Defense Date

2004

Document Type

Thesis

Degree Name

Master of Science

Department

Mechanical Engineering

First Advisor

Dr. Zoubeida Ounaies

Abstract

The main objective of this thesis is to efficiently disperse and align SWNTs in two different polymer matrices to obtain an orthotropic composite whose strength, stiffness and electrical properties depend on the orientation of the SWNTs. The SWNTs are successfully dispersed and aligned in a polyimide matrix and a polymer blend of UDMA/HDDMA. In-situ polymerization under sonication is used to disperse the SWNTs in polyimide matrix and sonication is used to disperse SWNTs in the UDMA/HDDMA matrix. In both cases, an electric field is used to align the SWNTs in the polymer matrices. In the polyimide, the SWNTs are aligned by electrospinning technique, and in (UDMA/HDDMA) the SWNTs are aligned by applying an AC electric field, while the composite is cured.The electrical and mechanical properties of randomly dispersed SWNT polyimide composites and SWNT/UDMA/HDDMA composite are measured. The dielectric constant and storage modulus of SWNT polyimide composite increased with SWNT concentration. Low percolation (0.06 wt%) and an increase of 113% in storage modulus with 0.2 wt% SWNTs, both indicate good dispersion of SWNTs in the polyimide matrix. The dielectric constants, conductivity for the unaligned SWNT/UDMA/HDDMA composite are isotropic. The electrical and mechanical properties of the randomly dispersed SWNT polyimide composite and SWNT/UDMA/HDDMA composite are used as references when analyzing the aligned counter parts. Different characterization methods are used to assess the alignment of the SWNTs in the polyimide and (UDMA/HDDMA) matrices. A variety of characterization techniques, i.e. microscopy, Raman spectroscopy, electrical conductivity, dynamic dielectric spectroscopy and dynamic mechanical analysis, indicate preferential alignment of SWNTs in two types of polymers: Polyimide and (UDMA/HDDMA). Optical microscope images showed alignment of the SWNTs in the UDMA/HDDMA composite. Inspection of the Raman spectra on aligned SWNT polyimide composite fibers and aligned SWNT/UDMA/HDDMA composite indicates a decrease in the intensity of the tangential peak of the SWNT with increase in the polarizer angle. The difference in the perpendicular and parallel Raman peaks indicate preferential alignment of SWNTs in both the polymer matrices. In the aligned polyimide composite, percolation transition is at 0.2 wt% SWNT concentrations when dielectric constant is measured parallel to the aligned SWNTs. But percolation transition is at 0.65 wt% SWNT concentrations when dielectric constant is measured perpendicular to the aligned SWNTs. Electrical measurements on aligned SWNT polyimide and UDMA/HDDMA composite are highly anisotropic. In both cases, the dielectric constant values parallel to the direction of SWNT alignment are higher than the values perpendicular to the direction of SWNT alignment. To analyze the resulting anisotropy in the dielectric constant, Bruggeman's effective medium approach is used. The effective medium theory predicts the effective dielectric constant of a composite with aligned anisotropic inclusions. The effective dielectric constant, perpendicular to the aligned inclusions and parallel to the aligned inclusions is estimated. The dielectric constant values of aligned SWNT polyimide and aligned SWNT/UDMA/HDDMA composites are compared to the experimental results. Both the values from the theory and experiment show anisotropy in dielectric constant. The theory indicated that the dielectric constant parallel to the aligned inclusions is highly influenced by the dielectric constant of the inclusion and the dielectric constant perpendicular to the aligned inclusions is highly influenced by the dielectric constant of the polymer matrix. Results from the different characterizing techniques indicate that SWNTs are successfully aligned in the polyimide matrix and (UDMA/HDDMA) matrix by electrospinning technique and by an AC electric field respectively.

Rights

© The Author

Is Part Of

VCU University Archives

Is Part Of

VCU Theses and Dissertations

Date of Submission

June 2008

Included in

Engineering Commons

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