Dynamic Mechanical Spectroscopy Analysis of Yielding in Poly (methyl methacrylate)

Abstract

Yielding of glassy poly(methyl methacrylate) (PMMA) was studied by means of dynamic mechanical spectroscopy (DMS). Specimens prepared by either slow cooling or quenching through the glass transition temperature Tg were uniaxially stretched up to various amounts of strain. For these stretched and also unstretched specimens, temperature dispersion curves of the dynamic viscoelasticity were measured by DMS. For slowly cooled specimens, the loss modulus E" at temperatures between β- and α-relaxations of the polymer increased progressively with increasing imposed strain. The strain-induced higher values of E" showed a frequency dependence similar to that of quenched specimens without stretching history. These results indicate that the strain-induced increase of E" reflects an extent of strain-induced structural change. Strain-induced increments of E" of quenched specimens were much smaller than those of slowly cooled specimens. This is probably because the quenched specimens are in highly non-equilibrium by nature. Yield point on stress-strain curves was observed more definitely for slowly cooled specimens compared to that of quenched ones. Thus, it is concluded that yielding in glassy polymers is a phenomenon caused by strain-induced structural change.