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 μm 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. From 10 to 80 Hz, all the spectral band intensities decrease linearly with ω − a where ω is the light modulation frequency and a varies from 0.48 to 1.00 for different constituent groups. The analysis of spectral intensity as a function of modulation frequency shows that peak intensity ratios of CH 3 , CH 2 , and 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 CH 3 , CH 2 and OH groups than the polymer bulk.