Nanoparticle Diffusion-dispersion-aggregation Interfacial Interaction And Mechanical Reinforcement Mechanism In Elastomer-based Composites:Insights From Molecular Dynamics Simulation

In the past several decades, with the rapid development of nano-technology, polymer-based nano-materials have been highlighted in the field of materials science and engineering, which possess great values of scientific research and potential of industrial application. Elastomer-based nano-materials, as one of polymer-based nano-materials, have been supporting and bearing the natural work of modern society and industry, attributed to its high elasticity. Accompanying that the two new concepts such as nano-materials and nano-compounding materials have been put forward and developed, carbon black filled and reinforced rubber system, as the main material of automobile tires, have been widely considered to belong to one kind of nano-compounding materials, and nano-reinforcement is regarded as being essential and necessary for the realization of highly efficient reinforcement of rubber. In the research of structure and property, elastomer-based nano-materials possess complicated microstructure, exhibiting characteristics of many length and time scales, many interactions and network structures, which results in that the investigation of the microstructure is far from the overall and fine characterization, attributed to the limitation of experimental research means, as well further leads to that the quantitative correlation between the microstructure and static/dynamic macro-mechanical properties is lack of systematic and comprehensive study. Obviously, these difficulties and challenges existing in basic scientific research will directly block the further industrial application of elastomer-based nano-materials.Based on the above research background and status, this work is mainly focused on the microstructure and mechanical property, carrying out deep and systematic research through molecular dynamics simulation. The main novelties are summarized as follows:First, elastomer three-dimensional network structure:The three-dimensional structure of polymer network was successfully constructed through molecular dynamics simulation. The effect of the cross-linking density on elastomer structure and dynamics was systematically investigated. The increase of the cross-linking density leads to some extent of the increase of the glass transition Tg, but with no prominent influence. For cross-linked system, the time-temperature superposition principle (TTSP) is applicable on the segmental length scale, but breaks down at the chain length scale. While for uncross-linked system, TTSP applies at both chain and segmental length scales. Moreover, with the increase of the cross-linking density, the cross-linked system becomes more fragile. Additionally the result indicates that internal energy contributes greatly to the elasticity.Second, diffusion-dispersion-aggregation of nanoparticles in elastomer:(ⅰ) The diffusion behavior of the nanoparticles in elastomer matrices was firstly explored, and the results indicated that when the radius of chain gyration Rg is smaller than the nanoparticle radius R(Rg<R), the Stokes-Einstein law can accurately describe the behavior of the nanoparticle diffusion, while for Rg>R, the Stokes-Einstein law does not validate, attributed to the nanoparticles experiencing the nano-viscosity rather than the macro-viscosity, and the diffusion coefficient of the nanoparticles is inversely proportional to the cube of the hydrodynamic radius. In the regime of Rg>R, the diffusion of the nanoparticles is independent of the chain length, but relies on the nanoparticle mass. This work provides theoretical explanation for the deviation observed in the experiment when using the Stokes-Einstein law to describe the diffusion of the nanoparticles in polymer melts.(ⅱ) The nanoparticle dispersion and aggregation behavior and mechanism in elastomer matrices, corresponding to the thermodynamic equilibrium state, was systematically investigated. The results indicate that at moderate interfacial interaction between nanoparticles and polymer, the nanoparticles exhibit a homogeneous dispersion. Further analysis indicates that at low interfacial interaction, the nanoparticles aggregate through direct contact, and at high interfacial interaction, the local aggregation of nanoparticles appears attributed to the adsorption of several nanoparticles onto a single polymer chain at the same time, but at moderate interfacial interaction, nanoparticles has a stable and good dispersion because of a thin adsorbed polymer layer on the nanoparticle surface. The dispersion of nanoparticles can be greatly improved by grafting polymer chains on its surface. Besides, the behavior of the nanoparticle aggregation accord with the Arrhenius relation, the aggregation rate of the nanoparticles decreases with the increase of the interfacial interaction, and increases with the decrease of the nanoparticle size. Generally this work provides significant practical guidelines about how to obtain a homogeneous nanoparticle dispersion and effectively hinder the aggregation of the nanoparticles.Third, interfacial interaction between nanoparticles and elastomer:(i) The effect of the nanoparticles on the chain and segmental relaxation behavior were systematically investigated, putting forward a new concept, namely time-temperature-concentration superposition principle (TTSPP). This result can be well applied to investigate the rheological and processing properties of polymer nanocomposites, for instance the viscosity of the system under a large external shear field exhibits the equivalent result for the viscosity at a higher temperature, or for the case at a much lower loading of nanoparticles. Meanwhile during the stress relaxation process after the tensile deformation, the time-temperature superposition principle (TTSP) applies for the unfilled system, but breaks down for the filled system. (ⅱ) The interfacial interaction manner and mechanism was deeply investigated, finding that the interfacial region is composed of partial segments of polymer chains. Meanwhile it is found that the thickness of the interfacial region is within the radius of chain gyration Rg, which is independent of the interfacial interaction and nanoparticle size. Further results indicate that for the interfacial interaction within the range of the hydrogen bond, the adsorbed polymer chains can still undergo adsorption-desorption process, and the dynamics of the interfacial chains is far from the corresponding glassy state, thereby denying the existence of the polymer glassy layer around nanoparticles. For the exploration of the bound rubber, it is found that the increase of the bound rubber as a function of the storage time results from the replacement of the short chains adsorbed on the filler surface gradually by larger ones, which provides theoretical support for the experimental interpretation of the increase of the bound rubber with the storage time.Fourth, Elastomer nano-reinforcement mechanism:Based on the above research work, the mechanical reinforcement of elastomer network contributed by three-dimensional spherical nanoparticles was deeply investigated. For spherical nanoparticles, by fixing the interfacial interaction, there exists an optimum filler volume fraction (around23%and32%). By characterizing the changes of the chain orientation and bond energy of polymer chains during the uniaxial tension, the molecular mechanism of rubber reinforcement results from the orientation and alignment of polymer chains induced by nanoparticles, and the finite limited chain extension at large deformation. The formed is controlled by the filler volume fraction, filler size and interfacial interaction, while the latter is dominated by the interfacial physical and chemical interactions. This work firstly elucidates the underlying mechanism of nano-reinforcement at the molecular level, which is very significant and beneficial for developing new nano-reinforcement technology and model.