Liquid crystalline properties of aromatic copolyesters modified with bisphenol–A
A series of thermotropic aromatic copolyesters was synthetized from 4,4'-dihydroxybenzophenone (DHBF), 4,4'-dihydroxydiphenylpropane (D) and isophthalic acid chloride (IF) at various DHB F/D ratios. It was found by the DSC method that these polymers exhibit liquid crystalline properties at DHBF content above 40 mole %. On increase in D content (D being a modifying component), the phase transition temperatures are lowered. Particularly considerable is the decrease in the isotropization temperature (Tt) what results in a narrowing of the temperature range of mesophase existence. The X-ray structural analysis revealed that copolyesters are characterized by a block structure and that the individual blocks crystallize separately. DSC diagrams of a copolyester containing 20 mole % of D had an additional endothermic peak between the crystal–mesophase transition temperature (Tm) and Ti. Below that endothermic maximum, samples had different morphological structure (large aggregates) than above it, although in both cases it is a fine-grained spherulite-like structure. The remaining copolyesters reveal in the range of mesophase existence a fine-grained structure only, without any defined symmetry of structures in the Hv soatering pattern. A comparison of the results of the present investigations with the previous ones, when the only agent modifying the structure of copolyesters was resorcinol (R), indicates that a larger effect on the changes of mesophase properties had factors disturbing linearity of macromolectiles (R) than those loosening the system (methyl substituents in D).
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