A Coarse‐Grained Model for cis‐Polyisoprene: Thermal Expansion and Glass Transition Temperature

Abstract

A coarse‐grained (CG) model of cis‐1,4‐polyisoprene (cis‐PI) is developed using systematic coarse‐graining strategy. The bonded interactions of the model are captured using analytical potentials with parameters derived to match structural probability distributions of atomistic system. The nonbonded interactions are presented by Lennard‐Jones potentials with parameters specified to match the density and thermal expansion behavior obtained from experiment. The CG force field is validated through the molecular weight dependence of chain dimension and Flory−Fox scaling relationship for the glass transition temperature of the polymer. The model developed provides much faster chain dynamics and simulation speed compared to the reference atomistic one.

A Temperature-Transferable Coarse-Grained Model for Poly(lactic Acid) Melts

“…47−49 In order for the CG model to be applicable at other temperatures, it is necessary to incorporate a temperature-correction term for its potentials. 21,24,50,51 Through our parametric study, we found that the order of influence of the four nonbonded parameters on density is σ BB > σ AA > ε BB > ε AA . To keep it as simple as possible, we chose to change only one parameter of the four.…”

“…It can be seen that the initial CG model (cyan symbols) underpredicts the all-atomistic density (black symbols) at temperatures below 500 K and overpredicts the density at temperatures above 500 K. This discrepancy in the density arises from the fact that there is no chemically specific coarse-graining potential capable of capturing the target properties across a broad range of temperatures. The accuracy of CG models is usually confined to a narrow temperature range close to their calibration state, a phenomenon referred to as the transferability issue. − In order for the CG model to be applicable at other temperatures, it is necessary to incorporate a temperature-correction term for its potentials. ,,, …”

“…The 12–6 Lennard-Jones (LJ) potential was used to describe nonbonded interactions among the CG beads. This selection is made due to its simplicity and computational efficiency in describing both intra- and intermolecular nonbonded interactions. ,, The nonbonded potential has the following form: U nonbond = 4 ε [ true( σ r true) 12 − true( σ r true) 6 ] + S LJ ( r ) Here, σ represents the distance at which the U nonbond between two beads is zero, and ε is the potential well depth. The polynomial term S LJ ( r ) shifts the LJ potential when distances fall below an inner cutoff R inner by a constant value, ensuring a gradual transition to zero energy from the inner cutoff to the outer cutoff R outer .…”

“…This selection is made due to its simplicity and computational efficiency in describing both intra-and intermolecular nonbonded interactions. 21,22,24 The nonbonded potential has the following form:…”

“…However, such large molecular systems as polymers can be computationally very expensive and may not be feasible to study using fully atomistic models. Due to the long relaxation times of real polymer chains, atomistic simulations of polymers are typically suitable only for systems with short chains, where the longest relaxation times are around hundreds of nanoseconds. − To extend the spatial and temporal scales of polymer simulations, coarse-grained (CG) models, in which several atoms are grouped into one CG bead, have been introduced. − Converting atomistic models to CG models by reducing the degrees of freedom enables the use of larger simulation time steps. This also allows the system to move more quickly through configurational space due to lower energy barriers and a smoother energy landscape.…”