Semicrystalline lamellar phase in binary mixtures of very long chain n-alkanes

Lamellar structure formed by melt crystallization of binary mixtures of monodisperse long n-alkanes C₁₂₂H₂₄₆, C₁₆₂H₃₂₆, C₁₉₄H₃₉₀, C₂₁₀H₄₂₂, C₂₄₆H₄₉₄, C₂₅₈H₅₁₈, and C₂₉₄H₅₉₀ was studied by small-angle X-ray scattering (SAXS) and thermal analysis. A semicrystalline form (SCF) was found to be the stable phase over a wide range of compositions in the temperature range between the melting points of the shorter alkane and that of the alkane with a length equal to the difference between the chain lengths of the two components. Lamellar structure of SCF is determined by Fourier synthesis of electron density profiles from SAXS intensities and by fitting to models. It was found that SCF consists of alternating crystalline and amorphous layers. The thickness of crystalline layers is defined by the extended chain length of the shorter alkane tilted at 35° to the layer normal. The shorter chains are fully crystalline and confined to the crystal layers. The longer molecules traverse the crystal layer and are only partially crystalline; their surplus length in the form of loose cilia makes up the amorphous layer. The structure is similar to the transient noninteger folded (NIF) form in pure long n-alkanes. Stable SCF solid solution was found in alkanes with length ratio up to 1.7:1 and a chain length difference of up to 100 carbons, having a crystallinity as low as 63%. Viability of a stable SCF is demonstrated by a simple thermodynamic calculation based on the above structural model. According to the C₁₆₂H₃₂₆-C₂₄₆H₄₉₄ phase diagram, the maximum molar fraction of C₂₄₆H₄₉₄ that SCF will accept is 0.44. Taking into account the 35° chain tilt, this means that the effective cross-sectional area of a chain crossing the crystalline-amorphous interface increases by a factor of 2.8.