Читать книгу The Rheology Handbook - Thomas Mezger - Страница 245

Immediately after the step-like removal of the stress, each one of the molecules is trying to return to a rest position bare of any deformation. Continuously, the molecules are recovering more and more from the previously occurred deformation, reducing it continuously by slow and compensating, creep motions within the superstructure. This is a delayed process which is also called “retarded”. For uncrosslinked polymers, the extent of re-formation may tend to zero if the creep phase was long enough, i. e., if a correspondingly great number of disentanglements occurred in the stress interval. For samples showing a chemical or physical network, the extent of partial re-formation corresponds to the elastic proportion. Fully crosslinked polymers are re-forming completely as shown in Figure 6.5 (no. 2) if the LVE range was not exceeded. Finally they will achieve the same shape as they displayed initially, before the stress was applied (as shown in Figure 6.6, no. 1).

Figure 6.6: Deformation process of a polymer molecule when performing a creep test (Figure from [6.1]); (1) at rest, before applying the stress (2) under stress after a certain period of time

In rheology, a delayed deformation or re-formation process after applying or removing a stress, respectively, is referred to as retardation . As a comparison: The term “relaxation” is used to describe the behavior at rest after applying strain (deformation), e. g. when performing relaxation tests (see Chapter 7).

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