Chain conformations on the surface of polyethylene single crystals

Abstract

New conformations for the shortest folds of polyethylene single crystals were determined from the principles: (1) the conformations of polymethylene chains are basically determined by the rotational isomeric approximation, (2) the crystal part is deformed where it is connected with the fold part, and (3) the conformation of the fold must be determined as the minimum of surface energy, not of conformation energy. It was shown that the surface energy, assuming the crystal state as the reference state, was composed of four terms: (i) the deformation energy of the crystal part, (ii) the sublimation energy of the fold part, (iii) the conformation energy of the fold part, and (iv) the interaction energy between folds. The conformations of the shortest fold were basically (GTGGTGGG) for (110)-folds and (G'G'TTGG) for (200)-folds. The setting angles of all the chains in the crystal part are rotated from their normal angle (41°) to 41 + a° at the boundary between the two parts. The fold part is deformed so as to fit in with the deformed boundary of the crystal part. The conformations with minimum surface energy were obtained at a = 30° for both (110)-and (200)-folds. Their surface energies were about 16 kcal/mole of fold (300 erg/cm2). The surface energies, assuming the liquid state as the reference state, were about 7 kcal/mole of fold (130 erg/cm²).