Molecular Dynamics Simulation Of Crack Initiation,Extension And Chemical Bond Fracture Evolution During Fracture Of Elastomeric Materials

Polymer nanocomposites have excellent electrical,mechanical and thermal properties and are used in a wide range of applications in aerospace,biomedical and industrial fields.Fracture behaviour is the most common mode of failure of actual components of polymer nanocomposites and therefore fracture performance is one of their important properties.However,characterising the relationship between microstructure and macroscopic stresses in the fracture process of materials through existing instrumentation is still very limited,and molecular dynamics simulations can be used to obtain information that cannot be measured experimentally,so that the fracture characteristics and mechanisms of materials can be analysed and explained at the molecular level,and the key factors affecting the fracture strength of materials can be identified.This topic uses a coarse-grained molecular dynamics simulation approach and addresses two aspects of the research.(1)Effect of end cross-linking on the fracture properties of polymeric nanocompositesEnd-chain polymer nanocomposites(PNC)were constructed by using nanoparticles(NPs)as cross-linking sites and functionalised end beads of molecular chains were attached to the surface of NPs.The effects of end-graft density,chain length and their polydispersity index(PDI)on the fracture properties of the grafted PNC were analysed.It was shown that the fracture energy rose and then decreased with increasing end-graft density or chain length,and gradually decreased with increasing PDI.More bonds were found to be broken on the main chain of the molecular chain than between the chain and the NPs,which was related to the number of bonds in the chain.Conversely,the proportion of broken bonds between chains and NPs was greater than that between chain majors due to the high stress of one NP or one terminal bead.At the same time,the proportion of broken bonds is greater in short chains than in long chains.As the strain increases,the number of holes first increases and then decreases.The subsequent re-increase in the number of pores is due to newly formed pores caused by the broken chemical bonds.This is evidenced by the fact that the strain at the second rise in the number of holes is approximately the same as the critical strain at the start of bond breakage.(2)Effect of double cross-linked networks on the fracture properties of polymeric materialsA double cross-linked system containing a reversible physical cross-linked network and a permanent chemical cross-linked network was constructed to quantitatively characterise the effect of the ratio of the physical and chemical cross-linked networks on the fracture properties of the material.The effects of the ratio of the double crosslinked network,physical cross-linked bead interactions and chemical cross-linked bond stiffness on the fracture properties of the material were mainly investigated.The material has the best fracture energy when the chemical cross-linked network and the physical cross-linked network have the same cross-link density.The introduction of chemical bonds can effectively increase the stiffness of the material,while the proper introduction of physical cross-linked bonds can increase the toughness of the material without changing the fracture energy.In a dual network system,the physical cross-linked network can act as a sacrificial network,where bond breakage occurs in the covalent bond in the tensile state as an irreversible process,while the physical cross-linked bonds in the physical cross-linked network are reversible.The chemical cross-linked network provides stress support and the introduction of the physical cross-linked network greatly improves the ductility of the polymeric material,increasing the toughness of the material without reducing the mechanical properties and preventing the material from macroscopic fracture.In summary,our results contribute to the design and manufacture of polymeric materials with high fracture properties,thus facilitating their potential applications.