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This manuscript focuses on our recent work involving photopolymenzations of monomers in a liquid crystalline environment. In particular, this effort concentrates on understanding the influence of a liquid crystalline medium and monomer segregation on polymerization kinetics and polymer structure. These studies are of considerable importance for polymer stabilized ferroelectnc liquid 1 (FLCs), because of the enormous potential impact on the area. Liquid crystals are a unique state of matter in which microscopic ordering is observed despite the fact that the material behaves as a liquid macroscopically. Thermotropic liquid crystals 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). 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 layer normal. Interestingly, the ordering within the LC may extensively influence the orientation of non-LC molecules placed within the LC matrix. It is this effect that changes the polymerization nature and kinetics. Additionally, chiral, smectic C LC molecules are commonly referred to as FLCs, and in the smectic C phase these FLCs exhibit a macroscopic dipole that makes 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 LC display, 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 different markets. In contrast, FLCs have extremely fast switching 2 but they are limited by susceptibility to mechanical 3 Current FLC displays require a great deal of external stabilization to maintain alignment, performance, and prevent damage from mechanical shock. Hence, the incorporation of a polymer network into a FLC material would potentially have a large impact by improving stabilization, especially if the polymer were incorporated in such a way that it did not have a significant, detrimental effect on the FLC electro-optic performance but rather, led to internal stabilization. These improvements could lead to dramatically expanded applications for FLCs.
The concept of a polymer stabilized FLC is to incorporate monomers in relatively small amounts
(typically less than 10 wt.%) and then photopolymerize them to form a polymer network swollen by the FLC, thereby stabilizing the FLC mechanically. This feature quickly leads us to the importance of using photopolymerizations for this application. Photopolymerizations have a number of 4 over
other polymerization techniques including high rates of polymerization, energy efficiency, facile spatial and temporal control of the polymerization and, most importantly for this application, the independence of the initiation rate from temperature. Basically, since the initiation is photochemically driven, the rate of initiation is independent of temperature. This feature is critically important because it allows the polymerization to be performed in any phase of the FLC. In addition to polymer stabilized FLCs, many other polymer/LC 56 have been developed, most of which utilize photopolymerizatiofls in one form or another.
The photopolymerization process and the formation of a crosslinked network during the
photopolymerization are complicated by many features including changing initiation efficiency, a
heterogeneous polymer 78 diffusion controlled propagation and termination reactions, the
attainment of a maximum double bond conversion less than 100%, and the coupling of the kinetics with volume relaxation of the ° These features have a dramatic effect on the structure of the polymer and the polymerization kinetics and have been studied extensively. Interestingly, in the liquid crystalline environment the photopolymenzation is further complicated by the nanoscale ordering of the monomers by the LC. These effects will be addressed in detail throughout the remainder of this manuscript.
