Carbon tetrachloride absorption in low-density polyethylene pellets: Concentration profiles by electron microprobe analysis

The concentration profiles of carbon tetrachloride in low-density polyethylene pellets after various periods of absorption (1–6 h) at 40°C have been measured in a new application of electron probe microanalysis using energy dispersive x-ray spectroscopy (EDAX). The concentration profiles were determined from an EDAX scan of chlorine content along the diameter of a cylindrical pellet (4-mm diameter) mounted in a scanning electron microscope. The maximum chlorine content seen in the EDAX scan was presumed to reflect the equilibrium surface concentration, eliminating the need to calibrate the system and enabling direct interpretation of the EDAX scan as a concentration profile. The concentration profiles revealed that absorption of CCl₄ in LDPE pellets took place with a sharp moving boundary, with the depth of penetration proportional to (time)^½ rather than to time as in case II diffusion. This behavior was attributed to a discontinuous relationship between diffusivity and concentration, resulting in a sharper than expected fickian diffusion profile. The EDAX technique, although requiring further refinement, appears to be a valuable method for assessing slow diffusion in thick polymer samples for those penetrants detectable by EDAX.