The nonlinear time-dependent response of semicrystalline polymers: Correlations between the configurational entropy and the rate of rearrangement of chains

Two series of tensile creep tests are performed on isotactic poly(propylene) in the sub-yield region of deformations at room temperature. In the first series, injection-molded specimens are used as produced, whereas in the other series the samples are preloaded (five loading-unloading cycles with the maximal strain 0.01). A constitutive model is derived for the viscoelastic and elastoplastic behavior of semicrystalline polymers. A polymer is treated as an equivalent transient network of chains bridged by junctions. Active chains separate from their junctions and dangling chains merge with the network at random times when they are thermally activated. The network is modelled as an ensemble of meso-regions (MRs) with various activation energies for detachment of chains from temporary nodes (a distribution function for the activation energies entirely determines the configurational entropy of the ensemble). Rearrangement of chains in the network reflects the viscoelastic response. The elastoplastic behavior is attributed to sliding of junctions with respect to their reference positions and to changes in the distribution of activation energies (driven by fine and coarse slip of lamellar blocks). Stress-strain relations for a semicrystalline polymer are determined by four adjustable parameters that are found by fitting the experimental data. It is demonstrated that the attempt rate for detachment of active chains from their nodes is proportional to the configurational entropy of the ensemble of MRs.