Rheological behavior of thermotropic flexible main‐chain polyesters based on 4,4′‐dioxy‐2,2′‐dimethyl azoxybenzene mesogens. I. Oscillatory shear behavior

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Abstract
The rheological properties of fractionated thermotropic nematic polyesters 4,4′‐dioxy‐2,2′‐dimethyl azoxybenzenenonanediyl (n = 7) (AZA‐9) and dodecanediyl (n = 10) (DDA‐9) magnified image have been determined under oscillatory shear in the nematic and isotropic phase. A markedly more pronounced shear thinning characterizes the polydisperse DDA‐9 polymer as compared to the fractionated polymer in the nematic state. This difference becomes less pronounced in the isotropic phase. In all cases, the elastic compliance J′ in the isotropic state is much lower than in the nematic state. The values of relaxation time λ, although lower in the isotropic state, are not very different in the nematic state. The loss tangent tan δ is significantly higher in the isotropic phase for lower molecular weights. For molecular weights of 13,000 and above, this difference becomes smaller and shifts to lower angular frequencies. For AZA‐9 and DDA‐9, η→0 the limit of the dynamic viscosity can be represented by η* = M with a value of α similar for both phases and approximately equal to 4.