Monte Carlo simulation of the crystal structure of the rotator phase ofnhyphen;paraffins. II. Effects of rotation and translation of the rigid molecules

摘要

Structures and molecular motions in the rotator phases ofnhyphen;paraffins are simulated by use of the Monte Carlo methodmdash;both rotational and translational degrees of freedom of the molecules being taken into account. Molecules of rigidtranshyphen;planar structure are assumed to be placed in a twohyphen;dimensional orthorhombic or orthohexagonal lattice with lattice parametersaandb. A model of the nearly continuous molecular motion is adopted in this work; each molecule is assumed to have 36 discrete orientations (in 10deg; intervals) and 10 discrete translations (in 0.128 Aring; intervals) around and along the molecular axis, respectively. Molecular packing modes in various lattice systems of differenta/bvalues are examined ranging from that of the ordinary orthorhombic phase (a/b=1.50) to that of the rotatorhyphen;II (Rhyphen;II) phase (a/b=31/2). It is found that the crystals in the rotator phases are generally composed of many domains within which molecules are packed more or less in order. In the rotatorhyphen;I (Rhyphen;I) phase, fairly large structural changes are observed with the change ina/bvalue; the structure of the Rhyphen;I phase reported in our previous MC work should be slightly modified. The structure of the Rhyphen;II phase, on the other hand, is found to be essentially the same as that of our previous MC work. Modes of the molecular motion in the Rhyphen;II phase are studied by monitoring the time evolution of the rotational and translational states of all molecules in the system. It is found that the molecules are making fairly active rotational oscillation around one of the six equivalent directions connecting nearest neighbor molecular pairs. The molecules are also making sporadic large rotations to the different directions. The translational motion of the molecules is found to be very active and evidently independent of the rotational motion. The overall molecular motions in the rotator phase are found to be described in terms of the movement of the boundaries of the ordered domains.