The amount and location of amorphous component in polyethylene single crystals

Polyethylene single crystals from dilute solutions usually have a density ρ which is less than that of the ideal crystal lattice (ρc = 1.000 g/cm3). This density defect can be formally assigned to an “amorphous” component (1-α) = (ρc-ρ)/(ρc-ρa) and may be caused by vacancies in the crystal lattice and/or by the surface disorder connected with chain folds. The two-phase concept of single crystal structure assumes a very nearly ideal lattice core and two amorphous layers on the fold-containing surfaces. It is supported by density and heat content data of annealed samples, by small-angle X-ray scattering, by wide-line NMR investigations, and by the results of fuming nitric acid treatment. The maximum thickness of the amorphous surface layer as a function of temperature may be estimated from the free energy requirement of such a layer. With regular chain folds one has a high concentration of gauche conformations yielding a high surface energy. The need for such a concentration disappears in the amorphous layer with loose loops. This gain in energy may be spent for surface melting. Still more important is the gain in entropy caused by the random distribution of loop lengths.