Anisotropic Nature of Open Volume “Defects” in Highly Crystalline Polymers

We assess the size and shape of the free volume holes in crystalline polyethylene fibers (crystallinity ∼85%) via the measurements of confinement momenta of parapositronium trapped within these holes. The holes show considerable anisotropy and a shape that is compatible with those created by chain folds buried in the crystal lattice. We estimate their sizes to be ∼8 and 5 Å in two perpendicular directions. These are also compatible with those argued from the lattice parameter and other geometrical considerations.