Helicoidal superstructure

Mesophase with a helicoidal superstructure of the director, formed by chiral, calamitic or discotic molecules or by doping a uniaxial nematic host with chiral guest molecules in which the local director n precesses around a single axis. 

Quite recently, Finkelmann and co-work-ers [232, 233] showed that an appropriate mechanical deformation of an SmC elastomer 50 yields a permanent macroscopically uniform orientation. This process also unwinds the helicoidal superstructure, and accordingly, frequency doubling is observed where the intensity of the SHG is directly related to the perfection of the uniform smectic layer orientation. The 22 < 23= 34 coefficients for a highly oriented sample were reported to be 0.1 pm/V and 0.15 pm/V, respectively. Taking into account that only 50% of the mesogenic units in the LC elastomer are active groups, these values are of the same order as those reported for low molar mass LCs containing similar chromophores [222]. 

These molecules (20) show an interesting phase behavior [51] and form helicoidal superstructures [52], Nonionic derivatives of aspartic and glutamic acid with stractures analogous to lecithins may contain perhydrogenated or perfluorinated apolar chains [53]. The A(,(V-diacylated compounds are synthesized in two steps (Scheme 19). The hydrophiUcity of the molecules (21) is due to the presence of an acyl-polyethyleneglycol-methylether [54]. 

The first chiral smectic C elastomer was synthesized by Zentel et al. in 1988 [13], [24]. The preparation of the network is similar to the synthesis of the cholesteric networks described above. The network was uniaxially stretched. An unwinding of the helicoidal superstructure is observed and an orientation of the director, as well as of the smectic layer normal roughly parallel to the stress axis occurs. A macroscopically uniform alignment of the sample was not observed as explained below. 

Nonlinear optical measurements on elastomers were performed by J. Benne et al. with mechanically aligned samples. Due to the alignment procedure, the helicoidal superstructure is untwisted and the two-step cross-linking process locks in the ferroelectric structure. Without applying external fields, these networks spontaneously produce frequency doubling and, from the SHG signal in a Marker-fringe experiment, the C2 symmetry of the elastomer is confirmed [35], [36]. 

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