Simulation of bicontinuous phase formation in additive-filled and shape-asymmetric diblock copolymers

Cornell University, Ithaca NY

Investigators

Abstract

CBET-0756248 Escobedo Intellectual Merit The goal of this project is to use molecular simulation to (1) quantify the impact of polymeric and nanoparticle additives on the onset and structure of bicontinuous phases in linear diblock copolymers (DBC), and (2) elucidate the effect of entropic disparities between blocks of DBC chains on the behavior of bicontinuous phases. The first goal is focused on understanding how additives with selective affinity for a given block will distribute and modify the structure of complex DBC bicontinuous phases (like the gyroid, double diamond, and plumbers nightmare phases where the minority component block forms two interweaving 3D networks); it is envisioned that a suitable choice of additive type, size, affinity, and concentration may suppress or stabilize a particular bicontinuous phase. A specific aim is thus to elucidate the design of optimal additives (e.g., in size and topology) that maximize the composition range of stability of a target bicontinuous phase. The existence of competing co-continuous phases (those whose minority block forms a single 3D network) will also be investigated. Our second goal is to systematically quantify the effect of disparities in block thickness and backbone flexibility on bicontinuous phase behavior. Athermal molecules having intrinsic disparities in thickness (shape) and stiffness can lead to asymmetrical packing interactions, i.e., an effective "repulsion" between opposite ends of the particles which could give rise to a phase behavior akin to that of conventional DBCs (that have an energetic inter-block disparity). There will be an investigation as to how to design systems where entropy, as opposed to energy, would be the main driving force underlying the assembly of different bicontinuous phases. Starting from the analysis of bicontinuous phases of pure DBCs via both on-lattice Monte Carlo simulations and continuum space Monte Carlo and molecular dynamics simulations, the following tasks are carried out: (i) determining the effect of selective additives (polymers and nanoparticles) of different sizes and structure on such bicontinuous phases, particularly in the particle-concentrated regime, (ii) simulating off-lattice coarse-grained models of DBC-like molecules with varying disparities in block affinity, flexibility, and thickness (pure and with additives) to determine how such changes affect the phase behavior and how they could be exploited to stabilize different bicontinuous phases. To map out reliable phase diagrams and improve ergodic sampling, several Monte Carlo methods are used and further developed; in particular, optimized expandedensemble techniques for measuring free-energies and for chemical potential equilibration. Broader Impacts This investigation provides phase diagrams that will serve as "road maps" which could not only be used to correlate simulations with experimental data but also to guide future experimental efforts toward more technologically targeted systems. Given Today's unprecedented ability to synthesize copolymers of precise architecture and composition as well as hybrid organic-inorganic materials and nanoparticles, a better microscopic understanding of the structure and phase behavior of fluids containing these building blocks could provide a sounder basis for rational design of new materials for future applications, including energy-storing devices like fuel cells. The close collaboration of the PI with an experimental group at Cornell provides the synergy between simulation and experimental efforts and that our findings will also be disseminated within the community of experimental polymer-chemists. Dissemination of results to industry is made through Cornell's annual Polymer Outreach Program symposium. The main educational outcome will be the training of a Ph.D. student who will also serve as a link with an experimental group at Cornell. In addition, it is expected that al least one undergraduate researcher from a different university will work on this project during a Summer via the REU program of CCMR (Cornell Center for Materials Research) and another Cornell undergraduate during two regular Semesters. Results of this investigation will be used in at least two classes: a new course on molecular simulations, and the advanced thermodynamics core course.