Study On The Electrical Properties And Dynamic Rheology Behaviors Of MWNTs/HDPE Composites

The performance of conductive polymeric composites are related not only to the components and structure of polymer material but also to the effect of exterior fields on the agglomeration structure during the preparation and application. In this thesis, the electrical and dynamic viscoelastic properties of multi-walled nanotubes (MWNTs) filled high-density polyethylene (HDPE) composites were investigated with the computer-controlled resistivity-temperature measurement system and the ARES, in order to reveal the mechanism for the variation and development of the agglomeration structure under the effect of thermal field.Electrical properties, especially the resistivity-temperature characteristic of MWNTs/ HDPE composites, were investigated. Results showed that composites filled with MWNTs have perfect conductivity and the electrical percolation threshold was about 3-5 wt%. A special resistivity - temperature characteristic, "V-PTC" characteristic, was found, which exhibited that as the temperature increasing, the resistivity of the composites first decreased, then increased abruptly when the temperature approached melting point (Tm) of the matrix. It should be noted that this phenomenon have not been found in polymer-based composites filled with other conductive particles, such as carbon black, graphite, carbon fiber, and so on. It was found that this V - PTC characteristic was due to the expanding of the matrix volume and the relaxation of the MWNTs entanglement. Maleic anhydride grafted styrene-(ethylene-co-butylene)-styrene copolymer (MA-g-SEBS) acts as a compatibilizer to MWNTs and HDPE, which improved the interaction between filler and polymer matrix, and thus almostly eliminated the V-PTC characteristic through restricting the movement of the filler.The dynamic rheological behaviors for MWNTs/HDPE composites were investigated. A obvious shear thinning phenomenon in η* vs co was found for the composites as MWNTs content beyond 3 %. A specific viscoelastic behavior in G' vs ω, i.e. so-called "the second plateau", appeared at low frequency region in case of φ >3%. This phenomenon could be regarded as a rheological threshold, and could be attributed to the formation of network structure due to MWNTs chain aggregation and the interaction between MWNTs and polymer matrix. A hill and dale was found at terminal region in the curves of tand ~ ω, which also was believed to be induced by formation of network structure. Results of dynamic frequency sweep undergoing different heating treatment time and dynamic time sweep approved previous postulation.The gelation rheology behaviors for MWNTs/HDPE composites have been studied, and the gel point was determined by Winter - Chambon method, critical gel exponent n and gel strengthen Sg were also calculated. It was found that the gel rheology behaviors appeared for particle filed polymer system were come from the interaction between particle and polymer matrix, or even itself (such as aggregate and entanglement). The particle morphology plays a important role for the occurrence of the gel rheology behaviors. Results showed that the gel point φg was temperature dependent, but n is almost temperature independent.Ultimately, the gel point was correlated with the rheological threshold and the electrical percolation threshold which was found for the composites containing MWNTs between 3 and 5wt%.