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Transient single relaxation time τSS of glassy polymers was evaluated by using a nonlinear single relaxation model during uniaxial compression processes in which the compression rate changed stepwise from ε̇n1 to ε̇n2. After the change of compression rate, stress showed an undershoot (ε̇n1 > ε̇n2) or an overshoot (ε̇n1 < ε̇n2) before reaching the same value as the constant-rate compression at ε̇n2. After a decrease of compression rate, τSS increased monotonously to the steady value at ε̇n2. When the compression rate increased, τSS showed a discontinuous increase right after the acceleration, then steeply decreased to the steady value at ε̇n2. Transient values of τSS after a decrease of compression rate as a function of the nonlinear flow rate ε̇η̄ of the dashpot in the model were almost the same function as the steady value of τSS, which was almost inversely proportional to ε̇η̄. After an increase of compression rate and also in the transient state under the constant rate compression, τSS ( ε̇η̄) showed a higher value than those in the steady flow state. This observation indicates that, when glassy structures are getting unstable due to deformation, the nonlinear flow requires larger stress than the steady flow.

Transient Mechanical Relaxation Behavior of Glassy Polymers Subjected to a Stepwise Change of Compression Rate