Effect of shear history on the steady shear flow behavior of a thermotropic liquid‐crystalline polymer

Abstract

AbstractThe steady shear stress (σ) and first normal stress difference (N1) of a thermotropic liquid‐crystalline polyester, poly[(phenylsulfonyl)‐p‐phenylene 1,10‐decamethylene‐bis(4‐oxybenzoate)] (PSHQ10), in both the isotropic and nematic regions were measured as a function of shear rate (γ), using a cone‐and‐plate rheometer. For the study, PSHQ10 was synthesized via solution polymerization in our laboratory. The PSHQ10 was found to have (a) the weight‐average molecular weight of 45,000 relative to polystyrene standards and a polydispersity index of 2, (b) a glass transition temperature of 88°C, (c) a melting point of 115°C, and (d) a nematic‐to‐isotropic transition temperature of 175°C. For the measurements of σ and N1 in the nematic region of PSHQ10, its initial conditions for the startup of shear flow was controlled by (a) first heating an as‐cast specimen to 190°C, (b) shearing there at γ = 0.085 s−1 for about 5 min, and then (c) cooling slowly down to a predetermined temperature (130, 140, 150, 160, or 171°C) in the nematic region. For each γ chosen, after start‐up of shear flow, we waited for a sufficiently long time until both the shear stress and first normal stress difference leveled off, giving rise to steady‐state values of σ and N1. Emphasis was placed on investigating the effect of shear history on σ and N1 of PSHQ10 in the nematic region. For this, the following experiments were conducted: (a) a fresh specimen was sheared continuously by increasing the γ stepwise, and (b) a presheared specimen was further sheared continuously by increasing the γ stepwise. We have found that fresh specimens exhibited ‘shear‐thinning’ behavior over the entire range of γ (0.008−0.27 s−1) tested, whereas the presheared specimens exhibited both zero‐shear viscosities and shear‐thinning behavior. When using fresh specimens, we found that N1 was positive over the entire range of γ (0.008–0.27 s−1) tested. However, when using presheared specimens we found that (a) at very low γ, N1 initially was negative and then became positive as shearing continued, and (b) at higher γ, N1 was positive over the entire duration of shearing. © 1994 John Wiley & Sons, Inc.