Yielding of quenched and annealed polymethyl methacrylate

The strain rate of polymethyl methacrylate (PMMA) in yielding under a constant load varies by a factor of approximately 50 with the thermal history. PMMA cooled rapidly from above to below the glass transition temperature yields at a high rate; isothermal innealing of the quenched PMMA below Tg reduces the yield rate. The rate of gain of flow resistance is maximal near 95°C annealing temperature (Tg = 110° C). We expect straining, like quenching, to perturb the glass structure. Accordingly the acceleration of the strain rate with increasing strain (strain-softening) observed in yielding under a constant load is attributable to the perturbation of the annealed glass by strain. Quenching and small plastic strains as well reduce the density of PMMA by 10−4 to 10−3 g/cm3.