EQUILIBRIUM THERMODYNAMICS OF HOMOPOLYMERS AND CLUSTERS - MOLECULAR-DYNAMICS AND MONTE-CARLO SIMULATIONS OF SYSTEMS WITH SQUARE-WELL INTERACTIONS

The thermodynamics of homopolymers and clusters with square-well inter actions of up to 64 particles are studied with constant-temperature di scontinuous molecular dynamics (DMD) simulations; for comparison Monte Carlo(MC) simulations are also reported. Homopolymers composed of mor e than five beads are found to exhibit two or more equilibrium transit ions. In the long chain limit, these multiple transitions correspond t o gas-to-liquid, liquid-to-solid, and solid-to-solid transitions. In p articular, the liquid-to-solid-like disorder-to-order transition for i solated 32mers and 64mers is strongly first order (bimodal energy dist ribution) at the reduced square-well diameter lambda = 1.5. As lambda decreases from 1.5 to 1.3, the bimodal distribution becomes unimodal. The use of Lindemann's rule for solids indicates that the structure fo rmed right below the liquid-to-solid transition temperature has a Soli d core but a liquid surface. Comparing the homopolymer results with th ose for square-well clusters indicates that the bonding constraint in homopolymers increases the temperatures of transitions but decreases t heir strength. The solid structure of an isolated 64mer is nearly iden tical to that of a cluster of 64 beads. Possible approaches to the exp erimental observation of the solid-state for an isolated chain are dis cussed. (C) 1997 American Institute of Physics.