Semicrystalline and superlattice structures in binary mixture of long chain n-alkanes C₁₉₄H₃₉₀ and C₂₉₄H₅₉₀

Binary mixtures of long chain n-alkanes from C₁₂₂H₂₄₆ to C₂₉₄H₅₉₀ have been found to form solid solutions despite their large chain length differences [Zeng and Ungar. Novel Layered Superstructures in Mixed Ultralong n-Alkanes. Phys. Rev. Lett. 2001, 86, 4875-4978]. In this article we describe a study of the binary mixture of C₁₉₄H₃₉₀ + C₂₉₄H₅₉₀ (50:50 w/w) using small angle x-ray scattering. The molecular chain length difference between the two components is 100 C-atoms, the largest so far studied. In accordance with the findings on some other binary mixtures, two types of lamellar structures are found: the semicrystalline form (SCF) at high temperatures (> 105 °C) and the triple-layer superlattice at low temperatures (< 95 °C). The SCF consists of alternating crystalline and amorphous layers: C₁₉₄H₃₉₀ molecules are fully crystallized in the crystalline layer while C₂₉₄H₅₉₀ molecules traverse the crystalline layer and are only partially crystalline; their protruding tails, or cilia, constitute the amorphous layer. The superlattice is a periodic 1-D array of triple-layer units: the two outer layers in the unit contain a mixture of C₁₉₄H₃₉₀ and C₂₉₄H₅₉₀ while the surplus tails of C₂₉₄H₅₉₀ coalesce and interdigitate in the center and form the third, thinner crystalline layer. In the superlattice form, the unusual diffraction order dependence of the linewidth is interpreted in terms of a particular type of stacking faults.