Semicrystalline and superlattice structures in an asymmetrically methyl-branched long-chain alkane

A series of lamellar structures have been found during melt-crystallization of an asymmetrically branched long-chain alkane, C₁₉₂H ₃₈₅CH(CH₃)C₉₉H₁₉₉, by small- and wide-angle X-ray scattering and DSC. The results show that methyl branches cannot be tolerated in the crystalline layer, consequently two semicrystalline forms are found to crystallize directly from melt depending on the crystallization temperature. In the semicrystalline form crystallized at higher temperatures (HTSCF) the longer arms, i.e., -C₁₉₂H₃₈₅, form the crystalline layers, while the shorter arms, i.e., -C₉₉H ₁₉₉, remain outside as cilia to form the amorphous layers. In the semicrystalline form that crystallizes at lower temperatures (LTSCF), the crystalline layers consist of complete short arms and incomplete long arms of the molecules, while the remaining segments of the molecules form the amorphous layers. The HTSCF is thermodynamically stable. HTSCF transforms into a double-layer ABAB superlattice upon further cooling, as the amorphous short arms crystallize and form crystalline B layers. The LTSCF is found to be metastable and transforms into a triple layer ABAABA superlattice upon annealing. The HTSCF and double-layer superlattice have unusually high chain tilt angle (up to 49°). This is the result of excessive steric overcrowding at the crystal-amorphous interface, caused by the disproportionately high number of cilia imposed by the topology which does not allow chain-folding.