Orientational ordering of flexible trimers on the square lattice

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Abstract
It is shown that a system of short flexible chain molecules interacting primarily via a local orientation‐dependent potential between different chain segments exhibit a phase transition between a disordered phase and an orientationally ordered phase. The effect of intramolecular and intermolecular interaction on the thermodynamics of small chain molecules each consisting of two flexible jointed segments (trimer) on the square lattice, has been studied using Monte Carlo methods. Systems consisting of up to 4840 chains have been considered. The intramolecular (configurational) energy of one trimer is ε B <0 if the two segments are colinear and zero otherwise. The intermolecular energy is ε S <0 for two parallel oriented, nearest neighbor segments of two trimers. No evidence for a phase transition is found for vanishing intermolecular interaction (ε S =0, ε B <0). But continuous phase transitions are found if local orientation‐dependent intermolecular interactions are included (ε S <0, ε B ≤0). Various quantities of interest as transition temperature, orientational order parameter and internal energy are discussed.