Structure to Properties Relations of Polyimide Foams Derived from Various Dianhydride Components

Dual Cross-Linked Networks Reinforced Polyimide Foams with Outstanding Piezoelectric Properties and Heat Resistance Performance

The application of traditional isocyanate-based polyimide (PI) foams is highly hindered due to limited flame retardancy, poor mechanical properties, and relatively single functionality. Herein, we propose an effective method to fabricate dual cross-linked polyimide/bismaleimide (PI-BMI) foams with outstanding heat resistance and enhanced mechanical properties by incorporating bis(3-ethyl-5-methyl-4-maleimidophenyl)methane (ME-BMI) as the interpenetrating network. The results show that the prepared PI-BMI composite foams exhibit enhanced mechanical properties with lightweight characteristics (23-80 kg·m-3). When the ME-BMI loading reached 120 wt %, the tensile and compressive strength of PI-BMI composite foam can reach 1.9 and 7.8 MPa, which are 9.6 and 63.3 times higher than that of pure PI foam, respectively. In comparison with PIF-0, the 10% heat loss temperature (Td,10%) of PIF-90 improved by 156 °C. Moreover, the PI-BMI foam piezoelectric sensor containing fluorine groups presents a short response time (14.22 ms), high sensitivity (0.266 V/N), and outstanding stability (10 000 cycles). Besides, the sensor can accurately monitor human activity in different states. This work provides a promising strategy for designing multifunctional PI foams, making them suitable for applications in aerospace and microelectronics.

Fabrication of Lightweight Polyimide Foams with Exceptional Mechanical and Thermal Properties

Lightweight polyimide foams (PIFs) with exceptional thermal resistance and compressive properties are fabricated by heating polyester ammonium salts (PEASs) which are prepared by copolymerizing 4, 4'-diaminobenzanilide (DABA), 4, 4'-diaminodiphenyl methane (MDA) and 3, 3', 4, 4'-benzophenone tetracarboxylic dianhydride (BTDA). Hydrogen bonds are formed between CONH and CO in the PI chains due to the addition of DABA and the melt viscosity of PEAS precursors increase with increasing content of DABA, which is advantageous to bind the foaming gases for cell expansion. The expansion ratio of PEAS precursors is increased from 633% to 1133% when the molar ratio of MDA/DABA is changed from 10:0 to 6:4. The compressive strength and modulus of PIFM9D1 (i.e., the molar ratio of MDA/DABA is 9:1, foam density: 120.8 kg m-3 ) reach as high as 0.59 and 15.0 MPa, respectively. The PIFs possess prominent thermal performance with the initial thermal degradation temperatures (under both nitrogen and air atmosphere) and glass transition temperatures (as assessed by DSC and DMA) exceeding 511 and 292 °C, respectively. The thermal conductivity of PIFs is lower than 0.049 W m-1 K-1 , which exhibits promising applications for serving as high-temperature thermal insulation materials in the fields of aerospace, marine, and nuclear sectors among others.

Novel Synthesis of Polyimide Foams with Aromatic and 1,6-Diaminohexane Imide Bonding

A novel type of polyimide foams (PIFs) with chemically inserted flexible aliphatic diamine (1,6-diaminohexane (HMDA)) segments was successfully synthesized and characterized in this research. The aliphatic HMDA segments were randomly incorporated in the long chain aromatic imide bonds. The obtained PIFs containing various HMDA contents (0 to 20 mol%) exhibited different morphologies such as lowered density and larger cell diameter (with higher HMDA content), and open cell ratio was increased as well. HMDA rendered flexibility to the copolymer leading to decreased rigidity. Compared to using 4,4 $\prime$ -oxydianiline (ODA) as the sole diamine source, incorporating low cost of HMDA would increase the PIF’s flexibility and improve its processibility while making the production more cost effective. Within some range of compromised thermal and mechanical properties, this proposed method could be feasible for industrial applications.

Structure of Industrial Sacrificial Fragile Cementitious Foams

Sacrificial fragile cementitious foams (SFCFs) act as a core material of the engineered material arresting system (EMAS) installed in airports to enhance the safe take-offs and landings of aircrafts. The foam structures and foaming mechanisms that greatly impact the collapse strength, specific energy, and arresting efficiency of SFCFs, however, have not been fully addressed. Herein, the engineering properties, chemical characteristics, and pore-skeleton structures of three batches of industrial SFCFs were experimentally investigated. Penetration tests showed significant differences in collapse strength and specific energy among the SFCFs with a similar density. Three-dimensional (3D) pore-skeleton structures were resolved by microfocused X-ray computed tomography. The pore-skeleton anisotropy was investigated to uncover the stages of differences in the SFCFs' engineering properties. The results demonstrate that the pore anisotropy rather than the porosity dominates the collapse of cementitious foams. Viscosity-associated nucleation and growth mechanisms were proposed to account for the featured pore-skeleton structures of the SFCFs. The findings would contribute to better pore structure controls of SFCFs toward the improved quality of EMAS.