Crystal and rotator phases of n-alkanes: A molecular dynamics study

Abstract

Normal alkanes have a simple molecular structure, but display a surprising variety of ordered phases, including an orthorhombic crystal, followed on heating by two partially ordered rotator phases RI and RII. These phases are interesting both because of the weakly first-order transitions that separate them, and because rotator phases are implicated in the nucleation of crystals in polyethylene. To understand this interesting and technologically important phenomenon, a clear picture of the rotator phase is essential. We conducted all-atom simulations of pure C(23) and mixed C(21)-C(23) normal alkanes. Among potentials we tried, only Flexible Williams gave good agreement with the experimental sequence of phases and transition temperatures. Physical properties of the simulated phases, including lattice dimensions and transition entropy between orthorhombic and rotator RII phase are in good agreement with experiment. We define order parameters for investigating pretransitional fluctuations in RI and RII phases; we observed only very short-range correlations in these phases, but slower temperature scans may be necessary to properly investigate these weakly first-order transitions.

Evidence for weakly bound electrons in non-irradiated alkane crystals: The electrons as a probe of structural differences in crystals

“…The distance from one chain of the molecule to the other is about 5 Å (the size of the unit cell for neighbouring C 23 H 48 was provided in Ref. 9). It can be assumed that the electron prefers location between chain ends of three molecules (higher value of interaction energy; Fig.…”

Molecular dynamics simulations of monodisperse/bidisperse polymer melt crystallization

“…There have been several studies of polymer crystallization using molecular dynamics in order to predict the effect of parameters such as polymer chain length, bond bending stiffness or cooling protocol . Jabbarzadeh et al have compared the crystallization kinetics for two different chain lengths upon quiescent and pre‐stretched regimes using a united‐atom force field for polyethylene.…”

Effect of the liquid crystal solute on the rotator phase transitions of n-alkanes

“…Detailed atomistic simulations of shorter chains by Ryckaert et al 30,31 conrmed the importance of translations in the rotator phases. Wentzel et al 35,36 performed atomistic molecular dynamics simulations of ordered phases in pure C 23 alkane and mixed C 21 -C 23 alkane systems, using several different all-atom potentials and conrmed the weak rst order character of the R II -R I transition. By contrast, in the pseudohexagonal R I phase, a dramatic increase in longitudinal chain motion is observed.…”