TRANSIENT AND STEADY RHEOLOGICAL BEHAVIOR OF THE THERMOTROPIC LIQUID-CRYSTAL COPOLYMER 73/27 HBA HNA

Transient and steady rheological measurements of shear and normal stresses are reported for a liquid crystal copolymer of 73 hydroxybenzoic acid/27 hydroxynaphthoic acid. Dynamic time sweeps indicate a sensitiveness of the rheological parameters to thermal history, probably related to polymerization phenomena. In the shear rate range 0.1-1 s-1 at T = 310-degrees-C the normalized stress overshoots vs strain curves nearly superpose. The steady values of the viscosity seem to follow the three region scheme of Onogi and Asada. The steady values of the first normal stress difference are positive in all the shear rate range analyzed. Constant stress measurements are also reported to prove that the low shear rate data are not significantly affected by the polymerization phenomena. Moreover, the constant stress data show that a true yield stress cannot be detected in the LCP studied.