(193f) Effect of Large Deformation on the Physical Age of Polymer Investigated By Multi?Step Nonlinear Creep

When a material is cooled into the glassy state physical aging occurs as the material relaxes towards equilibrium. It has been reported by Lee et al. (Science, 2009) that the characteristic relaxation time decreases by three orders of magnitude under a stress of ~25 MPa at T_g-19°C. An important question is will the material be rejuvenated, aged, or unchanged after removal of the large load as compared to the material that was not subjected to deformation? Three-step creep (i.e. load-unload-reload) experiments were used to discriminate between rejuvenation vs. accelerated aging, where the initial loading was sufficiently large to produce tertiary creep i.e. flow. Specifically, the response upon reloading would be slower than during the initial loading if accelerated aging were occurring vs. faster if rejuvenation is the operative process. Even more intriguing question is if the outcome remains the same when the unloading is only partial i.e. to non-zero stress. Initial results indicate that the picture is complex, where rejuvenation is the mechanism manifested for a DGEBA-DDS epoxy network upon large deformation; however, rejuvenation is switched to accelerated aging during partial unloading. The effects of the applied load, temperature below T_g and the sub-T_g annealing time prior to deformation will be discussed.

KEY WORDS: creep, physical aging, polymer