Reversal and activation of physical aging by applied deformations in simple compression and extension

The differential (incremental) storage modulus E′ was measured intermittently at 1 Hz during the stress relaxation of cylindrical specimens of polycarbonate subjected to finite static strains in both simple compression and extension. (In measuring E′, the amplitude of the applied sinusoidal strain was 0.2%.) Application of each static strain gave a value of E′/E′0 less than unity, where E′0 is the storage modulus at 1 Hz of the undeformed specimen. This behavior results from an increase in the mobility of short molecular segments; it signifies a partial erasure of the state of physical aging, a change also termed de‐aging or rejuvenation. After a static strain had been applied, E′ increased continuously, a reflection of physical aging that results from a progressive decrease in segmental mobility. Plots of E′/E′0 at an aging time of 100 s against the absolute value of the static strain show that simple extension de‐ages a specimen somewhat more than does a strain of the same magnitude in compression. This difference results from the increase or decrease in volume (or free volume) effected by strains in extension or compression, respectively. The dominant cause of de‐aging, however, is the deviatoric (shear) component of the strain tensor. The increase or decrease in volume produced by an applied strain modifies only slightly the segmental mobility. Copyright © 1988 John Wiley & Sons, Inc.