Lyotropic behaviour of liquid crystalline poly(ester amide) containing diamide links

The lyotropic behaviour of a poly(ester amide) (PEA), poly[bis (terephthala te butyramide) hexane] (PBTBH) containing aliphatic diamide links prepared through the amido diol route was studied by isothermal viscosity measuremen ts at different sheer rates and cross polarized light microscopic measureme nts in m-cresol, N-methyl pyrollidone (NMP) and concentrated sulphuric acid . The effects of polarity of solvent, shear rate and polydispersity on the threshold concentration were investigated. The viscosity values of the liqu id crystalline (l.c.) PEA were found to increase with increase in concentra tion, reach a maximum and then decrease sharply. The critical concentration at which the viscosity-concentration curve shows a maximum and the thresho ld concentration where maximum anisotropy was observed did not coincide. Th e threshold concentrations in the viscosity-concentration curve obtained fo r the PEA are 56, 54 and 50 wt%, respectively, in m-cresol, NMP and concent rated sulphuric acid. The present experiments show that the threshold conce ntration depends on the polarity of the solvent, shear rate and polydispers ity of the polymer. A narrow molecular weight distribution gives a sharper peak with a lower threshold concentration. PBTBH exhibited the typical shea r thinning region under low shear rate peculiar to l.c. polymer solutions. The Newtonian plateau and the second shear thinning region under higher she ar rate were also observed. Region I behaviour was confirmed by a number of observations, namely (a) the lack of scaling of transient viscosities duri ng startup with strain and (b) the strong dependence of D-0 on concentratio n. In Region I, the viscosity showed a mild dependence on shear rate. This corresponds to a decrease of texture size with shear rate approximately as D-1/2, as in the Wissbrun model. The formation of the l.c. phase was confir med by polarized light microscopy. Appearance of anisotropic inclusions in an isotropic phase started appearing prior to the occurrence of the critica l concentration. As the concentration of anisotropic phase increased, the w ell known 'phase inversion' took place and isotropic inclusions in anisotro pic phase started appearing as the system approached the threshold concentr ation. A fully anisotropic phase above the threshold concentration gave the typical nematic threaded texture under the polarized light microscope. A p olydomain microstructure was observed in the shear thinning region under lo w shear rate, which tumbled down to a monodomain structure on application o f higher stress. (C) 1999 Elsevier Science Ltd. All rights reserved.