||Semi-crystalline polymers comprise both amorphous and crystalline domains coupled in a rather complex manner. When the amorphous phase is in the rubbery state, the mechanical response is strongly dependent on the crystal rate, thus leading to mainly thermoplastic or elastomeric responses. In this study, finite deformation stress-strain behavior of polyethylene materials is modeled by considering these semi-crystalline materials as heterogeneous media in order to provide insight into the contribution of crystalline and amorphous phases on the overall material deformation resistance, i.e. intermolecular and network resistances. A hyperelastic-viscoplastic model is developed in order to represent the overall mechanical response of polyethylene materials under large deformation. An evolutionary optimization procedure based on a genetic algorithm is developed to identify the model parameters at different strain rates. The identification results show a good agreement with experimental data, demonstrating the usefulness of the proposed approach: the constitutive model, with only one set of identified parameters, allows reproducing the stress-strain behavior of polyethylene materials exhibiting a wide range of crystallinities, the crystal content becoming the only variable of the model.