Effect of cross-link concentration on the isotropic to smectic-A phase transition in smectic elastomers

We describe the isotropic to smectic-A phase transition in smectic elastomers within Landau phenomenological theory. We analyze the effects of external mechanical stress and cross-link concentration on the isotropic to smectic-A phase transition. Both the orientational and translational order parameters decrease with the increase of cross-link concentration. The first order isotropic to smectic-A transition in low-molecular-weight materials, can become weak first order transition in smectic elastomers. The effect of cross-link concentration on the strain birefringence and correlation length in the isotropic phase of the transition is discussed. (paper)