Monte Carlo simulation of the crystal structure of the rotator phase ofn-paraffins

Abstract

Structures of two rotator phases of n-paraffin, orthorhombic and hexagonal rotator phases recently found by Doucet et al., are simulated by use of the Monte Carlo method. Only subcell structures are considered; a layer structure and effects of chain ends are neglected. The molecules of n-paraffin are assumed to take a rigid trans-planar structure, and they are assumed to be packed in an orthorhombic or orthohexagonal lattice, with each molecule having 36 possible orientations around the chain axis. Interactions between nearest-neighbor chains are calculated from van der Waals interactions between nonbonded C–C, C–H, and H–H atom pairs. A Monte Carlo calculation, similar to that used in our previous work on the structure of the low temperature phase of PTFE, is applied to disordered packings of the molecules in the rotator phases. It is found that the crystals in the rotator phases are composed of many ordered domains within which the chains tend to parallel their zigzag planes. The average size of the ordered domains is calculated to be about 30 Å at 400 K. It is shown that this structure explains various experimental results which are not yet well understood.