Kosuke Kaneko

Electrorheological Effect of “Side-on” Liquid Crystalline Polysiloxane

Liquid crystals have attracted great attention in the field of engineering. Their application to the development of screens and displays is a representative example of their unique properties, because it demonstrates their excellent molecular-orientation ability. To develop further uses of liquid crystals, we have investigated the electrorheological (ER) effect, in which viscosity is controlled by the strength of an applied electric field. Since the discovery of the ER effect by Winslow in 1949, several ER fluids have been widely studied as smart materials for industrial uses. After extensive research on ER fluids, it has been found that side-chain liquid crystalline polymers (LCPs) show a relatively large change in shear stress upon applying an electric field, because the thermal motion of terminal mesogenic groups is independent from that of the polymer backbone. In addition, ER effects of the side-chain LCPs have been commonly observed at temperatures above the clearing point. This observation can be attributed to the enhanced order, which results from the formation of the liquid crystalline phase. From a practical viewpoint, side-chain LCPs exhibiting a smectic phase have a high viscosity in the phase, which causes a poor response to the application of an electric field. This limits the temperature range at which ER effects can be measured when LCPs showing a smectic phase are used as the ER fluids. It is widely known that the viscosity in the smectic phase is usually higher than that of the nematic phase, in accordance with the different order parameters of the various liquid crystalline phases. The generated shear stress is defined as the difference between the shear stress in the presence and that in the absence of an applied electric field. Therefore, a lower shear stress in the absence of an electric field is expected to enhance the ER effect. The shape of the constituent molecules in liquid crystals is a fundamental factor in determining the mesophase properties. Side-chain LCPs are generally classified into two kinds depending on whether the mesogenic groups are attached terminally or laterally to the polymer backbone via a flexible spacer, as shown in Figure 1. In “side-end” LCPs, in which the mesogenic groups are linked to the backbone through one of their end groups, the smectic phase is usually favored. In contrast, the nematic phase is favored in “side-on” LCPs, in which the mesogens are laterally attached. Although side-end LCPs have been widely used to investigate ER effects, there are no reports describing ER effects on side-on LCPs. In expectation of a large ER effect in the nematic phase, we synthesized a side-on liquid crystalline polysiloxane compound in this study. The phase-transition behavior of polysiloxane was determined by means of differential scanning calorimetry (DSC) and polarizing optical microscopy (POM), and its rheological properties were investigated in the nematic phase and the isotropic state. The synthetic route to the sideon liquid crystalline polysiloxane compound prepared herein is shown in Figure 2. Here, we describe the phase-transition behavior of a siloxane polymer. From POM observations we find that the siloxane poFigure 1. Structures of side-chain liquid crystalline polymers : a) side-end; b) side-on.

The phase transition behaviour and electro-rheological effect of liquid crystalline siloxane dimers

Symmetrical liquid crystalline dimers have been synthesised, composed of rod-like mesogenic units with polar groups and a siloxane core of varying length in the central region. All the dimers containing siloxane units have been shown to exhibit a smectic A phase, confirmed by polarising optical microscopy (POM), differential scanning calorimetry (DSC) and X-ray diffraction (XRD). Rheological properties under an applied electric field were investigated by rotational rheometer. The influence of the siloxane core length on rheological properties has been investigated, and in addition the behaviour of the liquid crystalline dimers in the smectic A phase under an applied electric field has been observed by POM.

A novel pre‐shearing technique for measurement of the electrorheological effect of a side‐chain liquid crystalline polysiloxane

A side‐chain liquid crystalline polysiloxane derivative possessing mesogens was synthesized. The mesogens included a large dielectrically‐anisotropic moiety. Phase transition behaviour was investigated and the electrorheological (ER) effect of the polymer considered. No ER effect was observed in non‐sheared polysiloxane derivative because of its high inherent viscosity. However, a large ER effect was observed after pre‐shearing. Pre‐shearing, i.e. shearing with a high shear rate before the measurements, arranges the terminal mesogens in the shearing direction. Furthermore, a dilution of the sample led to a larger ER effect exceeding 10 000 Pa. We thus were able to demonstrate a new approach for the appearance of the ER effect.

Study on an Electrorheological Effect of Side-Chain Polysiloxanes Including Fluorine Atoms by the Preshearing Method

Side-chain liquid-crystalline polysiloxane derivatives possessing fluorinated mesogens that have positive dielectric-anisotropy parallel to the long axis of mesogen were synthesized to study phase-transition behavior and electrorheological (ER) effect of the polymers with a preshearing method. About 2000 Pa increments in shear stress could be obtained after preshearing as an ER effect. In addition, shear rate dependence on shear stress as a function of temperature was investigated. As a result, it was shown that the preshearing treatment with high shear rate was important to generate a larger ER effect.

Electric‐Field‐Induced Viscosity Change of a Nematic Liquid Crystal with Gold Nanoparticles

The electro-rheological (ER) effect of a composite material consisting of a nematic liquid crystal (LC) and gold nanoparticles covered with mesogenic groups is discussed. The gold nanoparticles are covered by alkyl chains and liquid-crystalline compounds. The influences of the alkyl-chain length and the coverage by the alkyl chain and the mesogenic group on the miscibility of the nanoparticles with the LC are investigated by polarizing optical microscopy (POM). The presence of the gold nanoparticles in the nematic LC (5CB) leads to an enhanced ER response compared to that observed for 5CB. The prominent ER effect observed in this study is supported by the two mechanisms proposed, that is, the homogeneous and heterogeneous mechanisms. This study demonstrates the potential of a hybrid system consisting of an LC and gold nanoparticles to improve the ER effect.

Electro-rheological effect of blends composed of two liquid crystalline materials: composition dependence

The electro-rheological (ER) effect of blends composed of two liquid crystalline materials (polymer and oligomer) with varying volumes of each component is discussed in this article. The results indicated that the ER effect of each blend was observed at a different temperature range. The blends showed better ER response to an applied electric field compared with each neat sample. Also investigated is the behaviour of the liquid crystalline oligomer in the isotropic polymer under an applied electric field, by polarising optical microscope observation. This study demonstrates the importance of a host matrix in blend systems on ER effect.