Isothermal physical aging of poly(methyl methacrylate): Localization of perturbations in thermomechanical properties

Effects of isothermal physical aging on poly(methyl methacrylate) (PMMA) were investigated for isothermal aging temperatures ( T a ) from T g −13 to T g −128°C using a freely oscillating torsion pendulum (TBA). A single PMMA‐glass fiber specimen, the effects of the thermal history of which could be erased by heating above T g (=116°C, 0.7 Hz), was used for all experiments; this facilitated comparison of the unaged and aged specimen. The modulus was observed to increase linearly with the logarithm of isothermal aging time. Thermomechanical properties of the aged versus unaged specimen showed perturbations (e.g., increased modulus) principally in the vicinity of T a . This suggests that different intermediate portions of the relaxation spectrum are specifically involved in the process of aging for different values of T a .