Evaluation of mirage effect spectrometry for optical absorption depth profiling of photodegradation in thin poly(vinylchloride) films

Mirage effect (photothermal deflection) spectrometry was examined as a tool for the quantitative depth profiling of the optical absorption coefficient in thin films of poly(vinylchloride) (PVC) photolyzed under ultraviolet (U V) light. Profiles of the photoproduct of dehydrochlorination reactions wer e induced by controlled UV photolysis in test films on the scale of 10-100 mum, with total absorbance in the range 0.3-0.5 a.u. at 465 nm. Photolyzed test samples were prepared as laminate assemblies containing 10-25 individu al layers fused together under pressure, according to a methodology we prev iously developed. These assemblies were mechanically separated into individ ual layers after photolysis and spectrophotometrically analyzed. Phototherm al depth reconstructions of the optical absorption coefficient were made in these assemblies via inverse problem theory applied to the experimental ph otothermal deflection response, with the destructive layer-by-layer spectro photometric analysis performed in parallel as a reference measurement. Opti cal absorption coefficient profiles recovered by the mirage effect were in agreement with the reference spectrophotometric data to within a root-mean- square error of better than 15% of full scale, with a nearly quantitative r ecovery of the peak absorption, depth, and initial slope of the film's abso rption profile.