Hypersonic properties of nematic and smectic polymer liquid crystals

We have investigated the elastic and the optical properties of nematic and smectic side chain polymer liquid crystals at hypersonic frequencies. In contrast to classical liquid crystals, the polymer materials show a significant coupling between the static order parameter and the square of the elastic strain ${\epsilon }_3$. The inverse shear stiffness constants do not diverge. All elastic constants behave continuously at the nematic-isotropic transition ($T_{\mathrm{ni}}$). From certain optoacoustic properties and the refractive index functions, it follows that order-parameter fluctuations play no role at $T_{\mathrm{ni}}$, but that the glass transition dominates the dynamic elastic properties at hypersonic frequencies.