The polymer blend reference sample was created by spin coating a mixture of poly methyl methacrylate (PMMA) and polystyrene (PS) from solution. PMMA and PS (0.5 wt% each) were dissolved in toluene by vigorously stirring without heating. Once dissolved, the solution was directly spin coated (Spin150i Tabletop, APF Automation) on the cleaned and dried flat silicon carrier. With 60 µL of the mixture at 1750 rpm for 60 s, an average film thickness of 250 nm was achieved. The sample was annealed for 6 h at 115 °C to allow for local unmixing of both polymers. Finally, film thickness was verified using a profilometer (Bruker Dektak XT). Simulation. Simulations were conducted prior to the experiments to understand how the horizontal movement of the Si-ATR (respectively the stage) -orthogonal to the direction of the micromachined grooves -affects the focal spot position on the surface, whether moving the sample laterally would also lead to a shift in the focal spot position. IR focus position. The beam intensity and the displacement of the focal spot on the Si surface -relative to the POI -during scanning was determined using raytracing. Raytracing was conducted using Zemax Optic Studio (ANSYS Ltd., Canada, v22.3) in non-sequential mode. The model consisted of the final focusing lens (f=25 mm) and the Si sample carrier. The wavelength selected for the simulation was 6.246 µm (1601 cm -1 ). The center of the beam was determined from the maximum of the intensity distribution. Evanescent wave. The development of the evanescent wave at the Si/air interface at the top of the carrier was simulated with finite-difference time-domain (FDTD) using Ansys Lumerical (ANSYS Ltd. Canada, 2023 R1.1). Here we investigate the effects of two wavelengths relevant to our measurements, namely 5.780 µm (1730 cm -1 ) and 6.246 µm (1601 cm -1 ), carried out for positions of incidence (POI) IN1 and IN2 (see Figure ). A Gaussian beam with 15 µm diameter we directed within Si at an angle calculated from the interface geometry and Snell's law. AFM-IR parameters and data collection. The data generated by the AFM-IR system was collected using the Analysis Studio (v3.15; Anasys Instruments) instrument control software. For the measurements a mid-IR laser with s-polarization was used. IR maps -also called chemical imaging during this work -are maps collected at fixed wavenumber values to investigate chemical differences within the sample. Measurement parameters for chemical imaging were as follows: A phase locked loop (PLL) was set to follow the resonance of the cantilever at the eigenmode around 200 kHz utilizing resonance enhanced AFM-IR. The laser pulse width was set to 200 ns (~4% duty cycle). The selected area for imaging was 4 µm × 4 µm with a pixel resolution of 600 x 300 (xy) and a scan rate of 0.5 lines per second (0.5 Hz). The pump laser was attenuated using a wire mesh attenuator to adjust the laser power incident on the sample for each measurement environment and each probed absorption band: In air for the carbonyl (C=O) stretching band of PMMA (1730 cm -1 ) 12 mW pulse peak power were used. For C=C phenyl band of the PS (1601 cm -1 ) the peak pulse power was set to 18 mW due to the weaker nature of this band. In water, the pulse peak power for IR mapping the C=O stretching band was set to 190 mW, whereas for the weaker C=C it was set to 226 mW. After taking chemical images, spectra at different sample positions were recorded to spot spectral differences of the polymer blend. Here, the cantilever probe is held at a fixed position and a spectrum of a nanoscale domain is collected by sweeping the laser wavelength (1985 -900 cm -1 ). The spectra are recorded with the laser being tuned to the local contact resonance frequency of the cantilever. For the spectra, only one attenuator setting was selected for all wavelengths. In air the attenuation resulted in 14 mW (for 1601 cm -1 ) and 18 mW (for 1730 cm -1 ) pulse peak power respectively. whereas in water it was ten times higher due to viscous damping of the vibration. To account for the wavelength dependent intensity of the EC-QCL, spectra were normalized by an intensity spectrum collected on a power meter. AFM height images, PLL frequency, deflection images and IR images were acquired simultaneously and in trace and retrace direction. Data processing. The AFM-IR spectra and chemical images shown in this work were processed using python3 using the numpy numerical math library and matplotlib. Measurement data were exported from native NanoIR file format to xarray dataset by using the anasys-python-tools library. The spectra recorded in air and water were filtered using the Savitzky-Golay algorithm (2nd order polynomial, 17 smoothing points). Spectra were normalized to a maximum value of 1.