Elasticity of Self-Assembled Block Copolymers in Water and Oil Mixtures

Amphiphilic block copolymers self-assemble at water/oil interface and form different mesomorphic structures such as lamellar, micellar cubic, normal hexagonal and reverse hexagonal. Usually, these structures are polycrystalline and their elasticity depends on the orientation of their constituent’s single crystals. We provide a model to predict the elasticity and yielding of mesophases from their characteristic length and intermicellar interactions. Shear modulus of each structure has been calculated as a function of deformation (strain) applied in one direction. Zero shear modulus, G₀, depends on the inverse of intermicellar distance with a power-law model. The power-law index for each structure is about n+2 where n is the degree of confinement in mesophase. Rheological properties of different mesophases of Pluronic P84 in the presence of water and p-xylene are used as case study. Our model is in good agreement with experimental data in the linear viscoelastic region. However, the yield strain value of experimental data is slightly lower than that of the model. Frequency sweep measurements were done to further characterize each mesophase structure and cooperative model was used to fit the frequency sweep data of mesopahses.