Yield stress and flow measurements in ABA block copolymer melts

Abstract

AbstractA parallel‐plate constant‐stress rheometer is used to measure the yield stress τy, 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 τy exists, with values in the range 100 < τy < 500 Pa for these melts. A general trend is detected for τy to decrease as the casting solvent solubility parameter increases. A scheme for correlating the dependence of τy, 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 Ts (110 to 150°C) are also explored.