Insights into Uniaxial Tension and Relaxation of Nanorod-Filled Polymer Vitrimer Nanocomposites: A Molecular Dynamics Simulation

The mechanical properties of polymer nanocomposites (PNCs)dependsensitively on the structure (e.g., orientation, dispersion, and soon) of the incorporated nanofillers. Many studies have shown thatthe alignment of anisotropic nanofillers can improve the mechanicalproperties of PNCs. However, achieving this alignment typically requirescomplex preparation processes. To address this challenge, researchershave introduced dynamic covalent bonds to form reversible cross-linkedpolymer systems, which would lead to unique properties, such as self-healing,recyclability, and reprocessibility. In addition, inspired by theabove ideas, we introduce nanorods as fillers into a linear vitrimersystem to form nanorod vitrimer composites (NVCs). In NVCs, we caneasily manipulate the alignment of the nanorods due to bond exchangereactions (BERs) in the vitrimer matrices. By using coarse-grainedmolecular dynamics (CGMD) simulations, we systematically investigatethe factors affecting the nanorod orientation in NVCs. We find thatthe main factor affecting the nanorod orientation is the network rearrangementcaused by BERs. Specifically, the BER potential barrier (Delta E (sw)) directly determines the probability of BERs'occurrence. At the same time, the increase of the interfacial interactionbetween polymer chains and nanorods (epsilon(np)) confinesthe motion of the active beads, which slows down the rate of BERs.Additionally, the impact of temperature (T) and aspectratio (ld) on the orientation of nanorods during uniaxial stretchingor stress relaxation are also discussed. Finally, by combining uniaxialstretching and stress relaxation processes, we elucidate the orientationretention mechanism of NVCs and demonstrate the mechanical propertyenhancement phenomenon of the pre-oriented NVC systems. This workprovides a simple strategy for manipulating the nanorod alignmentin vitrimer matrices and uncovers guidelines for designing new functionalpolymer vitrimer nanocomposites at the molecular level.