|239. Eghbali, E.; Colombani, O.; Drechsler, M.; Müller, A.H.E.; Hoffmann, H.: Rheology and Phase Behavior of Poly(n-Butyl Acrylate)-block-Poly(Acrylic Acid) in Aqueous Solution, Langmuir 22, 4766-4776 (2006)|
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 neutralisation degree, α 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 α > 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% by weight, the solutions exhibit a highly viscoelastic behavior and have a yield stress value depending on α. The globular micelles are highly ordered in the bulk phase and the viscoelastic properties are a result of the dense packing of the micelles. 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.