Polymer network stabilized liquid

Polymer Network Stabilized Liquid Crystal Displays. The alignment of liquid crystals plays an important role in the operation of a display. The alignment is conventionally induced by the display cell surfaces, but by distributing the surface of a polymer network through out the bulk of the liquid crystal, new properties are possible, and the performance of conventional devices can be improved. This short section will mention some of the conventional liquid crystal display devices modified by these polymer networks. 

Polymer Network Stabilized Liquid Crystal Phase... 

Electro-optic materials can be made using liquid crystal polymer combinations. In these applications, termed polymer-stabilized liquid crystals [83,86], the hquid crystal is not removed after polymerization of the monomer and the resulting polymer network stabilizes the liquid crystal orientation. 

Good physical stability can, however, be obtained by developing a viscoelastic network in the continuous phase. The elastic component acts as a net that prevents the droplets from settling or creaming. Viscoelastic networks can be obtained with high-molecular-weight water-soluble polymers or lyotropic liquid crystals. 

Polymerization of reactive monomeric liquid crystals is one method for stabilizing the liquid-crystalline thin films. Another approach is to form chemical gels of liquid crystal molecules with low molecular weight by construction of a polymer network. This method has been investigated for the stabilization of ferroelectric liquid crystal displays. Guymon et al. reported that a polymer network produced by photochemical cross-linking accumu-... 

Nematic gels are very interesting systems, thus deserving further study. Actually, these systems are being studied experimentally for applications. Examples are polymer dispersed liquid crystal displays are sometimes dispersed not in a polymer, but in a polymer network. Displays by means of the polymer stabilized cholesteric texture change, are also achieved in crosslinked systems. In addition, the chiral smectic phase has been obtained in such systems as well. Other types of liquid crystal gels have been applied or are expected to be applied in such devices. 

In the past few decades the technological possibilities and interests have boosted research in systems in highly restricted geometries in almost every field of physics — recently down to lengthscales close to or even below the molecular level. In the field of liquid crystals, the importance of electro-optical applications which incorporate ordered liquid materials [1-3] has focused the research on LC systems with high surface-to-volume ratio [4]. In order to provide mechanically stable applications, liquid crystals are dispersed in polymers, stabilized by a polymer network, fill the cavities in porous materials, etc. [5,6]. The major technological interest concerns the scattering, reflective and bistable displays, optical switches, and others. 

A modification of the oriented polymer network systems are polymer stabilized liquid crystals (PSLC) 4) being studied in detail because of their application in flat-panel displays. In these materials, photopolymerizable diacrylate monomers are usually dissolved at a concentration less than 10% in non-reactive low-molar-mass liquid crystal solvents, commonly available, along with a small concentration of photoinitiator. Typically, the addition of small amounts of monomers and photoinitiator reduces the transition temperatures of the pure low-molar-mass liquid crystals slightly, suggesting that the order in the system is not dramatically altered by the addition of monomers or initiator. In application, this solution is aligned in a particular desired state and then photopolymerized. Photopolymerization is preferred to thermal free-radical polymerization, because photopolymerizations are very fast and because the temperature of photopolymerization can be controlled more easily to optimize processing of the display. 

In all of the devices mentioned above, the polymer network is used to influence the alignment of the liquid crystal, sometimes to stabilize order, other times to introduce disorder. Different types of network structure are desired in each case. Since these polymer networks seem to advantageously alter the electro-optic properties of many types of liquid crystal displays, it is imperative to understand the structure of these polymer networks for modifying, ftirther improving, and even developing entirely new types of liquid crystal displays. 

Figure 9 Schematic representation of the electro-optic properties of a reverse mode polymer stabilized liquid crystal as a function of the morphology of the polymer network.

Electro-Optic Properties of Polymer Stabilized Liquid Crystals. Polymer networks have been used to stabilize many of the liquid crystal display states in various types of displays quite advantageously. In this section, we present some recent work on correlating the material properties of the liquid crystal/polymer network composite to the electro-optic properties of the flat-panel displays specifically cholesteric texture displays (75) and simple nematic birefringent type displays (7(5). 

The direct correlation between the morphology of a polymer network and the observed electro-optic properties of these reverse-mode PSLC s is noteworthy. Desirable electro-optic response fi om these polymer- stabilized liquid crystals can be obtained by manipulating the structure of the polymer network inside the cells. 

Polymer stabilized liquid crystals are formed when a small amount of monomer is dissolved in the liquid crystal solvent and photopolymerized in the liquid crystal phase. The resultant polymer network exhibits order, bearing an imprint of the LC template. After photopolymerization, these networks in turn can be used to align the liquid crystals. This aligning effect is a pseudo-bulk effect which is sometimes more effective than conventional surface alignment. Several characterization techniques... 

---