Evaluation of space charge properties in polymers by using a scanning electron microscope

In order to try to improve dielectric properties of polymers, we have explored the space charge characteristics by using a scanning electron microscope (SEM). By controlling the conditions of charge injection close to the surface of the sample, it has been possible to evaluate on one hand the trapping ability of the material and on the other hand the corresponding detrapping potential. This latter parameter was connected to the space charge spreading during injection as seen by a decrease of the detrapping potential with increasing the size of the domain where the charges are injected. This size can be monitored by a careful control of the focalisation of the electron beam on the surface of the sample. Using this technic a same amount of charges can be injected into domains with different size. Moreover, the surface potential was found to be dependent on the structural parameters of the tested polymers such as branching ratio. To better characterize the space charge properties, the detrapping potential has been examined at different temperatures. These experiences have indicated that above a critical value a decrease of the sample temperature strongly affects the space charge spreading. Characterization of the evolution of the dielectric losses (tanδ) with the temperature showed that this critical temperature was close to the glass transition temperature of the sample. These results have suggested that the macromolecules motion plays a significant role in the space charge spreading