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