Development of orientation in PLCs

Equation (10.9) provides an elegant and simple view of the order parameter concept with the separation of the overall order parameters into the two parameters and We may consider the development of orientation in a PLC as being primarily the increase of from zero to a value closer to unity. We shall in the remaining part of this chapter use the concept order parameter for the description of orientation and... 

PLCs that have been oriented by a shear or elongational field develop a banded texture morphology when allowed to relax in the mesophase just above Ty, [58,59]. Under the microscope such textures appear as an alternation of dark and light striations perpendicular to the direction of orientation. They are erased on heating, but memory of their characteristic sinusoidal supramolecular organization must temporarily remain in the I phase since they can be recovered on cooling back into the mesophase [60,61]. 

As noted before, PLCs can be oriented in various kinds of fields, not only in shearing fields. Depending apparently on the techniques they had at their disposals, different groups have developed different techniques for orientation of specific polymers. Thus, magnetic fields are used to orient acrylates, while electric fields are applied to orient polysiloxanes. 

The general Landau theory, which was developed by de Gennes to describe critical phenomena in MFCs, has been appUed to elastic networks comprised of PLCs [66]. The Landau formalism also allows one to make contact with the theory used to describe conventional orientation phenomena in nonmesogenic polymer networks. In particular, a mechanical deformation via its associated stress field a influences g (and therefore Tc and 5c) analogously to external magnetic or electric fields. For a small (uniaxial) extension ratio X = e —, where e is the strain, the form of g in Eq. (5.16) is modified by the additional terms as follows ... 

Essentially all of the techniques developed to characterize MLCs can be applied to PLCs with the realization that phenomena that are dependent on reorientation processes in the liquid crystal must be considered on considerably longer time scales in a PLC. Underlying most of the physical measurements performed on liquid crystals is the relationship between the observed anisotropic properties of the mesophase and orientational order of the mesogen. For uniaxial nematic phases and idealized low molar mass mesogens (cylindrical molecules), this relationship is embodied in the following equation ... 

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