Definition: Polyimide (PI) is a polymer with imide rings (-CO-NR-CO-) in its main chain, known for its superior overall performance. It has excellent heat resistance, withstanding temperatures over 400°C, and can be used in the range of -200°C to 300°C for extended periods. Some types have no distinct melting point, offering high insulation properties, with a dielectric constant of 4.0 and a low loss factor (0.004–0.007).
Types: Polyimide can be categorized as:
1. Aliphatic, Semi-aromatic, and Aromatic polyimides.
2. Crosslinked and Non-crosslinked types based on molecular interactions.
Applications: Polyimide is used in aerospace, microelectronics, nanotechnology, and other fields. In the 1960s, it was recognized as a promising engineering plastic, essential for the development of modern microelectronics.
Condensation Type: Made by reacting aromatic diamines with aromatic dianhydrides or diacid esters. While useful for films and coatings, it is less common in composites due to the difficulty of solvent removal during manufacturing.
Addition Type: Developed to overcome the drawbacks of condensation polyimides, such as poor processing and brittleness. Notable types include:
1. Poly(bismaleimide): Easier processing with simpler synthesis.
2. PMR-Type Polyimides: Developed by NASA, using a mix of diacid esters, diamines, and other monomers for better processing in composite materials.
Subcategories: Includes HBP (hexabenzocoronene) type, soluble polyimides, polyamide-imide (PAI), and polyetherimide (PEI).
Thermal Stability: Aromatic polyimides start to degrade at temperatures around 500°C, with some types, like Kapton, reaching up to 600°C.
Low Temperature Tolerance: Polyimides maintain their mechanical properties even at temperatures as low as -269°C.
Mechanical Strength: Tensile strength exceeds 100 MPa for most types, with some films (e.g., Kapton) reaching 170 MPa, and others like Upilex S up to 400 MPa.
Chemical Resistance: Polyimides are resistant to weak acids and can be recycled using alkaline hydrolysis.
Solubility: Depending on the structure, polyimides can either be highly soluble or insoluble in organic solvents.
Dielectric Properties: Excellent dielectric strength (100-300 kV/mm), low dielectric loss (10-3), and high resistivity (10^17 Ω·cm).
Radiation Resistance: Retains 90% of its strength after 5×10⁹ rad electron irradiation.
Self-extinguishing: Polyimides are flame-resistant and emit little smoke when heated.
Biocompatibility: Some types, such as those used in medical devices, are non-toxic and withstand repeated sterilization.
Polyimides can be synthesized using a variety of methods depending on their application, with common routes involving the reaction of diamines with dianhydrides in polar solvents like DMF, DMAC, and NMP. Methods include:
1. Low-temperature condensation (produces soluble polyamic acid).
2. In-situ polymerization (for composites).
3. Direct polycondensation using high-boiling solvents like phenolic solvents.
Polyimides have a wide range of applications due to their remarkable properties:
1. Films: Used for electrical insulation and flexible solar cell substrates.
2. Coatings: High-temperature resistant coatings for electromagnetic wires.
3. Advanced Composites: Used in aerospace for high-performance materials, such as carbon fiber-reinforced composites in supersonic aircraft.
4. Fibers: Used in high-temperature filtration materials and fire-resistant fabrics.
5. Foams: Thermal insulation materials for high-temperature environments.
6. Engineering Plastics: Used in seals, gaskets, and structural components.
7. Adhesives: High-temperature resistant structural adhesives, especially in electronics.
8. Separation Membranes: Used in gas separation and dehydration of organic liquids.
9. Photoresists: Employed in semiconductor manufacturing with submicron resolution.
10. Microelectronics: Used for insulation, stress-buffering, and protection in microelectronic devices.
11. LCD: Used in orientation alignment layers in various types of liquid crystal displays.
12. Optoelectronic Materials: Used for waveguides and optical switches in communication technologies.
13. Humidity Sensors: Utilized for moisture-sensitive applications based on their linear thermal expansion.
Polyimides are widely recognized as a promising material with broad applications, particularly in insulation and structural components. However, their high cost remains a challenge. Future research focuses on reducing production costs through more efficient monomer synthesis and polymerization methods.
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