Properties of polymer liquid crystals: choosing molecular structures and blending

Molecular dynamics of liquid crystalline side-chain polymers: The dielectric relaxation behaviour of a siloxan polymer in the nematic and isotropic phases

Liquid crystals (LCs) are anisotropic fluids that combine the long-range order of crystals with the mobility of liquids1,2. This combination of properties has been widely used to create reconfigurable materials that optically report information about their environment, such as changes in electric fields (smart-phone displays)
 3
 , temperature (thermometers)
 4
 or mechanical shear
 5
 , and the arrival of chemical and biological stimuli (sensors)6,7. An unmet need exists, however, for responsive materials that not only report their environment but also transform it through self-regulated chemical interactions. Here we show that a range of stimuli can trigger pulsatile (transient) or continuous release of microcargo (aqueous microdroplets or solid microparticles and their chemical contents) that is trapped initially within LCs. The resulting LC materials self-report and self-regulate their chemical response to targeted physical, chemical and biological events in ways that can be preprogrammed through an interplay of elastic, electrical double-layer, buoyant and shear forces in diverse geometries (such as wells, films and emulsion droplets). These LC materials can carry out complex functions that go beyond the capabilities of conventional materials used for controlled microcargo release, such as optically reporting a stimulus (for example, mechanical shear stresses generated by motile bacteria) and then responding in a self-regulated manner via a feedback loop (for example, to release the minimum amount of biocidal agent required to cause bacterial cell death). Liquid crystals are used to self-report and self-regulate either continuous or transient release of droplets or microparticles trapped within them in response to thermal, chemical, mechanical or biological stimuli.

Self-reporting and self-regulating liquid crystals

It has been proven (see for instance [1–11] that dielectric spectroscopy is a useful tool to study the molecular dynamics of polymers. This is due to the fact that a broad dynamical range from the milli- to the Giga-Hertz region can be covered by this method in its modern form (see Chap. 2). Therefore motional processes which take place for polymeric systems on extremely different time scales can be investigated in a broad frequency and temperature range. Moreover the motional processes depend on the morphology and micromorphology of the system under investigation. Therefore information on the structural state of the material can be indirectly extracted by taking the molecular mobility as a probe for the structure.

Molecular Dynamics in Polymer Model Systems

An electro‐optical technique was used to measure the rotational viscosity of nematic liquid crystals, whose structures were chosen to investigate the effects of linking groups and lateral groups on rotational viscosity. These results are compared with the theory developed by Osipov and Terentjev. It is shown that the theory is useful in predicting rotational viscosity for specific molecular structures. The agreement between theory and experiment is found to be good for all the liquid crystals studied.

Rotational viscosity and molecular structure of nematic liquid crystals

A general expansion of the orientational distribution function in terms of the Wigner matrix is employed in order to calculate dipole moment correlation functions in liquid crystals. It is shown that the qualitative interpretation of dielectric relaxation spectra of liquid crystals is possible without explicit characterization of the molecular motions involved. Thus, a wide range of stochastic motions can be considered within the framework of the model. A fundamental influence of the orientational order of molecules in the liquid-crystalline state on the qualitative form of dielectric spectra is emphasized.

On Dielectric Relaxation in Liquid Crystals

Rigid-backbone polymers. Dielectric relaxation of isotropic and lyotropic solutions of poly(n-hexylisocyanate)

The order parameter and the temperature dependence of the Miesowicz viscosity coefficients of different mixtures of mesogenic 4-cyan-${4}^{\ensuremath{'}}$-octyloxybiphenyl (8OCB) and 4-butylbenzoate ${4}^{\ensuremath{'}}$-isothiocyanatephenyl (4TPB) are studied. Measurements were performed with the aid of a home-built Miesowicz viscometer. The data are analyzed by means of the relations of Kneppe et al. and critically compared with the predictions of available theoretical models.

Miesowicz viscosities study of a two-component thermotropic mixture

The synthesis route and phase properties for a new three-ring homologous series, 4-(trans,trans-4-alkylbicyclohexyl)benzene-isothiocyanates (nCCHBT) with n=2-7, are presented. The substances exhibit very broad range nematic phases exceeding 200 K, and low viscosity with a low activation enthalpy. The dielectric properties of the nCCHBT materials are compared with those of their long known two-ring analogues (nCHBT). The static permittivity, epsilons is considerably lower for the three-ring compounds. It is found that epsilons decreases with n at temperatures as low as Tr=T/TNI=0.7. The relaxation time, tau shows the odd-even effect, the amplitude of which reverses on going from Tr=0.7 to 1. The activation enthalpy for molecular rotation about the short axis alternates with n practically in the same manner for the corresponding members of both homologous series. It is argued that the present results are at odds with models assuming that the activation enthalpy consists of the viscosity and nematic potentia...

Phase transition and dielectric studies of a three-ring homologous series with the -ncs terminal group

In studying the dehydration of surface-moistened fumed silica Aerosil powders, we found a conductivity percolation transition at low hydration levels. Both the percolation exponent and the threshold are typical for correlated site-bond transitions in complex two-dimensional (2D) systems. The exponent values, 0.94--1.10, are indicative of severe heterogeneity in the conducting medium. The surface moisture at the percolation threshold takes on a universal value of 0.65 mg${}_{[\mathrm{H}2\mathrm{O}]}$/m${}^{2}$${}_{[\mathrm{silica}]}$, independent of the silica grain size, and equivalent to twice the first hydration monolayer. This level is just sufficient to sustain a quasi-2D, hydrogen-bonded water network spanning the silica surface.

Jozef K. Moscicki

Papers

On Dielectric Relaxation in Liquid Crystals

Rigid-backbone polymers. Dielectric relaxation of isotropic and lyotropic solutions of poly(n-hexylisocyanate)

Miesowicz viscosities study of a two-component thermotropic mixture

Phase transition and dielectric studies of a three-ring homologous series with the -ncs terminal group

Electric conductivity percolation in naturally dehydrating, lightly wetted, hydrophilic fumed silica powder.