Linear viscoelasticity and mixing of rubber

The exact mechanism of how the linear behaviour is maintained in the large deformation is not known and is the subject of future research. 

Another importance of linear viscoelasticity is that it provides a reference for non-linear behaviour that is, the latter is expressed as a deviation from the former with use of appropriate parameters. First is the universal parameter, the elongation ratio, a, which reduces the time-scale of nonlinear behaviour to that of linear behaviour. Next is the modulus shift factor, T(a), which indicates the degree of strain-hardening or strainsoftening. Finally, comparison of linearised elongation data with that of shear data indicates, if they disagree, the presence of strain-induced crystallisation or strain-induced association. All these deviations from linearity are related to the structure of rubber. 

Thus far, a considerable variety of gum rubbers have been investigated and the characterisation scheme defined in this book is satisfactory. However, the applicability of the scheme is limited to the behaviour where strain-dependence and time-dependence are separable. 

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