PIEZOELECTRICITY OF CHOLESTERIC ELASTOMERS .1. INFLUENCE OF THE HELICOIDAL PITCH ON THE PIEZOELECTRIC COEFFICIENT

Mechanical, optical, and electromechanical investigations were performed on a series of induced cholesteric elastomers. These elastomers are swollen with a chiral low molar mass liquid crystal and exhibit a mechanical coupling coefficient U that is of the same order of magnitude as that for nonswollen nematic elastomers. Above a certain threshold deformation, a macroscopically uniform orientation of the mesogenic side groups is achieved, with the helicoidal axis oriented parallel to the compression axis. Above this threshold deformation, the helicoidal pitch affinely deforms with elastomer compression. This deformation of the helicoidal superstructure is a basic requirement for piezoelectricity in cholesteric elastomers. Above a threshold deformation electromechanical investigations find a linear relation between voltage and deformation. While the direction of polarization of the elastomer with respect to the helicoidal axis is affected by the surface geometry, the absolute value of the effect remains unchanged. Under a given sample geometry a change of the handedness of the cholesteric helix and investigations with the corresponding nonchiral system prove that only piezoelectricity occurs. Other electromechanic effects like flexoelectricity or electrostriction are negligible. Temperature-dependent measurements find a linear relation between the piezoelectric coefficient and the cholesteric order parameter. Furthermore, the piezoelectric coefficient scales with the reciprocal pitch of the cholesteric elastomers.