The effect of polydispersity on the analysis of optical tracer diffusion experiments

Summary

The effect of intramolecular interference on the wave vector dependence of the average tracer diffusion coefficient in a dynamic light scattering experiment from an optically matched ternary system in which the tracer polymer reptates is examined using a Schulz molecular weight distribution and the Debye interference function for random coils. The average tracer diffusion coefficient in a system in which the tracer is polydisperse is shown to vary continually with q. This variation can be quite substantial for moderate polydispersities even in the regime q¯R_g ⁽ⁿ⁾<1. It is shown that a two parameter linear fit of 〈Γ〉 versus q² data will have a negative intercept and that the apparent q power law dependence of the average decay rate will be greater than 2.