SEMICRYSTALLINE AND SUPERLATTICE STRUCTURES IN BINARY MIXTURE OF LONG CHAIN n-ALKANES C₁₉₄H₃₉₀ AND C₂₉₄H₅₉₀

Abstract

Binary mixtures of long chain n-alkanes from C 122 H 246 to C 294 H 590 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 C 194 H 390 þ C 294 H 590 (50:50 w/w) using small angle x-ray scattering. The molecular chain length difference between the two components is 100 Catoms, 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 (.1058C) and the triple-layer superlattice at low temperatures (,958C). The SCF consists of alternating crystalline and amorphous layers: C 194 H 390 molecules are fully crystallized in the crystalline layer while C 294 H 590 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 C 194 H 390 and C 294 H 590 while the surplus tails of C 294 H 590 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.