Molecular-dynamics simulation of a glassy polymer melt: Incoherent scattering function

We present simulation results for a model polymer melt, consisting of short , nonentangled chains, in the supercooled state. The analysis focuses on th e monomer dynamics, which is monitored by the incoherent intermediate scatt ering function. The scattering function is recorded over six decades in tim e and for many different wave-vectors which range from the size of a chain to about three times the maximum position of the static structure factor. T he lowest temperatures studied are slightly above T-c,T- the critical tempe rature of mode-coupling theory (MCT) where T-c was determined from a quanti tative analysis of the beta- and alpha-relaxations. We find evidence for th e space-time factorization theorem in the beta-relaxation regime, and for t he time-temperature superposition principle in the alpha-regime, if the tem perature is not too close to T-c. The wave-vector (q-) dependence of the no nergodicity parameter! of the critical amplitude, and the alpha-relaxation time are in qualitative agreement with calculations for hard spheres. For q larger than the maximum of the structure factor the alpha-relaxation time T-q already agrees fairly well with the asymptotic MCT-prediction tau(q), s imilar to q(-1/b). The behavior of the relaxation time at small q can be ra tionalized by the validity of the Gaussian approximation and the value of t he Kohlrausch stretching exponent, as suggested in neutron-scattering: expe riments.