MECHANICAL CHARACTERIZATION OF MULTI-WALL CARBON NANOTUBE/POLY(METHYL METHACRYLATE) NANOCOMPOSITES: A METROLOGY COMPARISON STUDY

摘要

Carbon nanotubes (CNTs) possess remarkable properties that make them excellent candidates for various applications, including incorporation into polymer matrices as a nanoscale reinforcing material. Theoretically, it is expected that their combination of high strength, high elastic modulus, and high aspect ratios would translate to significant enhancements in the bulk properties of polymer composites. Experimentally, however, these property improvements have been very marginal. These lower than expected results have been mainly attributed to a combination of poor dispersion and alignment of CNTs in polymer matrices, as well as their chemically non-reactive surfaces. Consequently, a considerable amount of research has been aimed at addressing these issues by optimizing the processing of CNT/polymer composites, including the use of different dispersion and alignment methods, and chemically functionalized CNTs-in order to improve their adhesion to their constituent polymer matrices. Though some improvements have been reported, the results have been inconsistent. These inconsistencies have mainly been ascribed to the differences in polymer matrices and composite processing techniques. However, not much attention has been paid to the possible contributions arising from the differences in metrology. The objective of this study was to investigate how mechanical testing approach might lead to discrepancies in CNT/polymer composites properties. In the first part of this study, three mechanical testing techniques involving tensile testing, dynamic mechanical analysis (DMA), and nanoindentation were used to determine the elastic modulus of pristine poly(methyl methacrylate) (PMMA) in order to verify their feasibility. Then, these techniques were applied to MWNT/PMMA composites and the results were compared. At 10 wt. percent MWNT, the observed enhancements in the composite elastic modulus were 32 percent, 16 percent, and 7 percent from tensile testing, DMA, and nanoindentation, respectively. The reasons for the observed differences in the degree of enhancements measured by these metrologies will be explained in terms of the anisotropic mechanical properties of CNTs in conjunction with the mechanical testing approach. In the second part of our study, the effect of polymer molecular weight on the elastic moduli and glass transition temperatures (T_g) of CNT/polymer composites was investigated by tensile testing and DMA. The results showed that at room temperature, enhancements in elastic moduli with MWNT concentrations for the low and high molecular weight matrix composites were comparable. At high temperatures however, the enhancement in elastic moduli of the higher molecular weight composites was significantly greater (by more than a factor of 2) than that of the lower molecular weight composites. These results are consistent with the observed increases in the T_g of the composite systems. At 1 wt. percent MWNT, an average increase in T_g of 5.2 deg C was observed for the higher molecular weight composite, compared to a negligible increase of < 1 deg C for the lower molecular weight composite. These results suggest a stronger interaction between MWNTs and the higher molecular weight PMMA matrix.