Polyimides (PIs) are high-temperature engineering polymers originally developed by the DuPont™ Company. Polyimides exhibit an exceptional combination of thermal stability (>500 °C), mechanical toughness, and chemical resistance.It was based on pyromellitic dianhydride and 4,4’diaminodiphenyl ether.
The presence of n-π conjugation between non-pair electron of nitrogen atom and π electrons of the carbonyl group makes them resistant to chemical agents and moisture.
The linearity and stiffness of the cyclic backbone allow for molecular ordering. This phenomenon results in lower coefficients of thermal expansion (CTE) than those found for thermoplastic polymers having coiled, flexible chains.
Additionally, the morphology of long, linear ordered chains provides solvent resistance to the aromatic polyimides.
The comparatively rigid structure of polyimides provides high glass transition temperature (Tg > 300°C) and imparts good mechanical strength and high modulus.
The thermal stability of polymer can further be improved by incorporating aromatic rings on the backbone and/or side groups. In addition to such high thermal stability, the nature of the chemical structure consisting of rigid imide and aromatic rings always provides:
- Excellent mechanical toughness
- Excellent dielectric properties
- High chemical resistance
These result in thermoxidative stability, high mechanical strength, high modulus, excellent insulating properties, and superior chemical resistance.
Polyimide layers are suitable matrix for incorporation of metal, salts, chromophores as nanoscale particles to obtain of nanocomposite materials. It was discussed the possibility of use polyimides in materials chemistry and nanomaterials, one of these applications is the use for making biomedical implants for neurology, ophthalmology, biosensor device and chips which are a powerful tool in clinical diagnostics.
Another important trend is use in electronics and optoelectronics such as dielectric substrates and intermediate barrier layers, creating nanocomposite films with various nanosized particles such as dyes, metal, dielectric and other clusters.
Typical polyimide parts are not affected by commonly used solvents and oils — including hydrocarbons, esters, ethers, alcohols and freons. They also resist weak acids but are not recommended for use in environments that contain alkalis or inorganic acids.