Melting temperatures MCLCP

In the search for MCLCPs one may be inclined to select the monomers that give rise to the most rigid, extended structures. However, the requirement of a completely rigid structure must be mitigated by the requirement of polymers with accessible melting temperatures. The series of p-phenylene oligomers illustrates this point. 

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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