(637h) Integrated Strategy for Formulation of Realistic Potential Models for Simulation of Block Copolymers

Block copolymers (BCPs) have received much attention during the past few years because of their capability to self-assemble in various environments from high to low concentrations. Their ability to self-assemble makes them invaluable in producing different types of complex ordered structures for advanced materials. Therefore, BCPs are utilized extensively in nanostructured applications such as catalysts, organic photovoltaics, semiconductors, and optical coatings. Modeling BCPs is a challenging task as they have a vast variety of time and length scales important in their physical, structural, and dynamic characterization properties. Given the large time and length scales over which this self-assembly evolves, simplified mesoscale models are often employed because of their computational efficiency. However, these simplified mesoscale models possess some limitations that mediate their ability to realistically simulate BCPs. Chief among these are the use of parameters, such as the Flory-Huggins χ parameter, that do not account for the asymmetric interaction of the copolymer blocks and lack of a robust method to optimize the mesoscale energy parameters. While the commonly studied poly(styrene-b-methyl methacrylate) system reasonably approximates a block copolymer system in which the interactions of blocks with other blocks of their own type are relatively equal energetically (and thus this block copolymer is one we refer to as a relatively symmetric block interaction energy system), the vast majority of block copolymers deviate from this behavior. To address these limitations, in our work a coarse-grained mesoscale polymer model has been developed which utilizes interaction energy models and parameters that allow for the asymmetry of block interactions that occurs in many BCPs. Additionally, a detailed all-atom force field has been optimized for hydrocarbon polymers to allow atomistic simulations of block copolymers that reproduce with good accuracy the density, cohesive energy, and phase behavior of simple hydrocarbon block copolymers. This more detailed all-atom model has been used in conjunction with experimental data on block copolymer systems to parameterize more higher accuracy mesoscale homopolymer and block copolymer polymer models. This paper describes our approach to optimizing this all-atom force field to reproduce relevant bulk physiochemical and conformational properties and to properly account for long-range energy and pressure corrections. Specifically results of optimizing force fields for polystyrene and polyisoprene homopolymers will first be discussed. The block copolymer formed from these two homopolymers (i.e. poly(styrene-b-isoprene)) is known to possess a complex asymmetric phase diagram that has been experimentally well-characterized. Results of the ability of our models to reproduce this complex phase behavior will be discussed as a test case for our modeling approach and models.