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 mon omer in a chiral SmC* and SmA* liquid crystal by measuring the resulting ch anges in the temperature dependence of the spontaneous polarization. When t he 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 fo r increasing polymer concentration. When the polymerization is carried out in the SmA* phase, the polarization at the corresponding reduced temperatur es is decreased. This behaviour is due to a local stabilization of the resp ective phase, in which the polymerization is carried out, caused by the ela stic interaction of the polymer network with the liquid crystal. In the net work dominated regions the average tilt angle is approximately preserved af ter a transition to another phase (SmA* or SmC*) and this effect accounts f or the corresponding change in polarization observed in the experiment. The results are interpreted in a simple semi-phenomenological description base d on the effective stabilizing potential of the quadrupole symmetry.