High-Temperature Resistant Flexible Polyimide Foams Constructed via Copolymerization of 2,3,3′,4′-Biphenyltetracarboxylic Dianhydride/3,3′,4,4′-Biphenyltetracarboxylic Dianhydride and p-Phenylenediamine

Lightweight, high-temperature resistant copolymerized polyimide foams (CPIFs) were fabricated via a “stepwise heating-holding” thermal foaming process using 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) and 2,3,3′,4′-biphenyltetracarboxylic dianhydride (α-BPDA) as codianhydrides and p-phenylenediamine (PDA) as the diamine. Asymmetric α-BPDA rigidifies molecular chains to endow superior thermal resistance. The CPIFs possess a three-dimensional open-cell structure (open-cell content >99%), ultralow densities (<10 kg·m^–3), excellent heat resistance (T_g ranging from 373.8 to 405.6 °C), and high thermal stability (T_5% > 588 °C), substantially outperforming previously reported counterparts. Benefiting from the unique cellular architecture, the CPIFs exhibit thermal conductivities below 0.06 W·m^–1·K^–1 at room temperature and approximately 0.08 W·m^–1·K^–1 at 200 °C, and excellent sound absorption (NRC ≥ 0.4), along with outstanding flame retardancy (LOI > 44%, UL-94 V-0 rating). Moreover, the CPIFs remain flexible and thermal insulating even in liquid nitrogen. This work provides a robust strategy for high-performance polyimide foams integrating heat resistance, flexibility, thermal insulation, and acoustic absorption, promising for aerospace and extreme-environment applications.