The AFM-IR measurements in air and water were collected from a 4 µm x 4 µm area on the sample carrier, spin coated with the polymer blend, as described previously. The wavelengths selected for the chemical imaging are 1730 cm -1 for PMMA, corresponding to the carbonyl stretching (C=O) band and 1601 cm -1 corresponding to the C=C phenyl stretching of PS. Therefore, in Figure , areas of higher intensity (yellow) are PMMA richer, whereas the areas with less intensity are PS richer regions. This is also double checked with the IR image recorded at 1601 cm -1 (see, Figure ). Furthermore, this is verified with infrared spectra recorded at different locations. Within the sample, phase separation can be observed, although it can be seen that the blend is not constituent of distinct phases rather than a mixture of PMMA and PS. The infrared spectra in Figure furthermore underline the PMMA-(see Spectrum 1, 3 and 5) and PS-(see Spectrum 2, 4 and 6) phases. Generally, a predominance of PMMA is observable, whereas the intensity of PS is lower due to the weak nature of the aromatic ring breathing vibration (1601 cm -1 ). The vertical lines correspond to the specific absorption bands of the PMMA and PS, respectively at 1730 cm -1 for PMMA and 1601 cm -1 , 1494 cm -1 and 1452 cm -1 for PS. These will be discussed in further detail below. AFM-IR imaging and spectroscopy on Si substrate in water. IR images in water are depicted in Figure , where a) is recorded again at 1730 cm -1 and b) at 1601 cm -1 . The signal amplitude in these measurements is lower than in air due to the damping of the mechanical oscillation in water. Nevertheless, here again two polymer phases can be distinguished. Spectra for PMMA and PS in both, air and water are depicted in Figure . The spectra gathered in a PMMA rich phase of the sample are presented in Figure (top) and normalized for comparability. The most intense peak is a characteristic band of PMMA and is assigned to the carbonyl (C=O) stretching vibration. At 1146 cm -1 and 1264 cm -1 the C-O stretch forms additionally and is therefore a further indicator for PMMA. However, as previously discussed, the sample polymer-blend does not consist of two distinct single phases rather than a mixture of both phases. For this reason, in the spectra we can see features of PS like the weak band of the aromatic ring stretching at 1601 cm -1 . There is a good match between the spectra measured in air (dark blue line) and those measured in water (light blue line). Here, additional features are visible in the measurements in water, namely the broad band between about 1520 cm -1 and 1680 cm -1 . This is due to the absorption of water itself and caused by the evanescent wave reaching in the water above the sample surface and causing photothermal expansion and therefore being detected by the cantilever. Also, in Figure (bottom) spectra for a PS rich region are recorded and normalized for comparability. Here again the darker line corresponds to the spectra recorded in air, whereas the lighter line corresponds to spectra recorded in water. For imaging PS the aromatic ring breathing modes are used. These appear at 1601 cm -1 , 1494 cm -1 and 1452 cm -1 respectively. For the marker bands of the polystyrene, we can observe a good match between the measurements in air and water, for both, band position as well as normalized signal intensity. The broad absorption band due to water is also observable in the PS-rich region. Nevertheless, the 1601 cm -1 peak stands out.