Polymerization and Properties of Polymer-Stabilized Ferroelectric Liquid Crystals
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Introduction I would like to focus on our recent work involving photopolymerizations of monomers in a liquid-crystalline environment. This work is one of the many aspects of photopolymerizations that we are focusing on at the University of Colorado. In particular this effort concentrates on understanding the influence of a liquid-crystalline medium and monomer segregation on polymerization behavior and polymer structure. These studies are of considerable importance for polymer-stabilized ferroelectric liquid crystals1 (FLCs) because of the enormous potential impact on the area. I will briefly introduce liquid crystals (LCs), FLCs, and photopolymerizations. I will then discuss the observed electrooptic properties and how these properties change as the LC phase during polymerization is varied. Finally I will address how polymerization kinetics are affected by the LC phase and monomer segregation. This discussion will include results from x-ray diffraction, polarized infrared spectroscopy, and differential scanning calorimetry experiments. Liquid crystals are a unique state of matter in which microscopic ordering is observed even though the material behaves as a liquid macroscopically. TherMRS BULLETIN/SEPTEMBER 1997
motropic LCs may exhibit many different LC phases between the clearing point (i.e., the temperature at which the material becomes isotropic) and the freezing point (i.e., the point at which the material solidifies and becomes crystalline). Several of these phases are illustrated in Figure 1, which presents the nematic and two smectic phases. The nematic LC phase is characterized by ordering along a director with no layering of the LC molecules, and it is commonly used in many display applications. Smectic LC phases are characterized by a layering of the molecules: Molecules in the smectic A phase have no tilt on average and molecules in the smectic C phase have a certain average tilt with respect to the
cool heat
\xv/ \ /\ Nematic
Isotropic
cool I T heat n,z
heat cool
///I/ Smectic C
V//IX Smectic A
Figure 1. Some thermotropic liquid crystal (LC) phases.
layer normal. Interestingly, the ordering within the LC may extensively influence the orientation of non-LC molecules placed within the LC matrix, which changes the nature and kinetics of the polymerization. Additionally, chiral smectic C LC molecules are commonly referred to as FLCs, which exhibit a macroscopic dipole making them switchable and hence useful for display applications. Two types of displays exist: those using nematic LCs and those using FLCs in the smectic C phase. Nematic displays are currently the most common, and they have the advantage of possessing a high degree of mechanical stability. Unfortunately problems associated with the switching speed of nematic LC displays have limited their application in a number of markets. In contrast, FLCs have extremely fast-switching speeds,2 but they are limited by susceptibility to mechanical shock.3 Current FLC displays require a great deal of external stabilization to maintain alignment,
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