Subject Semi-dilute solution

Fig. 25 Viscoelastic behavior of a semi-dilute solution of DNA. Elastic modulus (G, filled symbols) and viscous modulus (G", open symbols) are plotted, together with their ratio G"IG = tan 5 (solid curve), for a 93 mg/mL DNA solution subjected to a heating-cooling cycle. The entanglement of DNA helices and, at high temperature, of single strands causes the almost monotonous increase of the dynamic moduli. Reproduced with permission from [110]...

In a semi-dilute solution, the interactions between polymers are screened but, if in a semi-dilute solution of monodisperse polymers, a very long polymer is introduced, the screening for this polymer cannot be complete. Indeed, for a scale of the same order as the size of the very long polymer, the semi-dilute solution may look practically homogeneous, but as was noticed by Flory51 and de Gennes,52 the very long chain is subjected to an excluded volume interaction our aim here is to calculate this effective interaction. 

The zero shear viscosity scales with Nf" to contrast Af dependence for isotropic polymers [20] So far, we have examined the dynamics of rod-Uke macromolecules in isotropic semi-dilute solution. For anisotropic LCP solutions in which the rods are oriented in a certain direction, the diffusion constant increases, and the viscosity decreases, but their scaling behavior with the molecular weight is expected to be unchanged [2,17], Little experimental work has been reported on this subject. The dynamics of thermotropic liquid crystalline polymer melts may be considered as a special case of the concentrated solution with no solvent. Many experimental results [16-18] showed the strong molecular weight dependence of the melt viscosity as predicted by the Doi-Edwards theory. However, the complex rheological behaviors of TLCPs have not been well theorized. 

Let us start with a semi-dilute solution where each monomer is subjected... 

In the preceding section, we have examined the frictional properties of semi-dilute solutions and gels and the analogy between the two systems. In this section, our interest is focused on the dynamics of concentration fluctuations of the polymer. Let us consider first the case of a gel. The polymer network fluctuating around its equilibrium position is subjected to two driving forces The osmotic force tends to equalize the concentration and the elastic force tends to keep the network in its position. The fluctuations are damped by the frictional force between the polymer network and the solvent. 

Even if nothing peculiar occurs at this critical concentration (C ) with respect to the solution properties, the medium is the subject of fluctuations of concentration while passing from the dilute regime to the semi-dilute one (Figure 4.10). 

Solid or semi-solid samples may require extraction with an aqueous solution to isolate the ionic components in a form suitable for IC. The actual procedures vary widely, depending on the type of sample. For example, meat and sausage products to be analyzed for nitrate and nitrite are first homogenized mechanically, extracted wth a 5 % borax buffer solution in a hot water bath, and then subjected to a precipitation with strong solutions of potassium hexacyano ferrate and zinc sulfate. The aqueous extracts are diluted further with de-ionized water and filtered through a membrane prior to injection. 

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