Liquid Crystals

Chapter 1.4.3 - Polymer-Dispersed Liquid Crystals

1.4.3.   Polymer-Dispersed Liquid Crystals

Just as the presence of dye molecules modifies the absorption characteristics of liquid
crystals, the presence of a material interdispersed in the liquid crystals of a different
refractive index modifies the scattering properties of the resulting “mixed”
system. Polymer-dispersed liquid crystals are formed by introducing liquid crystals
as micron- or sub-micron-sized droplets into a polymer matrix. The optical indices of
these randomly oriented liquid crystal droplets, in the absence of an external alignment
field, depend on the liquid crystal–polymer interaction at the boundary, and
therefore assume a random distribution (see Fig. 1.17a). This causes large scattering.
Upon the application of an external field, the droplets will be aligned (Fig. 1.17b),
and the system will become clear as the refractive index of the liquid crystal droplets
matches the isotropic polymer backgrounds.

Polymer-dispersed liquid crystals were introduced many years ago.13 There are
now several techniques for preparing such composite liquid crystalline materials,
including the phase separation and the encapsulation methods.14 More recently, optical
holographic interference methods15–17 have been employed successfully in making
polymer-dispersed liquid crystal photonic crystals (regular array of materials of
different refractive indices). Caputo et al.18 and Strangi et al.19 have also demonstrated
one-dimensional (1D) polymer/liquid crystal layered structures that exhibit
high diffraction efficiency as well as laser emission capabilities.

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