Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Mechanical and Materials Engineering

Supervisor

Dr. Liying Jiang

Abstract

The addition of carbon nanotubes (CNTs) in polymers to form conductive composites has been attracting great interest from research and industry communities due to their potential applications. Experiments and simulations have demonstrated that the addition of a very small amount of CNTs into polymers can significantly improve the electrical conductivity of the composites. Such significant improvement in the electrical conductivity is attributed to two conductivity mechanisms: nanoscale electron hopping and microscale conductive networks. Understanding and prediction of the overall electrical conductivity of the composites with the incorporation of the conductivity mechanisms that underpin the macroscopic electrical properties are essential for their engineering applications. One of the most promising applications of the conductive composites is for stretchable electronics. For such an application, it is naturally necessary to investigate the stretching effects upon the overall electrical conductivity of the composites. Furthermore, CNTs dispersed in polymers are usually not straight but rather have a certain degree of waviness due to the CNTs' large aspect ratio and low bending stiffness. It has been suggested that the waviness can have considerable effect on the electrical conductivity of the composites. Therefore, the investigation of CNT waviness effect is of great importance for the prediction of the overall electrical conductivity of the composites.

In this thesis, based on the micromechanics theory, a mixed micromechanics model with the incorporation of the nanoscale electron hopping and the microscale conductive networks is first developed to predict the electrical conductivity of the composites. The modeling results agree well with existing experimental data. It is found that both the electron hopping and the conductive networks contribute to the electrical conductivity of the composites while conductive networks become dominant to the electrical conductivity of the composites after percolation. It was also indicated that the sizes of CNTs have significant effects on the percolation threshold and the overall electrical conductivity of the nanocomposites. Based on the developed micromechanics model, stretching effects are then investigated by incorporating the stretching induced changes into the micromechanics model. The investigation found that the stretching, including uni-axial and bi-axial stretching, decreases the electrical conductivity of the composites in the stretching direction and the decrease is more evident for the bi-axial stretching compared to uni-axial stretching. It is also observed that the electrical conductivity is more sensitive to stretching for the composites with lower CNT volume fraction. Finally, we studied the CNT waviness effects upon the electrical conductivity of the composites under a uni-axial stretching. It is demonstrated that the waviness significantly decreases the electrical conductivity of the composites and the electrical conductivity is more sensitive to the waviness for the composites with lower CNT volume fraction and larger stretching strain. Reasons for the observed variations and phenomena are interpreted. The work in this thesis is expected to obtain increased understanding on the overall electrical conductivity of CNT-polymer composites from the theoretical perspective and provide useful guidelines for the design and optimization of the composites.


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