Networks liquid crystalline gels

In this chapter, we first describe the stmcture of networks, followed by the discussion of the simple classical models of elasticity and the more advanced theories such as the constraint and the tube models. We also give the molecular interpretation of coefficients obtained from the phenomenological theories. Some simulations relevant to mbberlike elasticity are then described, followed by a discussion of responsive gels because of their increasing interest to many groups. We then discuss the thermoelastic (force-temperature) behavior of networks, followed by the information on multimodal networks, liquid-crystalline (LG) elastomers, novel reinforcing fillers, and characterization methods. 

There have been a lot of studies of cholesteric films and gels in order to exploit their potential as specific optical media and as other functional materials. Most of the preparations were achieved by modification or improvement of previous attempts to immobilize the cholesteric structure of cellulose derivatives into the bulky networks either by crosslinking of cellulosic molecules with functional side-chains in the liquid-crystalline state [203], or by polymerization of monomers as lyotropic solvents for cellulose derivatives [204-206],... 

Before and after the works described above, contributions to the design and fabrication of similar multicomponent films or gels of cholesteric character, mainly based on HPC, EC, or their derivatives were also made [202, 219-224], Some of these [219,220,224] dealt with shear-deformed network systems preserving a unique banded structure, so that the disappearance and recovery of the optical anisotropy could be controlled thermo-reversibly. Special mention should be made of the successful preparation of two novel classes of solid materials maintaining cholesteric liquid-crystalline order. One consists of essentially pure cellulose only, and the other is a ceramic silica with an imprint of cellulosic chiral mesomorphy. 

Zhao, Y., Chenard, Y., and Paiement, N. Liquid crystalline anisotropic gels based tin azobenzene-containing networks. Macromolecules 33, 1049 (2000). 

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-... 

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. 

First, the lyotropic phase is used as a template for the preparation of a bicontinuous silica structure, from which the polymer is removed by calcination or extraction. In the second step the porous inorganic structure is filled with monomer and crosslinker which is polymerized to form a bicontinuous organic polymer network from which the silica template is removed by treatment with hydrofluoric acid. An example for the preparation of hierarchical structures is the synthesis of bicontinuous pore structures by using two templates simultaneously [115]. In this case a liquid crystalline lyotropic phase of an amphiphilic block copolymer is used as a template together with suspended latex particles. The sol-gel process with subsequent calcination leads to a bicontinuous open pore structure with pores of 300 nm and 3 nm. 

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. 

The main objective of this chapter is to review and outline the research studies and perspectives on liquid crystalline elastomers and LC anisotropic networks, with emphasis on recent interesting innovations on network-stabilized ferroelectric LC (FLC) gels, discotic columnar networks, and self-assembly hydrogen-bonded LC network. We will also present hybrid networks based on ladderlike polysiloxanes that have been developed in our group as advanced functional film materials. 

---