Relaxation modulus in the fitting of polycarbonate and poly(vinyl chloride) viscoelastic polymers by a fractional Maxwell model

The stress relaxation of two different polymers under a constant strain has been studied and approached by using a fractional Maxwell model in which the stress appears as a noninteger-order derivative of the strain. To obtain an accurate approximation of the experimental data for the model, two noninteger values for the derivative order are required. These values are related to two relaxation types. For short times, the derivative order is smaller and near zero, which indicates behavior close to the ideal elastic solid. For long times the derivative order is higher, showing more plastic behavior. In this work some classic models are revised and the fractional Maxwell model is used to fit the experimental data. Finally, the complex fractional modulus, the two derivative orders, and the relaxation times for samples of polycarbonate and poly(vinyl chloride) are obtained.[PUBLICATION ABSTRACT]