Issue
EPL
Volume 80, Number 3, November 2007
Article Number 38001
Number of page(s) 5
Section Interdisciplinary Physics and Related Areas of Science and Technology
DOI http://dx.doi.org/10.1209/0295-5075/80/38001
Published online 25 September 2007
EPL, 80 (2007) 38001
DOI: 10.1209/0295-5075/80/38001

On the momentum transfer dependence of the atomic motions in the $\mth{\alpha}$-relaxation range. Polymers vs. low$\hbox{--} $molecular-weight glass-forming systems

J. Sacristan1, F. Alvarez2, 3 and J. Colmenero1, 2, 3

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 in final form 3 September 2007; published November 2007
published online 25 September 2007

Abstract
By means of molecular dynamics simulations we have studied the momentum transfer (Q) dependence of the intermediate scattering function Fs(Q, t) of the low$\hbox{--} $molecular-weight counterpart of a glass-forming polymer. At Q $\geq$ 0.3 Å-1 we found a similar Q-dependence to that previously reported for many polymers. The relaxation time scales like $Q^{-
2/\beta}$, where $\beta$ < 1 is the parameter corresponding to a stretched exponential description of the slow decay of Fs(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.

PACS
83.10.Rs - Computer simulations of molecular and particle dynamics.
61.41.+e - Polymers, elastomers, and plastics.
61.12.Ex - Neutron scattering (including small-angle scattering).

© Europhysics Letters Association 2007