Monte-Carlo simulation of solid-solid transition to conformationally disordered state

CONDIS phases and transitions are of considerable importance with respect to the phase behavior of flexible chain molecules. Even low molecular weight compounds exhibit CONDIS phases where the molecules adopt highly disordered conformations as in the liquid phase but without loosing long-range order (CONDIS crystal). So far the understanding of these crystals may be of interest for the description of the melting process. In this paper we present a Monte-Carlo simulation of the CONDIS phase transition based on a simple three-parameter model for particles with one internal degree of freedom. The parameters themselves can be deduced in principle from atomistic calculations. It is shown that our calculations yield a solid-liquid as well as a solid-CONDIS crystal (solid) phase transition depending on the energy difference between both states of the particle.