid Crystals with Self-Assembled Fibers

Abstract

Physical gelation of liquid crystals with low molecular weight gelators leads to the formation of a new class of anisotropic gels that have great potentials for optical, electrical, and photofunctional materials. The liquid crystalline (LC) physical gels are microphase-separated anisotropic composites consisting of liquid crystals and self-assembled solid fibers. For these materials, the isotropic-anisotropic transitions due to liquid crystals and the sol-gel transitions due to gelators occur reversibly and independently. The thermal, optical, and electrical properties of the LC gels are tuned by the selection and combination of the components, which determine the microphase-separated structures. LC gels based on room temperature nematic liquid crystals show electro-optical switching on twisted nematic and light scattering modes. The electro-optical properties can be improved in the presence of fiber additives. Discotic liquid crystals that function as hole transport materials have been used as LC components of anisotropic gels. The discotic gels exhibit hole mobilities higher than those of the liquid crystal alone. Chemical modification of gelators with functional moieties is another versatile approach for functionalization of LC gels. Hydrogen-bonded gelators with photoswitchable azobenzene moieties have been developed and complexed with liquid crystals. The resultant photoresponsive LC gels show light-induced structural changes, which are applicable to rewritable information recording.

Functional Liquid‐Crystalline Assemblies: Self‐Organized Soft Materials

“…[24,58] The fibrous materials are simply formed in the liquid crystals by self-assembly of low-molecular-weight compounds 80-84, a process which is driven by hydrogenbonding or p-p interactions. [57,293,294] These materials are soft solids maintaining liquid-crystalline properties.…”

“…For example, liquid-crystalline physical gels are new composites consisting of liquid crystals and self-assembled solid fibers. [57,58] As a result of the formation of controlled phase-separated structures at the submicro-and micrometer level these gels show high performance in various functions.…”

Funktionelle flüssigkristalline Aggregate: selbstorganisierte weiche Materialien

“…[24,58] Sie entstehen dadurch, dass sich niedermolekulare Verbindungen durch Bildung von Wasserstoffbrücken oder p-p-Wechselwirkungen in Flüssigkristallen zu Fasern organisieren. [57,293,294] Diese Materialien sind weiche Festkörper, die flüssigkristalline Strukturen beibehalten.…”

“…Da intermolekulare Wechselwirkungen zwischen den Gelbildnern gebildet und wieder gebrochen werden, zeigen solche Gele reversible thermische Sol-GelUmwandlungen. [24,58,[295][296][297] In physikalischen flüssigkristallinen Gelen finden zwei Phasenumwandlungen unabhängig voneinander statt: 1) der Übergang zwischen flüssigkristallinem und isotropem Gel, der durch die Phasenumwandlung des Flüssigkristalls induziert wird, und 2) die Sol-Gel-Umwandlung, die durch die Assoziation und Dissoziation der Gelbildner zu Fasern hervorgerufen wird. Beide Phasenumwandlungen sind beobachtbar, wenn sie bei unterschiedlichen Temperaturen auftreten.…”

“…[54] Erst seit kurzem werden Selbstorganisationsprozesse beim Design von LC-Verbundmaterialien berücksichtigt. Flüssigkristalline physikalische Gele aus Flüssigkristallen und selbstorganisierten Fasern [57,58] [7,24,28] Biomoleküle wie Nucleinsäuren, Zucker, Proteine und Peptide enthalten Wasserstoffbrückendonor-und -acceptorreste, die molekulare Erkennung und Selbstorganisation ermöglichen. [59] Synthetische Flüssigkristalle mit Wasserstoffbrücken, z.…”