The effect on polymer dynamics of adding methoxy-terminated poly(ethylene oxide) (PEO) side-chains with different lengths and separations to an amorphous long-chain PEO backbone has been studied by Molecular Dynamics (MD) simulation at 293 K and 330 K. The study is seen as having a direct general relevance to the optimal design of ion-conducting polymer hosts for both Li-ion battery and polymer fuel-cell applications. The MD box used contains a long-chain PEO backbone to which side-chains comprising 3, 6, 7, 8, 9 and 15 EO units are added. The chosen separations between the side-chains are 5, 10, 15, 20 and 50 EO units. All potentials used to describe these systems are taken from earlier work (J. Mater. Chem., 13 (2003) 214). The overall mobility of the polymer host system is found to have both minima and maxima at both temperatures for side-chain lengths in the range 6-9 EO units. This is almost totally independent of side-chain separation at 293 K, while the situation is more complex at 330 K. (c) 2005 Elsevier B.V. All rights reserved.