Evaluation of Mirage Effect Spectrometry for Optical Absorption Depth Profiling of Photodegradation in Thin Poly(Vinylchloride) Films

Abstract

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 (UV) light. Profiles of the photoproduct of dehydrochlorination reactions were induced by controlled UV photolysis in test films on the scale of 10–100 μm, 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 individual layers fused together under pressure, according to a methodology we previously developed. These assemblies were mechanically separated into individual layers after photolysis and spectrophotometrically analyzed. Photothermal depth reconstructions of the optical absorption coefficient were made in these assemblies via inverse problem theory applied to the experimental photothermal deflection response, with the destructive layer-by-layer spectrophotometric analysis performed in parallel as a reference measurement. Optical 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 recovery of the peak absorption, depth, and initial slope of the film's absorption profile.