Interactions of Thermally Responsive Polyelectrolyte Latices with Low Molar Mass Organic Molecules Studied by Light Scattering

A latex of a thermally responsive polyelectrolytic copolymer of N-isopropylacrylamide, methyl methacrylate, and methacrylic acid was synthesized by emulsion polymerization. An aqueous latex of the copolymer and its mixtures with low molar mass organic compounds, ibuprofen, protocatechuic acid, and ephedrine, were studied by dynamic light scattering at temperatures from 20 to 80 °C. In this temperature range, pure copolymer latex particles gradually collapse with increasing temperature. The interactions of low molar mass compounds with the copolymer strongly depend on the ratio of hydrophilic and hydrophobic structural units in the polymer and in the low molar mass compounds. Ibuprofen and protocatechuic acid were strongly bound to the copolymer latex, causing a decrease in the particle size and a sharpening of the volume transition of the latex particles. Ephedrine is a water-soluble substance, and it was bound to the polymer particles below the lower critical solution temperature LCST. Binding of ephedrine to the polymer led to aggregate formation; the aggregates break down when temperature is increased above the LCST.