Does crystal thickness dictate yield kinetics in polyethylene?

There has been a longstanding effort to predict the resistance to plastic deformation of polyethylene via crystal plasticity-based micromechanical modeling, deriving the plasticity kinetics from the assumption that the strain carrier is a screw dislocation. This leads to the notion that the yield kinetics are dominated by crystal thickness, or more precisely the chain stem length within the crystal. There is in general a high degree of correlation between the crystallinity and crystal thickness, making it difficult to distinguish between these two microstructural attributes as the driving variable for plasticity. A wide range of crystallinity and stem lengths was obtained by varying crystallization via both cooling rate and various concentrations of short-chain branching, permitting the disentangling of stem length from crystallinity. The rate- and temperature-dependence of yield strengths were fit to a two-term stress-activated Ree-Eyring model. Chain stem length was measured via the Longitudinal Acoustic Mode in low wavenumber Raman spectroscopy. With this material series it was shown that crystallinity drives yield kinetics, rather than stem length. These insights will be placed in the context of a micromechanical model employing stress-activated crystal slip kinetics.