The Self-assembly And Emulsion Properties Of Amphiphilic Copolymers

In this paper, a series of amphiphilic random copolymers Poly(Styrene-co-Acrylic Acid) (P(St-co-AA)) were synthesized. The amphiphilic copolymers could aggregate into spherical micelles in selective solvents. The micelles could be used as particulate emulsifiers for toluene-water emulsions. The phase inversion characteristics of the emulsions were studied. The morphology of the emulsion interfaces were observed by solidification of the emulsions. The results showed that P(St-co-AA) micelles are high-efficiency particulate emulsifiers for Pickering emulsions, and the phase inversion of emulsion could be controlled by formulation variables or pH of the water phase. Very few studies on micelles acting as particulate emulsifiers have been published. As the synthetic process is simple, and all materials are commercially available, our approach may have potential industrial applications.First, using styrene as hydrophobic monomers and acrylic acid as hydrophilic monemers, a series of amphiphilic random copolymers P(St-co-AA) were synthesized by free radical copolymerization. The copolymer structures were examined by FTIR, GPC, DSC and 1H-NMR. The results shows that a series of P(St-co-AA) with similar molecular weight and molecular weight distributions but different ratios between St and AA units were obtained. These copolymers could self-assemble into micelles in selective solvents consist of DMF and water. TEM, DLS and SLS studies showed that polymer micelles were 102 nanometer scale spheres, and the mass distribution in each micelle was uniform. In the self-assembly process, excess amount of water in the selective solvents or low temperature of the solvents could lead the self-assembled micelles aggregate into big micelles. The micelles in aqueous solution are pH- sensitive, becoming more hydrophobic at lower pH, and more hydrophilic at higher pH.The hydrophilicity or hydrophobicity of the micelle surfaces were studied by XPS and contact angles of water drops on the surface of micelle chips under toluene. The polymer micelles could stabilize toluene-water emulsions even at very low concentration(0.5 mg/mL in water phase). Increasing salt concentration of water phase helped raise the emulsion properties of hydrophilic micelles. By changing the monomer ratios between St and AA units, and/ or, by changing the pH of the water phase, the hydrophilicity or hydrophobicity of the micelle surfaces would be changed, therefore, the transitional phase inversion of micelles stabilized emulsions happened. By changing the volume fraction between oil and water phase, the catastrophic phase inversion of emulsions happened, however, micelles with more hydrophilic surfaces could delay the catastrophic phase inversion. It is shown that emulsions stabilized by micelles prepared from nearly equimolar copolymers, exhibit a cycling pH-response: by adding NaOH and HCl, alternately, to the emulsion and re-homogenizing, the type of emulsion inverts repeatedly. In addition, St was introduced as oil phase, the St-water emulsions were solidified after the St was polymerized, and then observed by SEM. The pictures showed spherical particles with surface coated by micelles (for O/W emulsions) and porous structure with internal surface coated by micelles (for W/O emulsions), these results were direct evidences for the same kind of micelles stabiling both W/O and O/W emulsions.