The Study Of Nonlinear Conduction Of Polyethylene/graphite Nanosheet Nanocomposite Under External Fields

High-density polyethylene (HDPE)/Graphite nanosheet (GN) nanocomposite not only exhibits good electrical and mechanical properties, but also possesses excellent functionality. The nonlinear conduction behaviors depended on ambient pressure and temperature are studied and their intrinsic conducting mechanisms are investigated.The resulting exfoliated graphite nanosheets possess high aspect ratio with diameter ranging from 5 to 20 μ m and average thickness of 40 nm. Owing to this special morphology GN is very favorable to form conducting paths in the polymer matrix. As a result, very low graphite concentration is required to satisfy the critical percolation transition. Furthermore, the conductive nanosheets orient intensively in the composite leading to highly anisotropic properties.A notable nonlinear resistance-pressure conduction behavior was found for the nanocomposite. The piezoresistive behavior, including the negative pressure coefficient (NPC) and the positive pressure coefficient (PPC), is directly attributed to the change of conducting networks under stress. The transition from NPC to PPC effect is mainly due to the formation and destruction of conducting networks. There exists a critical pressure below which composite resistance decrease with the increase of pressure and above which resistance increases sharply with increase of pressure. HDPE/GN nanocomposite exhibits strong piezoresistive effect owing to the lower percolation threshold and the intensive orientation of GN in the composite. Examinations show that the tunneling theory could effectively account for the nonlinear resistance-pressure conduction behavior in the HDPE/GN system.The piezoresistive behavior of HDPE/GN nanocomposite is directly related to the variety and stability of conductive networks in the composite. Therefore, the pressure, filler content, compressive times, which can directly affect the conductive networks, play important roles in the piezoresistivity of the composites. The piezoresistive behavior of the composite also strongly depends on time applied due to the creep of the polymeric matrix and the elastic properties of the matrix and composite itself. On the other hand, the adding of another immiscible matrix changes the distribution of GN in the composite and the composite crystallites, thus also influence the piezoresistivity of the composite.The resistivity of HDPE/GN nanocomposite also strongly depends on temperature. In the vicinity of the polymeric matrix's melting point, the temperature dependent expansion and the crystalline melting of the matrix play a key role in the positive temperature coefficient (PTC) phenomenon. The disturbance in the continuity of the conducting paths is the intrinsic mechanism behind the changes in the composite resistance. Due to the lower filler loading, the conducting networks in the composite are very easy to be demolished and thus the HDPE/GN nanocomposite shows sharp electrical responses as a function of temperature. Investigations on three polymeric matrices which have different crystallinity show that the crystalline properties of polymer matrix greatly influence the PTC effect. The PTC intensity is much stronger if the crystalline property of polymer matrix is better. However, when the temperature is above the melting point of the matrix, the composite appearsnegative temperature coefficient (NPC) effect due to the rearrangement of GN.The PTC effect of HDPE/GN nanocomposite is closely related to the GN concentration. Near the percolation threshold, the PTC effect is strongest. The PTC intensity could be adjusted by adjusting the GN content. Heat treatment may improve the crystalline properties of the composite and greatly enhances the PTC intensity. Similar to the piezoresistivity, the PTC effect also strongly depends on time applied. Time dependence of PTC effect can be mainly attributed to the creep properties of polymer matrix and the movement of GN within polymer matrix.