Table 2. Tabulated optimized CG bending potentials (V angle ) between PS superatoms (monomers) in different systems. θ Vangle-melt Vangle-dilute Vangle-unconfined Vangle-confined Bond angledegrees (kJ/mol)
We have performed detailed molecular dynamics simulations to investigate the effects of solvation and confinement on the structure of polystyrene (PS) oligomers in four different environments, melt, concentrated solution, dilute solution and confined concentrated solution at 450 K and 1 bar, respectively. Local packing of the monomers and the solvent (toluene, good solvent) molecules were monitored by means of radial distribution functions (RDFs). We have also investigated bond, angle, and dihedral angle distributions of the monomers. End-to-end distances, radii of gyration and persistence lengths were calculated to characterize the static properties. The chain in the dilute solution was found to exhibit more stretched conformations. Dilution effect of the solvent was observed in the RDFs between the monomer centers. Only slight conformational changes in the polymers were observed by solvation. The effect of confinement was mainly seen in the density profiles, which showed an oscillatory behavior in the confined system.
ABSTRACT:We explore the limits of a purely structure based coarse-graining technique, the iterative Boltzmann inversion (IBI), in the coarse-graining of a confined concentrated polystyrene solution. In the first place, some technical considerations and challenges encountered in the course of the optimization process are represented. The concepts of the choice of the initial potentials and the cross-dependency of the interactions as well as the order of optimization are discussed in detail. Furthermore, the transferability of a previously developed CG confined polystyrene solution model, the "parent CG confined model", to different degrees of confinement at constant concentration and temperature is examined. We investigate if a CG force field developed for a confined polymer solution by IBI is sensitive to changes in the degree of localization or arrangement of polymers near the surfaces although the concentration is kept constant. For this purpose, reference atomistic simulations on systems of different confinement levels have been performed. The differences in the structure and dynamics of the chains are addressed. Results are compared with those of an unconfined (bulk) system at the same concentration. The chain dimensions and orientations as a function of the distance from the surfaces are also reported. To the best of our knowledge, this is the first computational study that investigates the structural behavior of polymers in close proximity of the surfaces in a concentrated polymer solution rather than in a melt. Transferability of the parent CG confined model is tested by employing the parent force field in CG simulations of the reference systems. Results indicate that the degree of arrangement of monomers and solvent molecules near the surfaces is an important factor that needs to be paid attention to when considering the application of a CG force field developed by IBI to different degrees of confinement.
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