Amphiphilic monomers bridge hydrophobic polymers and water

Abstract

Water dissolves a hydrophilic polymer, but not a hydrophobic polymer. Many monomers of hydrophilic polymers, however, are amphiphilic, with a hydrophobic vinyl group for radical polymerization, as well as a hydrophilic group. Consequently, such an amphiphilic monomer may form solutions with both water and hydrophobic polymers. Ternary mixtures of amphiphilic monomer, hydrophobic polymer, and water have recently been used as precursors for interpenetrating polymer networks of hydrophilic polymers and hydrophobic polymers of unusual properties. However, the phase behavior of the ternary mixtures of amphiphilic monomer, hydrophobic polymer, and water themselves has not been studied. Here we mix the amphiphilic monomer acrylic acid, the hydrophobic polymer poly(methyl methacrylate), and water. In the mixture, the hydrophobic polymer can form various morphologies, including solution, micelle, gel, and polymer glass. We interpret these findings by invoking that the hydrophobic and hydrophilic groups of the amphiphilic monomer enable it to function as a bridge. That is, the hydrophobic functional group binds with the hydrophobic polymer, and the hydrophilic functional group binds with water. This picture leads to a simple modification to the Flory–Huggins theory, which agrees well with our experimental data. Amphiphilic monomers offer a rich area for further study for scientific insight, as well as for expanding opportunities to develop materials of self-assembled structures with unusual properties.