LC polysiloxanes

Chien and Cada [42] have prepared optically active and photoactive SCLC copolymers, 15, with the 4-alkoxyphenyl-4 -alkoxycinnamate chromophore, with the intention of creating LC polysiloxane networks that could be used to prepare macroscopically oriented organic ferroelectric polymers for electro-optical devices. Optical activity was introduced into the polymer by the use of a chiral spacer. Those copolymers which were mesogenic exhibited properties characteristic of a Sc. phase. UV-irradiation of thin films of the polymers in their mesomorphic states at 90°C, led to a loss of the IR absorption at 1635 cm-1 that is due to the cinnamate double bond, and to cross-linking. Long-term irradiation led to... 

Figure 10. Synthesis of LC polysiloxanes by the polycondensation reaction between a mesogenic monomer containing allylic functions and oligodimethylsiloxanes.

A slightly different but widely used variant of this method is a hydrosilylation reaction between reactive poly-siloxanes and the mesogenic monomers of the olefin type, which can be used for LC polysiloxanes ... 

Furthermore, in FLCEs the macroscopic electric dipole moment provides a handle to apply a strong torque onto the director (see Fig. 14a). The resulting switching occurs on the cone of the so-called c-director, the projection of the director on the smectic layer plane (see Fig. 14a). Soon after the discovery of the potential of chiral smectic-C phases, the search for LC polymers with these phases started [130-132]. However, as ferroelectric switching is the final proof for the assignment of the phase, the more closely studied ferroelectric LC polymers were limited to several LC polysiloxanes, which have a low Tg and a relatively high switching speed [25, 66, 133-136] (see Scheme 1). These polymers form the basis for most of the FLCEs discussed here. 

Poths H, Andersson G, Skarp K, Zentel R (1992) Fast electroclinic switching in a ferroelectric LC-polysiloxane. Adv Mater 4(12) 792-794. doi 10.1002/adma. 19920041204... 

Finkelmann H, Kock H, Rehage G (1981) Investigations on LC polysiloxanes 3. Liquid crystalline elastomers - a new type of liquid crystalline material. Makromol Chem Rapid Commun 2 317-322... 

Cross-linked LC polysiloxanes can also be obtained by reactions of addition of cross-linking diolefin compounds to the polymer chain ... 

The dependences of the melting points on the length of the aliphatic spacers (Fig. 6.9a) and terminal alkyl substituents for two types of LC polysiloxanes (Fig. 6.9b) are shown in Fig. 6.9. Both series of polymers only form a smectic mesophase even with comparatively short lengths of the spacer (n = 3, 4) and alkoxy group (OCH3). 

The previously unexplained behavior in the orientation of cross-linked LC polysiloxanes was found in deformation of samples XXVIII and XXIX, which only differ with respect to the length of the aliphatic spacw ... 

A new brush-type CSP, the Whelk-0 1, was used by Blum et al. for the analytical and preparative-scale separations of racemic pharmaceutical compounds, including verapamil and ketoprofen. A comparison of LC and SFC revealed the superiority of SFC in terms of efficiency and speed of method development [50]. The Whelk-0 1 selector and its homologues have also been incorporated into polysiloxanes. The resulting polymers were coated on silica and thermally immobilized. Higher efficiencies were observed when these CSPs were used with sub- and supercritical fluids as eluents, and a greater number of compounds were resolved in SFC compared to LC. Compounds such as flurbiprofen, warfarin, and benzoin were enantioresolved with a modified CO, eluent [37]. 

It has been demonstrated on a number of polysiloxanes that recycled-flow NMR is superior to the use of relaxation reagents403. It levels spin-lattice relaxation, minimizes NOE, shortens the experiment time by a factor of 3-5 and gives spectra with improved resolution. However, as the flow method requires some additional equipment, it is unlikely to be used for isolated cases when quantitative 29 Si NMR is needed. On the other hand, when such spectra are run often, it is certainly worth consideration, especially when flow cells (probes) for LC NMR are available. 

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