2.2. Fourier-Transform Infrared (FTIR) and Nuclear Magnetic Resonance (NMR) Spectroscopy

The chemical structure of various PU samples was measured using a Thermo Fisher Scientific Nicolet iS10 ATR-FTIR spectrophotometer (Walthem, MA, USA), according to the previously published method [19]. The spectra were recorded between 4000 and 650 cm⁻¹ with a resolution of 8 cm⁻¹. Peak information from various PU sample spectra were analyzed using OMNIC^TM software.

The ¹H spectra of various PU samples were recorded on an Agilent NMR system at 400 MHz. PU samples were dissolved in dimethyl sulfoxide-d6 at 3 wt%, and DSS (3-(trimethylsilyl)-propanesulfonic acid, sodium salt) was used as the internal reference for the ¹H chemical shifts.