SIMULATION OF THE STRUCTURE AND PROPERTIES OF THE POLYETHYLENE CRYSTAL-SURFACE

The structures and thermodynamic properties of the (100), (010), and ( 110) lateral surfaces of extended-chain polyethylene crystals between 0 and 300 K were determined by minimization of vibrational free energy using consistent quasi-harmonic lattice dynamics. Slight rotations of the outermost chains from their corresponding orientations in the bul k were observed. These deviations from bulk structure were confined to the first three molecular layers (approximately 10 Angstrom) at the s urface. Surface free energy calculations for the crystal-vacuum interf ace found the (110) surface to be the most stable over the entire temp erature range modeled, with free energies ranging from 95.4 erg/cm(2) to 103.5 erg/cm(2) at temperatures of 0 and 300 K, respectively. Surfa ce free energies of the (100) and (010) surfaces were found to be at l east 15% higher than the (110) surface, with the (100) surface slightl y more stable than the (010) surface, over all temperatures considered . Surface free energy increases as the density of chains at the surfac e decreases. Intermolecular potential energy accounted for over 80% of the surface free energy at low temperatures. At higher temperatures, excess entropy accounted for almost half the surface free energy. The surface free energy was determined predominantly by deviations in latt ice mode frequencies from those of the bulk.