Depth Profiling of the Optical Absorption Coefficient in Ultraviolet-Degraded Poly(Vinylchloride) Films by Dual Beam Light Profile Microscopy

Abstract

Dual beam laser light profile microscopy (LPM) was applied in this work to the depth mapping of the optical absorption coefficient in photo-degraded poly(vinyl chloride) films. Depth profiles followed the absorption coefficient of a conjugated polyene photoproduct at visible wavelengths in photolyzed films of approximately 200 mum thickness. Both continuous and layered (separable laminate) films were studied. The absorption coefficient profiles reconstructed from photo-degraded thin films showed the classic concentration profiles seen in the literature for PVC degraded in the presence of oxygen and nitrogen atmospheres. In the case of single thin layers with continuous properties, the depth profiles were smooth and regular with minimum spatial noise. In the LPM of laminate structures, more optical anomalies were present because of the multiply interfacial structures that appeared in both the images and the reconstructed depth profiles. Notwithstanding, it was possible to profile the optical absorption coefficient at a level of error comparable to standard microtome methods. The latter was determined by comparing the LPM results to a destructive layer-by-layer analysis performed in parallel on the imaged materials. The dual beam LPM method should be generally useful for establishing polyene concentration profiles in industrial materials produced by photochemical, thermal, and chemical degradation mechanisms.

Light Profile Microscopy Based on Raman and Wavelength Resolved Luminescence Contrast

“…The imaging geometry used by the present work ( Fig. 1a) differs from past LPM configurations [7][8]10 in that the excitation beam is focused in the sample to a tight cylindrical beam of 10 lm radius (as opposed to the wider beams used in recent works) and a distance of collimation in excess of 2 mm. The line image is coupled to the input plane of an imaging spectrograph, which forms an image along two dimensions: the horizontal axis, x, which is the depth axis of the sample, and the vertical axis, y, which is aligned to the dispersion (wavelength) axis of the spectrograph.…”

“…In an LPM, the optical system forms an image of the excitation beam as it propagates through the sample: ''the beam is the object'', in essence. The following general equation of contrast has been put forth in past works 1,7,10 to describe the variation of the source beam irradiance, I(x), emitted with depth, x, in an inhomogeneous material:…”

“…Techniques of optical microscopy, which may entail modification of the sample, have had partial success. [3][4][5][6][7][8] Among these, light profile microscopy (LPM) [7][8][9][10] has provided a quasi-nondestructive method for the direct examination of structure in thin layers.…”

“…8 More recent works have demonstrated a dual beam depth profiling method that allows the depth determination of the optical absorption coefficient in nonhomogeneous structures. 9,10 The emission mechanisms used to provide contrast in these recent works include broadband luminescence and/or elastic scatter. 7 While this contrast has proved excellent for the depth imaging of sample morphology and physical microstructures (that may be otherwise invisible to reference imaging techniques), 10 such contrast provides very limited information about the sample composition and no possibility for layer identification.…”

Environmental effects of ozone depletion and its interactions with climate change: Progress report, 2005

“…The finding confirms synergistic effects of UV and temperature acting together causing increased photodamage to plastics. 170 New techniques allow more accurate modelling of photodamage at different depths in plastics exposed to UV radiation Recent improvements in depth profiling techniques, 171 especially those based on confocal microscopy, allow better quantification of the photodamage at different depths below the irradiated surface. The improved models allow for better design of plastic products as well as new stabilizer systems intended for materials destined for routine outdoor use.…”