Molecular dynamics simulations of the diffusion coefficient of systems of polydisperse chains were presented. Each system consisted of two lengths of chain of chemically identical flexible polymers. The mean square displacement of the center of mass of each species was measured as a function its length and volume fraction in the blend. The polymer lengths range from N = 10 monomers per chain to N = 90, about three times the entanglement length. The polymer species that comprised the bulk of the melt showed little change in behavior regardless of the length of polymer which made up the remainder. By contrast, when a species was the minority component, its motion was significantly affected by the length of the matrix chains. When a chain was immersed in a matrix of longer chains, its diffusion coefficient was smaller than its monodisperse value; conversely when a chain was in a blend of shorter chain its diffusion coefficient increased compared to a monodisperse melt. For chains shorter than the entanglement length, the diffusion coefficient compares well to theoretical predictions. The scaling exponent of the mean square displacement of the longest polymer was found to be sub-linear, unless blended with very short polymers. The scaling exponent seemed to be a measurement of the entanglements that the long polymers experience.

Molecular Dynamics Study of Diffusion in Bidisperse Polymer Melts. Barsky, S.: Journal of Chemical Physics, 2000, 112[7], 3450-6