Comparison of some mechanical and flow properties of linear and tetrachain branched “monodisperse” polystyrenes

Abstract

AbstractThe strength and flow properties of narrow distribution polystyrenes have been studied as functions of molecular weight and branching (tetrachain star‐type molecules). Most of the mechanical properties of linear and branched narrow distribution polystyrenes were essentially the same as those for a broad distribution general‐purpose polystyrene. For the narrow distribution linear polymers the value of a given property increased rapidly with molecular weight until ca. M = 1.6 × 105. Thereafter the magnitude of the property increased only slightly for rather large increases in M and asymptotically approached a maximum. The same was true of the branched polymers except that certain properties, e.g., tensile strength, were found to level off in value at higher molecular weights than for the corresponding linear polymers. Studies of apparent melt viscosity, η, made with a capillary rheometer showed that at low shear rates (\documentclass{article}\pagestyle{empty}\begin{document}$ (\dot \gamma < 10) $\end{document} sec.−1), the branched polymers exhibited much lower apparent values of η than linear polymers of the same total molecular weight. At high shear rates (\documentclass{article}\pagestyle{empty}\begin{document}$ (\dot \gamma \sim 2000) $\end{document} sec.−1), this difference essentially disappeared and η approached a constant value independent of \documentclass{article}\pagestyle{empty}\begin{document}$ \dot M(1.5 \times 10^5 - 4.5 \times 10^5) $\end{document} and branching. Melt viscosity data for both the linear and branched narrow distribution polymers when plotted in the form of reduced viscosities versus a function of \documentclass{article}\pagestyle{empty}\begin{document}$ \dot \gamma $\end{document} (as suggested by Bueche) fell upon one master curve.