As shown in Fig. 4, the characteristic peaks of both pure PURF and PEPA modified PURF become considerably weaker compared to those obtained at 300 °C, indicating the further decomposition at a higher temperature. Specifically, the absorption peaks of char of PURF, including 3360 cm−1 (–OH, stretch), 3050 cm−1 (–NH), and 2922 cm−1 (–CH) revealed the existence of hydrogen containing structures in the char of PURF even at 400 °C. However, these peaks were absent in the spectrum of the char formed by PEPA modified PURF suggesting that there was no hydrogen containing structure left in the char. Moreover, the spectra of different PURFs showed great differences at 1600–800 cm−1. The peaks at 1500 and 1363 cm−1 were aroused by the bending vibrations of CH and –CH3 in polyols respectively. Peaks in the range of 1300–1000 cm−1 presented wide absorptions, which can be ascribed to the soft segment structures. Furthermore, the peak at 1228 cm−1 in the spectrum of PEPA modified PURF was from the phosphate structure.