Phase behaviour of lyotropic side chain polymer liquid crystals

4 PHASE BEHAVIOUR OF LYOTROPIC SIDE CHAIN POLYMER LIQUID CRYSTALS 

Only relatively recently have lyotropic polymers been investigated with respect to their liquid crystal properties. In 1982 Finkelmann, Luhmann and Rehage synthesized a series of surfactants having 10-undecenoic acid as the hydrophobic region and a polyethylene glycol moiety as the hydrophilic region. The corresponding polymers were produced by addition of these surfactants to a polysiloxane main chain. The structure of the polymers is shown below  

This represents a major advance over previous work because polymers of well-defined molar mass could be obtained and because the hydrophobic and hydrophilic moieties are well separated. 

In contrast, the phase diagram of the polymeric surfactant shown in Fig. 5.15 displays three well-defined liquid crystal regions, namely lamellar (L j), hexagonal (Hi) and cubic (Vi). The two former phases are optically anisotropic and may therefore be easily identified by their characteristic textures. The narrow region of cubic phase, however, is optically isotropic and was identified by virtue of its high viscosity and shear birefringence. 

It has been suggested that the enlarged mesophase regions displayed by the polymer-water system are a consequence of a stabilization effect brought about by restriction of motion of the polymer side chains. However, in the process of polymerization the alkyl chain of the surfactant is effectively lengthened by one monomer unit of the original polymer 

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