Molecular Dynamics Simulations Of Mechanical Reinforcement And Interfacial Self-healing Of Elastomer Nanocomposites

With the rapid development of modern technology,elastomer materials have been widely studied and applied as a strategic material.Elastomer nanocomposites have obtained new or better properties on the basis of original properties due to the introduction of nanoparticles（NPs）.Therefore,how to design and prepare high-performance elastomer nanocomposites,as the frontier research direction in the field of materials science,has always been the key problem to be solved urgently.Based on this,it is important to deeply understand the relationship between the microstructure and macroscopic properties of materials,and to design and control the macroscopic properties of materials from the molecular scale.Due to the structural complexity of elastomer nanocomposites,it is difficult to quantitatively characterize the micro-structure of materials at the molecular level by traditional experimental characterization methods.Computer simulation technology is used to accurately track the evolution of the internal structure of the material in situ,which creates a link between the microstructure and the macroscopic performance.In this paper,the molecular dynamics（MD）simulation method is used to systematically study the static mechanical properties and interfacial self-healing properties of elastomer nanocomposites from the molecular scale.The main research results are as follows:（1）A novel kind of bimodal polymer end-linked network employing nanoparticles（NPs）as net-points has been designed and constructed through coarse-grained molecular dynamics simulation（CGMDS）.We systematically explore the effects of the molecular weight（length of the long polymer chains）,chain flexibility and temperature on the fine distribution of the spherical NPs and the resulting mechanical properties of the bimodal network.It is found that a more uniform dispersion of the NPs is realized with the increase of the length of the long polymer chains,the rigidity of short and long chains,and the temperature.There is a linear relationship between the average inter-particle distance of NPs and the arithmetical average of the root mean-squared end-to-end distance of long and short chains.By adopting the uniaxial deformation,the stress-strain curves and the bond orientation were obtained.The results illustrated that introducing the short chains into the uniform long chains network can notably improve the mechanical properties.The bond orientation behavior presented that the short chains were more prone to be oriented and stretched,which contributed to more stress in the process of stretching,while the long chains had a better extensibility.Furthermore,much stronger mechanical properties can be obtained by manipulating the chain stiffness and temperature.Interestingly,the bimodal end-linked network revealed a distinctively enhanced stress-strain behavior versus the temperature,which is opposite to that of traditional physically mixed polymer nanocomposites（PNCs）,attributed to a higher entropic elasticity and more uniform dispersion of NPs of the end-linked system at high temperature.A nonlinear relation for the stress at a fixed strain versus the temperature was obtained.Notably,it is indicated that the contribution of entropy accounts for most of the total stress,while the change of internal energy only accounts for a small part at the room temperature,which is consistent with the experimental observation of the classic rubber elastic theory.In general,this study demonstrates a rational route to precisely control the spatial dispersion of the NPs and effectively tailor the mechanical properties of PNCs.（2）In this work,via CGMDS,we focus our attention on investigating the welding interfacial structure,dynamics and strength by constructing the upper and lower layer of PNCs,by varying the polymer-nanoparticles interaction strengthε_NP-p.Remarkably,at lowε_NP-p,the NPs gradually migrate into the top and bottom surface layer perpendicular to the z-direction during the diffusion process,while they are distributed in the middle region at highε_NP-p,which is further confirmed by the results of non-bonding energies between the NPs-NPs and NPs-polymer,the second virial coefficient B₂ and the average number of neighbor NPs.The density distribution of NPs and polymer chains at different welding time t_w further support the above structural evolution process.Meanwhile,the dimensions of polymer chains are found to exhibit a remarkable anisotropy evidenced by the root-mean-square radius of gyration in the xy-（R_g,xy）and z-（R_g,z）component.The welding interfacial thickness increases the fastest at low,attributed to the high mobility of polymer chains and NPs.Lastly,although the mechanical properties of PNCs at highε_NP-p is the strongest because of the presence of the NPs in the bulk region,the welding efficiency is the greatest at lowε_NP-p.Generally,this work could provide a fundamental understanding of the interfacial welding of polymer nanocomposites,in hopes of guiding to design excellent self-healable PNCs.（3）The amphiphilic nanorods were introduced into immiscible elastomer blends by means of coarse-grained molecular dynamics simulation,focusing on how the nanorods induced and affected the adhesion at the system interface,the mechanical strength and fracture toughness of the mixture.We systematically explored the length diameter ratio of nanorods,the filling fraction of nanorods,the stiffness and flexibility of molecular chains,the interaction strength between molecular chains and nanorods,and compared the effects of nanorods and flexible block molecular chains on the properties of the system.Nanorods can accurately move to the two-phase interface and heal the interface with two immiscible elastomer composites,playing an effective role in interface self-healing.The results show that there is a threshold for the volume fraction of the nanorods with a fixed aspect ratio l/d of 6.If the volume fraction is higher than the strength threshold,the uniaxial tensile strength can be higher than that of the bulk system;When the volume fraction is higher than the toughness threshold,the system can be toughened effectively.When the number of fillers is fixed at120 and the interaction strength is 10.0,the uniaxial tensile stress curve increases with the increase of aspect ratio l/d;In the process of triaxial tension,there is a threshold,which makes the system show a stronger toughening effect than the original system.In addition to the volume fraction and aspect ratio of nano rod,the interaction strength between polymer chain and nanorod is also an important factor affecting the strength and toughness.The interaction strength increases,and the mechanical strength of the system increases.The interaction strength affects the dispersion state of nanorods and then affects the together to determine the toughness of the systems.When the strength is high enough,the negative impact of non-uniform dispersion of nanorods on toughness is weakened and eliminated.With the increase of polymer chain stiffness,the mechanical strength and toughness first increased and then decreased.Comparing nanorods with flexible block polymer chains of the same length,it is found that nanorods have the best performance in terms of mechanical strength and energy dissipation.In general,this study systematically studies the effects of various factors on the interfacial self-healing,strength and toughness of immiscible elastomer materials,and provides a new guiding idea for the design,preparation,recovery and utilization of high-performance multiphase polymer blends in experiments.