Elastic properties of polymer interfaces: aggregation of pure diblock, mixed diblock, and triblock copolymers

Block copolymers adsorbing to an interface between two immiscible homopolymers modify the elastic constants of this interface. Within self-consistent field calculations for Gaussian chains, we determine how the bending constants vary in dependence on the block copolymer concentration and architecture. Four phenomena are discussed. (i) When a tricritical or isotropic Lifshitz critical point is approached in a ternary mixture by varying the concentration of diblock copolymers or changing temperature, the elastic constants vanish. We determine the corresponding power laws, and show that the de Gennes-Taupin criterium for the stability of lamellar phases against undulations and the Ginzburg-Landau criterium for bulk fluctuations yield identical predictions for the validity of the mean-field approximation. (ii) Addition of a small amount of diblock copolymers modifies the bending rigidity. If the diblock copolymers are comparable in length to the homopolymers, adsorption of the diblocks reduces the bending rigidity. If the diblocks are much longer, they increase the bending rigidity. Only for an extreme ratio of chain lengths (>100), the predictions for polymers tethered to an infinitely thin, impenetrable sheet become accurate. (iii) Mixtures of short and long symmetric diblock copolymers are studied, as well as mixtures of two asymmetric diblocks, which are obtained by exchanging the long and short ends. The saddle-splay modulus is found to be the same in both mixtures, while the bending rigidity is significantly smaller in the latter case. (iv) The role of the block copolymer architecture is studied by comparing the effect of triblock copolymers with the effect of diblocks with the same overall length and composition. We propose that triblock copolymers are a very efficient way to control the spontaneous curvature of an interface.