Flexible MCLCP

All Liquid Crystal Polymers are characterised by the fact that they contain stiff meso-genic groups, often inserted in flexible chain systems (so called "spacers") and connected to them by linking functional groups the mesogenic unit is inserted either in the main chain or in the side chains or (in exceptional cases) in both. We shall discuss MCLCPs and SCLCPs. A schematic representation of common structures of LCPs is displayed in Fig. 6.14 (Jansen, 1996). An example of a SCLCP with disc-like mesogens is displayed in Fig. 6.15 (Franse et al., 2002, 2004). 

The degree of flexibility and the struetural eomposition both determine the mesomorphie properties of MCLCPs. 

From the point of view of molecular architecture, as schematically shown in Figure 9.3, two types of LCP have been developed (1) main-chain LCPs (MCLCPs), having the monomeric liquid crystals (i.e., mesogenic group) in the main chain of flexible links, and (2) side-chain LCPs (SCLCPs), having the monomeric liquid crystals attached, as a pendent side chain, to the main chain. 

LCPs combine the mechanical properties of polymers with the order of LCs. The low molar mass mesogenic units, which form a LC phase, consist of a rigid core. If this core is extended, so-called main chain liquid crystalline polymers (MCLCPs) are obtained. A second method to obtain LCPs is to connect low molecular weight LC (LMWLC) units via flexible spacers. This approach allows the preparation of side chain liquid crystalline polymers (SCLCPs) (Box 1). 

There are different polymer structures that exhibit LC phases. The extension of the mesogen s rigid aromatic core leads to main chain polper systems. Other approaches are based on the spacer concept. Here rigid cores that are common in LMWLCs are decoupled by flexible alkyl chains. The mesogenic units can be placed either in the main or in the side chain. Also combined MCLCP/SCLCPs have been prepared (figure 3). 

Because of the structural similarity between semi-rigid thermotropic polymers and LCPs, i.e. the presence of rigid moieties separated by flexible spacers, all polymers of this kind are commonly referred to as the main chain liquid crystal polymers (mcLCP). 

Completely rigid rod-like molecules such as poly(4-oxybenzoyl) or poly( p-phenylene terephthalate) tend to be highly crystalline and intractable, with melting points above the decomposition temperature of the polymers (>450°C). The problem of thermotropic MCLCP design is to disrupt the regularity of the intractable para-linked aromatic polymers to the point at which mesomorphic behaviour is manifested below the decomposition temperature and the materials can be processed in fluid yet ordered states. The disruption must not, however, be taken to the stage where conventional isotropic fluid behaviour is preferred. These requirements that the polymer must retain some rod-like nature but at the same time be melt-processable below 400-450°C have limited thermotropic MCLCPs mainly to polymers based on the linear ester or ester/amide bonds. With polyester/ polyesteramides, disruption is normally achieved by the th ee copolymerization techniques outlined in Fig. 8.1, i.e. frustrated chain packing, flexible spacers and non-linear links. 

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