1 INTRODUCTION The molecular structure of polymers and the crystallization conditions strongly act on the semi- crystalline structure and the solid-state rheology. The spherulitic organization results from static crystallization condition during extrusion process and in the core of injected parts. This rheology depends also on the deformation path (traction, shear, compression …). Under traction the non- affine deformation of the spherulite begins in equatorial zones and propagates to the polar zones [1]. This process depends on the deformation path which explains the difference of rheology observed. Decoupled molecular models are able to predict the main features as a function of deformation path with mechanical models of crystalline and amorphous phases [2]. They consider critical shear-stress on shear planes inside crystals and a finite extensibility chain model (Langevin dumbbell) for the amorphous phase [2]. The rheological behaviour must be searched at the molecular level to predict such complex rheology. A coupled molecular model is necessary to physically describe the deformation mechanism and the rheology. Molecular chains go through thecrystalline and amorphous phases in the initial configuration with a physical path which must include all the key points of the organization. A traction of the equatorial zone is applied by a molecular dynamics model with a full atom description of the polymer chains. 2 MOLECULAR MODEL 2.1 Model construction The physical modelling of semi-crystalline polymer is a difficult task as an effect of numerous physical constrains which must be applied. Two linear polyethylene chains M= 108 656 g.mol -1 (46 570 atoms) must completely contribute to the amorphous ABSTRACT: The plastic deformation of a semi-crystalline polyethylene is predicted by a molecular dynamics method. A full-atom model is able to predict the correct amorphous and crystalline phases. Two high molecular weight molecules are involved in these two phases with 3D-periodic conditions. This is one of the first fully coupled model including the main features of the molecular organization. A tensile experiment is applied on the molecular system perpendicular to the lamellae surface. The deformation mechanism inside the amorphous and crystalline phases, and the force evolution, are detailed. This analysis is able to check the effect of details of molecular structure in the amorphous phase on the solid-state rheology. Moreover, a cavitation appears very early in the amorphous phase for all the simulations. Key words: Molecular dynamics, polyethylene, semi-crystalline, elastic deformation, plastic deformation Molecular Dynamics prediction of elastic and plastic deformation of semi-crystalline polyethylene B. Monasse 1 , S. Queyroy 1 , O. Lhost 2 1 Ecole des Mines de Paris PARISTECH, CEMEF, UMR 7635, BP 207, 06904 Sophia-Antipolis Cedex, France 2 Ineos rue de Ransbeek, Bruxelles (Belgique) URL: www-cemef.cma.fr e-mail:bernard.monasse@ensmp.fr; severine.queyroy@ensmp.fr