Rheology of poly(vinyl chloride) melts. II. Shear rate‐dependent properties

Abstract

AbstractFour samples containing 40, 60, 80, and 97 wt‐% of poly(vinyl chloride), the rest being plasticizer and stabilizer, were tested by using the Weissenberg Rheogoniometer in the steady‐shearing mode at temperatures between 155 and 235°C and rates of shear \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma = 0.01 - 400 $\end{document} sec−1. The viscosity η versus \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} follows Graessley's theoretical dependence for infinitely entangled system. The primary normal‐stress difference coefficient ψ versus \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} is well described by the same theoretical function, used with the square of its argument. The temperature dependence of η0 and ψ0 shows discontinuities at T = Tb. The numerical values of Tb can be calculated from the theory of the melting point depression due to diluent. The activation energy of viscous flow Eη below Tb is 5–9 times as large as above this temperature. The activation energy of normal stress is found to be Eψ ≈ 5Eη. The characteristic relaxation times τo, ψp, calculated from superposition of η versus \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} and ψ versus \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} data, respectively, onto Graessley's master curves, and τN, computed from zero shear parameters η0 and ψ0, differ in their sensitivity to the melting of microcrystalline regions. It is postulated that in the systems investigated, aggregates with long lifetimes are being formed, increasing the effective molecular weight and introducing changes in the effective polydispersity.