Collapsed chains as models for filler particles in a polymer melt

There is no evidence for spontaneous aggregation of impermeable filler particles, but sufficiently permeable filler particles can aggregate. The parameters used in the simulations insure that the aggregation cannot be energetically driven. Matrix chains that fill space within a permeable filler particle have severe restrictions placed on their available conformations. The reduction in the conformational entropy of the matrix chains can be alleviated if the permeable filler particles interpenetrate, or aggregate. Then fewer matrix chains must enter the permeable filler particles in order to maintain the density of the system. The simulation detects no aggregation of impermeable filler particles because it is not necessary for matrix chains to enter completely collapsed particles.