Physical aging of poly(methyl methacrylate) in the nonlinear range: Torque and normal force measurements

Cylindrical specimens of poly(methyl methacrylate) (PMMA) were quenched from above the glass transition and subsequently tested in torsion. Torque and normal‐force relaxation responses were recorded simultaneously. The experiments were performed at 40, 60, and 80°C, at increasing aging times and at deformations ranging from γ = 0.0027 to γ = 0.074. It was found at 40 and 60°C, that, contrary to the classical picture of aging, the small‐deformation torque‐relaxation curves could not be superposed by any combination of vertical and horizontal shifts. On the other hand, at 80°C, and in the nonlinear deformation range even at lower temperatures, the relaxation curves are superimposable simply by horizontal shifts, but as γ increases they involve decreasing values of the double‐logarithmic shift rate. At the same deformation level, however, the shift rate for the normal force is significantly higher than is that for the torque over most of the γ range investigated.