Multiscale Analysis Of Mechanical Properties Of Rubber Nanocomposites

Rubber material as an important engineering material,has been widely used in dynamic working conditions such as tires,conveyor belts and vibration damping bearings due to its high elastic and unique viscoelastic properties.Single rubber materials can no longer fully meet the rapidly developing requirements of application,and they must be modified by nano-reinforcement technology.For rubber nanocomposites,the intervention of nanoparticles leads to the formation of a multi-scale,multi-interaction,multi-network structure inside.In-depth understanding of the structure-property relations of rubber materials is the key to prepare the next generation of advanced materials.With the rapid development of computer performance,computational simulation based on"theoretical models"has become a new paradigm in materials science research.In order to predict the possible structure of a material from given chemical composition and its response properties in the service environment,multi-scale simulations have emerged.Multiscale methods take full advantage of the efficiency of macroscopic scale and the accuracy of microscopic scale,and the key issue is the connection of different scales.In this paper,a multiscale simulation approach was adopted to carry out an in-depth study on the structure-property relations of rubber nanocomposites,starting from the microscopic molecular structures to the application of macroscopic rubber products.(1)At the microscopic level,all-atom(AA)molecular dynamics was used to construct silica/SBR interface model,and a comprehensive analysis of the interfacial structure,dynamics and mechanical properties was carried out to gain insight into the effect of physical interaction or chemical bonding on the interface between silica/rubber.The range of the interphase can be determined using the variation of molecular chain density and proximity structure along the filler surface.Characterization of interfacial interactions,interfacial conformations(trains,loops,tails,free),mean square displacements and interfacial tensile revealed that when the binding energy of the interface is greater than the intramolecular interaction of the matrix,the increasing adsorbed atoms along the filling surface lead to the decrease of flexibility and dynamics in polymer chains,ultimately contributes to an increase in interfacial mechanical properties.Interfacial chemical bonding can reduce the structural differences between the interphase and the bulk as well as the anisotropy of the system.Most importantly,the introduction of chemical bonding leads to 4-5times greater interfacial mechanical properties than the ungrafted coupling agent system.In addition,it was found that the molecular chain conformation is less dependent on temperature,while the interaction energy and dynamics are more affected by temperature.(2)At the mesoscopic level,coarse-grained(CG)molecular dynamics was applied to study the static and dynamic enhancement mechanism of graphene-filled natural rubber(NR).The CG force fields of the nanocomposites took full account of the molecular properties of the different components and were derived from AA simulations,which can reproduce the key structural and mechanical properties in the AA model.The effective CG force fields of NR and graphene were obtained by inverse Boltzmann iteration and strain energy conservation,respectively.Meanwhile,the CG force field of the interphase was obtained by proposing an energy matching method during the graphene pull-out testing.In addition,the interlayer distance distribution of graphene sheets,the entanglement of molecular chains of NR and the conformation of polymer chains on the graphene surface were calculated to quantitatively describe the dispersion of graphene,the entanglement network of rubber matrix and the graphene-rubber network structures.For different graphene filling fractions,three stages of entanglement dynamics,chain confinement,and sufficient transient network can be distinguished for rubber molecular,which corresponds to the three states of dispersion,dispersion and aggregation of the filler.The deformation of graphene facilitates the increase of overall elastic modulus,and the enhancement of matrix chains mainly comes from the increase of interaction enthalpy and the loss of conformational entropy.For the dynamic response in the rubbery state,the initial high enhancement at high frequencies is caused by the slow dynamics of the trapped chains on the graphene surface,while at low frequencies,the gained conformational freedom leads to a relatively low enhancement efficiency that increases exponentially with increasing filling fraction.Finally,the lower percolation threshold,the better dispersion,and the wider accelerated stiffering regime can be obtained in the model incorporated with larger graphene.Finally,a constitutive model derived from microscopic physical and chemical property was used to describe the viscoelastic behavior of rubber materials.All parameters in the constitutive model were obtained through CG molecular dynamics,and this equation can be directly used in the continuum finite element model,thus enabling the connection from mesoscale to fine scale.The viscoelasticity of the rubber was divided into viscous and hyperelastic parts,which were described by a non-affine network model and a modified tube model.(3)Macroscopic aspects,the intrinsic constitutive equations are vitally important to the design and optimization of rubber materials,processing processes and product structures in finite element analysis.A nonlinear viscoelastic algorithm for rubber materials was presented based on the parallel rheological model(PRF),which consists of an elastic element and multiple viscoelastic elements in parallel.The hyperelastic constitutive parameters of the elastic element were obtained by fitting uniaxial tensile stress-strain curves,and the parameters of the PRF model of the viscoelastic element were transformed from the Prony series and optimized by continuous iteration.and the validity of the algorithm was confirmed by stress relaxation experiments.Then,the transient temperature rises and rolling resistance of solid tires were predicted based on the thermomechanical coupling analysis combined with the nonlinear viscoelasticity algorithm mentioned above.The energy generation inside the tire depends mainly on the viscoelastic energy dissipation of the rubber material.A rolling resistance of 3.9 N with a temperature rise of 0.14 ~oC was calculated by PRF model.