Rheology of polyacrylate systems depends strongly on architecture

The relation between polymer architecture of polyacrylic acid (PAA) derivatives and the rheological properties is examined in aqueous systems with and without surfactant. The capability of PAA and its derivates to swell above a critical pH, due to the ionization of the acrylic acid groups, gives completely different properties at low and high pH, both for cross-linked and linear polymers. Four different PAA derivatives of different nature were studied. Two versions are chemically cross-linked and two are linear. In two versions, there exist both long and short hydrophobic groups, while in the other two versions, only short hydrophobic groups are present. The role of chemical cross-linking and of long hydrophobic groups, giving a physical cross-linking, is emphasized. The pH-dependent sol-gel transition observed above a certain polymer concentration, as well as the magnitude of the shear viscosity, is strongly affected by the modification of the polymer architecture. Chemical cross-linking has a very large effect on the sol-gel transition induced by increasing the pH, while physical cross-linking, due to the presence of long hydrophobic groups, plays a more moderate role and is more pronounced in the linear PAA derivatives. It was also found that the onset of polymer swelling occurs at a higher pH for the cross-linked versions of the polymers and for the polymers with long hydrophobic groups. The swelling capacity of the PAA derivatives at low pH, when associated with anionic surfactant, was also explored. The results show a similar rheological behavior of the solutions when pH is increased above 6.5 as when anionic surfactant is added above a certain concentration.