Anisotropic surface stress in polymer crystals: Example of polyethylene

The model of sharp folds at the surface of lamellar single crystals of polymers lead to high surface stress. The layer of folds is analogous to an epitaxial layer. Weak strains of the crystalline layers, when they are possible, relax these surface stresses. The principal properties of the most studied polymer, polyethylene, which are not observed in nonpolymeric materials, are shown to be deformation modes for relaxing the surface stress. From experimental observations it is inferred that in polyethylene, in the (110) and (100) fold domains, there are, respectively, an extension and a contraction along the growth faces. The observed twinning and martensitic modes appearing in polyethylene single crystals are the modes activated by the tensile surface stress due to the intrafold interaction. The idea proposed in this paper that surface stress activates some deformation modes can be easily extended to other lamellar polymer crystals.