Polyethylene yielding behaviour: What is behind the correlation between yield stress and crystallinity?

Yielding in semi-crystalline polymers is generally attributed to crystallite thickness. However, a better correlation is found between yield stress and crystallinity degree. In this paper, it is attempted to clarify this correlation by investigating a set of linear and branched polyethylenes. The polymers were crystallized in order to obtain a wide range of crystallinities and crystallite thicknesses. The influence of these parameters on the yielding behaviour is then studied. A new method that correlates the neck width and the Stress Transmitters (ST) density is proposed and enables to evaluate the ST concentration for each material as close as possible from the initiation of plasticity. The density of ST is found to be dependant on the content of co-unit and on the crystallisation conditions.To study specifically the initiation of the crystallites shearing, a threshold stress σth is introduced. σth appears to be proportional to the crystallite thickness (Lc) at equivalent crystallisation conditions, while it is well correlated to the crystallinity (Xc). This relationship with Xc is explained describing σth with both parameters Lc and ST.