Microphase separation under constraints: a molecular thermodynamic theory for polyelectrolytic amphiphilic model networks in water

Abstract

The aqueous aggregation behavior of covalent model networks comprising amphiphilic ABA triblock copolymers with hydrophilic ionic mid-blocks and hydrophobic non-ionic end-blocks was studied by formulating a molecular thermodynamic theory, which considers the Gibbs free energies of the various possible morphologies of the networks: the spherical, the cylindrical, the lamellar and the disordered (unimers). The appropriate expressions for the elastic, mixing, electrostatic and interfacial components of the Gibbs free energy were developed for each of the four cases, and the prevailing morphologies were identified by numerical free energy minimization. The results are presented on a phase diagram with axes the degree of ionization of the hydrophilic block and the content in hydrophobic units. At very low degrees of ionization (1%), an increase in the hydrophobic content from 0.83 to 99.17% leads successively to the following structures: spheres, cylinders, lamellae, and shrunk unimers, in this order. An increase in the degree of ionization above 10% completely suppresses the formation of cylinders and lamellae. At a 50% degree of ionization, a variation in hydrophobicity from 0.83 to 99.17% results in the formation of only spheres, swollen unimers, spheres again, and shrunk unimers, in this order. The change in morphology of the networks with respect to their degree of ionization and hydrophobicity is reflected in their degrees of swelling which present discontinuities at the points of the morphological transitions. The phase behavior of networks comprising amphiphilic statistical (random) copolymers of hydrophilic ionizable units and hydrophobic units is known from the work of Dušek and Tanaka and presents only shrunk and swollen unimers. Thus, the constitution of the networks of block rather than random amphiphilic copolymers enriches their phase diagram with spherical, cylindrical and lamellar morphologies.