(348d) Coarse-Graining Protocol for Mesoscale Simulations of Nanostructured Polymers

We suggest a straightforward technique for obtaining parameters for quasiparticles that can be used in coarse-grained molecular dynamics (CGMD) and dissipative particle dynamics (DPD) simulations of nanostructured polymeric materials.

The molecule of interest is first dissected into fragments of approximately similar size. Based on the chemistry of the fragments, we choose reference compounds that represent the corresponding sections of the molecule. We fit the parameters that describe the conservative interactions of these fragments to simple thermodynamic properties of the solutions of reference compounds, in particular, to the infinite dilution activity coefficients γ_inf of the corresponding binary solutions. This means that γ_inf  for model solutions composed of quasiparticles impersonating different fragments of the molecules of interest have to match the activity coefficient of real  solutions of representative compounds. The latter should be taken from experimental data when available or obtained with thermodynamic models, such as UNIFAC or COSMO-RS.

While fitting coarse-grained parameters to thermodynamic properties of reference systems is by no means new, our approach has several important advantages. Firstly, it is applicable to a variety of coarse-grained methods, including DPD, lattice MC and coarse-grained MD; (2) this scheme requires only standard and inexpensive calculations that do not require extensive experience in molecular simulations; (3) the reference correlations between the parameters of the coarse grained models and γ_inf are determined only by the form of the coarse-grained potentials and therefore have to be calculated only once and apply to parameterization of any system.

We verify our coarse-graining scheme against experimental data on aggregation numbers and critical micelle concentrations of several non-ionic, zwitterionic and ionic surfactants and demonstrate its advantages and limits of applicability. Then, we apply our scheme in DPD simulations of surfactant–polymer systems. SEM studies suggest that Carbopol forms a foam-like gel, which is destructed as the surfactant concentration increases. Our simulations shed light on this phenomenon. We demonstrate that surfactant molecules form micelles inside the polyacrylic acid structure without any appreciable interactions with the polymer. At the same time, surfactants interact intensively with hydrophobic linkers that may cause polymer network rapture. Using DPD simulations, we show how the gel nanostructured morphology and the disjoining pressure in the polymer foam films depend on the chemical composition and surfactant concentration.