Mechanics Aspects of Glassy Polymers

In view of the presence of the free volume Vf, the macro-molecular polymeric chains still retain a certain degree of freedom of motion that allows for some configurational changes under loads. These changes may be viewed as occurring against viscous resistance and are therefore time dependent. 

Consequently, the strain response to the application of, say, a unit step load is related by a time dependent creep compliance D(t). Inversely, the stress associated with the application of unit strain is given by the relaxation modulus E(t). These are shown schematically in Fig. 2.3. 

When excited by thermal agitation, both creep and recovery quicken with temperature. In many cases, thermal effects are focused on time alone and can be accounted for by means of a temperature dependent shift factor a iT), thereby D(t) D( ), where = (T) = tlajiT) and analogously for E(t). This states that the time-dependent response at all temperatures is essentially the same. 

Viscoelastic response exhibits reversal effects that differ qualitatively from elastic behavior. Consider, for instance, the following cases (Fig. 2.4). 

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