Coarse-graining polymers with the MARTINI force-field: polystyrene as a benchmark case

We hereby introduce a new hybrid thermodynamic-structural approach to the coarse-graining of polymers. The new model is developed within the framework of the MARTINI force-field (Marrink et al., J. Phys. Chem. B, 2007, 111, 7812), which uses mainly thermodynamic properties as targets in the parameterization. We refine the MARTINI procedure by including one additional target property related to the structure of the polymer, namely the radius of gyration. The force-field optimization is mainly based on experimental data. We test our procedure on polystyrene, a standard benchmark for coarse-grained (CG) polymer force-fields. Our model preserves the backbone-ring structure of the molecule, with each monomer represented by four CG beads. Structural properties in the melt are well reproduced, and their scaling with chain length agrees with the available experimental data. The time conversion factor between the CG and the atomistic simulations is nearly constant over a wide temperature range, and the CG force-field shows reasonable transferability between 350 and 600 K. The model is computationally efficient and polymer melts can be simulated over length scales of tens of nanometres and time scales of tens of microseconds. Finally, we tested our model in dilute conditions. The collapse of the polymer chains in a bad solvent and the swelling in a good solvent could be reproduced.