Nanoparticle Dispersion-Aggregation, Polymer Interfacial Behaviour, Materials Property In Elastomer-Based Composites: Insights From Molecular Dynamics Simulation

Currently, polymer nanocomposites develop quickly and become a hot area of research because of the academic and industrial interest. This is because polymer nanocomposites have excellent rheological, optical, electrical, thermal, and mechanical properties, which indicates that it can be applied in the real life. Among them, an elastomer-based nanocomposite plays a very important role in the modern industry, which is attributed to its high elasticity. Carbon black, montmorillonite or carbon nanotube filled elastomer materials are considered one kind of nanocomposites. The nano-sized particle is necessary to realize the efficient reinforcement of rubber. In the research of structure and property, elastomer-based nanocomposites possess complicated microstructure, which owns some characteristics of many length and time scales. Thus, it is very difficult to obtain the overall and fine characterization for this complex microstructure, attributed to the limitation of experimental research means. The relationship between the microstructure and material properties is lack of systematic and comprehensive study. Obviously, these existing difficulties will directly block the further industrial application of elastomer nanocomposites.Based on the research background above, the purpose of this work is focused on dispersion and aggregation of nanofiller, interfacial microstructure and mechanical, rupture and conductive property through molecular dynamics simulation. We clarify the inner connection between the microstructure and property. The main novelties are summarized as follows:First, Microstructure and intercalation dynamics of polymer chain within nanoclay:By employing molecular dynamic simulation, we have systematically investigated the microstructure and intercalation dynamics of polymer chain within nanoclay. Results indicate that the intermediate polymer-clay interaction, low polymer molecular weight, large interlayer distance can speed up the polymer intercalation. And the chain intercalation process exhibits an Arrhenius-like temperature dependent behavior. The appropriate surfactants can promote the chain intercalation into nanoclay. The polymer within nanoclay shows exhibits an obvious layering and orientation behavior. Our work can assist in the rational design to achieve the exfoliation of nanoclay in polymer-nanoclay nanocomposites.Second, The dispersion and aggregation kinetics of nanorods in elastomer matrix:There exits the best nanorod dispersion in the elastomer matrix at the intermediate polymer-nanorod interfacial interaction. At a low interfacial strength, the nanorods aggregate directly; while at high interfacial strength, nanorods form a local bridging structure through by one or two layers polymer chains. Meanwhile, we observed that the aggregation and dispersion processes exhibit an Arrhenius-like temperature-dependent behavior. The effect of nanorod volume fraction, cross-linking density, cross-linking reaction rate and shear rate on the aggregation process of nanorods has been investigated. This works provides some guides to achieve uniform nanorod dispersion in the elastomer matrix; meanwhile, it can help to inhibit the aggregation of nanorods during the usage of materials.Third, Microstructure and dynamics of interfacial polymer close to nanofiller:(i) We systematically investigate the static and dynamic properties of polymer melts in the presence of nanosheet, including density, the bond, segment and chain orientations, and the mean square radius of gyration. They all exhibit different behavior compared with bulk polymer. The interfacial range close to nanofiller is about 5σ. The dynamic analysis, including spatial distribution of the population of beads, interfacial beads exchange rate, desorption dynamics, translational and orientation mobility, indicates that the "glassy polymer layer" exists for strongly attractive interfacial interaction. Actually, the results obtained here could provide some insights into the polymer-nanosheet interfacial behavior.(ⅱ) We systematically investigate dynamic properties of the interfacial elastomer melts for systems filled with three kinds of fillers (nanosphere, nanorod and nanosheet). Results indicate that the nanosheet has the strongest confinement on the mobility of interfacial polymer beads, next is the nanorod and nanosphere. In addition, the mobility of interfacial beads increases with the decrease of filler size. Meanwhile, for the systems filled with high loading filler, the mobility of the interfacial beads will be slower than that for the systems with just one filler because of the effect of many particles. It is found that the curvature of nanofiller and the total force that is exerted on the polymer beads by the filler that determine the mobility of the interfacial polymer beads. The results obtained here could provide some insights into the polymer-filler interfacial behavior, especially for systems filled with different shapes of filler.Fourth, Elastomer nanocomposites mechanical and rupture property:(ⅰ) Our results indicate that there exists an optimum nanorod volume fraction for rubber reinforcement. A yielding point occurs at strong interfacial strength and high filler volume loading, which is attributed to the breakage of a strong nanorod network via polymer chains. By characterizing the chain orientation and bond energy during the tensile process, the reinforcement results from the orientation and alignment of polymer chains induced by nanorods at small strain, and the finite limited chain extension at large strain. The former one is decided by the interfacial interaction and volume fraction; while the latter one is dominated by the interfacial chemical couplings. This work helps to understand the reinforcement mechanism of nanorod filled composites at the molecular level, which is useful to develop the materials with high mechanical property.(ⅱ) During the rupture process of nanorod filled nanocomposites, by focusing on the free volume fraction, size and number of voids, Van der Walls (VDW) energy, polymer mobility and orientation, we make a correlation between these microstructure and stress-strain curves. In particular, the nucleation of voids upon deformation occurs preferentially near the ends of polymer chains. Then we systematically study the effects of the interfacial interaction, temperature, chain length, volume fraction of nanorods and cross-linking density on the rupture behavior. With increase of interfacial interaction, there are two rupture modes, namely mode A (no bundles) and B (bundles). Actually, this work provides a fully understanding for rupture process, which is useful to develop the materials with high rupture property.Fifth, Conductivity mechanism of nanorod filled elastomer nanocomposites:(ⅰ) We systematically investigated the conductive properties of nanorod-filled elastomer nanocomposites by focusing on the effects of the interfacial interaction, the aspect ratio of fillers and the external shear field. By analyzing the nanorod network, we make a connection between these parameters and conductive property. We found that there is not a simple relationship between the dispersion state of nanorods and conductive property. In addition, our results show the evolution of conductivity network under the shear field. Our work provides assistance for better design to obtain high conductivity materials, especially for the nanorods filled polymer nanocomposites.(ii) The destruction and recovery process of a nanorod conductive network in the elastomer matrix is investigated under the shear field. At an intermediate functional ization extent of polymer, the conductive property of polymer nanocomposites can be significantly emhanced. Under the shear field, the anisotropy of the conductive probability appears, which is attributed to the orientation of nanorod. The relationship between homogeneous conductive probability and shear rate can be described by a semi-empirical equation. Meanwhile, an empirical formula is obtained for the dependence of the anisotropy of the conductive probability on the orientation of nanorods. Furthermore, the conductivity stability of polymer nanocompoistes increases with increasing nanorod volume fraction. During the recovery process after stopping the shear field, it can be fitted well by a model combining classical percolation theory and time-dependent nanofiller aggregation. In summary, this work clearly presents some interesting results to help further understand the destruction and recovery processes of the nanorod network during the shear field.