Contraction of a polyelectrolyte upon dilution, light scattering studies on a polycation in basic and acidic water-acetone mixtures

A polycation poly(methacryloylethyl trimethylammonium methylsulfate), PMETMMS, with molar mass M-w = 26 x 10(6) has been studied by viscosity and light scattering in a mixture of water and acetone. Various amounts of a base (sodium hydroxide) or acids (hydrochloric, sulfuric, or phosphoric acid) have been added to the samples. PMETMMS shows typical polyelectrolyte behavior of the reduced viscosity in water-acetone mixtures with acetone mass fraction, gamma, below 0.80. When gamma greater than or equal to 0.80, PMETMMS undergoes a reversible coil-to-globule transition. The collapse can also be induced by dilution of the polymer solution, keeping the solvent composition constant, gamma = 0.70. Correlation functions measured using dynamic light scattering show a decrease of the relaxation time as well as the narrowing of the size distribution of the polymer with dilution. It has been shown that a compact structure instead of an extended coil is formed in a very dilute solution. Base or acid added into the solvent increases the value of the critical polymer concentration at which the collapse occurs. Ina medium with low dielectric constant, base or acid molecules strongly interact with the charged groups of the polycation. Addition of NaOH into the solvent (less than 6 x 10(-4) mol/L) decreases the degree of ionization of the polycation. However, the addition of acid (0.3 x 10(-4) mol/L) introduces counterions that not only screen the positive charges of the polymer chain but also form inter- and intramolecular links. The latter effect becomes more profound with increasing the counterion valency. Upon dilution of the polymer solution, the number of the contacts between the polymer and added base/acid increases, but the number of inter- and intramolecular ion pairs of the polycation decreases. The balance of inter- and intramolecular interactions changes; and at high dilution, the intramolecular interactions dominate, owing to the poor quality of the solvent.