Structural Design Of Polylactic Acid Conductive Composites

Polymer composites, which have be one of the most interesting and researched areas in nanotechnology and composites, have demonstrated their potential as high-performance and multifunctional materials. Due to their ease of processing, tunable properties and wide range of applications, conductive polymer composites are one of the most important and interesting areas in polymer composite research, which has remained active for several decades. The electrical properties of conductive polymer composites arise from their conductive networks. Moreover, the addition of more conductive filler to a particular type of polymer matrix often further enhances the electrical conductivity of the resulting composite, but this procedure often leads to poor process ability and deteriorated mechanical properties for relative high filler contents. Therefore, it is the most important to enhance the conductivity by adding a small amount of filler and by controlling its distribution and network formation in the matrix.In the work, polylactide (PLA) conductive polymer composites were prepared by melt mixing. Then the morphology of different conductive networks structure and the conductive behavior were studied by morphological characterization and electrical conductivity measurements, further explicated dominant factors that influenced conductive behavior. Conductive polymer composites with high conductivity and high performance were ultimately achieved by establishing a relationship between the morphology of composites and processing technology or bulk properties. Carbon nanotubes (CNT) and carbon black (CB) filled PLA hybrid composites were fabricated using a melt mixing method. The effects of the CB and CNT content on the room temperature resistivity of the nanocomposites were examined. The room temperature resistivity of the composites decreased significantly with increasing CNT content, the incorporation of a small amount of CNT into the PLA/CB composites significantly improved the electrical properties of the hybrid nanocomposites. Tensile and impact test results revealed that CNT and CB reinforced hybrid composites exhibited better tensile strength and impact strength than only GF or CNT reinforced composites. According to all these results, it can be concluded that simultaneous usage of CNT and CB in the composites increased the reinforcing ability of nanotubes in polymer composites. Moreover, the influence of processing technology was explored through a series of characterizations.To reveal the possibility of the migration of PCL to CF, the morphology of the PLA/CF/PCL composite mixed for different CF loading was examined. It was observed that many fibers are "welded" together by the minor PCL phase, and a continuous PCL-CF network was formed throughout the PLA matrix. CF loading greatly affected PCL-CF compound in a PLA matrix during melt mixing, with low CF content, it was too little to build network structure. The surface microstructure (mainly amount and distribution of CF) of PCL-CF was a key factor determining the electrical conductivity of the composite, and was dominated by the PCL content in PCL-CF composites. Above a critical value, the PCL-CF network was connected by electrically conductive contact points and thus was able to sustain electron transmission in the whole system. Self-welded network structure led to a good conductivity and mechanical properties. The conductive network was built by the contact and overlapping of the CF coated PCL phase. The preferable distribution of CF was originated from several factors including interfacial tension and viscosity. The PCL with a lower viscosity shows a rapid increase, but a larger difference in viscosity between PLA and PCL results in a stronger self-welded network structure.PLA foams were fabricated by autoclave system and compression molding. The morphology PLA foams were characterized and discussed. The results showed the cellular structures of PLA with autoclave system were more easily controlled. The cellular structures obtained from various ranges of saturation time and saturation pressure were investigated by using scanning electron microscopy (SEM), the average cell diameter increased with the saturation pressure decreased, while the cell density and the volume expansion ratio increased. But the average cell diameter first decreased and then increased with the extension of saturation time. Compared with neat PLA foam, PLA/CB nanocomposite foams showed much higher cell density and smaller cell size due to CB serving as the heterogeneous nucleating agents. Foam characteristics and electrical conductivity of the PLA/CB compounds were investigated. It has been demonstrated that foaming PLA/CB composites resulted in the formation of a conductive network at lower CB concentrations, with foams showing the potential for use in conductive high-performance lightweight composite systems.