The effect of annealing on the elastoplastic and viscoelastic responses of isotactic polypropylene

TL;DR

This study investigates how annealing affects the elastoplastic and viscoelastic behavior of isotactic polypropylene through tensile and creep tests, and develops a heterogeneous network model to explain the observed responses.

Contribution

It introduces a novel model treating semicrystalline polymers as heterogeneous transient networks to explain their complex mechanical responses.

Findings

Annealing alters the elastoplastic response of isotactic polypropylene.

The developed model accurately fits experimental stress-strain data.

Creep behavior varies with thermal treatment and is explained by network dynamics.

Abstract

Observations are reported on isotactic polypropylene (i) in a series of tensile tests with a constant strain rate on specimens annealed for 24 h at various temperatures in the range from 110 to 150 C and (ii) in two series of creep tests in the sub-yield region of deformation on samples not subjected to thermal treatment and on specimens annealed at 140 C. A model is developed for the elastoplastic and nonlinear viscoelastic responses of semicrystalline polymers. A polymer is treated an equivalent transient network of macromolecules bridged by junctions (physical cross-links, entanglements and lamellar blocks). The network is assumed to be highly heterogeneous, and it is thought of as an ensemble of meso-regions with different activation energies for separation of strands from temporary nodes. The elastoplastic behavior is modelled as sliding of meso-domains with respect to each other driven by mechanical factors. The viscoelastic response is attributed to detachment of active strands from temporary junctions and attachment of dangling chains to the network. Constitutive equations for isothermal uniaxial deformation are derived by using the laws of thermodynamics. Adjustable parameters in the stress-strain relations are found by fitting the experimental data.