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Viscoelastic Properties of p-n Alkyl Benzoic Acid Mesogens

S. Sreehari Sastry, A. Bindu Madhavi and Ha Sie Tiong

Dynamic rheological behaviour, such as storage modulus (G') and loss modulus (G'') are measured as a function of frequency at different temperatures in nematic region and also studied linear viscoelastic behaviour of p-n alkyl benzoic acid (nBA) mesogens. Different zones are exhibited by the samples, - a plateau zone at low frequencies, a transition zone at high frequencies and a zone entire plateau showing for crossover frequencies and relaxation times. The dependence of the elastic (G′) and loss (G′′) moduli on frequency is strong. The observed crossover points of G′, G′′ in the study of frequency range is indicating that nematic phase of nBA has behaved like both viscoelastic solid and liquid for small deformations (<1% strain). A typical shear thinning behaviour of viscosity has been displayed when the samples are subjected to shearing at a constant temperature. This typical shear thinning has approached slowly moving to Newtonian viscosity at high shear rates showing progressive development of nematic molecular orientation along flow direction on increasing shear rate. Rheological behaviour of the nBA nematic phases is in general consistent like the other reported compounds except the one thermotropic liquid crystals. The higher shear force and lower viscosity ratio is observed in these compounds in which pentyl benzoic acid has high ratio indicating the high molecular orientation may be due to low chain entanglement.