Transient shear behavior of a thermotropic liquid crystalline polymer in the nematic state Available to Purchase

Transient shear experiments show that a thermotropic liquid crystalline polymer (TLCP) in its nematic state behaves linear viscoelastically as long as the shear does not exceed a characteristic stress of 2×10⁴ Pa or a characteristic strain of approximately γ=2, whichever is reached first. The relaxation modes, as determined in dynamic mechanical experiments, are sufficient for describing the linear viscoelastic behavior in step strain and start‐up at constant rate. Beyond this linear region, the behavior deviates drastically from that of conventional homopolymers: Negative first normal stress differences are measured during steady shear at rates below 0.5 s⁻¹; long relaxation modes are more affected in a large step strain experiment than short relaxation modes [thus no factorization of G(t,γ) into separate time and strain functions]; no zero shear viscosity is detected in the accessible range; the normal stress relaxes faster than the shear stress; in a start‐up experiment at constant shear rate, the shear stress and the first normal stress difference both go through a maximum at approximately γ=2. The material, a random copolyester of 73 mol % 4‐hydroxybenzoic acid and 27 mol % 6‐hydroxy‐2‐naphthoic acid (Vectra A900 of Hoechst–Celanese), was shown to be stable at the experimental temperature of 290 °C for at least 1 h if first preheated to 320 °C.