Polyimide Aerogels with Excellent Thermal Insulation, Hydrophobicity, Machinability, and Strength Evolution at Extreme Conditions

Abstract

High performance polyimide aerogels (PIA) are highly desirable as a part of thermal protection systems for aircraft. However, the large drying shrinkage and insufficient thermal insulation of PIA limit their practical applications in the aerospace field. Herein, the strategy of two-step prepolymerization and polycondensation route was proposed to fabricate PIA with ultralow drying shrinkage, outstanding thermal insulation, hydrophobicity, machinability, compressive strength in cryogenic/high-temperature atmospheres, flame resistance, and fantastic dimensional stability. In more detail, the drying shrinkage rate of PIA series can be lowered to 1.84%, mainly owing to the strengthened skeletons by chemical bond formation and physical entanglement of molecular chains. Excellent thermal insulation is reflected upon keeping an ink drop state as long as 265 s despite placing it on a cold surface with a temperature of −196 °C. PIA-O2B2 are imparted with hydrophobic properties (with water contact angle of 135°) depending on the hydrogen-bonding interaction between polyimide networking chains and 1H,1H,2H,2H-perfluorodecyltriethoxysilane that was introduced. In addition, the exceptional flame retardant is endowed completely, which is attributed to the evidence of the presence of aromatic ring structures in constructing the aerogel. The high-temperature shrinkage rate of PIA series is as low as 4.81% due to the relative small free volume of the molecular chains. Our study would provide a method for designing ultralow drying shrinkage and superb thermal insulation in cryogenic conditions for PIA in aerospace uses.

Mechanically Robust Nanoporous Polyimide/Silica Aerogels for Thermal Superinsulation of Aircraft

“…Material dimensions were measured before and after thermal treatment calculate shrinkage rate along the uniaxial direction. The shrinkage rates reported in previous work were 5.53, 5.74, and 4.81% in the LD, WD, and HD, respectively, over a thermal treatment time of 10 min . In comparison, the shrinkage rates of PIAs-A2.6 herein were 8.85, 9.43, and 9.70% in the LD, WD, and HD, respectively, over a thermal treatment time of 40 min (Figure S4e).…”

“…A larger temperature difference suggests the material exhibits a better thermal insulation performance between the cold surface and the hot surface. Compared with previous work, 32 PIAs-A2.6 displayed a thermal superinsulation performance mainly resulting from their nanopore structure, which can be clearly seen from the FESEM data in Figure 2j,k. The thermal conductivity of PIAs-A2.6 was measured to investigate the thermal superinsulation performance.…”

“…The shrinkage rates reported in previous work were 5.53, 5.74, and 4.81% in the LD, WD, and HD, respectively, over a thermal treatment time of 10 min. 32 In comparison, the shrinkage rates of PIAs-A2.6 herein were 8.85, 9.43, and 9.70% in the LD, WD, and HD, respectively, over a thermal treatment time of 40 min (Figure S4e). This result is mainly due to the introduction of Si−O−Si bonds, making the structure of PIAs-A2.6 more stable and resistant toward high temperature, and is consistent with the FESEM, mechanical properties, and TG data.…”

“…The produced polyimide aerogel samples are described by the abbreviation PIAs-Ax, where Ax represents the APTES content, and PIAs-A0 represents the blank as described in previous work. 32 The content of APTES in PIAs-A2.6 is 1.28 wt %, including solvent. The hydrophobic-treated PIAs-A2.6 is abbreviated as PIAs-A2.6-H.…”

“…Then, the wet gels were dried in a supercritical CO 2 fluid dryer with a drying temperature of 60 °C and a drying pressure of 16 MPa for 8 h to obtain PIAs. The produced polyimide aerogel samples are described by the abbreviation PIAs-A x , where A x represents the APTES content, and PIAs-A0 represents the blank as described in previous work . The content of APTES in PIAs-A2.6 is 1.28 wt %, including solvent.…”