Phase Transition of Cubic-Lattice Polymer Systems

Abstract

Monte Carlo simulations of a lattice polymer system of chains of 20 segments each, comprising a total of 441 chains on a 21 X 21 X 21 cubic lattice (and hence filling ca. 95.2% of the total volume), are performed to study the details of the disordered vs. the ordered states of bulk polymers of great length as a function of chain flexibility. Assuming an energy ϵ > 0 for each transverse (“gauche”) bond connection over that of a collinear (“trans”) bond, we find a definite first-order phase transition to a nearly perfectly ordered state upon cooling the system from a disordered state stable at high reduced temperatures T (=kBT/ϵ), as demonstrated by discontinuities in the internal energy (conformational order) and the macroscopic orientational order parameter. The appearance of hysteresis in the transition upon heating from the ordered state further confirms the first-order character of this transition. The key features of the Monte Carlo results are matched closely by the predictions of mean-field theory, except that in disordered states close to the transition the chains exhibit appreciable conformational perturbations. This is manifested in enhancement of more extended conformers and significant local orientational correlations, the latter being limited primarily to near-neighbor segments. Relevance of these results to the molecular configurations and phase transitions of real polymers is discussed. © 1984, American Chemical Society. All rights reserved.