LOW-DENSITY POLYETHYLENE DEPTH PROFILE ANALYSIS BY PHOTOACOUSTIC-SPECTROSCOPY

Near-infrared photoacoustic spectra of polyethylene (1 mm slab) were taken in the modulation range 10-240 hz, which corresponds to thermal diffusion layers in the 56-11 mu m range. Thick-layer spectra are very similar to polyethylene film transmission spectra, but large differences are observed between the spectra taken at various modulation frequencies. The analysis of spectral intensity as a function of modulation frequency shows that peak intensity ratios of minus CH//3, equals CH//2, and minus OH groups, relative to that of methylene groups, increase as thinner, closer-to-surface polymer layers are sampled. From this we conclude that near-to-surface layers of solid polyethylene are richer in minus CH//3, equals CH//2 and minus OH groups than the polymer bulk.