Modifizierung von Polypropylen : Einfluss von Struktur- und Prozessparametern auf die photochemische Immobilisierung von Arylaziden

Abstract

The aim of this work was the study of the influence of various parameters on photochemical modification of polypropylene with aryl azides. Azido benzoic esters with aliphatic alcohols of different chain length (A2, A6, A12) were synthesized to study the effect of a more hydrophobic structure, which should increase the affinity to the top layers of the PP surface and the solubility in the surface-near layers of the bulk, and consequently the amount of sorbed azide prior to irradiation. In addition the influence of process parameters such as immersion time, azide solvent, preswelling in toluene and irradiation intensity was investigated. FT-IR spectroscopy was used to determine the relative amount of sorbed azide as well as the amount of covalently bonded arylamine. Real-time FT-IR spectroscopy was employed to monitor azide photolysis. The modified films were characterized by XPS, TOF-SIMS, FT-IR microscopy and DSC. It was found that an increase of chain length resulted in a significant increase of azide uptake. Compared to A2 twice as much of the more hydrophobic aryl azides were sorbed and enrichment effects due to enhanced adsorption were observed. Prolongation of immersion time resulted in a further increase of azide uptake due to diffusion within the first 30 min, followed by a decrease due to desorption and partial dissolution of surface-near layers of the films. After photochemical immobilization significant drawbacks of the tailored azide structure were revealed. The relative amount of covalently bonded aryl amine strongly decreased with increasing initial azide concentration and high amounts of unreacted azide were found. Grafting yields and rate of photolysis could be enhanced by increasing radiation intensity, but complete conversion at high azide concentration was only achievable by two-step-irradiation. Determination of the crystallinity showed that grafting was not only restricted to surface-near layers, but that also bulk modification had taken place due to the strong swelling effect of toluene and diffusion of azides deeply into the bulk. Using a mixture of acetone and n-hexan as azide solvent resulted in the formation of homogenous azide films prior to irradiation, high conversion even at high azide concentration and grafting yields of up to 50% without any change of crystallinity.