Liquid Crystal Elastomers and Light - Liquid Crystal Elastomers: Materials and Applications - page 100

primarily diminish the influence of the network. In unswollen nematic LCEs, the
orienting effect of the network is comparable to that of electric fields of magni-
tude E

p

Y=e o De m, so the effect of practically achievable electric

fields is small. Swelling diminishes the aligning effect of the network, and, in the
case of nematic solvents, this allows applied electric fields to reorient the nematic
director [ 12 ].

10V =m

2.2 Polydomain LCEs, Gels, and LC Networks

The network structure of LCEs is determined by processing during sample pro-
duction. Side-chain nematic LCEs with siloxane main chain [ 13 ] are typically
partially polymerized in a centrifuge, from which they are removed in the
solvent-swollen isotropic state. As the solvent evaporates, the mesogen number
density increases, and the sample locally undergoes the isotropic-nematic transi-
tion. Polymerization continues as the sample is baked, until it is complete and the
structure is locked in. If there is no symmetry breaking to establish a preferred
direction, the director of the emerging nematic regions is random, and a
polydomain sample with randomly oriented nematic domains results. The domain
size is determined by competition between the Frank director distortion energy and
the random aligning effects of the network [ 14 ]; the resulting domain size length
scale l d is of the order l d

p

K=Y 10 7 m. Since the domain size is comparable

to an optical wavelength, polydomain samples scatter light and appear opaque.
Some structural information can be extracted by large angle light scattering

[ 15 , 16 ].
Monodomain samples produced by this method are stretched slightly while the
solvent evaporates; this symmetry breaking is sufficient to align the emerging
nematic domains all in the same direction. It is interesting to note that such
transparent monodomain samples transform to scattering polydomain structures
when swollen with solvents that weaken the aligning effect of the network, and, as
the solvent concentration increases, become transparent again in the isotropic
phase. Figure 2 such swollen networks are formally gels, dispersions of liquids in

Fig. 2 Monodomain LCE swollen in chloroform vapor, which forms nematic monodomain
(0 min), polydomain (12 min) and isotropic structures (14-80 min) as the solvent evaporates