Spacer chains, chiral nematics

PEIs derived from AT-(4-carboxyphenyl) trimellitimide and aliphatic spacers (11) are not thermotropic, irrespective of whether the spacer is chiral or not. If the spacers are semi-aliphatic and contain a benzene ring, then thermotropic PEIs may be formed with both a smectic and nematic phase. If chiral spacers are then used a chiral smectic A or C phase may additionally be obtained. Such phases may be ferroelectric, which is extremely rare for main-chain polymers [26]. Examples of chiral and non-chiral spacers used in the copolymers (12) are ... 

The texture of polymeric chiral liquid crystalline phases. The chiral liquid crystalline phases include the chiral smectics and the chiral nematic or cholesteric phase. Poly(7-benzyl-L-glutamate) and derivatives of cellulose are popular examples of polymers that form a chiral mesophase. Side-chain type copolymers of two chiral monomers with flexible spacers of different, lengths and copolymers of one chiral and the other non-chiral mesogenic monomers may also form a cholesteric phase (Finkelmann et al., 1978 1980). In addition, a polymeric nematic phase may be transformed to a cholesteric phase by dissolving in a chiral compound (Fayolle et al., 1979). The first polymer that formed a chiral smectic C phase was reported by Shibaev et al. (1984). It has the sequence of phase transition of g 20-30 Sc 73-75 Sa 83-85 I with the Sc phase at the lower temperature side of Sa- More examples of Sc polymers are given by Le Barny and Dubois (1989). 

The concept of mechanical field induced orientation can easily be transferred to nematic elastomers with oblate chain conformation, i.e., side chain end-on elastomers with an even number of spacer atoms. In order to achieve a monodomain structure, a globally oblate chain conformation has to be established. This can be achieved by uniaxial compression or biaxial stretching of the polydomain elastomer which induces a uniform homeotropic alignment of the nematic director perpendicular to the film plane. Up to now, this orientaticMi technique has only been realized experimentally for chiral nematic elastomers [72]. 

It is also known that in side-chain LC polymers the copolymerization of optically active monomers with mesogenic monomers, in the same manner as the mixing of optically active compounds with nematic low molecular weight compounds, can induce the formation of a cholesteric mesophase. Therefore, it is expected that inclusion of chiral spacers in main chain liquid crystal polymers, which would be nematic... 

Aromatic imide groups are known to be nearly planar, rigid, polar, and thermostable. However, aromatic imide structures are also known to be non-mesogen in nature. Poly(ester-amide) (PEI) derived from N- (4 -carboxyphenyl) trimellitimide and aliphatic spacers are not thermotropic, whether the spacer used is chiral or not. Semi-aliphatic spacers are observed to exhibit both a smectic and a nematic LC phase in the resultant thermotropic PEIs. The semi-aliphatic chiral spacers exhibit both chiral smectic phase (A or C ) and cholesteric phase. Such a chiral smectic LC-phase, which may be ferroelectric in nature, is extremely rare for LC-main-chain polymers [27]. It is a particular advantage of polar imide mesogens to favor the formation of layer-structures when combined with non-polar species [28]. 

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