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  • Online publication date: August 2012

9 - Probe diffusion



This chapter examines the diffusion of mesoscopic rigid probe particles through polymer solutions. These measurements form a valuable complement to studies of polymer self- and tracer diffusion, and to studies of self- and tracer diffusion in colloid suspensions. Any properties that are common to probe diffusion and polymer self-diffusion cannot arise from the flexibility of the polymer probes or from their ability to be interpenetrated by neighboring matrix chains. Any properties that are common to probe diffusion and to colloid diffusion cannot arise from the flexibility of the matrix polymers or from the ability of matrix chains to interpenetrate each other. Conversely, phenomena that require that the probe and matrix macromolecules be able to change shape or to interpenetrate each other will reveal themselves in the differences between probe diffusion, single-chain diffusion, and colloid single-particle diffusion.

In a probe diffusion experiment, one examines the motions of dilute mesoscopic particles dispersed in a polymer solution. In some systems, a single relaxation is found. In others, probe motions involve multiple relaxation processes. Probe diffusion is sensitive to the probe radius R, matrix polymer molecular weight M and concentration c, solution viscosity η, solvent viscosity ηs, and other variables.

The literature examined here includes three major experimental approaches, namely (i) optical probe diffusion studies, largely made with quasi elastic light scattering spectroscopy (QELSS), to observe diffusion of dilute probe particles, (ii) particle tracking studies in which the detailed motions of individual particles are recorded, and (iii) true microrheology measurements of the driven motion of mesoscopic probes.

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