The surface activity and the rheological properties of aqueous solutions of the amphiphilic block copolymer poly(n-butyl acrylate)-block-poly(acrylic acid) (PnBA-b-PAA) were studied as a function of the degree of neutralization, alpha, of the poly(acrylic acid) block. Although the block copolymer spontaneously forms spherical micelles having a stretched PAA corona and a collapsed PnBA core in water for alpha > 0.1, the solutions do not exhibit any surface activity at this degree of neutralization. Cryo-TEM micrographs show that the radii of the hydrophobic core of the largest micelles are as long as the length of the hydrophobic chain. The micelles, however, have a broad size distribution, and on average, as shown by SANS, the micelles are only about half as long. At concentrations as low as 1 wt %, the solutions exhibit highly viscoelastic behavior and have a yield stress value depending on alpha. The globular micelles are highly ordered in the bulk phase, and the viscoelastic properties are a result of the dense packing of the micelles. The addition of salt or cationic surfactants dramatically decreases the viscosity of the solution. The observed properties seem to be due to electrostatic interactions between the PAA chains of the micelles.

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Summary The phase diagram and the properties of the aqueous phases of the AB block copolymer poly-n-butylacrylate-block-polyacrylic acid (nBuA100-b-AA150) has been studied as a function of the neutralisation degree α with NaOH. Aqueous solutions of the block copolymer with α < 0.1 are turbid and separate into two phases while solutions with α > 0.1 are transparent and single phase. The viscosities of such solutions increase with α and pass over a maximum at α = 0.5. Solutions with a concentration of Cp ≥ 1% and at α = 0.5 are highly viscoelastic and have a yield stress value. Cryo-TEM micrographs show that globular micelles are present in the viscoelastic solutions. The micelles have a polydisperse size distribution. The radius of the hydrophobic core varies from about 100 Å to 200 Å what is the length of the fully stretched hydrophobic block. The rheological properties of the solutions are explained on the basis of the globular core–shell micelles where the polyelectrolyte chains form a corona around the hydrophobic block. Most of the counter ions are in the condensed and free state inside the corona. Some of the counter ions are outside of the corona and form an electrical double layer whose thickness depends on α. The viscoelastic properties of the solutions are thus given by the dense packing of the charged globular micelles. The decrease of the viscosity for α > 0.5 comes about by the shrinkage of the thickness of the electrical double layer. With increase of excess salt the double layer and the corona also shrink and the viscoelastic properties break down. With adsorption of zwitterionic surfactants the viscoelastic properties also disappear. Cryo-TEM micrographs of the block copolymers at α = 0.5 with adsorbed zwitterionic surfactants show that the polyelectrolyte chains of the block copolymers are wrapped around small globular micelles of the zwitterionic surfactants. The supra-molecular structures look like raspberry-like micelles. The block copolymers show a very unusual amphiphilic behavior. For a neutralisation degree of α > 0.1 the compounds do not seem to adsorb at the water surface even though the compounds form micelles in the bulk solution. The micellar solutions have the same surface tension of water. The acidic form of the compound however is surface active.