Concentration-dependent switch between chain association and dissociation of oppositely charged weak polyelectrolytes in solution

Abstract

Joint solutions of oppositely charged weak polyelectrolytes are considerably less studied than their strong counterparts; as a result, their thermodynamic understanding is still unsatisfactory. This shortcoming hampers the development of a general picture about the physical properties of these mixtures, which further hampers their use to design new materials. To close this gap, we investigate the ternary system ethanol/polyacid/polybase (polyacid: methacrylic acid containing copolymer; polybase: N,N-dimethylaminoethyl methacrylate containing terpolymer) with respect to its demixing and viscometric behavior. Complete homogeneity can only be reached if the total polymer concentration remains below 0.005 g dL−1. The locations of the tie lines in the phase diagram reveal that the interpolymer contacts are favorable at low polymer concentrations but unfavorable at high concentrations; viscosity measurements corroborate these findings. Adding either acid or base to the solvent extends the region of homogeneity, where low concentrations of HCl split the two-phase region into two separate parts. This phenomenon is rationalized in terms of concentration-dependent changes in the degree of ionization of the polybase.