Yield stress and flow measurements in ABA block copolymer melts

Abstract A parallel‐plate constant‐stress rheometer is used to measure the yield stress τ y , and the post‐yield flow curve T ( \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} ), where τ is shear stress and \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} is shear rate, for microphase‐separated triblock copolymer melts. Five polymer samples, all styrene‐butadiene‐styrene but with differing composition ratios and molecular weights, are tested at 125°C. Specimens are prepared by casting sheets from solutions made with different solvents. The τ( \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} ) is found usually to be sigmoidal, for the range 10 −5 < \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} < 10 −3 s −1 , representing different stages of microstructural degradation in flow. Measurements indicate that a true τ y exists, with values in the range 100 < τ y < 500 Pa for these melts. A general trend is detected for τ y to decrease as the casting solvent solubility parameter increases. A scheme for correlating the dependence of τ y , on composition and molecular weight is proposed for the various polymers. For selected samples, the effect of mechanical history (sequence of stress application) and a temperature variation that crosses T s (110 to 150°C) are also explored.