Engineering Ordered Bicontinuous Networks Formed By Block Copolymers With Additives Using Molecular Modeling

Unlike other simpler morphologies that AB diblock copolymers (DBCs) can form, ordered bicontinuous phases are made of two interweaving network structures of the minority phase A in a matrix of the majority phase B. These network structures are attractive for applications involving ordered nanoscale porous materials including solar cell active membranes, filters, catalysts, and nanolithographic templates. Key challenges in obtaining these structures successfully are: (i) the need for very precise chemistries due to their very limited region of stability in pure block copolymer melts, (ii) the limited tunability of the morphology feature size for specific applications, and (iii) their proclivity for defect formation. The use of additives could provide more handles to tailor feature size and to enhance the stability of bicontinuous phases (e.g., by alleviating the packing frustration of the short A-blocks which creates an entropic penalty that hampers the stability of such phases). We used molecular modeling to delineate phase diagrams, provide design guidelines for lithographic applications, and to explore the nucleation behavior of one of these phases from a disordered melt. We have used different strategies to modify the stability of bicontinuous phases by exploring the effect of distinct additives: (i) A cosurfactant (short DBC) that straddles the interface and alleviates packing frustration in both A and B-domains, (ii) two solvents selective to each phase that swells both domains unevenly (for potential nanolithographic applica- tions), and (iii) an A-selective homopolymer that swells only one of the domains but provides additional configurational entropy to access bicontinuous phases beyond those found in pure DBC melts. A combination of theory and molecular simulations is used to study these systems. Self-consistent field theory is fast and used to calculate free energies of prespecified morphologies but fails to include molecular fluctuations. Coarse-grained molecular simulations are slower and require more sophisticated techniques for calculating free energies but can capture molecular fluctuations and more realistically describe defects and kinetically trapped phases. Bicontinuous phases in the DBC + homopolymer system (namely the Gyroid, Double diamond and Plumbers Nightmare) are particularly challenging because they possess large unit cells with hundreds of molecules (and thousands of monomers) per unit cell, and the observed morphology depends strongly on simulation box size, which is unknown a priori. Accurate free energy estimates are required to ascertain the stable phase, particularly when multiple competing phases spontaneously form at the conditions of interest. A variant of thermodynamic integration was implemented to obtain free energies and hence identify the stable phases and their optimal box sizes. Clear evidence was found of phase coexistence between bicontinuos phases, consistent with previous predictions for the same blend using Self-consistent field theory. Our simulations also allowed us to examine the microscopic details of these coexisting bicontinuous phases and detect key differences between the microstructure of their nodes and struts