Curvature elasticity of diblock copolymer monolayers

Abstract

We study the curvature elastic properties of monolayers of diblock copolymers adsorbed at the interface of two incompatible solvents which are also selective solvents for the two blocks. At saturation, the interfacial free energy is minimized with respect to contributions from the chain conformation free energy, the interfacial tension, and the two-dimensional translational entropy of the chains. For a curved interface, this minimization leads naturally to curvature elasticity. The three elastic coefficients, the spontaneous radius of curvature, the bending modulus, and the Gaussian bending modulus, as functions of the molecular weights, the interfacial tension, the interaction parameters, etc., are obtained for a number of cases. Our study employs the theory for grafted chains recently developed by Milner et al. to obtain the chain conformation free energy which takes into account the nonuniformity of the chain-end distribution. This improvement not only affects the overall prefactor of the free energy but it changes the relative values of the three elastic coefficients as well. We consider the cases of both a swollen monolayer and a monolayer consisting of a melt of copolymer chains, as well as an interesting case where one of the blocks is in the swollen condition and the other block is in the melt condition. Because the chains in the melt and the swollen conditions have distinctively different scaling behaviors, the mixed case displays some features that are different from either the swollen and the melt cases.