Modern Polymer Spect..

2.4 Srvuc me-Dependenr Alignment 37
In the case of the laser irradiation experiments (see Section 2.6), the polarization
direction of the incident laser beam was chosen as reference direction. Contrary to
the former publications we have changed the reference direction for the calculation
of the photo-induced anisotropy due to a better understanding of the underlying
orientation mechanism. We assume that the mesogens are aligned preferentially
perpendicular to the polrivizniion iiectov [34, 351 and not to a polnvizntion plane [15,
361 of the irradiating laser. Thus, we obtain negative S-values, whereas for the lastmentioned
case the order parameter values would be positive, according to the
Maier-Saupe theory.
2.4 Structure-Dependent Alignment of Side-Chain
Liquid-Crystalline Polyacrylates on Anisotropic
Surfaces
For the characterization of the anisotropic properties of liquid-crystalline (LC)
materials it is necessary to induce a well-defined orientation of the mesogenic
groups. Although spontaneous orientation phenomena can be regularly observed in
limited domains, the necessary prerequisite for the application of LC-materials as
optical construction elements, for example, is the preparation of uniform textures
over larger areas. Such structures can be prepared by the alignment of LC-systems
on anisotropic surfaces (e.g., glass, polymer films). The origin and theory of the
interaction and the induced anisotropy has been treated in several books and
articles [37-441. Here, the attention is primarily directed towards the influence of
the spacer length in a homologous series of nematic side-chain liquid-crystalline
polyacrylates on their alignment on an anisotropic polyimide surface.
2.4.1 Materials and Experimental
The polymers had a molecular weight M, (determined by gel permeation chromatography)
of about 5000 [45] and their structural formula is shown alongside their
nematic-isotropic transition temperatures (determined by polarization microscopy)
[45] in Figure 2-4.
For the preparation of the substrate surface, KBr-plates have been spin-coated
with a thin layer of polyiinide (polyimide-kit ZLI 2650, Merck, Darinstadt, Germany).
Upon drying and curing this coating to the final thickness of about 1 pm, the
surface anisotropy was induced by unidirectional rubbing with a polymethacrylimide
hardfoam-roll. The LC-polymers were sandwiched between the SUrfdCetreated
KBr-plates above their clearing temperatures and finally annealed 3 "C
below these temperatures for several hours. FTIR polarization spectra with a resolution
of 4 cm-' were recorded by accumulating 20 scans.