Advanced Thermoplastic Nanocomposite Melt Processing Using an Improved Supercritical Carbon Dioxide Pretreatment for the Nanomaterial

Polymer nanocomposites have been proposed as lightweight replacements for traditional composite materials in various applications. Montmorillonite (MMT) and carbon nanotubes (CNTs) are two nanofillers which have accrued significant interest in the past 20 years due to their superior mechanical and electrical properties, respectively. However, efficient dispersion of the nanofiller and damage to CNTs prevent widespread utilization of these materials in polymer nanocomposites. Novel methods of nanocomposite generation combining the use of supercritical carbon dioxide (scCO2) with melt compounding have been investigated to overcome these issues. The focus of this work is on developing the scCO2 treatment of nanomaterial for thermoplastic nanocomposite generation. First, the supercritical carbon dioxide aided melt blending method was applied to nanoclay nanocomposites of Nylon 6/ and organoclay where the polymer may interact with the nanoclay surface. Second, the effect of scCO2 processing of CNTs was investigated with special consideration to the processing variables. Finally, a study was carried out to analyze the electrical conductivity of polycarbonate nanocomposites generated using CNTs deagglomerated by scCO2 processing.

The initial focus of this dissertation is the use of supercritical carbon dioxide (scCO2) as a processing aid in the generation of nylon 6 nanocomposites in which the nylon 6 may interact with the nanoclay surface. Wide-angle X-ray diffraction, transmission electron microscopy, rheology, and tensile tests were carried out to investigate the effect of processing with scCO2 on the final composite morphology and properties. It was observed that mechanical properties of composites prepared with the scCO2 aided melt blending method were similar to or higher than those reported in the literature for samples prepared with twin screw compounding. At 7.6 wt% nanoclay the modulus value reaches 4.75 +/- 0.194 GPa which is one of the highest increases (1.7 GPa) reported for these materials processed at intermediate concentrations. Beyond 7.6 wt% the improvement due to scCO2 processing matched that of direct blending.

The next objective of this work is to develop a method for the deagglomeration of commercially available multi-walled carbon nanotubes (MWCNTs) by manipulating processing variables and observing carbon nanotube aspect ratios after deagglomeration. High levels of deagglomeration of Baytubes C 150 P and Nanocyl NC-7000 MWCNT agglomerates were observed, resulting in 30 fold and 50 fold decreases in bulk density, respectively, with median agglomerate sizes < 8 um in diameter. These results were obtained while retaining the aspect ratio of the as-received nanomaterial, irrespective of the MWCNT agglomerate morphology. It was found that the supercritical temperature and pressure of 40 deg C and 7.86 MPa were the optimal temperature and pressure for maximum deagglomeration without damaging the MWCNTs.

The final goal of this work is to apply the scCO2 aided melt blending process to generate polycarbonate/ carbon nanotube (CNT) nanocomposites with enhanced electrical conductivity and improved dispersion while maintaining the aspect ratio of the as-received CNTs. Different degrees of scCO2 processed Baytubes C 150 P CNT were benignly deagglomerated with scCO2 resulting in 5 fold (5X), 10X, and 15X decreases in bulk density from the as-received CNTs. The CNT were melt compounded with polycarbonate using single screw melt extrusion and compression molded into plaques. A surface conductivity of 4.8 x 10-8 +/- 2.0 x 10-9 S was observed for samples prepared with the scCO2 aided melt blending at 15X scCO2 processing. Electrical percolation thresholds as low as 0.83 wt% were observed for composites prepared with 15X CNTs using the scCO2 aided melt blending method, while concentrations as high as 1.5% are required without scCO2 processing. The percolation concentration in nanocomposites prepared with 15X scCO2 processing and single screw extrusion is competitive with values reported for similar nanocomposites generated using twin screw melt compounding in the literature. Optical microscopy, transmission electron microscopy, and rheology were used to investigate the dispersion and mechanical network of CNTs in the nanocomposites. The dispersion of CNTs generally improved with scCO2 processing compared to direct melt blending but was found to be significantly worse than that of twin screw melt compounded nanocomposites from the literature. Because the percolation thresholds are similar with substantially different extents of dispersion, the importance of maintaining longer CNTs during nanocomposite generation is emphasized.