Changes in Elastic and Viscoelastic Properties of Poly(Methyl Methacrylate) by Physical Aging

The influence of physical aging on mechanical properties of glassy polymers was investigated in this paper. After annealing above T-g to release the previous thermal and stress history, the polymethyl methacrylate (PMMA) samples were quenched to 27 degrees C, aged for various times (t(a)), and were then stretched at the same temperature by two ways: (1) step stresses with four different magnitudes varying from 15MPa to 30MPa; (2) constant rate stretch up to fracture. The physical aging effect was monitored by measuring the initial instantaneous elastic modulus (E) and the fracture strength (sigma(f)) from the stress-strain curves as a function of t(a) up to 1368h. It is shown that both E and sigma(f) of the material increase with aging time and approach to their asymptotic values, which satisfy the KWW rule, while the isochronous creep compliance decreases with log t(a) in a linear manner within the aging time range considered in this paper.