Relation between molecular structure and electrorheological effects in liquid crystalline polymers

The influence of the molecular structure of homogeneous fluids comprised of liquid crystalline polysiloxanes (LCSs) diluted in polydimethylsiloxane (DMS), on the increase in their viscosity upon application of an electric field (electrorheological effect, or the ER effect) was investigated, particularly in relation to the degree of polysiloxane polymerization, the composition of their mesogenic side chains, and the composition of alkyl spacers inserted between these two components. A large ER effect was observed in LCSs having spacer length of 3 carbons (C3) and diluted with DMS, even though little such effect was observed with no DMS dilution. The ER effect generally increased with increasing DMS dilution and with increasing mesogenic group content, main chain length, and spacer length. At the same time, however, the miscibility of the DMS solvent with the LC polymer decreased, resulting in instability and difficulty in measuring the shear stress at temperatures up to 40°C or higher. When spacers containing an ether bond were employed, on the other hand, little or no polymer-solvent phase separation occurred, and the ER effect was strong and stable down to temperatures of 30°C or lower. With all of the LCSs, the ER effect was observed at temperatures above their isotropic phase transition temperature as measured by DSC.