Elastic and dielectric effects
An empirical relationship between electrostrictive, dielectric,
and elastic constants was already established
by Eury et al.20 for a variety of dielectric materials,
including glasses, ceramics, single crystals, and polymers.
The findings of these researchers indicated that
the hydrostatic electrostriction coefficient (Qh) is linearly
related to the ratio of the elastic compliance to
the dielectric permittivity, s/0p (where 0 is the permittivity
of the vacuum and p is the dielectric constant
of the material). The electrostrictive coefficients
Qijkl relate the strains Sij to the square of the polarization
PkPl and are also related to the strain coefficients
Mijkl through Qijkl (kl 0)2 Mijkl. Hence, in the
case of our polyurethanes, the following relationship