Size-dependent optical properties of polyethylene powders in far-IR region: On the way to universal matrix

Similar to potassium bromide (KBr), which is used in mid-IR as a matrix for pellet preparation, polyethylene (PE) is widely applied in far-IR because it is an inert material without intensive absorption lines in this range. However, despite the material itself being transparent, the transmission of pellet made of PE powder strongly depends on the wavelength tending to decrease almost to zero in a short-wave part of the far-IR range. Heating of the pellet to PE melting temperature significantly improves its transparency, but this approach can only be applied to thermally stable compounds. In this work 8 types of PE powders of different mean particle size (MPS) ranging from 43 to 3 µm have been synthesized. Far-IR spectra of PE pellets made of these powders at room temperature have been measured and the inverse correlation between overall far-IR pellet transparency and MPS of its PE powder has been found. The imaginary part of the effective refractive index of the studied pellets obeys light-scattering scale invariance rule, indicating that the transmission of PE pellets is limited by light scattering by PE grains. It has been shown that PE powders with MPS of ~4 µm or less are suitable as a matrix for pellet preparation in far-IR spectroscopy without the necessity of pellet melting.