Molecular dynamics study of diffusion in bidisperse polymer melts

Molecular dynamics simulations of the diffusion coefficient of systems of p olydisperse chains are presented. Each system consists of two lengths of ch ain of chemically identical flexible polymers. The mean square displacement of the center of mass of each species is measured as a function its length and volume fraction in the blend. The polymer lengths range from N = 10 mo nomers per chain to N = 90, about three times the entanglement length. The polymer species that comprises the bulk of the melt shows little change in behavior regardless of the length of polymer which makes up the remainder. By contrast, when a species is the minority component, its motion is signif icantly affected by the length of the matrix chains. When a chain is immers ed in a matrix of longer chains, its diffusion coefficient is smaller than its monodisperse value; conversely when a chain is in a blend of shorter ch ain its diffusion coefficient increases compared to a monodisperse melt. Fo r chains shorter than the entanglement length, the diffusion coefficient co mpares well to theoretical predictions. The scaling exponent of the mean sq uare displacement of the longest polymer is found to be sublinear, unless b lended with very short polymers. The scaling exponent seems to be a measure ment of the entanglements that the long polymers experience. (C) 2000 Ameri can Institute of Physics. [S0021-9606(00)52106-0].