Phase transitions frustrated smectics

For a composition fixed close to the apex of the parabola (e.g., x = 0.4, where x is the weight ratio of 60CB/80CB) with decreasing temperature we meet the nematic, the smectic A, and again the nematic phase. The parabolic shape of the phase diagram is accounted for in terms of modern phase transition theory with a competition of the orientational and translational orders taken into account [32]. A microscopic explanation is based on the frustration phenomenon taking into account the competition between the steric and dipole-dipole interactions [38-40]. 

Figure 17 Effect of the DP (P ) on phase transitions in fractions of LC poiymers 6. G is the giassy state SmC is the smectic (ribbon-iike) C phase SmA is the smectic A phase /Vis the nematic phase /Vre is the reentrant nematic phase and / is the isotropic meit. Adapted from the papers Frustrated Behavior of a Side-Chain Liquid Crystaiiine Poiyacryiate pubiished in Macromoi.Chem.Phys., 2001, v. 202, p.297-303 Fig. 2.

Most experiments on LC elastomers have so far used single crystal elastomers made via the two-step crosslinking process, which involves stretching in the LC state. There is increasing evidence that this situation represents a special thermodynamic state - smectic elastomers made in such a way are well aligned but their layer positions are frustrated due to the random crosslink distribution. Evidently, there is room for experiments on nematic and smectic elastomer samples oriented in different ways, for example by photo-crosslinking. In such a way, any memory of the aligning procedure imprinted in the samples will be avoided (at least partially) and new features of phases and phase transitions could be revealed. 

Figure 15. Heat capacity of 80PCB0B near the nematic to smectic A1 phase transition. The smooth curve represents a fit to the data with Eq. (6) based on critical parameters in agreement with the three-dimensional XY model. The index 1 in SmA refers to the monolayer structure of this frustrated smectic compound [65].

At a normal chiral nematic to smectic A transition, the helical ordering of the chiral nematic phase collapses to give the layered structure of the smectic A phase. However, for a transition mediated by a TGB phase, there is a competition between the need for the molecules to form a helical structure due to their chiral packing requirements and the need for the phase to form a layered structure. Consequently, the molecules relieve this frustration by trying to form a helical structure, where the axis of the helix is perpendicular to the long axes of the molecules (as in the chiral nematic phase), yet at the same time they also try to form a lamellar structure, as shown in Fig. 21. These two... 

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