Temperature-Transferable Coarse-Grained Models for Volumetric Properties of Poly(lactic Acid).

A new coarse-grained (CG) model, for which each monomer is mapped as one bead at its center of mass, was developed for simulating the volumetric properties of the polylactide (PLA) bulk. The three bonded CG potentials are first parametrized against the strain energies of the dimer, trimer, and tetramer, and the nonbonded CG potentials are then optimized to match the melt densities of the decamer. With the derived CG potentials, molecular dynamics (MD) simulations are found to reproduce thermal expansion and glass transition.

Critical Thicknesses of Free-Standing Thin Films of Molten Polymers: A Multiscale Simulation Study.

The free-standing thin films of melted poly(ethylene oxide) have been extensively simulated by the chemically specific coarse-grained (CG) molecular dynamics (MD) method. It is revealed that if the polymer thin film becomes thinner than some critical value, it would initially turn into a fiber, accompanied by an increase in the free surface area and a decrease in surface tension. A simple but efficient scheme is proposed to determine the critical interfacial thickness and the film thickness from the non-intrinsic density and pressure profiles, and the ratio of the two thicknesses is defined as the interfacial fraction.

Multiscale modeling of thermomechanical properties of stereoregular polymers.

Multiscale coarse-grained (CG) models are expected to play the critical roles in molecular simulations of complex polymers. However, this poses a great challenge for accurately simulating their thermomechanical properties, for which excellent representability and transferability are required for the CG potentials. In this work, virtual sites and elastic network bonds are introduced to improve the structural and volumetric property-based CG models including explicit electrostatic interactions, which is exemplarily applied to the iso- and syndio-tactic poly(methyl methacrylate).

An Enhanced Scheme for Multiscale Modeling of Thermomechanical Properties of Polymer Bulks.

While multiscale modeling significantly enhances the capability of molecular simulations of polymer systems, it is well realized that the systematically derived coarse-grained (CG) models generally underestimate the thermomechanical properties. In this work, a charge-based mapping scheme has been adopted to include explicit electrostatic interactions and benchmarked against two typical polymers, atactic poly(methyl methacrylate) (PMMA) and polystyrene (PS). The CG potentials are parameterized against the oligomer bulks of nine monomers per chain to match the essential structural features and the two basic pressure-volume-temperature (PVT) properties, which are obtained from the all-atomistic (AA) molecular dynamics (MD) simulations at a single elevated temperature.

Tacticity Effects on the Bulk Modulus of Poly(methyl methacrylate) Explored by Coarse-Grained Simulations.

It has long been realized that stereochemistry is quite important to the mechanical properties of relevant polymers, but it still remains far from complete understanding. In this work, the tacticity dependences of the bulk modulus () of two stereoregular poly(methyl methacrylate) bulks have been extensively studied. Both equilibrium (EQ) and nonequilibrium (NE) molecular dynamics (MD) simulations are employed with chemically specific coarse-grained models.

A multiscale scheme for simulating polymer Tg.

All-atomistic (AA) molecular dynamics (MD) is considered as one of the desirable methods for studying glass transition temperatures (Tg) of specific polymers. However, heavy computational efforts are generally required, and the simulated Tg values are not always in good agreement with the experimental data. In this work, a multiscale scheme is proposed: first, the structural and volumetric properties based multiscale modeling is employed to parameterize the coarse-grained (CG) potentials against the AA simulations of an oligomeric melt; with the CG potentials, MD simulations are then carried out on a serial of oligomer bulks and polymer systems of interests, for which the dynamical Tg values are determined.

Re-examining the procedure for simulating polymer T using molecular dynamics.

In this work, the poly(ethylene oxide) bulk as one example has been iteratively heated and cooled back using MD simulations to examine the effects of thermal history on the resulting T. It is demonstrated that, after the system is equilibrated once at the high temperatures, the simulated T does not exhibit a systematical shift with the thermal history, and the averaged T compares well with that for the single procedure, that is, adequately equilibrating at the highest temperature and cooling with the same rate to the lowest temperature. Additionally, the continuous and stepwise processes lead to almost identical T, density and volumetric expansive coefficients at both the glassy and rubbery states at 300 K and 1 atm.

Melt-phase behavior of collapsed PMMA/PVC chains revealed by multiscale simulations.

Single- and double-chain models of three stereoregular polymers, iso- and syndiotactic poly(methyl methacrylate) and isotactic poly(vinyl chloride), were extensively simulated using systematic coarse-grained (CG) potentials. It was found that, in vacuum, all of these long chains collapse in a two-stage process from their fully extended configurations into coils, and the two chains in each double-chain model ultimately become intertwined. Strong intermolecular interactions were found to occur between two chains of the same polymer ("like pairs"), which helps to explain the high densities of single-component melts.

Multiscale simulations of the structure and dynamics of stereoregular poly(methyl methacrylate)s.

The chain tacticity of a polymer is a key influence on its structure and dynamics, which ultimately determine its properties. While they have great potential to elucidate the influence of chain tacticity, all-atom molecular simulations are often restricted to short chains and small systems. In this work, two typical stereoregular poly(methyl methacrylate) melts were investigated via multiscale simulations.

Simulated glass transition of poly(ethylene oxide) bulk and film: a comparative study.

Stepwise cooling molecular dynamics (MD) simulations have been carried out on the bulk and film models for poly(ethylene oxide) (PEO) to understand glass transition of amorphous polymer films. Three types of properties--density, energy, and dynamics--are computed and plotted against the temperature for the two systems. It has been confirmed that all these properties can reveal glass transition in both PEO bulk and film systems.