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

(Received 21 June 2004; final revision received 22 July 2004) Synopsis In this work the rheological properties of the thermotropic liquid crystalline polymer 1,4 hydroxybenzoic acid(HBA)/ 2,6hydroxynaphatoic acid (HNA) 73/27 produced by Ticona have been investigated over the molecular weight range 30 000-> 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 (critical strain) significantly decreases (from about 35% to about 5%) as the applied frequency increases, independently of the molecular weight. When the results are plotted in terms of G*/G0* (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 increases (the critical stress is about 700 Pa at the frequency of 1 rad/s, the critical stress is about 2000 Pa at the frequency of 100 rad/s for Mw = 51 000), 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* profiles curves at different frequencies 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 the frequency the storage modulus tends to level off and dominates the loss modulus. On the other hand, the higher molecular weight samples (Mw = 51 000 and Mw > 51 000) 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 frequencies. 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. © 2004 The Society of Rheology. (DOI: 10.1122/1.1795194)