Probe sedimentation molecular weight

We begin with two experimental methods, sedimentation and electrophoresis, that measure the driven motion of polymer chains and colloidal particles. In each method, an external force is applied directly to particular molecules in solution, and particle motion is observed. The forces are buoyancy and the Coulomb force. Light pressure ( optical tweezers ) has also been used to move particles this method appears in Chapter 9. Chapter 2 presents phenomenology associated with sedimentation by polymers and sedimentation of particulates through polymer solutions. The sedimentation rate of polymers in homogeneous solution, and the sedimentation of particulate probes through polymer solutions, both depend on the polymer concentration and molecular weight and the size of the particulates. 

At elevated concentrations, hydrodynamic and other interactions between sedimenting molecules become important. Two sorts of sedimentation measurement, involving respectively a binary and a ternary system, then suggest themselves. First, s in binary polymer systems may depend on polymer concentration and molecular weight. Second, the sedimentation rate of colloidal particles or probe polymer molecules through a solution of a second polymer, as might occur in ternary systems, may depend on the second polymer s properties. 

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. 

Koenderink, et al. examined the motion of perfluorinated hydrocarbon spheres through xanthan solutions(72). Depolarized QELSS spectra were measured at a series of angles and fitted to second-order cumulant expansions. The spheres had radius 92.5 nm the xanthan molecular weight was 4 MDa. Koenderink, et al. measured solution viscosity, shear thinning, storage and loss moduli, translational and rotational diffusion coefficients Dp and Dr of the probes, and probe sedimentation coefficient s, and made an extensive and systematic comparison... 

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. 

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