The aggregation phenomena in aqueous solutions of hydrophobically modified (HM) chitosan, containing 4 mol % of n-dodecyl side chains, were studied by viscometry and fluorescence spectroscopy with pyrene as a probe. The results are compared with those for unmodified chitosan. Surprisingly, fluorescence data reveal the appearance of intermolecular hydrophobic aggregates both in chitosan and in HM chitosan. Nevertheless, these polymers exhibit quite different rheological properties: upon the formation of aggregates the viscosity of HM chitosan sharply increases, while that of unmodified chitosan raises only slightly. The aggregation models for both chitosan and its hydrophobic derivative were proposed. It was shown that in solutions of HM chitosan two types of hydrophobic domains exist: hydrophobic domains typical for different associating polymers with hydrophobic side chains and hydrophobic domains inherent to chitosan itself.

The behavior of poly(acrylic acid) (PAA) partly neutralized with tetramethylammonium hydroxide in methanol was studied by the viscometry, 1 H NMR spectroscopy, light scattering, and conductivity methods. It was shown that, when the degree of PAA neutralization increases from 0.1 to 0.2, a transition from a polyelectrolyte to ionomer state occurs. This transition is caused by the formation of ion pairs due to the binding of counterions with the charged groups of the polymer chain and is accompanied by a macromolecule collapse. Collapsed macromolecules aggregate. At degrees of neutralization above 0.4, the restored polyelectrolyte state is accompanied by polymer swelling. The data obtained are qualitatively explained.

The influence of tetramethylammonium chloride on a polyelectrolyte-ionomer behavior of poly(acrylic acid) (PAA) partly neutralized with tetramethylammonium hydroxide in methanol was studied by the viscometry and conductivity methods. It was shown that the addition of a small amount of this low-molecular-weight salt decreases the size of macromolecules in the polyelectrolyte state. When the salt concentration rises, the tendency toward formation of ion pairs is enhanced and the ionomer state becomes thermodynamically more advantageous than the polyelectrolyte state. As a result, macromolecules are transformed into the ionomer state at degrees of polymer neutralization corresponding to the polyelectrolyte behavior of salt-free solutions.