On the momentum transfer dependence of the atomic motions in the -relaxation range. Polymers vs. lowmolecular-weight glass-forming systems

¹  Donostia International Physics Center - Paseo Manuel de Lardizabal 4, 20018 San Sebastián, Spain
²  Departamento de Física de Materiales (UPV/EHU) - Apartado 1072, 20080 San Sebastián, Spain
³  Unidad de Física de Materiales (CSIC-UPV/EHU) - Apartado 1072, 20080 San Sebastián, Spain

Received: 9 February 2007
Accepted: 3 September 2007

Abstract

By means of molecular dynamics simulations we have studied the momentum transfer dependence of the intermediate scattering function of the lowmolecular-weight counterpart of a glass-forming polymer. At Q ≥ 0.3 Å^-1 we found a similar Q-dependence to that previously reported for many polymers. The relaxation time scales like , where β < 1 is the parameter corresponding to a stretched exponential description of the slow decay of F_s(Q, t). Moreover, at Q higher than about 1 Å^-1 a crossover towards an approximate Q^-2 scaling is observed. Thereby this behavior is not a consequence of the connectivity of polymer chains but a general feature of glass forming systems. Differences among polymers and low molecular glass-forming systems emerge at lower Q's, where the connectivity of the macromolecules prevents normal diffusion.