Block copolymers have the unique ability to microphase separate into nanoscale periodic structures such as spheres, cylinders, gyroids, and lamellae. This phenomenon allows block copolymers to form reproducible features at size scales ranging from nanometers to hundreds of nanometers in both structured films and bulk materials. This natural propensity to microphase separate allows block copolymers to produce nanostructured materials that are useful in applications such as advanced separation membranes, photovoltaics, semiconductor devices, and next-generation batteries. In di-block copolymers, this morphological behavior is determined by a series of physiochemical properties of the polymer including the relative volume fractions of the blocks, total degree of polymerization, and differences in the energy of interaction of the blocks (e.g. as described by parameters such as the Flory-Huggins χ parameter). From a theoretical viewpoint, mean field theory has been the technique most commonly used to calculate phase diagrams representing block copolymer phase behavior. However, the incorporation of many experimentally relevant issues such as polydispersity, homopolymer impurity, and density or cohesive energy density differences have not been studied at the same level of detail as idealized block copolymer systems through theory or simulation due to challenges in incorporating such effects into the models that have been utilized. The goal of this work is to provide an understanding of the influence of such experimentally relevant block copolymer characteristics on the resulting phase behavior of real block copolymer systems. This challenge has been addressed by utilizing molecular dynamics simulations of block copolymers in both bulk and thin film form to understand the phase behavior of such systems while incorporating effects such as polydispersity, homopolymer density differences, and homopolymer cohesive energy differences.
There has been some limited previous work on utilizing simulations to understand the phase behavior of non-idealized block copolymers. For example, previous simulation work using both self-consistent field theory (SCFT) and dissipative particle dynamics (DPD) have been used to show that polydispersity can have a significant impact on BCP phase behavior. Unfortunately, a technique such as SCFT places limitations on the types of effects that can be probed in such block copolymer phase studies. Furthermore, the majority of these previous simulation efforts have utilized a Flory-Huggins (i.e. χ parameter) description of block interactions and thus are incapable of properly accounting for differences in factors such as block cohesive energy density between the polymer blocks. In the work reported here, coarse-grained molecular dynamics (MD) simulations are employed to determine the effects of polydispersity on the phase behavior of a BCP model that closely resembles PS-b-PMMA. In addition, studies on the impact of differences in the physiochemical properties of the constituent block (e.g. block density differences or cohesive energy density differences) will also be reported. It will be shown that the influence of these various factors can dramatically shift the expected phase diagram and behavior for such realistic block copolymer systems.
