PHOTOTHERMAL IMAGING OF DEHYDROCHLORINATION IN POLY(VINYL CHLORIDE) FILMS

Impulse (time domain) laser photopyroelectric effect spectrometry uses the time dependence of heat diffusion in an irradiated sample to infe r optical and thermal property depth distributions in the sample. This method was used in conjunction with a recently reported method of inv erse problem theory to recover the depth dependence of optical absorpt ion in photodegraded thin films of poly(vinyl chloride) (PVC) on a dep th scale of a few tens of microns. The thin films were photodegraded u nder broadband ultraviolet light prior to photothermal analysis to ind uce subsurface profiles of visible absorption arising from the depth d ependence of light initiated dehydrochlorination reactions in the film s, Optical depth profile reconstructions could be made in samples that had undergone mild degradation only and where the absence of signific ant thermal effusivity changes accompanying degradation could be confi rmed. Reconstructed optical transmission profiles were compared with o ptical transmission micrographs and were shown to resolve features tha t arise from the depth dependence of photo-induced dehydrochlorination reactions in the films. The extent of these latter reactions is contr olled by the depth dependence of the oxygen concentration in the films .