Analysis of polycarbonate degradation at melt/FeCr-alloy interfaces as a function of the alloy composition by means of combinatorial thin film chemistry

Interfacial reactions at the polycarbonate (PC)/FeCr-alloy interface during melt contact were studied as function of the Fe:Cr ratio within the alloy. Thin Fe/Cr films with lateral composition gradients were deposited by magnetron sputtering; the analysis of the films was done with microscopy and X-ray photoelectron spectroscopy (XPS). The local interfacial polymeric film formation could be therefore directly correlated with the Fe:Cr ratio. The local thickness and structure of the formed polycarbonate residue was analyzed by means of imaging ellipsometry, atomic force microscopy as well as Fourier-transform infrared spectroscopy under grazing incidence and XPS. Moreover, confocal fluorescence microscopy of the PC melt/alloy interface could reveal the formation of minor degradation products in the interphase region. The results show that already an Fe:Cr ratio of 2 : 1 leads to a strong inhibition of the thermal degradation in comparison to the unalloyed iron, and that in general, the enrichment of chromium in the passive film leads to an effective suppression of interfacial PC degradation. The data contributes to improving the mechanistic understanding of the role of iron during this process. Additionally, a critical concentration of chromium in the alloys used for PC processing can be deduced.