Main-Chain Chiral Smectic Polymers Showing a Large Electroclinic Effect in the SmA* Phase

The synthesis and characterization of a main-chain smectic liquid-crystalline polymer system designed for development into electromechanical actuators is described. The chemical structure is chosen to provide a large electroclinic effect in the SmA* phase, with large concomitant layer shrinkage (a rare combination). The polymers are prepared by acyclic diene metathesis polymerization (ADMET) of liquid-crystalline α,ω-dienes. Oligomers with a degree of polymerization of ∼10−30 are obtained using Grubbs first-generation catalyst, while oligomers with a degree of polymerization of ∼200 are obtained using Grubbs second-generation catalyst. All polymer samples show the following phase sequence: I − SmA* − SmC* − Glass. X-ray analysis of polymer powder samples demonstrates the desired layer shrinkage at the SmA* − SmC* transition. The polymers form well-aligned fibers by pulling from the isotropic melt, and X-ray analysis of fibers in the SmA* phase shows that in the bulk of the fiber the layers are oriented perpendicular to the fiber axis, while at the surfaces there appears to be a thin sheath where the layers are parallel to the fiber/air interface. The desired layer shrinkage with tilt at the SmA* − SmC* transition in these fibers is seen as well, and in the SmC* phase the fibers exhibit an interesting conical chevron layer structure. Electro-optic investigation of aligned thin films of the polymer, prepared from quenched fiber glasses using a novel technique, exhibit a large electroclinic effect, with substantial degradation of alignment quality upon field-induced tilt. This degradation in alignment quality, coupled with the layer shrinkage at the SmA* − SmC* transition demonstrated by X-ray scattering, strongly suggests the desired layer shrinkage with electroclinic tilt is in fact occurring in the polymer films.