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Thesis Format

Integrated Article

Degree

Doctor of Philosophy

Program

Physics

Supervisor

Dr. John R. de Bruyn

2nd Supervisor

Dr. Jeffrey L. Hutter

Co-Supervisor

Abstract

This thesis consists of three projects concerning the electrical and mechanical properties of polymer nanocomposites. We study the effect of nanoscale filler particles on the polymer dynamics at different length and time scales. In the first study, poly(ethylene oxide) (PEO)-multiwalled carbon nanotubes (MWCNT) nanocomposites with a MWCNT concentration ranging from 0 to 5 wt\% were prepared by both melt-mixing and twin-screw extrusion. Their electrical properties were studied over a wide range of frequency and temperature using a dielectric spectrometer. A percolation transition is observed at which the electrical conductivity of the nanocomposites increases by several orders of magnitude. The percolation threshold concentration p_c is very well-defined in the twin-screw extruded material, but less so in the melt-mixed nanocomposites. We identify two different dielectric relaxation processes in our PEO/MWCNT nanocomposites, which we attribute to polymer dynamics at different length scales. The second project is a study of the mechanical properties of PEO/MWCNT nanocomposites made by melt-mixing. We used a rotational shear rheometer to perform measurements during thermal cycling. Our results show that there are three main mechanical relaxation times in the nanocomposites, all of which are much slower than the relaxation times observed in dielectric data. One of these processes is due to the reptation of polymer chains. Another is due to the relaxation of PEO chains whose motions are restricted by MWCNT. The third one is related to the sample preparation process. In the last project, we used dielectric spectroscopy to investigate the electrical properties of polystyrene (PS)-MWCNT nanocomposites made using twin-screw extrusion. Our data suggest that the percolation threshold for these nanocomposites is between 4 and 5 wt\%, but the transition only occurs once the sample has been heated above 330 K. In most cases, the dielectric spectrum did not show any relaxation features. A dielectric relaxation was only observed for a MWCNT concentration of 5 wt\%, and the relaxation peak disappeared when the sample was heated above 330 K due to the high electrical conductivity of the sample. Our studies showed several examples of polymer dynamics influenced by the presence of MWCNT on time scales ranging from microseconds to hundreds of seconds.

Summary for Lay Audience

Polymer nanocomposites are a novel class of composite material made by adding nanometer-sized filler particles to a polymer. The properties of the nanocomposites can be enhanced over those of the pure polymer by choosing the right filler. For example, conducting materials can be made from an insulating polymer by adding a conductive nanofiller such as carbon nanotubes, and the conductivity can be tuned by changing the concentration of the filler particles. The main motivation for this work is to use measurements of polymer nanocomposites to learn about the motion of the polymer molecules at different length scales. In particular, we want to study how the presence of nanotubes affects the polymer dynamics. Here we study nanocomposites made by adding a small amount of carbon nanotubes to poly(ethylene oxide) and polystyrene. We were able to examine the distribution of nanotubes in the polymers using a scanning electron microscope. Our dielectric data showed evidence of microsecond-scale polymer dynamics in the nanocomposites. The mechanical measurements showed the presence of slow polymer dynamics occurring over time scales ranging from one-tenth of a second to few hundred seconds. Pure poly(ethylene oxide) and pure polystyrene are both insulators. Our data showed that nanocomposites based on these polymers become conducting when a few weight percent of carbon nanotubes was added to the polymer. For example, the electrical conductivity of poly(ethylene oxide) with 5\% carbon nanotubes added was a factor of $10^{8}$ higher than that of the pure polymer. The conductivity increase for the polystyrene nanocomposite was even higher - a factor of $10^{11}$.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

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