The effect of a polymer network on smectic phase structure as probed by polarization measurements on a ferroelectric liquid crystal

We have studied the effect of a polymer network, formed from an achiral monomer in a chiral SmC* and SmA* liquid crystal by measuring the resulting changes in the temperature dependence of the spontaneous polarization. When the polymerization is performed well into the SmC* phase, an increase of the spontaneous polarization with respect to the original values of the liquid crystal mixture is observed in the vicinity of the SmC*-SmA* transition for increasing polymer concentration. When the polymerization is carried out in the SmA* phase, the polarization at the corresponding reduced temperatures is decreased. This behaviour is due to a local stabilization of the respective phase, in which the polymerization is carried out, caused by the elastic interaction of the polymer network with the liquid crystal. In the network dominated regions the average tilt angle is approximately preserved after a transition to another phase (SmA* or SmC*) and this effect accounts for the corresponding change in polarization observed in the experiment. The results are interpreted in a simple semi-phenomenological description based on the effective stabilizing potential of the quadrupole symmetry.