Chain length dependent polymorphism in even number n-alkanes: line profile analysis of synchrotron powder X-ray diffraction data

Abstract

Normal alkanes form a basic building block unit in systems such as liquid crystals, oils, proteins, lipids and plastics and, as a result, short-chain alkanes, owing to their simplicity, are often used as simple representative models for probing the physical nature of the more complex hydrocarbon systems, nAlkanes crystallize to form thin plates with regular faces in which the chain direction is more or less perpendicular to the lamella surface. A combination of single-crystal and powder X-ray analysis has enabled some of these systems to be characterized; three solid-state structures have been identified [1, 2]: triclinic, 12 ~ 36 and n(odd). Prior to melting, n-alkanes are known to exhibit solid-solid phase transitions to crystalline rotator phases [3, 4]. In the low temperature phase (phase I) the chains adopt an overall trans configuration, after which the material may transform to a rotator phase (phase II) in which the chains undergo hindered rotation about the main carbon axis. Such a transformation can be accompanied by the introduction of gauche bonds [5, 6] into an otherwise all t rans structure: with each set of gauche bonds conferring different types of structural disorder (ukinks", chain twisting and chain folding). Such molecular disorder is innate to crystals of polyethylene [7] as well as being present in the longer chain linear hydrocarbons (n > 100) [8, 9]. In previous studies [10] we employed high resolution X-ray powder diffraction using the Daresbury synchrotron radiation source (SRS) to determine the unit cell parameters for a number of homologues taken from the series C13H28-C60H122. However, as well as being able to determine the unit cell parameters from the X-ray powder data, it is also possible to extract, by line profile analysis, other physical information such as crystallite size or lattice strain by observing the line broadening behaviour [11-14]. In this study we present a strain analysis of the [00I] plane for a series of even n-alkanes (C18H38-C60H122) in order to attain further under-