Linear thermo-viscoelasticity of polypropylene

Observations are reported on isotactic polypropylene in tensile relaxation tests in a wide interval of temperatures (from −20 to 120 °C) that involves α- and β-relaxation points. Constitutive equations are derived for the linear viscoelastic response of semicrystalline polymers. A polymer is modeled as an equivalent transient network of chains, whose rearrangement is governed by the Eyring equation with a fictive temperature. The stress–strain relations involve four adjustable parameters that are found by fitting the experimental data. It is demonstrated that (i) the dimensionless fictive temperature reaches its maximum at the α-relaxation point, and (ii) the rate of rearrangement increases with temperature following the Arrhenius law outside the β-relaxation region.