Polyelectrolyte complexes measurement

The reaction between poly-4-vinylpyridine and PAA in water-ethanol (1 1 by volume) solutions has been investigated by calorimetry,2). This reaction proceeds without the release of H+ or OH- ions. As the heat of dissociation of the polyacid and the heat of formation of ionic bonds between macromolecular components are near zero, the protonation heats of PVPy at different pH both in the presence or absence of PAA have been measured. It has been found that in neutral solutions the heats of polyvinylpyridine protonation in the presence of PAA considerably exceeds the corresponding values in the absence of PAA, i.e. a considerable portion of pyridine rings is protonated in the polyelectrolyte complexes (Fig. 12). This may be caused only by the cooperative trasfer of the proton from the PAA carboxy group to the pyridine ring. Similar reactions cannot occur between low molecular model substances and neither when only one component is a polymer. 

The osmotic pressure ti of a polyelectrolyte solution is complex, since it depends largely on the amount of salt added. In order to determine the molecular weight of the polyelectrolyte by measuring the osmotic pressure it is important to add a small amount of salt and plot nfc against the concentration c at a constant salt concentration. 

Polyelectrolytic complexes have been prepared by the reaction of chitosan with sodium carboxymethyldextran. Water-soluble precipitates were formed whose composition was affected by the reaction pH, order of mixing of the polyelectrolytes, and the molar ratio of amino-sugar to Na+ ion. There was no measureable interaction between carboxy- and hydroxy-groups in the polysaccharides. The number of binding sites in the complex prepared at pH 3.0 was lower than that in the complex prepared at pH 6.5 but both were able to suppress the coagulation of blood. 

Carriere D, Dubois M, Schonhoff M, Zemb T, Mdhwald H (2006) Counter-ion activity and microstractuie in polyelectrolyte complexes as determined by osmotic pressure measurements. Phys Chem Chem Phys 8 3141—3146... 

One way to study polyelectrolyte complex formation in solution is using dynamic light scattering (DLS) titrations on micelle-forming systems. Here, a solution of polyelectrolytes is titrated to a solution of oppositely charged polyelectrolytes, at least one of the polyelectrolytes should have a neutral hydrophilic block, and after every addition the intensity and hydrodynamic radius are measured. When a pH electrode is fitted into the measuring cell, the pH can be followed during the titration. In this titration, nanoparticles will be formed (instead of insoluble polyelectrolyte complexes) with a polyelectrolyte core and a neutral corona. 

Fig. 15 AFM measurement in which the interfacial tension of a polyelectrolyte complex is measured [75, 78]...

The stoichiometry of polyelectrolyte complexes is an important property, determined by viscosity measurements in solution (dilute enough such that precipitation does not occur) or by conductance measurements. For non-neutrahzed acid-base complexes (e.g., PSSA/PVBTAOH), a minimum in conductance occurs at the stoichiometric equivalence point as the charged groups are neutralized. For neutralized acid-base combinations (e.g., NaPSSA/PVBTACl), a maximum in conductance occurs as the interacting acid-base pairs release salt (e.g., NaCl), which is ionized in water yielding high conductance. This was well demonstrated by Michaels et al. [924] for the examples noted. Poly(4-vinyl pyridine-HCl)... 

An example of the use of conductance to estimate the complex stoichiometry is illustrated in Fig. 4.27 for poly(vinyl amine)/poly(acrylic acid) (PVAm/PAA). The minimum in conductance depends on the formation of the complex. In the forward curve, PAA was added at predetermined increments to a PVAm solution, followed by 60 s stirring and conductance measurements. The reverse curve involved PVAm addition to a PAA solution. The minimum in conductance depends on the path chosen to prepare the complexes, indicating that tmcom-plexed polymer maybe trapped during the polyelectrolyte complex phase separation process. As the minimum points are on both sides of the equimolar position, this complex appears to be equimolar. 

Figure4.27 Conductance measurements on PVAm/PAA polyelectrolyte complexes (unpublished data of L. M. Robeson)...

The aim of this review is to demonstrate the potential of surface forces measurement as a novel means for investigating surfaces and complex soft systems by describing our recent studies, which include cluster formation of alcohol, polyion adsorption, and polyelectrolyte brushes. 

In conclusion one can say that SEC is a very powerful method for polymer characterization, especially in combination with other composition sensitive or absolute calibration methods. A big advantage is also that the sample amount is fairly small, typically 10 mg. For more complex polymers, such as polyelectrolytes, enthalpic effects often become dominant and also for rather high molecular weight polymers chromatographic methods such as field-flow fraction (FFF) techniques might be more suitable. For fast routine measurements linear columns are often used. 

---