Spectroscopic diagnostics of morphological changes arising in thermal processing of polypropylene

Polypropylene is considered as a representative thermoplastic matrix for advanced composite materials that have some advantages in various engineering applications. Wide-range infrared optical properties of polypropylene are important for combined heat transfer modeling in these composite materials, which are semitransparent in a considerable part of the whole spectral range. This study is focused on optical properties of polypropylene in the visible and near-infrared ranges because the measurements in these ranges exhibit a stronger effect of the processing temperature used in the material manufacturing. The experimental study is based on spectral measurements of both the normal-hemispherical reflectance and transmittance of polypropylene samples. The main characteristics of volumetric absorption and scattering are identified using the inverse problem solution based on the modified two-flux approximation, which is sufficiently accurate to determine the hemispherical characteristics of the radiation field in the range of the problem parameters. In particular, the effect of a relatively strong scattering is observed at the absorption peaks in the near-infrared range. An approximate theoretical model based on spectroscopic data is developed to estimate morphological changes arising in thermal processing of polypropylene at different temperatures.