The modulus and yield stress of glassy poly(methyl methacrylate) at strain rates up to 103 inch/inch/second

Abstract

AbstractSpecimens of poly(methyl methacrylate) (PMMA) were compressed at nominally constant strain rates ranging from 10−5 to 103 in./in./sec. at 0, 22, 50, and 115°C. A plot of stress at a small fixed strain (2%) and constant temperature versus logarithm of strain rate is sigmoidal in shape and, furthermore, time‐temperature superposition could be used to construct a master curve of stress versus temperature‐compensated strain rate. It is suggested that the sigmoidal curve is a manifestation of the β transition in PMMA, and this is supported by the measured value of activation energy and the strain rate value at the point of inflection on the curve. By contrast, yield stress varies linearly with log \documentclass{article}\pagestyle{empty}\begin{document}$ \mathop {\rm \varepsilon }\limits^{\rm.} $\end{document}. Time‐temperature superposition could not be applied. Rationalizing on the basis of the high stress form of the Eyring equation, yielding is by a yet unspecified molecular mechanism in which activation volume has the order of magnitude of a monomer unit but increases with increasing temperature.