Investigation Of The Microstructure And Physics Property For Polycarbonate/Multi-walled Carbon Nanotube Composites

Since carbon nanotubes (CNTs) were first discovered in1990, due to its excellent physical properties, it has become a potential material in many fields, especially in the field of polymers filled with CNTs, which have been received increasing attentions. Conductive polymer composites have also attracted intense attention in the past two decades due to their excellent properities:with the optics and electricity properties of organic semiconductor, wirh easy-shaped properties and mechanical of organic polymer, and their extensive application in electronics, biomedical devices, thermal interface material, automotive, aerospace as antistatic, electromagnetic shielding, and electrically conductive materials.Although significant progress in mechanical, thermal, electrical and rheological properties has been made in developing nanocomposites with different polymer matrices, a general understanding for the mechanism has not yet emerged. In other words, how the addition of nanofillers can affect the microstructure of polymer? And how the microstructures can affect the macropropeties? A major direction in the further development of nanocomposites is finding out the simple structure-property relationship.In this dissertation, positron annihilation lifetime spectrum, combining with other experimental techniques were used to study the effects of multi-walled carbon nanotube (MWCNTs) on the free volume and the glass transition temperature, and the relationship between conductive, rheological and viscoelastic properties and the microstructure of nanocomposites. Moreover, the effects of temperature and free volume on the viscoelastic properties of polycarbonate (PC)/MWCNT composites were investigated. The main results are as follows:1. Investigation of the rheological and conductive properties of PC/MWCNT composites by positron annihilation techniquesThe microstructure, rheological and conductive properties of multi-walled carbon nanotube (MWCNT)/polycarbonate (PC) composites were investigated by positron annihilation lifetime spectroscopy, positron annihilation coincidence Doppler broadening (CDB), oscillatory rheometry and electrical resistivity for different MWCNT contents. A ten orders of magnitude increase in electrical conductivity was achieved with very small quantities of MWCNTs. CDB was used to determine a percolation threshold value, which was in good agreement with the electrical conductivity and rheological measurements. The results showed that with increasing MWCNT content, the composites underwent a phase transition from insulating to conducting at room temperature, which was attributed to the formation of a MWCNT network. The effect of MMCNTs on the microstructure of MWCNT/PC composites has been studied by positron annihilation lifetime measurements. The results showed that the fractional free volume decreased because of the MWCNTs and the formation of conductive network. The effects of MWCNT filler on the atomic scale free volume and mechanical property of MWCNT/PC composites were also discussed.2. Investigation of the glass transition and viscoelastic properties of PC/MWCNT composites by positron annihilation lifetime spectroscopyThe influences of free volume and temperature on the viscoelastic properties of MWCNT/PC composites were investigated by positron annihilation lifetime spectroscopy (PALS) and dynamic mechanical analysis (DMA). Three methods, including PALS, DMA and differential scanning calorimetry were used to determine the glass transition temperature (Tg) of PC/MWNT composites, The experimental results indicated that the higher the MWNT contents, the lower the Tg, which attributes to the large free volume hole and the enhanced polymer mobility in PC/MWNT composites with higher MWNT contents. The effect of MWNTs on viscoelastic property has been investigated in detail. A direct linear relationship between fractional free volume and viscoelastic property has been obtained using the Williams-Landel-Ferry equation based on free volume theory, which indicates that the free volume plays an important role in determining the viscoelastic property.