The warmer colors, i.e., higher Gram-Schmidt signals (higher total integral for all wavenumbers), within the bulk of SBR W168h in Figure are interpreted as the presence of higher amounts of hydrocarbon structures. In contrast, the cooler colors in the heat map, near the surface (UV irradiated side), are indicative of chemical changes, i.e., modification due to weathering, caused mainly by reaction with oxygen. This also serves as a first proof of the formation of 'skin' and 'core' regions across the sample thickness, due to distinct chemical and, possibly, physical differences. From the microscopic image in the background, a thick skin region for this specimen is observed to be up to about 95 µm from the surface. This demarcation is noted as the moderately modified yellow-green separation line in the Gram-Schmidt plot. Of course, a much more modified, thinner skin can be present immediately at the surface due to the gradient, but analyses of these regions are excluded here. The yellow-green blob on the right-end could be resulting from the topography effects of the surface and highlights the after all qualitative nature of the observation of the skin with this method. Microscopic images of NR weathered up to the same duration demonstrated a skin thickness of about 60 to 80 µm. The IR microscopy measurements were used as a control to probe the desired regions ('skin' and 'core') through ATR FTIR measurements. Overlays of absorption spectra for NR and SBR weathered to various durations can be found in Figure . A general trend of peak broadening with the weathering duration can be observed in the core-specific spectra of both, NR and SBR.