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

Abstract

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. By rescaling the dihedral and nonbonded potentials with temperatureindependent factors, the glass transition temperature (T g ) is also satisfactorily restored with little modifications on the volumetric expansive coefficients at both rubbery and glassy states. Therefore, the finally optimized CG potentials exhibit excellent temperature transferability, as rationalized by the Simha-Boyer relation. Furthermore, it is confirmed that the dihedral torsions and nonbonded interactions play key roles in glass transition. Also, the simulated bulk moduli and conformational properties in a wide temperature range compare well with the referenced data. The proposed multiscale scheme has great potential in simulating thermo-mechanical properties of PLA and other polymers.

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

“…For the short to medium chain lengths (10 < N < 100) studied here, our coarse-grained models satisfactorily reproduce the R g and R e values of the atomistic chains. For PLA chains with N = 10 and at 500 K, the simulated R g and R e from our CG model ( R g = 0.63 ± 0.01 nm, R e = 1.61 ± 0.03 nm) are also consistent with those of the CG PLA model recently developed by Wu ( R g = 0.635 ± 0.003 nm, R e = 1.64 ± 0.01 nm). Both the ρ-tuned and ρ–δ-tuned models yield almost the same R g and R e values for all considered chain lengths.…”

“…The selection of a coarse-grained mapping approach is arbitrary and relies on how well it fulfills the intended properties of the model developed. The one-bead-per-monomer , or two-bead-per-monomer , CG models for PLA have been proposed in the past. In this work, we chose a two-bead-per-monomer mapping scheme.…”

“…For PLA, there are only few CG models of this polymer presented in the past. − In 2016, we developed a CG model to predict the glass transition temperature ( T g ) and conformational properties of PLA . For our model, each monomer unit was represented by one CG bead, and the CG potentials were derived using the iterative Boltzmann inversion (IBI) method .…”

“…Later on, their CG model has been refined for investigating the mechanical and structural properties of strained pure PLA and its blends with oligolactic acid . Recently, Wu has proposed a one-bead-per-monomer CG model for predicting volumetric properties of PLA . The bonded CG potentials were parametrized against the strain energies of PLA oligomers from atomistic simulations, and the nonbonded CG potentials were optimized to match the melt densities of the polymer.…”