Polyelectrolyte chain contraction

Effects of Ionic Strength. Figure 13 illustrates the effect of NaCl concentration on intrinsic viscosity for each terpolymer. Of course, this experiment should demonstrate only the effects of added electrolyte on individual chain contraction or expansion. The chains with sufficient monomer pairs exhibit increases in viscosity as expected with addition of NaCl. The best chain expansion is seen for the 5-5 sample, which is rapidly solvated with increasing ionic strength. The 5-10 sample shows some typical polyelectrolyte behavior because it has an excess of macroanions at pH 7. 

To verify the predictions of eqn [69] for the osmotic coefficient, Figure 16 shows a universal plot of the reduced osmotic coefficient yo ° /yR as a function of the normalized polymer concentration cjc. For the rodlike chains, we define the overlap concentration c as monomer concentration in the cylindrical zone 4NI nL ), which is an overlap concentration for rodlike polyions. All points collapse onto the universal curve as predicted by eqn [69] for rodlike polyelectrolyte solutions (see Figure 16(a)). However, the size Re of flexible chains is a function of the polymer concentration because polyelectrolytes contract with increasing polymer concentration. To collapse all points into one universal curve and to take into account the chain contraction in Figure 16(b), the reduced osmotic coefficient yo ° /yR is plotted against the ratio of... 

Random coil conformations can range from the spherical contracted state to the fully extended cylindrical or rod-like form. The conformation adopted depends on the charge on the polyion and the effect of the counterions. When the charge is low the conformation is that of a contracted random coil. As the charge increases the chains extend under the influence of mutually repulsive forces to a rod-like form (Jacobsen, 1962). Thus, as a weak polyelectrolyte acid is neutralized, its conformation changes from that of a compact random coil to an extended chain. For example poly(acrylic acid), degree of polymerization 1000, adopts a spherical form with a radius of 20 nm at low pH. As neutralization proceeds the polyion first extends spherically and then becomes rod-like with a maximum extension of 250 nm (Oosawa, 1971). These pH-dependent conformational changes are important to the chemistry of polyelectrolyte cements. 

Polyelectrolyte contraction can be followed by determining the Kj as a function of mobile phase ionic strength (104). In practice, however, the mobile phase ionic strength must be sufficiently high to ensure that the chain is in a contracted state. In this way, small changes in ionic strength, which may be inadvertently introduced during mobile phase preparation, will not affect the elution behavior of the sample. Also, if the ionic... 

The solution properties of polyelectrolytes in general are markedly different from those of polyelectrolyte solutions with added salts. These differences are very strikingly revealed in their viscometric behaviors. Viscosity, as pointed out in the previous section, is related to the size of polymer molecules and therefore is affected by molecular expansion. When a small amount of a simple salt, such as sodium chloride, is added to a dilute polyelectrolyte solution, the ionic strength of the solution outside of the polymer coil is increased relative to the strength of the solution inside of the coil. Consequently, some of the mobile electrolyte diffuses into the polyion coil and the thickness of the ionic atmosphere around the polymer chain is reduced. This effect produces a significant contraction of the polyion coil and is reflected in decreased values of the viscosity. 

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