A Coagulation Method to Prepare Single-Walled Carbon Nanotube/PMMA Composites and Their Modulus, Electrical Conductivity, and Thermal Stability

A coagulation method that provides better dispersion of SWNT in the polymer matrix has been used to produce single-walled carbon nanotube (SWNT)/poly(methyl methacrylate) (PMMA) composites. Optical microscopy and SEM show improved dispersion of SWNT in the PMMA matrix, which is a key factor in composite performance. Aligned and unaligned composites were made with purified SWNT with different SWNT loadings, from 0.1 to 7 wt%. Comprehensive testing shows improved elastic modulus, electrical conductivity, and thermal stability with addition of SWNT. The electrical conductivity of a 2 wt% SWNT composite decreases significantly (\u3e105), when the SWNT is aligned and this result is discussed in terms of percolation

Increased Flexural Modulus and Strength in SWNT / Epoxy Composites by a New Fabrication Method

A new method for preparing SWNT/epoxy nanocomposites has been developed which involves high shear mixing of the epoxy resin and SWNT and heat treating the mixture prior to introducing the hardener. The glass transition temperature of the epoxy resin is unaffected by the presence of nanotubes. An improvement of 17% in flexural modulus and 10% in flexural strength has been achieved at 0.05 wt% of nanotubes. These improvements in flexural modulus and strength are attributed to good dispersion of the nanotubes and grafting of epoxy resin to SWNT by an esterification reaction

Effect of nanotube alignment on percolation conductivity in carbon nanotube/polymer composites

Percolation conductivity of a stick network depends on alignment as well as concentration. We show that both dependences exhibit critical (power-law) behavior, and study the alignment threshold in detail. The highest conductivity occurs for slightly aligned, rather than isotropic, sticks. Experiments on single wall carbon nanotube composites are supported by Monte Carlo simulations. These results should be broadly applicable to percolating networks of anisotropic conductors

Interfacial in situ polymerization of single wall carbon nanotube/nylon 6,6 nanocomposites

An interfacial polymerization method for nylon 6,6 was adapted to produce nanocomposites with single wall carbon nanotubes (SWNT) via in situ polymerization. SWNT were incorporated in purified, functionalized or surfactant stabilized forms. The functionalization of SWNT was characterized by FTIR, Raman spectroscopy and TGA and the SWNT dispersion was characterized by optical microscopy before and after the in situ polymerization. SWNT functionalization and surfactant stabilization improved the nanotube dispersion in solvents but only functionalized SWNT showed a good dispersion in composites, whereas purified and surfactant stabilized SWNT resulted in poor dispersion and nanotube agglomeration. Weak shear flow induced SWNT flocculation in these nanocomposites. The electrical and mechanical properties of the SWNT/nylon nanocomposites are briefly discussed in terms of SWNT loading, dispersion, length and type of functionalization

An infiltration method for preparing single-wall nanotube/epoxy composites with improved thermal conductivity

Recent studies of SWNT/polymer nanocomposites identify the large interfacial thermal resistance at nanotube/nanotube junctions as a primary cause for the only modest increases in thermal conductivity relative to the polymer matrix. To reduce this interfacial thermal resistance, we prepared a freestanding nanotube framework by removing the polymer matrix from a 1 wt % SWNT/PMMA composite by nitrogen gasification and then infiltrated it with epoxy resin and cured. The SWNT/epoxy composite made by this infiltration method has a micron-scale, bicontinuous morphology and much improved thermal conductivity (220% relative to epoxy) due to the more effective heat transfer within the nanotube-rich phase. By applying a linear mixing rule to the bicontinuous composite, we conclude that even at high loadings the nanotube framework more effectively transports phonons than well-dispersed SWNT bundles. Contrary to the widely accepted approaches, these findings suggest that better thermal and electrical conductivities can be accomplished via heterogeneous distributions of SWNT in polymer matrices

Nanoparticle Networks Reduce the Flammability of Polymer Nanocomposites

Synthetic polymer materials are rapidly replacing more traditional inorganic materials such as metals and natural polymeric materials such as wood. Since these novel materials are flammable, they require modifications to decrease their flammability through the addition of flame-retardant (FR) compounds. Recently, environmental regulation has restricted the use of some halogenated FR additives, initiating a search for alternative FR additives. Nanoparticle fillers are highly attractive for this purpose since they can simultaneously improve both the physical and flammability properties of the polymer nanocomposite. We show that carbon nanotubes can surpass nano-clays as effective FR additives if they form a jammed network structure within the polymer matrix, such that the material as a whole behaves rheologically like a gel. We find this kind of network formation for a variety of highly extended carbon-based nanoparticles: single and multi-walled nanotubes, as well as carbon nanofibers

Relationship Between Dispersion Metric and Properties of PMMA/SWNT Nanocomposites

Particle spatial dispersion is a crucial characteristic of polymer composite materials and this property is recognized as especially important in nanocomposite materials due to the general tendency of nanoparticles to aggregate under processing conditions. We introduce dispersion metrics along with a specified dispersion scale over which material homogeneity is measured and consider how the dispersion metrics correlate quantitatively with the variation of basic nanocomposite properties. We then address the general problem of quantifying nanoparticle spatial dispersion in model nanocomposites of single wall carbon nanotubes (SWNT) dispersed in poly(methyl methacrylate) (PMMA) at a fixed SWNT concentration of 0.5 % using a \u27coagulation\u27 fabrication method. Two methods are utilized to measure dispersion, UV-Vis spectroscopy and optical confocal microscopy. Quantitative spatial dispersion levels were obtained through image analysis to obtain a \u27relative dispersion index\u27 (RDI) representing the uniformity of the dispersion of SWNTs in the samples and through absorbance. We find that the storage modulus, electrical conductivity, and flammability containing the same amount of SWNTs, the relationships between the quantified dispersion levels and physical properties show about four orders of magnitude variation in storage modulus, almost eight orders of magnitude variation in electric conductivity, and about 70 % reduction in peak mass loss rate at the highest dispersion level used in this study. The observation of such a profound effect of SWNT dispersion indicates the need for objective dispersion metrics for correlating and understanding how the properties of nanocomposites are determined by the concentration, shape and size of the nanotubes

Nanotube networks in polymer nanocomposites: Electrical and thermal properties, rheology, and flammability

The superior electrical and thermal conductivity, mechanical properties, and high aspect ratio of single wall carbon nanotube (SWNT) have made them potential fillers for polymer nanocomposites. In this dissertation, we have developed new fabrication methods for SWNT/polymer nanocomposites and explored the relationship between the properties and structures of these materials. A coagulation method was developed to prepare SWNT/poly(methyl methacrylate) (PMMA) nanocomposites, providing uniform nanotube dispersion. Optical microscopy, Raman imaging, SEM, and AFM were employed to determine the nanotube dispersion at different length scales, from 100 μm to 1 nm. The resulting SWNT/PMMA nanocomposites exhibit improvement in elastic modulus and thermal stability. The addition of SWNT also significantly improves flammability of the polymer matrix, and nitrogen gasification forms a self-supporting nanotube network covering the whole nanocomposite. Both nanotube concentration and alignment significantly influence electrical conductivity of the SWNT/PMMA nanocomposites; the electrical conductivity exhibits percolation behavior with increasing nanotube loading or isotropy due to the formation of an electrically conductive nanotube network. The thresholds for alignment percolation shift to more anisotropic as the loading increases. Near the concentration threshold, the highest conductivity, σmax , occurs in slightly anisotropic nanocomposites. These experimental results were further supported by two-dimensional simulations. The SWNT/PMMA nanocomposites show rich rheology; they exhibit solid-like non-terminal behavior at low frequencies due to the formation of a hydrodynamic nanotube network. A rheological threshold was found to be significantly smaller than the conductivity threshold. We understand this difference in terms of the smaller nanotube-nanotube distance required for electrical conduction, as compared to impeding polymer motion. The filler dispersion, alignment, size, and shape and the molecular weight of polymer matrix have strong influence on the rheological behavior of the nanocomposites. We compared thermal conductivities with electrical conductivities of these nanocomposites. The electrically conductive nanocomposites have the same thermal conductivities as pure PMMA, indicating that the electrically conductive nanotube network is not thermally conductive. A large interfacial thermal resistance between the nanotubes and the polymer matrix is the cause for the poor performance in thermal conductivity. To avoid the interfacial thermal resistance, an infiltration method was used to make composites with more enhanced thermal conductivity