Physical aging time scales and rates for poly(vinyl acetate) stimulated mechanically in the T-g-region

The physical aging behavior of poly(vinyl acetate), PVAc, in the T-g-region has been investigated using dynamic mechanical analysis [DMA). Tests in shear and flexure modes were performed for samples quenched from a stabilization temperature of 40 degrees C to aging temperatures 35.0 degrees C and 32.5 degrees C (down-jumps], and from equilibrium at a stabilization temperature of 32.5 degrees C to aging temperature 36.2 degrees C (up-jump). Volume recovery was investigated for the same thermal histories using mercury-in-glass dilatometry. The time scales for the evolution of the storage and loss moduli during physical aging have been compared with the volumetric time scales. Aging rates, mu, have been calculated in the frequency and time domains. The evolution of dynamic mechanical properties during aging is found to be different from the volumetric behavior. In the case of the temperature down-jump, the evolution of the volume ceases earlier than that for the storage and loss moduli. In the up-jump experiment, the opposite is found. Aging rates for the storage and loss moduli are analyzed. A suggestion is offered that the evolution of dynamic mechanical properties during physical aging is retarded by the cyclic mechanical stimulus, and that this effect (rejuvenation) is more noticeable the higher is the damping; consequently rejuvenation in the T-g-region can be caused by small strains in dynamic mechanical analysis.