Electronic Thesis and Dissertation Repository

Degree

Master of Science

Program

Physics

Supervisor

Dr. John R. de Bruyn

2nd Supervisor

Dr. Jeffrey L. Hutter

Joint Supervisor

Abstract

Gels based on polyvinyl alcohol (PVA) can be formed by repeated freezing and thawing of a solution of the polymer. PVA cryogels have applications as biomaterials, including artificial tissue and drug delivery systems. The mechanical and electrical properties of polymeric materials can be changed significantly by adding a small amount of nanometer-sized particles. In this work, the dielectric properties of PVA solutions and gels were studied in the frequency range from 10 micro Hz to 1 MHz as a function of temperature, using a dielectric spectrometer. Comparison of the dielectric constant of a PVA solution to that of water indicates that --OH groups on the polymer make a large contribution to the dielectric response. The real and imaginary parts of the permittivity $\varepsilon$($\omega$) decrease during the cooling phase of a freeze-thaw cycle due to a reduction in the mobility of the dipoles, and do not completely recover upon thawing. Measurements of the loss tangent indicates the presence of three major relaxation processes in the PVA cryogel. One of them was similar to the process observed in water dielectric. The other two relaxation processes are identified as with $\alpha$ and $\beta$ relaxation of the PVA molecules. Similar to water, all the relaxation peaks in the loss tangent move to lower frequency as the temperature is decreased due to a slowing of the relaxation processes. We observed a critical temperature at which the permittivity drops suddenly during the cooling portion of the freeze-thaw cycles. We identify this with the formation of micro crystals of PVA in the cryogel. Adding carbon nanotubes to the PVA solution raises this critical temperature. We interpret these results in terms of the structural changes that take place within the PVA gel in the process of gel formation. Overall, we have shown that dielectric spectroscopy data can be used to obtain useful information about PVA cryogels and nanocomposites.

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