Semicrystalline Lamellar Phase in Binary Mixtures of Very Long Chain n-Alkanes

Abstract

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.

Novel Layered Superstructures in Mixed Ultralongn-Alkanes

“…3), as well as with increasing proportion of the longer alkane in the mixture [17]. The semicrystalline form (SCF) is found in a broad range of compositions with molar fraction of the shorter alkane between ഠ0.5 and 1.…”

“…A clear transition occurs at around 105 ± C with melting at 130 ± C. Wide-angle scattering (WAXS) shows the crystal unit cell to be that of polyethylene [16], remaining unchanged in the transition. An identical but temperature-shifted reverse series of diffractograms is obtained on cooling the sample from the melt at 2 ± C min 21 ; the temperature of both transitions is lowered by 10 ± C [17]. Similar behavior is observed in a wide range of compositions, as well as in other binary mixtures involving alkanes C 122 H 246 , C 162 H 326 , C 194 H 390 , C 210 H 422 , C 246 H 494 , C 258 H 518 , and C 294 H 590 .…”

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

“…[11]. The beam was monochromatized and doublefocused onto the detector having a cross-section of 2 £ 0.3 mm 2 in the sample plane.…”

“…The electron density profile can thus be directly reconstructed from the diffraction pattern via Eq. [11,12] High Temperature Form Figure 1 shows a series of SAXS diffractograms recorded during heating of the 50:50 w/w mixture of C 194 H 390 and C 294 H 590 at 58C/min.…”

“…In common with the previous work on long alkane mixtures [11] the "rectangular" model is attempted first and the intensities were calculated from the model according to the procedure introduced in the experimental section. At the same time, using Eqs.…”