Linear thermo-viscoelasticity of isotactic polypropylene

Observations are reported on injection-molded isotactic polypropylene in torsion oscillation tests at various temperatures from room temperature to T= 150 °C, as well as in torsion relaxation and bending creep tests in the range of temperatures between 30 and 80 °C. Constitutive equations are derived for the viscoelastic response of a semicrystalline polymer at three-dimensional deformations with small strains. A polymer is treated as an equivalent transient network of strands connected by junctions (entanglements, physical cross-links on the surfaces of crystallites and lamellar blocks). The network is thought of as an ensemble of meso-regions with various activation energies for rearrangement of strands. The distribution of meso-domains is described by a generalized Gaussian function. The time-dependent behavior of the ensemble reflects separation of active strands from their junctions and merging of dangling strands with the network. The rearrangement events occur at random times, when the strands are excited by thermal fluctuations. Stress-strain relations involve five material constants that are found by fitting the experimental data. Good agreement in demonstrated between the observations and the results of numerical simulation. The study concentrates on the effect of temperature on the adjustable parameters.