Masters Theses

Date of Award

5-1992

Degree Type

Thesis

Degree Name

Master of Science

Major

Chemistry

Major Professor

David C. Jay

Abstract

Scanning electron microscopy (SEM) is a relatively new technique that has the potential to be very useful for studying polymers and other insulating materials. It offers much of the resolution of the transmission electron microscope (TEM) with the ease of use of the optical microscope (OM). When operated at low voltages (≤ keV) it is possible to minimize the problems associated with sample charging and therefore view unprepared, bulk insulating samples in the SEM without the need for metal coatings which obscure surface fine structure. Changes in a variety of operating parameters can be utilized to decrease the problems associated with charging. Among these are increasing the angle of electron beam incidence on the sample surface, decreasing the beam current, and working at lower magnifications. However, while increasing sample tilt is an effective means of increasing contrast and decreasing the effects of charging, decreasing beam current degrades the ultimate resolution attainable and working at lower magnifications may prevent the study of surface fine structure that is important in understanding structure-property relationships. Therefore, the use of an SEM operating at low voltages with a sufficiently high emission current is a viable alternative to viewing metal coated samples. In the work presented here, electron emission from thirty five polymer samples was studied. Thirty three of these were conventional polymers in that they exhibited no significant electrical conductivity. The other two samples were HQ doped conductive polyaniline.Two methods of determining the electron emission were employed depending on whether the sample was conductive or not. For nonconductive samples, the scan square method was used to determine at what accelerating voltage the total electron emission was equal to one (E2). For the two conductive samples, electron emission was studied by measuring the ratio of current conducted through the specimen to the beam current. The difference between these two currents is the number of electrons emitted from the surface of the polymer. Schemes found in the literature for the prediction of electron emission were tested and some schemes based on the chemical properties of the polymer were proposed. It was found that E2 was a strong function of the chemical composition of the polymer and that properties such as the density, crystallinity and/or the conductivity had little or no discernible effect upon E2. A computer program was written which allows the calculation of electron trajectories through electrostatic fields. Calculations were performed in order to determine the effects of sample charging on SEM performance. Also, a field emission electron gun was modeled as a test for the method. It was found that sample charging had significant effects on secondary electron detector efficiency with negative charging causing the greatest problems.

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