Investigations on the distribution of polymer additives in polypropylene using confocal fluorescence microscopy

Within this work, a fluorescence microscopy approach for the investigation of the distribution of polymer additives in polypropylene is presented. The fluorescent whitening agent 2,5-bis-(5-tert-butyl-benzoxazol-2-yl)-thiophene was used as a model compound representing other groups of polymer additives. So far, methods reported in the literature such as UV and IR microscopy offer a high spatial resolution, however, suffer from poor sensitivities, thus not allowing them to analyze samples with low additive concentrations typically used in engineering materials. Using the fluorescence microscopy technique, it was shown that independent from the applied concentrations (0.1-1.7wt%), additives are distributed on a spherulitic scale with the majority being found at the spherulite boundary and only traces in the center. Furthermore, it could be demonstrated that the additive distribution is affected not only by the spherulite sizes but also by the cooling rate of the polymer melt leading to more or less pronounced additive distribution patterns.