Complex layered nanostructures in crystalline and semicrystalline ultralong alkanes

Abstract

Lamellar structure and its formation from melt has been studied in ultra-long monodisperse alkanes using realtime versions of small-angle neutron and X-ray (SANS and SAXS) and low-frequency-shift Raman scattering. Pure and binary linear unlabeled and deuterium endlabeled alkanes were examined, as well as an asymmetrically methyl-branched alkane, C192H385CH(CH3)C99H199. It was possible to study the structure and the transformation of the initial transient Non-Integer Folded (NIF) form into a variety of secondary phases, whose formation depends on temperature [1,2]. These secondary phases include the extended-chain form and two mixed folded-extended forms: the double-layer and the triple-layer superlattice. The semicrystalline form, which features in pure alkanes as the transient NIF, becomes a stable high-temperature form in either binary mixtures of linear alkanes [3] or in pure branched alkanes. The rich layer polymorphism in the asymmetrically branched alkane was found to be due to the anchoring of the methyl branch at the crystal surface. Two semicrystalline and two superlattice forms were found [4] see Figure. The exceptionally high chain tilt angle (46°) was caused by the inability of surface overcrowding to be alleviated by chain folding. The study provides new insights into the complex problem of chain folding in crystalline polymers, exactly 50 years after its discovery.