Probing the Chain and Crystal Lattice Orientation in Polyethylene Thin Films by Near Edge X-ray Absorption Fine Structure (NEXAFS) Spectroscopy

The chain and the crystal unit cell orientation of linear low density polyethylene (LLDPE) were measured with near edge X-ray absorption line structure (NEXAFS) spectroscopy. A strongly attractive substrate, silicon, and a weakly attractive substrate, mica, were used. For a 100 nm thick LLDPE film on the silicon substrate, the crystals exhibit an edge-on lamellar morphology, with the chains predominantly parallel to the substrate, and the orthorhombic unit cell < a, b, c > in the following approximate orientation: b and c are in the film plane with b along the crystal fibril direction and c perpendicular to the fibril direction and a perpendicular to the him plane. On the mica substrates, LLDPE films with thickness below 180 nm completely dewet the surface and form isolated droplets, while a film 366 nm thick crystallizes as spherulites with most of the chains perpendicular to the substrate before annealing and with a twisted lamellar structure after isothermal crystallization at 60 degrees C. The results demonstrate that the combination of electron yield NEXAFS and high spatial resolution transmission NEXAFS is a powerful tool to measure the crystal orientation in the polymer thin films on a small length scale.