A Thermotropic Main-Chain Random Copolyester Containing Flexible Spacers of Differing Lengths. 2. Rheological Behavior

Abstract

The rheological behavior of a thermotropic main-chain random copolyester containing flexible spacers of 6 and 12 methylene groups (PSHQ6-12) was investigated. This copolyester undergoes only two thermal transitions: (i) glass transition at ca. 92 °C and (ii) nematic-isotropic transition at ca. 192 °C. Such a very broad range of temperatures between the glass transition temperature and the nematic-isotropic transition temperature allowed us to take rheological measurements in the transient/steady-state shear flow mode or in the oscillatory shear flow mode over a wide range of temperatures in the nematic region. The initial conditions (i.e., the initial morphology) for transient shear flow were controlled by first heating an as-cast specimen to the isotropic region, shearing there at a rate of 0.1 s -1 for ca. 10 min, and then cooling slowly down to a preset temperature (140, 150, 160, 170, or 180 °C) in the nematic region. A fresh specimen was used for each transient shear flow experiment, and the shear stress growth (σ + (t,γ)) and first normal stress difference growth (N 1 + (t,γ)) were recorded as functions of time (t) for various shear rates (γ) and temperatures. In the nematic region, PSHQ6-12 exhibited a single overshoot in σ + (t,γ), but multiple overshoots in N 1 + (t,γ) with its peak value greater than that of σ + (t,γ). In the isotropic region, however, PSHQ6-12 exhibited Newtonian behavior with negligible overshoot in both σ + (t,γ) and N 1 + (t,γ). Only positive values of the steady-state first normal stress difference (N 1 ) were observed in PSHQ6-12 for all temperatures and shear rates investigated. Also conducted were intermittent shear flow experiments after cessation of start-up shear flow, showing evidence of structural recovery in PSHQ6-12 as determined by the overshoot in N 1 + (t,γ). The steady-state shear viscosity of PSHQ6-12 exhibited three regions at 140 and 150 °C: (i) a shear-thinning region at very low γ, (ii) a Newtonian region at an intermediate range of γ, and (iii) another shear-thinning region at higher γ. But PSHQ6-12 exhibited a very mild shear-thinning behavior as the temperature was increased to 180 °C, approaching the isotropic region. Logarithmic plots of N 1 vs shear stress (σ) for PSHQ6-12 were found to vary with temperature in the nematic region but to be independent of temperature in the isotropic region. A similar temperature dependence was also observed in logarithmic plots of the dynamic storage modulus (G' ) vs dynamic loss modulus (G ). We conclude that plots of log N 1 vs log σ or plots of log G' vs log G may be used to determine the isotropization temperature of thermotropic liquid-crystalline polymers.