Functionalized carbon nanotubes for polymer based nanocomposites


The extraordinary properties of carbon nanotubes (CNTs) attracted the attention of the scientific community over the past years, leading to research in areas such as electronics, sensors, medical, aerospace and automotive industries. Among the various possible applications, the incorporation of CNTs in polymer matrices presents particular interest. The high aspect ratio of CNTs allows the enhancement of composite properties at very low content, when compared to other traditionally used materials. However, its physical form of very fine powder formed by long and entangled nanotubes, as well as their high surface chemical inertia, limits their dispersion in the matrix material, leading to the formation of aggregates that adversely affect the composite properties. Functionalization of CNT surface is a solution to overcome the CNTs chemical inertness and enhance the compatibility with the matrix. The adequate tailoring of the chemical functionalization may improve the dispersion and interface of the CNTs in a polymer matrix, and thus it is expected to improve the final composite properties. One of the objectives of this thesis was to implement a functionalization method that did not induce structural damage on the CNTs, increasing the CNT surface reactivity and helping their dispersion into a range of polymer matrices. The functionalization method studied was based on the 1,3-dipolar cycloaddition reaction of azomethine ylides, and was carried out under solvent-free conditions using a specific amino acid as reagent. The reaction was investigated by thermal analysis, X-ray photoelectron spectroscopy and electron microscopy techniques. The results demonstrated the modification of CNTs with reactive groups through a simple procedure that was scaledup, and may be compatible with current industrial processes. The modified nanotubes were dispersed in different polymeric matrices that could react with the functional groups introduced. Two melt mixing methods generating flow with different characteristics were studied, and the dispersion of carbon nanofibers and nanotubes in different polymers was analyzed. Namely, the effect of conventional twin-screw extrusion and the application of a converging/diverging flow sequence to the composite melt were compared. The simple application of a converging/diverging flow sequence demonstrated an interesting improvement of nanofiller dispersion in the polymers studied, compared to twin screw extrusion. The effect of processing polycarbonate/CNT composites on a prototype equipment was investigated. It was observed that CNT dispersion improved and the electrical resistivity decreased with composite processing, both for composites with non-functionalized and functionalized CNTs. CNTs were melt mixed with polypropylene and poly(lactic acid) using twin screw compounding under various operating conditions. The CNT dispersion was studied by optical microscopy and the effect of the different types of functionalization and processing conditions on the composites properties was evaluated. The composites formed by melt mixing the polymer with nanotubes that were modified by grafting polymer molecules to their functionalized surface presented improved tensile properties and good CNT/polymer interface. The composites electrical conductivity tended to be lower compared to composites with non-functionalized CNTs, but the differences may be minimized. This thesis provides insight into the joint effect of CNT chemical functionalization and polymer melt blending conditions on CNT dispersion in polymer matrices, and on its influence on the electrical and tensile properties of the resulting composites.


Rui M. Novais

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