Abstract
A coarse-grained (CG) model for atactic polystyrene is presented and studied with classical molecular-dynamics simulations. The interactions between the CG segments are described by Mie potentials, with parameters obtained from a top-down approach using the SAFT-γ methodology. The model is developed by taking a CG model for linear-chain-like backbones with parameters corresponding to those of an alkane and decorating it with side branches with parameters from a force field of toluene, which incorporate an “aromatic-like” nature. The model is validated by comparison with the properties of monodisperse melts, including the effect of temperature and pressure on density, as well as structural properties (the radius of gyration and end-to-end distance as functions of chain length). The model is employed within large-scale simulations that describe the temperature–composition fluid-phase behavior of binary mixtures of polystyrene in n-hexane and n-heptane. A single temperature-independent unlike interaction ene...
Highlights
As a consequence of its relatively simple chemical structure, polystyrene (PS) has become a ubiquitous model system for fundamental studies aimed at understanding the behavior of more-complex macromolecules and supramolecular assemblies
In spite of impressive advances in computer power, the computational expense incurred by retaining such a detailed description still limits the system size of the computational cell, which scales exponentially with the molecular weight of the polymer. This becomes relevant as some macroscopic properties require large systems to properly explore the several coexisting bulk phases. This difficulty can be alleviated through coarse graining; the atomistic detail and the degrees of freedom of the model are reduced by representing groups of atoms as simple segments or “superatoms”, where the interactions are modeled with effective force fields.[8]
Following the SAFT-γ methodology, the estimation of binary-interaction parameters is typically carried out by minimizing an objective function constructed as the difference between an equation of state (EoS) prediction and experimental information related to fluid-phase equilibria, e.g., isothermal pressure−composition or isobaric temperature−composition data, or to other thermodynamic properties such as volumes of mixing.[37,53−56] in the SAFT theory we employ, it is implicit that the chains representing the molecules are linear
Summary
As a consequence of its relatively simple chemical structure, polystyrene (PS) has become a ubiquitous model system for fundamental studies aimed at understanding the behavior of more-complex macromolecules and supramolecular assemblies. Following the SAFT-γ methodology, the estimation of binary-interaction parameters is typically carried out by minimizing an objective function constructed as the difference between an EoS prediction and experimental information related to fluid-phase equilibria, e.g., isothermal pressure−composition or isobaric temperature−composition data, or to other thermodynamic properties such as volumes of mixing.[37,53−56] in the SAFT theory we employ, it is implicit that the chains representing the molecules are linear In this sense, there is a clear mismatch between the theory and the simulations so that this procedure is not directly appropriate for our current study. It is gratifying that, collectively, these results are entirely consistent with the “hourglass” phase diagram, illustrated in Figure 17a, observed experimentally by Cowie and McEwen.[39]
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