Polyesters MCLCP

The transition temperature and transition heat for polyester MCLCP vary with the thermal treatment conditions. By changing the sample mass, heating rate and thermal treatment conditions, satisfactory results from DSC measurement for the rigid-rod MCLCP are usually obtainable. In some cases, self-polycondensation of polyesters proceeds continuously on thermal treatment by DSC. Thus, the relative molecular mass increases and a closer packed structure is obtained, resulting in more evident transition peaks. As the transition peaks on the DSC curve for the rigid-rod MCLCP are relatively small, careful inspection is necessary for their identification. When the sample is annealed at a temperature slightly below the isotropic transition temperature, more distinct peaks are frequently obtained. 

Preparation of Main Chain Liquid Crystalline Polymers (MCLCPs) Table 1. Representation of some typical structures of polyesters. 

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. 

Low-molecular-weight liquid-crystalline compounds have been known for about 100 years. However, main chain liquid crystal polymers (MCLCPs) have attained prominence only in the last 15 years. In 1956 Flory predicted lyotropic behaviour and this theoretical prediction was well demonstrated in the synthetic polymer area with the discovery by Kwolek of the aramids, e.g. poly(/ -phenyleneterephthamide). This led to interest in thermotropic main chain LCPs and, although aromatic polyesters which are thermotropic were described during this period in patents issued to and Carborundum Co., their liquid-crystalline... 

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. 

Fig. 8.13. DMA of a MCLCP containing kinks compared to an isotropic aromatic polyester. (-----) normalised DMA stiffness HBA.36 IA.32 HQ.32 (— —)...

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