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Yaxin An1 Karteek Bejagam1 Sanket A. Deshmukh1

1, Virginia Tech, Blacksburg, Virginia, United States

The self-assembly process of polymers and peptide amphiphile usually involves large time (~microseconds) and length (~micrometers) scales. Modeling of such a process using all-atom (AA) classical molecular dynamics (MD) simulations is a challenge. Coarse-graining is a technique to overcome these challenges, in which a group of atoms is represented by a bead. In this work, the coarse-grained (CG) models of the monomer’s analogues of polystyrene (PS), i.e. ethylbenzene, and that of poly(acrylic acid) (PAA), i.e. propionic acid, are developed firstly to reproduce their corresponding experimentally measured properties at 300 K. A 2:1 mapping scheme is used to map a ethylbenzene molecule to be a C2E bead and three BZ beads, while a propionic acid molecule was represented by a C2E bead and a COOH bead. The C2E bead represents the ethyl group in ethylbenzene and propionic acid, and three BZ beads constitute the phenyl group, and the COOH bead represent the carboxylic acid group. The bonded interaction parameters are obtained by matching the bond length distributions with all AA mapped trajectories. The nonbonded parameters of the C2E and BZ beads were taken directly from our previously developed hydrocarbon and benzene models, while the nonbonded parameters of the COOH bead was optimized to reproduce the density (ρ), self-diffusion coefficient (D), enthalpy of vaporization (Hv), and surface tension (γ) of propionic acid at 300 K. The ρ, Hv, D, γ of CG ethylbenzene and propionic acid models were in good agreement with their corresponding experimental values. The free energies of hydration (ΔGhydr) of ethylbenzene and propionic acid are within 10% of their experimental values. Finally, CG PS and PAA polymer models with 30 monomer units were constructed. The conformation of these polymer chains in a CG water model was studied at 300 K by MD simulations. Results of these CG MD simulations were in good agreement with those of the AA simulations.

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