Based on the compositions shown in Table 1, a certain quantity of OOBA was added to the SMPU/DMF solution containing approximately 3.0 g of PEG-based SMPU resin. Under strong mechanical stirring at 80 °C for 2 h, SMPU and OOBA were sufficiently mixed to obtain a homogenous solution-phase mixture. Then, the SMPU-OOBAm composites for the following tests were obtained by casting the mixture onto a Teflon pan, which was placed at 80 °C for 24 h and further dried at 80 °C for 24 h. The samples were coded as P1–P5 (SMPU-OOBAm, m = 0.2, 0.4, 0.6, 0.8, and 1.0, which is the molar ratio of OOBA/MDI).