The viscoelasticity of thermotropic liquid crystalline polymers: effects of the chemical composition

In this work, the rheological properties of the wholly aromatic random copolyester HBA/HNA (60/40), the commercial copolyesteramide Vectra B950, and the semiflexible commercial copolyester Rodrun 3000, PET/HBA 40/60, were investigated. All the thermotropic liquid crystalline polymers (TLCPs) show linear viscoelastic behavior at small strain amplitudes. The strain sweep experiments have clearly indicated that the onset for non linearity significantly decreases as the applied frequency increases in all the TLCPs here studied, independent of chemical composition and flexibility. In the case of the Vectra B950, a strain-hardening effect occurred at a strain of about 40% for all the applied frequencies. The frequency behavior showed that the HBA/HNA 60/40 TLCP is characterized by the leveling off of G′ and by a dominant elastic behavior at low frequencies. The inclusion of the flexible PET determines the disappearance of the plateau value for G′ with a viscous response at low frequencies.Paper presented at the Annual Meeting of the European Society of Rheology, Grenoble, April 2005.