Order and Disorder in Liquid-Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 193

atoms, SANS indicated a reversible inversion of the backbone anisotropy at the
phase transition from nematic to smectic-C. In the high temperature nematic phase,
a weak prolate anisotropy, R g// / R g ⊥

1.2, was measured. At the phase transition
from nematic to smectic-C, the backbone anisotropy continuously changes from
weakly prolate to spherical (isotropic) and then to strongly oblate: R g// / R g ⊥ 0.5.

Intuitively for a side-on type of linking it seems difficult to impose smectic layering
and to confine the backbone in between these layers. In fact, neutron diffraction
measurements of these polymers show the polymer backbone to be partly distri-
buted in the middle of the mesogenic layers [ 51 ] . The observed inversion of the
backbone anisotropy in the side-on smectic system can be related to the high
flexibility of the polysiloxane chain and the long spacer. The intrinsically con-
flicting preferred orientations of mesogenic cores, backbones, and aliphatic spacers
in these polymer molecules leads to strongly disordered smectic layering.

3 Shape Anisotropy and Orientational Order in Nematic
Elastomers

3.1 Structure and Diversity

LC polymers can be covalently crosslinked to form a 3D network leading to a LC
elastomer (Fig. 5 ). Since the synthesis of the first LC elastomer based on a polysilox-
ane backbone by Finkelmann et al. [ 52 ] , a number of different types of elastomers

Fig. 5 Representation of an
end-on side-chain smectic LC
elastomer