Structure And Properties Of Conductive Nanoparticle/Polymer Composites:a Molecular Dynamics Simulation Study

Polymer matrixes filled with conductive nanoparticles can dramatically improve their electrical properties. The conductive nanoparticle/polymer composites can be used to fabricate electromagnetic shielding materials, antistatic materials and positive temperature coefficient materials, which are widely applied to electron devices in military and civilian fields. Once the content of conductive nanoparticles reaches the critical value (percolation threshold), the composite changes from an insulator to a conductor. Low conductive percolation threshold indicates that low content of conductive fillers is needed, which is valuable for the modification of mechanical and processability of composite, and for reducing the cost. At present, the mechanism and model of conductive composite still cannot well analyze and predict the electrical properties. The conductive network is also hard to be characterized by the experimental methods. Computer simulation can be used to describe the material structure on a micro scale and explain or even predict the electrical behavior of composite. However, the effect of polymer matrix was neglected in the previous simulations. This work mainly focuses on the structure, electrical and mechanical properties of polymer nanocomposite, using molecular dynamics simulation. The main works and novelties are summarized as follows:1. Relationship between the nanoparticle dispersion and the conductivity of nanocomposite is researched. As polymer-nanoparticle interaction increases, the dispersion rises first and then drops, whereas the conductivity increases monotonously. By increasing the grafting density of chain on the nanoparticle surface, the dispersion is increasingly better, whereas the tendency of conductivity is showed as’M’type. In addition, the nanoparticles becomes much better dispersive while the conductivity rises at first then decreases with the increase of cross-linking of polymer chain. By increasing the content of immiscible component in polymer blend, the dispersion decreases monotonously, whereas conductivity rises first and then drops. Combining the four types of nanocomposites, we find that the best dispersion does not benefit the highest conductivity. To improve the electrical properties of polymer nanocomposites, a proper increase of the dispersion of nanoparticles in polymer matrix is necessary, but an unlimited increase is harmful for the electrical properties.2. Comprehensive electrical and mechanical properties of polymer nanocomposite are studied by grafting chains on nanoparticle surface, where the effect of grafting density, graft chain length, and matrix-graft interaction are investigated. In the situation which matrix-graft interaction equals to matrix-matrix interaction, as the grafting density increases, the nanoparticle dispersion and mechanical properties of nanocomposite both improve, whereas the tendency of conductivity is’M’type. As the graft length increases, the nanoparticle dispersion improves monotonously, whereas both the electrical and mechanical properties rise first and then drop down. If the matrix-graft interaction is greater than matrix-matrix interaction, increasing matrix-graft interaction and graft length can increase the glass-transition temperature, resulting in the enhancement of mechanical properties. As the matrix-graft interaction increases, the nanoparticle dispersion improves monotonously, whereas conductivity rises first and then drops. As the graft length increases, the nanoparticle dispersion rises first and then fall. It is noted that aggregation and overdispersion of nanoparticles are harmful to conductivity.3. Selective localization of nanoparticles in a continuous phase of immiscible block copolymer is an effective strategy to reduce the percolation threshold. The flexibility of polymer chain can influence the microstructure and electrical properties of nanocomposites. For the flexible copolymer, the lamellae structure can be observed in a relative wide range of component ratio. As the nanoparticle content increases, the range of component ratio which lamellae structure exists in narrows down and eventually disappears. If the filler-rich phase is three-dimensional continuous, the conductivity increases with the decrease of the component ratio of this phase. If the filler-rich phase is the dispersed phase or present lamellae structures, the conductivity is low. For the semiflexible copolymer, the probability of appearance of lamellae structure reduces with increasing stiffness of rigid chain and then the order-disorder transition is affected. The configurational entropy and the enthalpy varies with the stiffness of rigid chain, thus the continuous-dispersed phase transition of nanoparticle-rich phase is changed.4. The conductive stabilities responded to deformation of nanorod/polymer composites are studied. High polymer-nanorod interaction or cross-linking density leads to high degree of alignment of nanorod during the process of uniaxial tensile, and to lower recovery of nanorod orientation in the subsequent relaxation process. The depletion attraction, which is induced by polymer chains, has an effect on the movement of nanorod and the variation of conductive network. After deformation, the three-dimensional conductive network is destroyed. The smaller the aspect ratio of nanorod, the higher conductive stability responded to deformation. As the nanorod content increases, the conductive stability drops first and the rises, and the minimum corresponds to the percolation threshold. By increasing the strain, the three-dimensional conductivity monotonously decreases, whereas conductivity in stretching direction rises firstly and then drops down, exhibiting a strong anisotropy in the stability of conductive property.To sum up, this work investigates the influence of configurational entropy of polymer, accumulation of entropy of nanoparticle and interfacial enthalpy on the dispersion and orientation of nanoparticles, electrical and mechanical properties of nanocomposites. The factors influencing the microstructure are investigated, and the relationships among the microstructure, electrical and mechanical properties are established. This work provides theoretical guidances for designing the polymer nanocomposites with low conductive percolation threshold, high electrical and mechanical properties.