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

Transient single relaxation time tau(SS), of glassy polymers was evaluated by using a nonlinear single relaxation model during uniaxial compression processes in which the compression rate changed stepwise from <(epsilon)over dot>(n1), to <(epsilon)over dot>(n2). After the change of compression rate, stress showed an undershoot (<(epsilon)over dot>(n1) > <(epsilon)over dot>(n2)) or an overshoot (<(epsilon)over dot>(n1) < <(epsilon)over dot>(n2)) before reaching the same value as the constant-rate compression at <(epsilon)over dot>(n2). After a decrease of compression rate, tau(SS) increased monotonously to the steady value at <(epsilon)over dot>(n2). When the compression rate increased, tau(SS) showed a discontinuous increase right after the acceleration, then steeply decreased to the steady value at <(epsilon)over dot>(n2). Transient values of tau(SS) after a decrease of compression rate as a function of the nonlinear flow rate <(epsilon)over dot>((eta) over bar) of the dashpot in the model were almost the same function as the steady value of tau(SS), which was almost inversely proportional to <(epsilon)over dot>((eta) over bar). After an increase of compression rate and also in the transient state under the constant rate compression, tau(SS) (<(epsilon)over dot>((eta) over bar)) 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.