Overaging under stress in polymer glasses? Larger yield stress despite faster segmental dynamics!

Understanding the relationship between physical aging, structural relaxation time and yield stress in polymer glasses is crucial for modeling and long-term prediction of mechanical properties of these materials. If two identical polymer glasses with the same thermal history are aged under the same conditions, except in one case the polymer glass is held under stress and in the other is aged without stress, the stress-aged sample can develop a larger yield stress than the quiescently-aged sample.  The increased yield stress of the stress-aged sample has been interpreted as accelerated aging or “overaging”. A similar feature was initially reported for Nylon 6,10 by Kramer. However, it is important to test whether the yield stress properly indicates the age of a polymer glass.  If so, then the segmental relaxation time of an overaged glass should be longer than the corresponding quiescently-aged sample. Direct measurements of structural relaxation time using a probe reorientation technique enable us to address this issue. Here using PMMA glasses we examine a case where the stress-aged sample, despite having the larger yield stress value, has a shorter structural relaxation time in comparison to the quiescently-aged sample.