A2 Creep and Stress Relaxation

Topic

Subjecting this polymer to a sustained load—rather than an instantaneous load that is subsequently removed—causes its molecular chains to reptate and slide past one another (a process described in detail by another model in this set). Consequently, this reptation and sliding produce a time-dependent deformation known as creep compliance; this value continues to increase as long as the sustained load is maintained, rather than stabilizing at a fixed value as would occur in a purely elastic material. Prolonging the duration of the sustained load causes further chain reptation and sliding, leading to a continued increase in creep compliance.

Directly measuring this creep compliance over the decades of service life the polymer might experience in practice is not feasible. This is where the time-temperature superposition principle comes into play—quantified by the WLF equation, which is described in greater detail by another model in this set. This principle allows for the prediction of the polymer's creep behavior over those decades based on much shorter tests conducted at temperatures other than the service temperature, because an increase in temperature and the passage of time have equivalent effects on chain reptation and sliding. Performing these shorter tests at a temperature higher than the service temperature means—according to the time-temperature superposition principle—that the observed creep behavior corresponds to a service duration at the actual operating temperature that is far longer than the actual duration of the test itself. For this reason, this model—characterized by the sustained load and creep compliance with which it was introduced—also describes the complementary case for the same polymer: when a constant deformation is imposed instead of a sustained load, that same reptation and chain sliding result in stress relaxation—as described by the Maxwell model (another model in this set)—whereby the stress required to maintain the constant deformation progressively decays over time. The fact that reptation and chain sliding constitute the underlying physical cause for both creep compliance under sustained load and stress relaxation under constant deformation means that, despite being measured in opposite ways, both phenomena share the same time and temperature dependencies already jointly described by the other two models in this set.

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