Preparation of Liquid Crystalline Elastomers - Liquid Crystal Elastomers: Materials and Applications - page 37

4.1.2 Coupling to LC Properties
In this section some selected examples for the orientation due to surface effects and
magnetic and electric fields will be presented. Elastomers oriented using the
anisotropic physical properties of the liquid crystalline phase structure are usually
restricted to thin films, except for lyotropic samples and when extremely high fields
are available. Typically, a thin film of a linear LC polymer or a reactive mixture of
low molar mass LC and crosslinker is aligned by using surface effects, magnetic
fields, or electric fields and subsequently photo-crosslinked. This method has been
widely used for side chain polymers, which show low viscosities, allowing good
orientation at moderate field strengths. Main chain polymers exhibit much higher
viscosities and thus it is usually very difficult to achieve good alignment.
An advantage with respect to the methods discussed in Sect. 4.1.1 is that a
homogeneous or homeotropic orientation of the mesogens can be chosen indepen-
dently of the chain conformation. For the orientation in an electric or magnetic field
it has to be considered that the mesogens must have a suitable anisotropy of the
dielectric constant and the diamagnetic susceptibility, respectively.
The orientation achieved by the methods discussed in this section is sometimes,
especially in the case of cholesteric elastomers, much better than for the thick films
prepared by coupling to a mechanical field.
The orientation of the LC polymer films is usually carried out between treated
glass slides. Consequently the orientation process, the existence of defect structures
and variations in orientation, as well as boundary effects can be directly visualized
by means of polarizing optical microscopy.
Orientation Due to Surface Effects
In order to minimize the energy at its surface, thin films of monomeric and
polymeric material often form nicely ordered liquid crystalline monodomains on
surfaces. This happens spontaneously especially for smectic-A and cholesteric
films and can be promoted by coating of the surfaces and by annealing the sample.
Annealing is usually done a few Kelvin below T LC,i , often after slowly cooling from
the isotropic phase, so that the mesogens are mobile enough to organize themselves
at the surface.
Urayama et al . were able to synthesize a nematic elastomer with homeotropic
orientation of the mesogens employing surface effects. They started from a mixture
of a reactive acrylate-monomer, the crosslinker 1,6-hexanediol diacrylate, a
photoinitiator, and a non-reactive nematogen that is necessary to broaden the
temperature range of the nematic phase. This mixture was sandwiched between
two polyimide coated glass plates that were separated by spacers and induce a
normal alignment of the mesogens. After photo-polymerization the resulting film
was detached from the glass substrate by immersing the cell in dichloromethane.
The swelling in dichloromethane also washes out unreacted and nonreactive