Fast macromolecules control mutual diffusion in polymer blends

Abstract

The tracer diffusion coefficient D^* of small concentrations of tagged linear polymers seems well understood on the basis of the reptation model^1–6, but the mutual diffusion coefficient D which characterizes diffusion at higher concentrations remains controversial^7–17. Such diffusion is important in blends of miscible, yet chemically dissimilar, polymers where it controls the kinetics of phase separation and the welding of polymer interfaces. The controversy centres on predicting D from the D^*s of the components of the blend. While the 'slow theory'^7,8 predicts that D is controlled by the diffusion of the slower-moving component, 'fast theory'^9,10 says it is controlled by the diffusion of the faster-moving one. Here we report experiments in which we measure the mutual diffusion coefficient as we increase the tracer diffusion coefficient of the faster-moving component in the blend by decreasing its molecular weight. The strong increase in D we observe demonstrates that the 'fast theory' prediction is correct.