2.2.3. Comparison of Different Polymers by FTIR Spectroscopy

The Fourier transform infrared (FTIR) measurements were performed on the Nicolet 6700 spectrometer (Thermo Scientific, Madison, WI, USA) in transmission mode using the KBr method. First, the stock solutions of the different PS microspheres (PS-100, PS-200, PS-NH2-100, and PS-NH2-200) were diluted with distilled water to a concentration of 0.1 mg/mL. Next, aqueous PS solutions were frozen with liquid nitrogen and lyophilized in a freeze dryer (Christ Alpha 1–2 LD plus with double–chamber, Osterode am Harz, Germany) for 24 h under 0.013 mbar pressure. The obtained powder samples were mixed and ground in an agate mortar with dry KBr at a mass ratio of 1:100. The powders

pressed into KBr pellets were subjected to FTIR analysis. The FTIR spectra of different PS microspheres were collected at room temperature in the range 4000–400 cm−<sup>1</sup> with a wavenumber resolution of 4 cm−<sup>1</sup> .
