Simulations of dense one-component melts of coarse-grained polyethylene (PE) and polyoxyethylene (POE) chains have been modified so that some of the parent chains are collapsed intramolecularly to become filler particles. All intermolecular pairs of beads (filler-filler, filler-matrix, matrix-matrix) interact via exactly the same Lennard-Jones (LJ) potential. Filler particles are obtained by an increase in the strength of the minimum in their intramolecular LJ potential. The response of the mean square radius of gyration, < s(2)>(matrix), of the free matrix chains to disordered arrangements of the filler particles is evaluated for differences in the compactness and mobility of the filler particles, and for various relationships between the mass of matrix chains and filler particles. Even with this simple model, where distinctive surface-matrix interactions are completely suppressed, the response of < s(2)> matrix to the filler particles is complex. It depends on the completeness of the collapse of the chains that represent the filler particles, the mobility of these particles, their concentration, and the relationship between the number of beads in the matrix chains and filler particles. Expansion of the matrix chains is seen when the particles and matrix are both represented by a small number of beads, but contraction of matrix chains can be observed when both species contain a large number of beads. The mobility of the filler particles is likely to be an important issue when they are small, i.e. nanoparticles, and the matrix is not glassy. (c) 2006 Elsevier Ltd. All rights reserved.