Liquid Crystal Network Gels

For simplicity, we restrict our consideration to nematic polymers of prolate shape. This includes (i) main chain nematic polymers and (ii) side chain nematic polymers, but in such a way that they preferentially align with the backbone, thus giving the features of (i). This has been called the Nni phase. The other prolate possibility in the Nn phase is that the backbone takes a prolate shape while the side chains are forced to have oblate symmetry. We shall only discuss the nematic polymers of (i), but the method and the conclusions are quite general. 

There are three combinations of the two components of nematic network gels (1) an isotropic network swollen by a nematic solvent, reminiscent of the polymer dispersed liquid crystal systems (PDLC). This case (1) was discussed by Brochard (1979) and Ballauff (1991) (2) a nematic network swollen by an isotropic solvent was actually studied experimentally by Carudo et al. (1992) and theoretically by Warner and Wang (1992a) (3) both components can order at a temperature above the glass transition. Actually the first two systems are special cases of the last one which has been experimentally investigated (Zentel, 1986 Barnes et al., 1989 Kishi et al., 1994) and theoretically studied (Wang Warner, 1997). 

We now start the case (2) to focus on the new effects of nematic gels, and then discuss the general case (3). 

---

Refraction of light Polymer dispersed liquid crystals (PDLC), gel and polymer network displays... 

Another consequence of the addition of fatty alcohols to cationic surfactants is the formation, under the right conditions, of liquid crystal and gel networks [41-45] that can greatly increase viscosity and confer stability upon the emulsion. Formation of such liquid crystals has been observed even at low concentrations [44,45] the ready formation of these structures, along with low cost, improved stability, and compatibility with cosmetic ingredients are important reasons why long-chain alcohols are so ubiquitous in conditioning formulations. 

For conditioners, as discussed previously in Section II.B.l, mixtures of fatty alcohols and quaternary ammonium compounds form liquid crystals and gel networks that can greatly increase viscosity. 

Similarly, when an aerogel is impregnated with a nematic liquid crystal, the gel network randonuiess destroys the crystal long-range order and transforms the liquid crystal into a glassy state [179]. 

Usually the surfactant concentration in ointments and creams is significantly lower than in surfactant gels. Ointments are non-aqueous preparations, whereas creams result from ointments by adding water. The microstructure of both ointments and creams may consist of liquid crystals, as long as a liquid crystalline network or matrix is formed by amphiphilic molecules. In a liquid crystalline matrix, it is easier to deform the system by shear such formulations show plastic and thixotropic flow behavior on shear. In comparison to systems with a crystalline matrix which are usually destroyed irreversibly by shear, those with a liquid crystalline matrix exhibit a short regeneration time of... 

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. 

The liquid crystalline polymer has since developed far beyond imagination that a decade ago. The liquid crystalline polymer family has so far included the main chain-, side chain-, and crosslinked- (i.e. network or elastomer) types, and their solutions and gels. The liquid crystal phases cover nematic, cholesteric and smectics. Although the science of the liquid crystalline polymer is not fully mature, it has attracted significant research interests and has already made tremendous progress. As investments and human resources continue, the liquid crystalline polymer is expected to have an even brighter future. 

Xie P, Zhang R. 2005. Liquid crystal elastomers, networks and gels advanced smart materials. J Mater Chem 15 2529 2550. 

Amphoterics can also be used to stabilize emulsions by immobilizing the oil droplets in a network to retard or prevent coalescences (109). Chun (110), in a conference paper presented in 1978, proposed using an amphoteric surfactant (see Figure 15.29) in combination with a fatty amphophile, such as cetyl alcohol, to form a gel network (similar to that of liquid crystal formation) to further improve emulsion stability. A number of commercial skin preparations have used this technique to stabilize treatment lotions. 

Anisotropic PLC gels are blends of a PLC network and a miscible MLC, and the first reports appeared in the early 1990s [138-140]. Systems of this kind are also referred to as liquid crystal-liquid crystalline network composite systems. 

---