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

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.