For AFM-IR images a 500 nm × 500 nm area was scanned with a line rate of 0.1 Hz (lateral speed 100 nm s -1 ) and a resolution of 400 pixels per line at 200 lines. For chemical imaging the distinct carbonyl-stretching band of PMMA (C=O) at 1730 cm -1 was selected and pulse peak power was set as 15 mW. The polarization of the laser source is set to 0 degrees in respect to the sample plane. The laser repetition rate was kept at the frequency of the contact resonance of the cantilever using a phase-locked loop (PLL).