Effects of Ionic Additives and Ionic Comonomers on the Temperature and Pressure Responsive Behavior of Thermoresponsive Polymers in Aqueous Solutions

The cloud points of the aqueous solutions of poly(N-vinylisobutyramide) (PNVIBA) and poly(N-isopropylacrylamide) (PNIPAM) were measured down to a subzero temperature under elevated pressures, and the effects of ionic additives and the ionic comonomer were studied. The polymers with higher molecular weight showed lower transition pressure (Pt) and smaller ellipsoids, due to larger absolute values of ΔCp and Δβ. Addition of 0.02% sodium dodecyl sulfate (SDS) into aqueous solutions of these polymers apparently extinguished the clouding phenomenon, but even under these conditions the dynamic light scattering measurement showed the chain collapse of the random coil preceding the interchain aggregations. Pt and transition temperature (Tt) were highly dependent on the addition of inert anions. Less lyotropic ions showed lower Pt and Tt, producing smaller ellipses as a result, while additions of SCN- and I- caused a slight increase in both Pt and Tt and therefore larger ellipses. Introduction of ionic comonomer (4-pentenoic acid) into PNIPAM (less than 20%) made the transition P−T curves pH dependent and Pt was much more influenced by the charged comonomer than Tt. Anionic residues less than 5% were sufficient to screen the cloud point (20 °C and <400 MPa).