3D Printing of Lightweight Polyimide Honeycombs with the High Specific Strength and Temperature Resistance

Lightweight structures are often used for applications requiring higher strength-to-weight ratios and lower densities, such as in aircraft, vehicles, and various engine components. Three-dimensional (3D) printing technology has been widely used for lightweight polymer structures because of the superior flexibility, personalized design, and ease of operation offered by it. However, synthesis of lightweight polymeric structures that possess both high specific strength and glass transfer temperature (T_g) remains an elusive goal, because 3D printed polymers with these properties are still very few in the market. For example, 3,3',4,4'-biphenyl tetracarboxylic dianhydride (BPDA) and p-phenylenediamine (PDA)-type (UPILEX-S type) polyimides show exceptional thermal stability (T_g up to ≈400 °C) and mechanical properties (tensile strength exceeding 500 MPa) and are the first choice if extremely high temperatures of 400 °C or even higher (depending on the duration) are required, which hampers their processing using existing 3D printing techniques. However, their processing using existing 3D printing techniques is hampered due to their thermal resistance. Herein, a 3D printing approach was demonstrated for generating complex lightweight BPDA-PDA polyimide geometries with unprecedented specific strength and thermal resistance. The simple aqueous polymerization reaction of BPDA with water-soluble PDA and triethylamine (TEA) afforded the poly(amic acid) ammonium salt (PAAS) hydrogels. These PAAS solutions showed clear shear thinning and thermo-reversibility, along with high G' gel-state moduli, which ensured self-supporting features and shape fidelity in the gel state. Postprinting thermal treatment transformed the PAAS precursor to BPDA-PDA polyimide (UPILEX-S type). The resulting layer-by-layer deposition onto lightweight polyimide honeycombs in the form of triangular, square, and hexagonal structures showed tailorable mechanical strength, exceptional compressive strength-to-weight ratio (highest up to 0.127 MPa (kg m^-3)^-1), and remarkable thermoresistance (T_g approximately 380 °C). These high-performance 3D printed polyimide honeycombs and unique synthetic techniques with general structures are potentially useful in fields ranging from automotive to aerospace technologies.