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