Tuning The Properties Of Multiple Networks In Rubber Materials Via Molecular Dynamics Simulations And A Development Of The Simulation Method

As an important strategic resource,rubber materials have attracted much attention because of its unique entropic elasticity and recoverability.Understanding the relationship between structure and properties of rubber materials is the key to prepare the next generation of advanced rubber materials.In order to accelerate the processes of design,preparation,deployment and application of new rubber materials,the "materials genome initiative" emerges with the urgent requirement.The complicated and hierarchical structureproperty relationship of rubber materials are revealed by high-throughput computational simulations and integrated into the high-throughput database to further direct the preparation of new rubber materials.Materials preparation are stepping into the computational materials by design age.In this paper,a series of molecular dynamics simulations were employed to reveal the relationship between the microstructure and static mechanical properties of several special covalent bond networks and non-bonded interaction networks within rubber materials in the view of molecular scale.At the same time,some perspectives were proposed to solve the problems in quantitatively calculating the static mechanical properties of rubber materials by molecular dynamics simulation.The main results are as follows:(1)Through coarse-grained molecular dynamics simulation,we have successfully designed the chemically cross-linked fixed junction and the slidering junction(SR)systems.Firstly,we investigated the dynamic properties such as the mean-square displacement,the bond-to-bond and end-to-end autocorrelation functions as a function of the cross-linking density,consistently pointing out that the SR system exhibits much lower mobility compared with the fixed junction system at the same cross-linking density.This can be further validated by a relatively higher glass transition temperature for the SR system compared with that of the fixed junction system.Then the effects of the crosslinking density on the stretch-recovery behavior for the SR and fixed junctions systems were examined.Although the chain orientation of the SR system is higher than that of the fixed junction system,the tensile stress is smaller than the latter.It can be inferred that much greater chain sliding can occur during the stretch because the movable ring structure homogeneously sustain the external force in the SR system,which,therefore,leads to much larger permanent set and higher hysteresis during the recovery process compared with the fixed junction one.Based on the stretch-recovery behavior for various cross-linking densities,we obtain the change of the hysteresis loss,which is larger for the SR system than that of the fixed junction system.Lastly we note that the relatively bigger compressive stress for the SR system results from the aggregation of the rigid rings compared with the fixed junction system.(2)This subject also considers the properties of a special kind of double sacrificial multiple networks(MNs),which are composed of one chemically cross-linked covalent network,one non-covalent hydrogen bond network and one non-covalent coordination network.The effects of the chemical crosslinking density,number of the coordination beads and the coordination interaction strength on the structural characteristics of this MNs are examined.Larger concentration of coordination beads and stronger coordination interactions are shown to result in a larger coordination network which partly replaces the hydrogen bond network.We evaluate the toughening mechanism of the MNs and determine the key parameters underlying the toughness of the MNs during the tri-axial deformation.In systems with strong coordination interactions,the enhanced toughness is shown to arise from the long and oriented polymer fibers induced by physical cross-linking sites in coordination network.Further,it can be concluded that the characteristic sequential breakage of the different sacrificial non-covalent networks occurs during deformation,with the hydrogen bond network sacrificed to dissipate energy during initiation of strain,followed by the coordination interaction network at much larger magnitudes of tensile strain.(3)Two methods are proposed to calculate the mechanical properties based on a fully atomistic model of crosslinked styrene-butadiene rubber(SBR)network via molecular dynamics simulation.Firstly,in the stepwise deformation and relaxation method,by keeping the tensile velocity constant and regulating the simulation time that each deformation period takes,sufficient relaxation occurs in the consecutive deformation,which enables each deformation period followed by a corresponding relaxation period.The engineering stress-strain curve finally obtained by this stepwise deformation and relaxation method reflects similar and typical features of elastomers compared with the experimental results.The second method divides the whole deformation process into several discontinuous parts.Sufficient relaxation also occurs in several independent SBR systems at the specific strain and the relaxed stress plateau is employed to represent the stress that SBR system exhibits at specific strain after allowing the system to relax.The second method could also successfully characterize the typical mechanical behavior of elastomers and improve the computational capability on the level of a large-scale system,compared with the traditional uniaxial tensile method and the first method.To sum up,compared with the excessively large values obtained by traditional method,the methods we proposed exhibit more realistic static mechanical behavior and merely exceeds around one order of magnitude than the experimental results.