The linear viscoelastic behavior of a series of molecular weights of the thermotropic main-chain liquid crystal polymers HBA/HNA 73/27

In this work the rheological properties of the thermotropic liquid crystalline polymer 1,4 hydroxybenzoic acid (HBA)/2,6 hydroxynaphatoic acid (HNA) 73/27 produced by Ticona have been investigated over the molecular weight range 30000- > 51000. The thermotropic main-chain liquid crystal polymers (TLCPs) show linear viscoelastic behavior at small strain amplitudes, as detected in strain sweep experiments performed at a constant frequency of 1, 5, 10, 30, 50, and 100 rad/s. The onset for nonlinearity (gamma(C)) significantly decreases (from similar to35% to similar to5%) as the applied frequency (omega) increases, independently of the molecular weight. When the results are plotted in terms of G*/G*(0) (the complex modulus normalized to the corresponding linear viscoelastic value) as a function of the stress amplitude, the different curves do not seem to collapse upon each other in the large frequency range investigated in this paper. Indeed, the stress onset for nonlinearity increases as the applied frequency (omega) increases (sigma(C) similar to 700 Pa at omega = 1 rad/s to sigma(C) similar to2000 Pa at omega = 100 rad/s for M,, = 51000), opposite to the trend shown by the strain onset. The behavior of these TLCPs seems, therefore, intermediate between having a characteristic strain and stress level for the occurrence of nonlinearity. The nonlinear onset dependence on the frequency has been, then, analyzed by horizontally shifting the normalized G*(gamma) profiles curves at different omega values and a master curve has been obtained. The frequency behavior of these TLCPs has also been studied showing that the lower molecular weight samples are characterized by a prevalent viscous behavior at high frequencies (G" > G'), while decreasing W the storage modulus tends to level off and dominates the loss modulus. On the other hand, the higher molecular weight samples (M-w = 51000 and M-w > 51000) show a linear viscoelastic behavior that resembles that of conventional flexible polymers with a terminal regime at the lower frequencies and a transition to the rubber-like region at higher w. Our results support, then, the hypothesis suggested in the literature [Romo-Uribe (2001)] of the existence of entanglements in the HBA/HNA 73/27 thermotropic copolyester, whose contribution increase with increasing the molecular weight. (C) 2004 The Society of Rheology.