Multiblock Copolymer Solutions in Contact with a Surface: Self-Assembly, Adsorption, and Percolation
Résumé
Amphiphilic copolymers are often used as compatibilizing or stabilizing agents, either in solution or at surfaces. In the special case of multiblock copolymers the connectivity of the blocks combines with the antagonistic behavior of the different types of blocks. Here we report on the behavior of solutions of amphiphilic multiblock copolymers with a large number of blocks and a low fraction of solvophobic monomers in contact with an attractive surface. Using lattice Monte Carlo simulations, the influence on the structures of the solvent quality and the type of surface from noninteracting to strongly attractive to the solvophobic monomers can be assessed. In the presence of a surface bulk micelles are formed that are not different in size and shape from the micelles observed in the absence of a surface. When increasing the surface attraction, solvophobic monomers tend to adsorb either as isolated blocks or forming surface micelles. Evidence is given of a surface concentration threshold above which surface micelles can form due to microphase separation. These surface micelles are in equilibrium with bulk micelles, some of which are connected to the surface through a path of either hydrophobic and/or hydrophilic blocks or hydrophobic cross-links, or both. The size distributions of bulk and connected micelles are similar. With increasing surface concentration surface micelles get organized due to the steric repulsion between core-shell surface micelles. Moreover, these organized surface micelles percolate. The connected micelles form a concentrated layer parallel to the attractive surface. In addition, these systems are governed by two very different time scales: The fast one leads to micellar self-assembly in the bulk and at the surface while the slow one prevents the system from reaching equilibrium in the course of the simulations and corresponds to the transfer of copolymers from the bulk to the attractive surface.