Rheology, properties and microstructure development of polymer/carbon nanotube composites in microinjection molding process

Carbon nanotubes (CNTs) are great materials with superior electrical and thermal conductivity, mechanical properties and high aspect ratio. In this dissertation, we intended to create a predictive understanding of the coupling of microinjection molding process to the development of the performance properties of carbon nanotube/polymer composites, focusing on the rheology, microstructure and properties of microparts. To explore the CNTs feature and to enable their processing, the nanocomposite preparation was optimized through a set of rheological measurements and microstructure characterization.At the next step we investigated the effect of flow field and deformation rate on the nanotube alignment and on the properties of PC/MWCNT nanocomposites. The obtained results of SEM, TEM and Raman spectroscopy revealed that the nanotubes are preferentially aligned in the flow direction, particularly at large injection rates. Rheological measurements corresponding to high shear rate conditions showed drastic changes in the viscoelastic behavior. The complex viscosity significantly decreased and percolation threshold notably rose. High degrees of nanotube alignment also resulted in a significant increase in the electrical percolation threshold. For compression molded samples the percolation threshold is found to be about 3 wt% nanotube content as the electrical conductivity rises suddenly by more than 10 decades. However, the longitudinal flow in the dog-bone shaped cavity resulted in a very high value for the percolation threshold, about 9 wt% of nanotube loading.The electrical conductivity and percolation behavior of PC (polar amorphous polymer)/MWCNT nanocomposites and iPP (non-polar semi-crystalline)/MWCNT were analyzed and compared. In spite of the different levels of dispersion, the nanotubes in both nanocomposites were connected and a percolated network was formed. The crystallinity of the PP/MWCNT nanocomposites as a function of nanotube content was found to go through a maximum at 2 wt% loading while the overall rate of crystallization increased. We observed for the highly sheared microinjected PP/MWCNT samples the formation of well oriented crystals however, the overall crystallinity was only slightly affected by strain. The electrical conductivity of the nanocomposites was improved by the presence of the crystalline structure even in high sheared samples. The percolation threshold for compression molded (non-processed) samples was found to be around 1 wt% for PP nanocomposites. Similarly, the conductivity values after percolation were about one order of magnitude higher in the case of PP nanocomposite. Although a high degree of nanotube alignment in the microparts resulted in a significant increase in the electrical percolation threshold, the level of alignment was less in the crystalline polymer. In the crystalline material the orientation of crystalline phase also decreased with the incorporation of nanotubes.It was also shown that the PP nanocomposites exhibited mechanical properties significantly enhanced by nanotube loading this effect was small in the case of the PC nanocomposites.We first examined the rheological behavior of polycarbonate (PC)/ MWCNT nanocomposites in light of interactions between CNTs and polymer chains or between CNTs themselves. To understand the percolated structure, the nanocomposites were characterized via a set of rheological, electrical and thermal conductivity measurements. The rheological measurements revealed that the structure and properties were temperature dependent the percolation threshold was significantly lower at higher temperature suggesting stronger nanotube interactions. However, when the measurement temperature was high enough the percolation threshold tend to reach a plateau (0.3 wt%) The nanotube networks were also sensitive to the steady shear deformation particularly at high temperature. Following preshearing, the elastic modulus decreased markedly indicating that the nanotubes were aligned in the flow direction. Consequently, they interconnected minimally leading to a remarkable increase in the percolation threshold.Finally the effect of carbon nanotube on the morphology and electrical conductivity of nanocomposites of MWCNT and PP/CBT blend were investigated and it was found that a double percolation threshold is the basic mechanism for the conductivity of such nanocomposites. It was also shown that the nanotubes affected the morphology of the blends by increasing the viscosity of the filler-rich phase. Consequently the CBT domains became smaller and more elongated changing from droplets into strips, which are more favorable to form a continuous phase. (Abstract shortened by UMI.)...