Probe sedimentation matrix

In ternary mixtures at small matrix concentration, larger probes sediment more rapidly. However, with increasing matrix concentration the sedimentation coefficient of a larger probe decreases more rapidly than does the sedimentation coefficient of a smaller probe, so at large c large probes sediment more slowly than smaller probes. The a of Eq. 15.1 increases approximately linearly with the hydrodynamic radius of the probe. The a also increases with matrix molecular weight, even when the matrix polymer is much larger than the probes. Indeed, a polymer matrix solution can hinder probe sedimentation when the matrix sediments more rapidly than the probe does. 

From limited results, s(c) of sedimenting probes in a polymer matrix generally follows a stretched exponential in c. With a small matrix polymer, s(c) of a probe chain simply tracks the solution viscosity. In solutions of large matrix polymers, s and rj do not show the same concentration dependence. With probe spheres, 5(c) may track the solution viscosity or may show re-entrant behavior. The agreement of 5(c) with a stretched-exponential form is less outstanding when re-entrance is observed. The literature describes too few probe polymer pairs to be able to say if re-entrant behavior is common or rare. 

At fixed polymer concentration, p/fio decreases with increasing matrix molecular weight. Rodbard and Chrambach found that p is not independent of M, consistent with many other results on probe electrophoresis and sedimentation. Nonlinear (in E) mobility behavior was observed, namely the probe mobility increased at larger applied fields. The nonlinearity was more pronounced at larger polymer concentrations. The dependence of p upon E could be said to be shear thinning, but if so the relevant shear rate (for example, involving a thin layer around each probe) must be quite large, because direct measurement at lower shear rates found no dependence of the macroscopic on /c. 

Chapter 2 treats the sedimentation of polymer molecules and the sedimentation of probe particles and tagged polymer molecules through a solution of matrix polymers. What generalizations can be obtained from the extant phenomenology First, s c) for polymers in a homogeneous binary polymer solvent system almost always follows a stretched exponential in polymer concentration. For probe colloids or chains sedimenting through a polymer matrix solution, s(c) also has a stretched-exponential dependence on matrix polymer concentration. 

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