A multiphase material model considering strain-induced crystallisation in polymers

The crystallisation ability of polymers under high stretches is a challenging phenomenon in material science. Multiphase approaches help scientists to distinguish between different structures, which evolve or vanish during experimental studies. Here, the observed phenomenon is the evolution of crystals during a uniaxial tensile test. The current paper introduces an approach considering three different phases in one fibre of polymer. The free energy of the material considers the contribution of the mixing entropy. The derivation of the mixing entropy is explained in detail, making use of different assumptions and approaches, e.g. consideration of ideal mixtures. In the end, this multiphase approach for the free energy is implemented in a constitutive model for rubbery materials stretched to high strains. It is able to depict the strain-induced crystallisation phenomenon observed via a uniaxial tensile test, prooved by stress-stretch and crystallinity-stretch simulations.