Introduction

In recent years considerable attention has been given to liquid crystal/polymer mixtures. The polymer may be added to increase the inherent mechanical strength and/or enhance certain electro-optic properties. Two different types of these liquid crystal mixtures have been designated as polymer dispersed liquid crystals (PDLCs) and polymer stabilized liquid crystals (PSLCs). PDLCs are characterized by a large percentage of polymer or polymer network (usually >10%) which disperses the liquid crystals into droplets. The properties of these systems are governed largely by surface interactions between the polymer and liquid crystal and behave quite differently than the neat liquid crystal. PSLCs, on the other hand, have a relatively small amount of polymer (<5%) and are primarily used for stabilization. In these systems the electro-optic behavior of the LC is not influenced significantly, whereas the mechanical strength is increased dramatically. Most PSLC and PDLC systems studied to date involve nematic or cholesteric liquid crystals and despite their versatility and potential applications, their usefulness is limited by the relatively slow response times of these liquid crystals.

Ferroelectric liquid crystals are likely candidates to improve upon existing technology, as they may exhibit response times up to 500,000 times faster than systems currently used in high contrast computer displays. They have found limited applicability, however, due to their extreme susceptibility to mechanical shock. To combine the unique properties of PDLC and PSLC systems with those of FLCs, both polymer stabilized FLCs and polymer dispersed FLCs have been developed. These systems demonstrate enhanced resistance to mechanical shock relative to typical FLC displays and show considerable potential for a wide array of applications.

PSFLC Experiments in our Laboratory

Our group has focused on adding a small amount of various diacrylate monomers to an FLC mixture. This monomer is then polymerized in situ forming a polymer stabilized ferroelectric liquid crystal (PSFLC) or FLC gel. The goals of our work include the development of a polymer stabilized FLC system that is resistant to substantial mechanical shock, but still functions and behaves similarly to a simple ferroelectric liquid crystal display. We also want to understand the interactions present in such a system and therefore be able to optimize the electro-optic and polymer properties.

To do this, we are taking a two-pronged approach. The first involves examining the effect of the monomer and polymer network on the behavior of the ferroelectric liquid crystals. The second entails exploring the effects of a ferroelectric liquid crystal environment on the polymerization behavior of cross-linking monomers.

Figure 1. Optical Micrographs (X 200) of 2% HMDA in W82,W7 at 56 degrees C (a) before polymerization, (b) fifteen seconds into polymerization, and (c) after polymerization. The phase transitions of the FLC are depressed upon addition of monomer. During polymerization the transitions return to those observed in the neat FLC.

To analyze the effects of the monomer and polymer network on the FLC, we have looked at a variety of properties including the phase behavior and electro-optic characteristics before and after polymerization. Both polarizing microscopy and electrical response have proven invaluable to help us elucidate such changes induced in the FLC. As observed by polarizing microscopy and differential scanning calorimetry, the phase behavior changes significantly upon addition of monomer, but the polymer seems to have little effect (see Figure 1). On the other hand, the electro-optic characteristics, such as ferroelectric polarization and optical response time, may change significantly depending on the concentration and nature of the monomer or polymer.

The inherent ordering of a ferroelectric liquid crystal provides a fascinating medium in which we can also study polymerization characteristics. Our group has devoted considerable energy to understanding the changes induced by polymerizing diacrylate monomers to form PDFLCs. We have monitored a variety of different polymerizations in FLC media using differential scanning calorimetry and have seen some fascinating results. Polymerization rate, double bond conversion, and kinetic constants change dramatically when polymerization occurs in an FLC (see Figure 2). These changes in behavior have been studied as functions of concentration, FLC and monomer type, and the LC phase in which polymerization occurs.


Figure 2. Polymerization rate as a function of time for 2% HMDA in ethylene glycol diacetate and in W82,W7 at various polymerization temperatures.

Our research has already helped us tremendously in understanding the scope and potential of PSFLCs. Many systems have been found that exhibit dramatically increased mechanical strength, without sacrificing the desirable electro-optic properties of the FLC. In addition, the polymerization behavior during the formation of these same systems is significantly changed when compared with bulk and solution polymerizations. Through these studies and continuing work we hope to expand the understanding of the unique and promising properties of PSFLCs.

For more information on this project, contact Christopher Bowman.

 

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