Reactive Molecular Dynamics with Material-Specific Coarse-Grained Potentials: Growth of Polystyrene Chains from Styrene Monomers

Abstract

We have developed a reactive molecular dynamics (RMD) scheme to simulate irreversible polymerization of realistic polymer systems in a coarse-grained resolution. We have studied the chain propagation of styrene to polystyrene. For monodisperse polystyrene samples, we reproduce the results of equilibrium MD simulations: density, end-to-end distance, radius of gyration, and different geometrical distribution functions. The RMD simulations on polydisperse systems should be considered as case studies intended to understand the influence of different tuning parameters of the RMD approach on calculated polymer quantities. The parameters for the irreversible polymerization include the number and position of the initiator units (I*) as well as capture radii r(I) (r(P)) defining the geometrical conditions for chain initiation (propagation) and a characteristic delay time τ(r) separating two reactive MD time steps. As a function of the r(I) (r(P)) and τ(r), it is possible to model polymerization processes both in the limit of almost unrelaxed and fully relaxed samples. The strong influence of the spatial localization of the I* on the polymer size distribution is discussed in detail. The RMD results are used to formulate optimized computational conditions for the simulation of irreversible polymerizations, to explain observed trends in the polydispersity index, and to suggest experiments that might lead to an unexpected polymer size distribution.

A kinetic chain growth algorithm in coarse-grained simulations

“…Here we employ the existing numerical CG potential [28] derived by the IBI method [29] to describe the interactions between monomers of PS chains and styrene beads. As another preset parameter, the capture radius is set to 0:65 nm, in consideration of the size of styrene as around 0:47 nm obtained from radial distribution function (RDF) [2,28] and the choice of 0:4 $ 0:55 nm as the capture radius adopted by Farah et al in a similar model. To account for the stereochemistry of atactic PS chains, two bead types are defined according to the absolute R and S configuration of the styrene monomer given by the direction of the phenyl ring against the backbone.…”

“…In this model, each styrene unit is represented by one CG bead. [2] This issue will be explained in the following. A simulation box with size 40340340 nm 3 is constructed.…”

“…In our previous model, [20][21][22] for a specific active center, we simply set a capture radius. Based on this, Farah et al's "distance-based" protocol [2] appears plausible since closer reactants are more likely to overcome the energy barrier of reaction. Subsequently, we decide whether the monomer will connect with the active center simply by a preset reaction probability.…”

“…[2] In this work, we employed two parameters to control the polymerization process, that is, the characteristic delay time separating two successive reaction attempts, and the capture radius defining the geometrical conditions for chain propagation. To solve the problem of coupling MD-driven dynamics with "reactions" on the CG level, many techniques of treating both the particle dynamics and the reactive processes with force fields were developed.…”

A coarse-grained molecular dynamics – reactive Monte Carlo approach to simulate hyperbranched polycondensation

“…A capture radius criterion similar to the work of Farah et al 33 has been employed in our study to initiate and control chemical reactions. In contrast to past simulation approaches, 33,[36][37][38]78 several rene-ments have been considered in the present reactive model. Thus the probability of a reaction is correlated with the structure of the reacting molecules.…”