Molecular Dynamics Simulation Of The Structure And Properties Of Vitrimers Elastomer Based On A Dynamic Covalent Polymer Network

Recyclability and reprocessability of permanently cross-linked polymeric materials have received considerable scientific and technological attention in view of the environmental pollution and sustainable development.By introducing dynamic covalent bonds,vitrimers are emerging as a promising attempt to address this pressing challenge.However,traditional experimental methods are difficult to accurately and comprehensively characterize the microstructure changes,and there is still a lack of thermodynamic and kinetic understanding of the bond exchange reactions（BERs）of vitrimers at the molecular level.Herein,by employing coarse-grained molecular dynamics simulations,we successfully constructed a model vitrimer system composed of linear chains.In this work,the structure and properties were investigated.The details are as follows:（1）bond exchange dynamics and mechanical properties of vitrimer materialsFirstly,the constructed model can maintain a constant cross-link density during the bond exchange process,and the mechanism of its bond exchange reaction was association type,which is consistent with Arrhenius and first-order reaction kinetic relationship.In addition,the relaxation kinetic behavior of the system with different potential barriers(ΔΕ_sw)and different temperatures is also consistent with the Arrhenius equation.The results of the stress-strain curves for the system in isovolumetric uniaxial stretching experiments show that the presence of a moderateΔΕ_sw can yield the maximum tensile stress,and the microstructural changes during stretching,such as the degree of molecular chain orientation,the trend of mean square displacement and mean square radius,are consistent with the stress-strain curves.At low potential barriers,the topology of the system is continuously rearranged due to bond exchange,and the relaxation results show that the system can achieve rapid and complete relaxation in a short time range;asΔΕ_sw increases,the topology gradually freezes and the relaxation rate gradually becomes slower.At a moderateΔΕ_sw,the system exhibits the lowest dynamic hysteresis loss.（2）self-healing and extrusion processing performance of vitrimer materialsThe self-healing behavior of differentΔΕ_sw systems was investigated by triaxial stretching.At theΔΕ_sw less than 10,the self-healing efficiency could reach more than 95%,and then decreased with the potential barrier.This is because the lower theΔΕ_sw,the easier it is to achieve the rearrangement of the interfacial topology,which leads to the fusion of the interface and the interpenetration of molecular chains at the interface.Since the system with a potential barrier of 10 exhibited good mechanical properties and excellent self-healing efficiency,fixing the potential barrier and increasing the healing temperature by extending the healing time was found to significantly improve the self-healing efficiency.The simulation results of the extrusion reprocessing process showed that theΔΕ_sw,extrusion force and extrusion temperature showed a linear positive correlation with the extrusion rate.