Structural and dynamical properties of thermoplastic polyurethane/fullerene nanocomposites: a molecular dynamics simulations study

Abstract

In this work, the structural and dynamical properties of thermoplastic polyurethane (TPU)/fullerene (C₆₀) nanocomposites are investigated using atomistic molecular dynamics simulations, focusing on the glass transition, thermal expansion, polymer mobility, polymer–C₆₀ interactions, and diffusion behavior of C₆₀. The results show a slight increase in the glass transition temperature (T_g) with increasing C₆₀ weight fraction (wt%), attributed to hindered polymer dynamics, and a remarkable reduction in the coefficient of thermal expansion above T_g. Results of the mean squared displacement and the time decay of bond-reorientation autocorrelation indicate that the mobility of TPU hard segments is more restricted than that of soft segments, owing to the electrostatic attractions and the π–π stacking between isocyanate groups and C₆₀ molecules. Analysis of TPU–C₆₀ interaction energy reveals that the electrostatic interactions are weakened with an increase in the C₆₀ wt%, while the van der Waals contributions become more significant due to the TPU–C₆₀ interfacial characteristics. Further analysis shows that the translational and rotational diffusion of C₆₀ are both increasingly suppressed with the increase of C₆₀ wt%, indicating a violation of Stokes–Einstein (SE) and Stokes–Einstein-Debye (SED) relations, presumably due to the polymer chain-mediated hydrodynamic interactions arising from chain bridges between neighboring C₆₀ particles. This is highlighted by a stronger decoupling of translational–rotational diffusion and a lower ratio of translational–rotational diffusion coefficient (D_T/D_R) with increasing C₆₀ wt%. This work elucidates an atomistic understanding of the structure and properties of polymer/C₆₀ nanocomposites.