Mechanical MCLCP

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). 

Dielectric measurements of mcLCPs are very sparse and were essentially performed as auxiliary to mechanical studies, reflecting principally interest in the mechanical properties of new materials rather than in peculiarities of the molecular dynamics caused by the liquid crystallinity. 

We have to note at this point that despite the observed tendency of the polyesters studied to form the liquid-crystalline glassy state, no effort has been made so far, to study oriented samples of solid mcLCPs. As measurements of Davis and Ward have shown in the past, such studies may provide us with more significant information on different mechanisms governing the a- and j8-processes than can be extracted from measurements on unoriented materials. 

Mechanical properties, particularly tensile strengths and stiffness, depend upon the degree of orientation achieved. This is limited to some extent by the fabrication method and type of article produced, as shown schematically in Fig. 8.15. Thus, a compression-moulded unoriented LCP has mechanical properties similar to that of a conventional isotropic polymer. On injection moulding, tensile bars of MCLCPs generally show superior mechanical moduli to that of conventional glass-fibre-reinforced isotropic thermoplastic (Fig. 8.16),... 

As discussed in Section 8.4.2.3 MCLCPs exhibit a significant fall-off in modulus with temperature and, although this can be influenced by control of the component parts of the MCLCP, this is obviously undesirable in an engineering resin. However, this is partly offset by the exceptionally high stiffness and strength of MCLCPs at room temperature and useful mechanical properties are retained at high temperatures (>200°C). 

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