Isotropization 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. 

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 (— —)...

The rheology of MCLCPs is complex. In general, the shear viscosity of MCLCPs is much lower than that of conventional polymers at a comparable molecular weight, and the transition from the isotropic state to the liquid-crystalline state is generally accompanied by a significant decrease in melt viscosity. At the onset of nematic behaviour, the melt viscosity of the MCLCPs is three decades less in order of magnitude than that of a similar but non-mesogenic polymer. 

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),... 

MCLCPs also exhibit very low mould shrinkage and minimal sinkage and warpage compared to conventional isotropic polymers (Fig. 8.21), ... 

Fig. 8.22. Differences in behaviour between MCLCPs and isotropic polymers and summary of the MCLCP key properties.

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