PREMELTING OF LONG ETA-PARAFFINS IN CHAIN-EXTENDED LAMELLAE - AN ELECTRON-DIFFRACTION STUDY

Electron diffraction is used to study the thermally-induced structural changes in n-paraffins for which chains are too long to permit transition to a hexagonal layer packing. Monolamellar crystals of n-C50H102, n-C60H122, n-C82H166, and n-C94H190 obtained from dilute solution undergo a lattice expansion upon heating so that the lateral orthorhombic unit cell axial ratio approaches a limiting value a/b congruent-to 1.55. Near the melting point, a structure with an oblique chain packing begins to form irreversibly. Crystallization from the melt forms the monoclinic structure described earlier by Khoury. Diffraction patterns from heated epitaxially-crystallized samples demonstrate that the longitudinal chain motions responsible for this transition occur well below the melting point, producing a slightly thickened lamellar structure which, on average, resembles a pure paraffin with one more chain methylene unit. This structure incorporates the expected interfacial disorder due to conformational defects.