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.