A polyethylene chain investigated with replica exchange molecular dynamics simulation: Equilibrium lamellar thickness and melting point, ordering and free energy

Abstract

In the present work, a polyethylene chain with N=200 CH2 units was simulated using replica exchange molecular dynamics (REMD). Simulations were performed in a broad temperature range and for intra-chain interactions varying from the fully interacting to the ideal spring chain.Our work demonstrates that REMD is a very efficient method to obtain equilibrium data. It is found that the coil-to-globule transition is dominated by the vdW energy, whereas the globule-to-folded chain transition is accompanied by transitional behavior in the torsion and vdW energies. Our data clearly show that for the chain length considered here, the chain folded crystal to globule transition is a continuous transition. Nevertheless, we can establish with good accuracy the equilibrium transition temperature for the chain folded crystal to globule transition.A set of orientational order parameters was used to investigate the order in the polymer chain. At the globule-to-folded chain transition an abrupt change in the value of the order parameter is observed, whereas there is no or almost no change in the value of the order parameter at the coil-to-globule transition temperature. The (apparent) order in the disordered globular and coiled states indicated by some studied order parameters is related to the definition of the order parameter and depends on the chain length of the polymer.Below the equilibrium melting temperature the (largest principal component of the) radius of gyration and the equilibrium lamellar thickness of the folded chain crystal decrease with increasing temperature, which gives support to the theory of Muthukumar but is opposite to the prediction of classical crystallization theories. The agreement between simulations and theory may hint to universal behavior of the relative equilibrium thickness versus the relative super cooling.