| Elastomer nanocomposites(ENCs) have been highlighted in the research field of materials science and engineering due to their excellent performance. However, to realize the full potential of ENCs, the nanofillers must be thermodynamically stabilized in the elastomer matrix. Besides, considering that the microstructures influence the macroscopic properties, controlling the mircrostructure of ENCs provides an indirect approach to control the properties of ENCs. However, the complicated microstructures of ENCs involve many interactions and different scales of length and time, leading to the investigation of microstructures being far from fine and overall due to the limitation of traditional experimental tools, as well as the quantitative correlation between the microstructure and mechanical properties being inaccurate. Therefore, computer simulation will play an ever increasing role in understating and predicting property enhancements of ENCs.In this work, coarse-grained models of ENCs are established. By earring out the equilibrium and un-equilibrium molecular dynamic simulations, the microstructures and the macro-mechanical properties of ENCs are studied. The main novelties are summarized as follows:First, the dispersion mechanism of polymer grafted nanoparticles:The previous research work about the dispersion of polymer grafted nanoparticles was focused either on the dispersion state of nanoparticles, which was studied mainly by experimental tools, or on the microscopic structures of grafted polymer brushes which most simulations and theoretical calculations did. Especially, most simulation systems at present were simplified as one or several nanoparticles grafted with polymer chains. This work simulated such a system which was composed of48nanoparticles blended with hundreds of polymer chains, which was more realistic than what has been done before by simulation tools and theoretical calculations. Moreover, for this many-particle system, it was very suitable to directly characterize the dispersion state of grafted nanoparticles, such as intuitive snapshots, total filler-filler interaction energy and other detailed approaches. By combining the dispersion state to the conformation of grafted chains and matrix chains, the dispersion mechanism was studied. The results indicated that the dispersion of nanoparticels was mainly controlled by the excluded volume effect of the polymer brushes and the matrix effect.Besides, the effects of the grafting density, the grafted chain length and the matrix chain length on the dispersion of the nanoparticles were investigated. Results showed that increasing grafting density or grafted chain length led to better dispersion owing to larger excluded volume, however, increasing the matrix chain length led to aggregation of nanoparticles, attributed to both a progressive loss of the interface between the grafted brushes and the matrix and the overlap between brushes of different nanoparticles, intrinsically driven by entropy. Meanwhile, it was found that there existed an optimum grafting density for the dispersion of the nanoparticles.Additionally, taking into consideration the practical situation that the grafted chains and the matrix chains are usually not chemically identical, the effect of the thermodynamic (Flory-Huggins) interaction parameter between the host and grafted chains was also examined, which has not been touched on yet through simulation and theoretical tools.Second, the strain-induced non-linear behavior of ENCs:This simulation work firstly established the correlation between the micro-structural evolution and the strain-induced non-linear behavior of ENCs. Previous research work was focused either on the thermodynamics and kinetics of ENCs under quiescent condition through equilibrium simulations and theoretical calculations, or on the reinforcement mechanism through experimental tools and non-equilibrium simulations. Up to now, no simulation work has been performed to characterize the micro-structural evolution during the deformation process, and explain the microscopic mechanism for the non-linear behavior of ENCs by establishing the correlation between the micro-structural evolution and mechanical properties. This work employed the coarse-grained model to simulate big systems on large length and time scales, which were composed of60480beads in total. Non-equilibrium molecular dynamic simulations were carried out, including the uniaxial tensile and shear tests. By designing different initial dispersion states of nanoparticles before deformation, the stress-strain behaviors were investigated. By fitting the simulated stress-strain data, the elastic modulus-strain relation was obtained by taking the derivative of the fitted stress-strain curve. The phenomenon that the elastic modulus decreased non-linearly with the increase of the strain and reached a low plateau at large strain was observed, which was quite similar to the so-called "Payne effect" for ENCs.Based on the detailed characterization and analysis of the micro-structural evolution under deformations such as the number of neighboring nanoparticles, coordination number(CN) of nanoparticles, root mean-squared average force exerted on each nanoparticle along the tension direction, local strain, chain conformations(bridge, dangle, loop, interface bead and connection bead), and the total interaction energy of nanoparticle-polymer and nanoparticle-nanoparticle, it was indicated that the break-up of the network or clusters formed through direct contact of nanoparticles accounted for the non-linear behavior of the aggregation case. The strain amplification effect occured at the linking points and the strain diminution effect happened inside the nanoparticle clusters. For the dispersion case, the elastic modulus was dominated by the nanoparticle network formed through the bridging of adsorbed polymer segments among nanoparticles.Besides, this work also investigated the effects of nanoparticle-nanoparticle interaction, nanoparticle-polymer interaction and the volume fraction of nanoparticles on this non-linear behavior of ENCs. Results showed that the initial elastic modulus increased faster than linearly with increasing the volume fraction of nanoparticles.Lastly it was found that for the dispersion case, further increasing inter-particle distance or grafting nanoparticles with polymer chains can effectively reduce the non-linear behavior due to the decrease of the physical network density.Third, the static and the dynamic mechanical properties of ENCs filled with grafted nanoparticles:The dynamic properties of ENCs filled with polymer grafted nanoparticles was firstly explored by performing the un-equilibrium molecular dynamic simulations. This work also adopted a new model of nanoparticles for polymer grafting. The nanoparticle cores were modeled as solid spheres with96virtual surface beads distributed evenly on the surface. These virtual surface beads were introduced for the purpose of fixing the grafted chains on the surface of nanoparticles, which was in accordance with the practical situation. The effects of grafting density and the grafted chain length on the static and dynamic properties were investigated. By intuitive snapshots, radial distribution function of nanoparticles and the conformation of grafted chains and matrix chains, the microsture of ENCs were examined. By carrying out uniaxial tension, the static property of ENCs such as stress strain behavior was investigated, and by carrying out periodic oscillatory shear, the dynamic properties of ENCs were investigated. Results showed that increasing grafting density and grafted chain length improved the dispersion of nanopartcicles, and due to the resulting decrease of direct contact between nanoparticles, the strain-induced non-linear behavior of elastic modulus and the dependence of shorage modulus on the shear amplitude were both reduced. It was indicated that stronger interface interaction between grafted chains and matrix chains was the origin of mechanical reinforcement as well as the reduction in non-linear behaviors of ENCs. |
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