A study of the factors affecting the diffusion of chlorinated hydrocarbons into polyisobutylene and polyethylene-co-propylene for evanescent wave sensing

Two polymers, polyisobutylene (PIB) and polyethylene-co-propylene (60% ethylene) (E/Pco), have been considered as preconcentration media for the detection of chlorinated compounds using mid-IR evanescent wave spectroscopy. In order to optimize and predict sensor response factors affecting the diffusion of analytes into the polymer films has been examined using a silver halide sensing fiber coupled to a FTIR spectrometer. A Fickian diffusion model was used to quantify the diffusion process. The diffusion model calculated a diffusion coefficient based on such parameters as refractive index of the polymer cladding and light guiding core, the polymer cladding thickness and the principal analyte wavelength of detection. The diffusion of analyte isomers was investigated. The diffusion of chlorinated benzene isomers was employed to determine whether the position of chlorines on the ring effects diffusion. Physical steric and molecular size effects were found to dominate the diffusion of these compounds. Solution composition was a fundamental issue in sensor design. The polarity of the sample stream was another diffusion determinant. The polarity of the matrix solution was altered by the addition of organic solvents. Rate of diffusion was seen to increase with decreasing matrix polarity. E/Pco was found to be less susceptible to polarity modification. The results obtained can be employed as a basis for the selection of a polymer for particular industrial applications and also as a means of improving the speed of sensor response.