Molecular-dynamics simulations of the thermal glass transition in polymer melts:: α-relaxation behavior

We present molecular-dynamics simulations of the thermal glass transition in a dense model polymer liquid. We performed a comparative study of both constant volume and constant pressure cooling of the polymer melt. Great emphasis was laid on a careful equilibration of the dense polymer melt at all studied temperatures. Our model introduces competing length scales in the interaction to prevent any crystallization tendency. In this first manuscript we analyze the structural properties as a function of temperature and the long time or cr-relaxation behavior as observed in the dynamic structure factor and the self-diffusion of the polymer chains. The ct relaxation can be consistently analyzed in terms of the mode coupling theory of the glass transition. The mode coupling critical temperature T-c, and the exponent gamma defining the power law divergence of the alpha-relaxation time scale, both depend on the thermodynamic ensemble employed in the simulation.