Chain unfolding in single crystals of ultralong alkane C₃₉₀H₇₈₂ and polyethylene: An atomic force microscopy study

Ultralong alkanes (C_nH_2n+2, n > 150) are generally considered as the most appropriate model for polyethylene, PE, and studies of these compounds can shed light on fine structural features of polymer crystals and the mechanisms of polymer crystallization. High-temperature atomic force microscopy was applied for comparative studies of solution-grown single crystals of C₃₉₀H₇₈₂ (the longest available alkane) and PE. While the structures of single crystals of C₃₉₀H₇₈₂ and PE are similar, the details of their thermal behavior are quite different. The structural transitions in C₃₉₀H₇₈₂ crystals follow the typical oligomer-type integral chain-folding scheme. Upon annealing, the alkane crystal undergoes a complete series of transformations corresponding to stepwise unfolding from the folded-in-five conformation toward the fully extended-chain crystal. The chain unfolding in PE crystals is a continuous and slower process. The morphological evolution of single crystals on heating is found to be dependent on the presence of solvent traces. Melting of the alkane crystals on graphite is accompanied by spreading of the alkane onto the substrate leading to the formation of a thin epitaxial layer. The complete melting of this film occurs at 185 °C, i.e., approximately 55 °C above the melting point of the bulk material.