Photothermal imaging of dehydrochlorination in poly(vinyl chloride) films

Abstract

Impulse (time domain) laser photopyroelectric effect spectrometry uses the time dependence of heat diffusion in an irradiated sample to infer optical and thermal property depth distributions in the sample. This method was used in conjunction with a recently reported method of inverse problem theory to recover the depth dependence of optical absorption in photodegraded thin films of poly(viny1 chloride) (PVC) on a depth scale of a few tens of microns. The thin films were photodegraded under broadband ultraviolet light prior to photothermal analysis to induce subsurface profiles of visible absorption arising from the depth dependence of light initiated dehydrochlorination reactions in the films. Optical depth profile reconstmctions could be made in samples that had undergone mild degradation only and where the absence of significant thermal effusivity changes accompanying degradation could be confirmed. Reconstructed optical transmission profiles were compared with optical transmission micrographs and were shown to resolve features that arise from the depth dependence of photo-induced dehydrochlorination reactions in the films. The extent of these latter reactions is controlled by the depth dependence of the oxygen concentration in the films.

Analytical Methods

“…160,161 The laser mirage effect, a method of photothermal beam deflection, measures heat conduction from the surface of an optically heated sample into an adjacent fluid medium. The nondestructive character of the technique permits long-term studies to be performed on the same sample of slowly evolving processes such as natural weathering.…”

“…The nondestructive character of the technique permits long-term studies to be performed on the same sample of slowly evolving processes such as natural weathering. 161 10.5.6 MASS, MS Mass spectroscopy is seldom used alone. 160 Depth profiling measurements are carried out by the technique of impulse laser photopyroelectric effect spectrometry.…”

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

“…This solution has a spatial low-pass character that strongly smooths interfacial structures and that may also be strongly influenced by small determinate errors. Depth profiling by photothermal methods 20,21 therefore requires careful control of experimental conditions to obtain a meaningful result.…”

Depth Profiling of Optical Absorption in Thin Films via the Mirage Effect and a New Inverse Scattering Theory. Part I: Principles and Methodology

“…Unlike competitive techniques, phototherm al methods offer advantages of being potentially noncontact, single-ended, and robust in the presence of moderate light scattering levels. Previous phototherm al depth pro® ling studies include thin-® lm optical characterization 12,13 and depth pro® ling of humidity in starch m edia 15 using the laser photopyroelectric effect, and tissue absorption depth pro® ling using photothermal infrared radiometry. 16 W hile these techniques have their speci® c advantages, a primar y drawback in both cases is incompatibility with inspection at a solid/liquid interface, especially with aqueous media.…”