Role of torsional potential in chain conformation, thermodynamics, and glass formation of simulated polybutadiene melts

F Demydiuk and M Solar and H Meyer and O Benzerara and W Paul and J Baschnagel, JOURNAL OF CHEMICAL PHYSICS, 156, 234902 (2022).

DOI: 10.1063/5.0094536

For polymer chains, the torsional potential is an important intramolecular energy influencing chain flexibility and segmental dynamics. Through molecular dynamics simulations of an atomistic model for melts of cis-trans-1,4-polybutadiene (PBD), we explore the effect of the torsions on conformational properties (bond vector correlations and mean-square internal distances), fundamental thermodynamic quantities (density, compressibility, internal energy, and specific heat), and glass transition temperature T-g. This is achieved by systematically reducing the strength of the torsional potential, starting from the chemically realistic chain (CRC) model with the full potential toward the freely rotating chain (FRC) model without the torsional potential. For the equilibrium liquid, we find that the effect of the torsions on polymer conformations is very weak. Still weaker is the influence on the monomer density rho and isothermal compressibility kappa(T) of the polymer liquid, both of which can be considered as independent of the torsional potential. We show that a van der Waals-like model proposed by Long and Lequeux Eur. Phys. J. E 4, 371 (2001) allows us to describe very well the temperature (T) dependence of rho and kappa(T). We also find that our data obey the linear relation between 1/root kBT rho kappa T and 1/T (with the Boltzmann constant k(B)) that has recently been predicted and verified on the experiment by Mirigian and Schweizer J. Chem. Phys. 140, 194507 (2014). For the equilibrium liquid, simulations result in a specific heat, at constant pressure and at constant volume, which increases on cooling. This T dependence is opposite to the one found experimentally for many polymer liquids, including PBD. We suggest that this difference between simulation and experiment may be attributed to quantum effects due to hydrogen atoms and backbone vibrations, which, by construction, are not included in the classical united-atom model employed here. Finally, we also determine T-g from the density- temperature curve monitored in a finite-rate cooling process. While the influence of the torsional potential on rho(T) is vanishingly small in the equilibrium liquid, the effect of the torsions on T-g is large. We find that T-g decreases by about 150 K when going from the CRC to the FRC model. Published under an exclusive license by AIP Publishing.

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