Analysis of the evolution of microcreep during physical aging and mechanical deformation in poly(methyl methacrylate) using a microstructural model

The evolution of poly(methyl methacrylate) (PMMA) during physical aging at 90°C is followed by torsional microcreep tests. On the aged specimen a longitudinal stress is applied which induces a strain of 5 percent after 30 days of creep. The torsional microcreep tests are performed during the longitudinal creep in order to compare the structure evolution of PMMA caused by straining with its evolution measured during aging. The microcreep, for the first 800s, follows a reversible logarithmic law. In this stage the mobile defects achieve their activated form which is perfectly reversible when unloaded. The physical aging reduces this logarithmic part of microcreep. This is due to the decrease of either the number or the volume of the mobile defects. Beyond a critical elongation ϵ = 1 percent, the longitudinal straining has just the opposite influence, i.e., the logarithmic part of microcreep increases. This critical elongation ϵ = 1 percent corresponds to the beginning of the steady state longitudinal creep. The transient that precedes this steady state has no detectable influence on the structure of the specimen.