Chain-Length Dependence of the Segmental Relaxation in Polymer Melts: Molecular Dynamics Simulation Studies on Poly(propylene oxide)

Performing molecular dynamics simulations for an all-atom force field, we study the segmental or, equivalently, α relaxation of poly(propylene oxide) chains consisting of N = 2−100 monomer units. In particular, we analyze the dependence of the α relaxation on molecular weight and temperature on the basis of incoherent intermediate scattering functions. For all studied chain lengths N, the temperature dependence is well described by a Vogel−Fulcher−Tammann behavior and time−temperature superposition is obeyed at sufficiently low temperatures T . When the molecular weight increases, time scale and stretching of the α relaxation smoothly increase until they saturate at N = 30−40, where the characteristic ratio reaches the limiting value C ∞ and, hence, the chains start to show Gaussian conformation. The temperature-dependent α relaxation times collapse onto a master curve in a fragility plot, indicating that the high-temperature fragility of the model polymer is independent of the degree of polymerization. We determine to which extent the observed dependence of the segmental relaxation time on T and N can be traced back to excess free volume near chain ends within free-volume theory.