(660h) Parametrization of Coarse-Grained Models for Real Polymer Systems to Simulate Block Copolymers

Block copolymers (BCPs) have attracted much attention due to their capability to self-assemble into various complex ordered nanostructures including spheres, cylinders, gyroids, and lamellae because of chemically dissimilar blocks. Thus, BCPs are utilized in fields such as catalysts, drug delivery, optical coatings, organic photovoltaics, and semiconductors. However, optimizing the conditions to produce defect-free ordered morphologies is still challenging. Application of molecular dynamics (MD) simulations can improve our understanding of BCP self-assembly, optimization of the conditions for obtaining a well-defined morphology of a BCP, and reduction of the time and material required to examine these properties and design relationships experimentally.

Although many different MD models have been created to describe and predict the morphology of BCPs, the vast majority of them utilize coarse-grained polymer models and the parameters chosen for those models are most commonly optimized through coarse-grained simulations themselves to produce the desired or expected properties for the polymer system of interest. Given the large size of this model parameter space, multiple coarse-grained model parameter sets can generally be found to produce similar model property predictions for the various limited set of properties chosen for model validation. Furthermore, the vast majority of such coarse-grained MD models can only somewhat accurately reproduce the behavior of ideal BCPs with symmetric phase envelopes. Such models generally cannot predict the complex phase behavior and morphology of BCPs such as poly(styrene-b-isoprene), or as it is referred to later PS-b-PI. To produce a unique coarse-grained model parameter set that includes the fundamental behavior of BCPs with complex inter-block interactions, this work has focused on performing atomistic simulations of such BCP homopolymer pairs and optimizing coarse-grained model parameters to reproduce the significantly richer behavior produced by the atomistic models. In this work, both atomistic and coarse-grained force-fields for polystyrene and polyisoprene have been developed to capture the structural and thermodynamic properties of these polymers including radius of gyration, characteristic ratio, density, cohesive energy, coefficient of thermal expansion, and glass transition temperature.

This presentation will describe our approach which first focuses on optimizing the atomistic force-field to accurately reproduce various homopolymer properties and then second optimizing a coarse-grained model that can faithfully reproduce both the relevant bulk physiochemical polymer properties and the more detailed conformational properties available from atomistic simulations. Specifically, atomistic and coarse-grained force fields for polystyrene and polyisoprene homopolymers have been produced and validated with a variety of available data and properties for these homopolymers. These homopolymer models have then been used to construct a coarse-grained BCP model for PS-b-PI, and the ability of that model to predict the complex phase behavior of PS-b-PI has been studied and will be discussed. Best practices learned for the simulation and mapping of such force fields will be discussed.