CHARACTERIZATION OF ISOLATED POLYETHYLENE CHAINS IN THE SOLID-STATE

The study of polyethylene (PE) in the perhydrotriphenylene inclusion compound permits a characterization of the conformation and chain motions of the isolated polymer chain. Our solid state C-13 NMR results indicate that isolated chains of PE adopt an all-trans planar zigzag conformation similar to that of bulk crystalline PE. Variable-temperature spin-lattice relaxation measurements demonstrate that PE in the inclusion compound undergoes considerable local motion compared to the chains of the bulk material. The activation energy associated with this motion is 1.1 kcal/mol. This low-energy barrier suggests that the solid-state motions are not the result of trans-gauche conformational averaging. Several chain motions with low barriers are proposed to account for the increased flexibility of the isolated chains. These include rigid-rod rotation about the long axis of the molecule, librational motions at each carbon-carbon bond, and the translational movement of a "twiston", a conformational arrangement of ca. 12 bonds that introduces a 180-degrees twist into the polymer chain. These motions are not identified in crystalline polyethylene at room temperature but may be related to the alpha-c relaxation process that occurs in crystalline polyethylene at elevated temperatures.