Electric birefringence and zero-shear viscosity studies of the conformational changes of polyacrylic and polymethacrylic acids with pH and concentration

The conformational transitions of polyacrylic (PAA-205 and PAA-250) and polymethacrylic (PMA-100) acids have been investigated by means of electric birefringence and zero-shear viscosity measurements over a wide pH and concentration range. The conformation of the polyelectrolytes has been found to depend on both the pH and the polymer concentration. At low pH the polyelectrolytes have a hypercoiled sphere conformation. At pH ca. 6, the polymer coils begin to open, while at higher pH (ca. 8) the chains are stretched out due to the electrostatic repulsion of the anionic groups. A further increase in the pH leads to a recoiling of the polyelectrolytes attributed to an increase in the ionic strength of the solution. At constant pH an increase in the polymer concentration causes a recoiling of the polyelectrolyte chain in the semidilute concentration region. The particle lengths have also been estimated, providing further evidence to the dependence of the molecule sizes on both the pH and polymer concentration. Moreover, the particle lengths have been found to increase with increasing molecular weight of PAA. Suggestion is made that opening of the polyelectrolyte chains occurs in a progressive way.