Enhancing Processability of Phenylethynyl-terminated Imide Oligomers and Properties of Cured Polyimides via — CF3 Incorporation

Abstract

Advanced aerospace technologies demand polymer-based wave-transparent materials with low dielectric constant (D_k), low dissipation factor (D_f), and high heat resistance for hypersonic vehicles. Thermosetting polyimides (PIs) from phenylethynyl-terminated imide (PETI) oligomers are promising candidates due to their excellent thermal stability; however, their application is limited by high D_k and D_f at high frequencies and processing difficulties. Here, a series of PETI oligomers were designed and synthesized by polycondensation of 9,9-bis(3,4-dicarboxyphenyl)fluorene dianhydride containing a Cardo structure, and six diamine monomers, respectively. The diamines were divided into three pairs by backbone flexibility, with each pair comprising a non-fluorinated diamine and its trifluoromethyl (—CF₃) substituted analog. The effects of chain segment flexibility and — CF₃ substitution on the processability of the oligomers and the properties of the cured PIs were systematically investigated. Results show that oligomers with — CF₃ groups exhibit enhanced solubility and improved viscosity stability at 270 °C compared to their non-fluorinated counterparts. Moreover, the corresponding cured resins show lower D_k and D_f values while maintaining good thermal stability and mechanical properties. Notably, the BPAF-FA system exhibits favorable properties: an oligomer solubility of 40 wt% in NMP, and a minimum melt viscosity of 5.61 Pa·s; D_k=2.953, D_f=0.0098 (at 50% RH), T_g=403 °C. Mechanistic analysis indicates that these improvements result from the synergistic effects of the intrinsic characteristics of — CF₃ groups (steric hindrance, electron-withdrawing effect, low polarizability, and hydrophobicity) and their role in influencing chain flexibility, symmetry, and molecular packing. These findings provide valuable guidance for the design of high-temperature-resistant, low-dielectric wave-transparent composite materials.