The diffusion of mercury vapor in plastics can be studied by a radiotracer method using the Hg isotope. Hg has a half-life of 46.9 days and emits beta particles of 208 keV and gamma radiation of 279 keV. The gamma activity of 203Hg can be measured easily by a NaI(Tl) scintillation detector.

The diffusion studies are as follows. Discs are cut from the plastic samples and they are placed onto the plane top of glass vessels containing a drop of mercury labeled with 203Hg. The diameters of the plastic discs and the top of the glass ves­sels have to be the same and they have to be fitted tightly to avoid the escape of mercury from the vessel. Mercury evaporates in the glass vessel and introduces into the plastic disc. After a given experimental time, the plastic disc is sliced by a microtome into layers that are about 10 micrometers thick. The gamma activities of the plastic slices are plotted as a function of the square of the distance measured
from the surface connecting to the mercury vapor. The virtual diffusion coefficient can be determined using Eq. (9.34).

In Figure 9.5, the logarithm of gamma activities (ln I) is plotted as a function of the square of the distance measured from the surface connecting to the mercury vapor (t = constant). From the slope of the straight line, the virtual diffusion coeffi­cient is determined if the time of diffusion is known. The virtual diffusion coeffi­cients of different plastic samples are very similar, around 10_10 cm2 s_1.

During the migration of mercury in plastic samples containing sulfur, a chemical reaction takes place between mercury and sulfur, influencing the profile of the ln I versus x2 plot. As a result of this chemical reaction between mercury and sulfur, a maximum is observed at a given distance (Figure 9.6). The place of the maximum

depends on the sulfur concentration in the plastic and the time of migration. This phenomenon is analogous to the principle of chromatography.