Simulation of mechanical properties of carbon nanotubes and their interaction with polymers

Carbon nanotubes are fullerene-related structures, which are entirely composed of carbon atoms. We present molecular mechanics (MM) simulations of mechanical properties of single-walled carbon nanotubes (SWNT) and molecular dynamics (MD) simulations of interactions between polymers and SWNT. A SWNT is usually thought of as a perfect graphene sheet wrapped up into a cylinder. However, in practical applications defects on SWNT are very common. In our study, a molecular mechanics approach is applied to study the mechanical properties of "zigzag" and "armchair" SWNT with or without defects under tensile and compressive load. Three kinds of defects are studied: defects caused by chemical functionalization, topological defects, and vacancies. In our study, the influence of defects on mechanical properties of SWNT is investigated. Our study shows that the elastic modulus of SWNT is slightly influenced by these defects, while the buckling and fracture strength are greatly affected. The buckling strength of all the SWINT deteriorates with increasing defect concentration. The defects greatly decrease the fracture strength of SWNT, and we use the Weibull strength distribution to describe the fracture data of defected SWNT. Furthermore, a weakest link model is used to bridge the scale gap between the length of SWNT in simulations and that of SWNT in experiments. Carbon nanotubes have exceptional mechanical properties and large aspect ratio, which make them ideal candidates as ultra-strong reinforcers for composites. Good interfacial binding is a necessary condition for successful load transfer across the interface between carbon nanotubes and polymer matrix. A series of MD simulations of the interaction between polymers (PS, PPA, PrnPV and PPV) and SWNT are carried out, and the results indicate that the interaction between carbon nanotubes and polymers is strongly influenced by the specific monomer structure. Among the four polymers, the intermolecular interactions are strongest for conjugated polymers with aromatic rings on the polymer backbone, as these rings are able to align parallel to the nanotube surface and thereby provide strong interfacial adhesion. In the presence of well-separated SWNT, different polymer chains get disentangled and align along the SWNT to cover the nanotube surface ("wrapping"). This phenomenon is most pronounced for PmP'V which combines certain flexibility in the backbone structure, flexible side-chains and strong interaction with the nanotube surface. Declaration of originality I hereby declare that unless stated otherwise in the text, the research recorded in this thesis and the thesis itself was composed and originated entirely by myself in the Institute for Materials and Processes, School of Engineering and Electronics & Centre for Materials Science and Engineering, The University of Edinburgh. Some of the material used in this thesis has been published in the following paper: M. Yang, V. Koutsos, M. Zaiser, Interaction between polymers and carbon nanotubes a molecular dynamics study, Journal of Physical Chemistry B, 109, 2005, 10009-10014