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History of polyimides

In the mid-20th century, with the development of aviation and aerospace technology, the need for heat-resistant, high-strength and lightweight structural materials became very urgent. - Natta catalysts were another major achievement in polymer science at this time.
Dozens of aromatic heterocyclic polymers were published, but only a few were accepted by industry, with polyphenylene sulfide, polyethersulfone, polyetherketone, liquid crystalline polymers and polyimides being the most notable in terms of performance-to-price ratio.
Polyimides are a group of polymers containing imide rings in the main chain, with those containing phthalimide structures being particularly important.
The linear imide structure is not easy to synthesize and, more importantly, its low thermal stability makes it impractical.
Although these polymers had been reported as early as 1908, the nature of the polymers was not fully understood at that time and therefore did not receive much attention.
It was not until the mid-1940s that some patents appeared, but the real development of polymers as a material began in the 1950s.
At that time DuPont filed a series of patents and in the mid-1960s commercialized the first polyimide films (Kapton) and varnishes (Pyre ML), thus beginning an era of prosperous polyimide development.
Polyimides are particularly valued in comparison to other aromatic heterocyclic polymers because
This is due to
  1. their outstanding combination of properties
  2. the multiple synthetic routes available
  3. the possibility of processing in a variety of ways
  4. the extremely wide range of applications.

Multiple routes of synthesis

According to incomplete statistics, more than 400 dianhydrides and thousands of diamines have been used to synthesize polyimides, so at least thousands of polyimides have been synthesized and studied.
At the same time, polyimides have multiple routes of synthesis and can therefore be selected for a variety of applications, a synthetic versatility that is difficult to match for other polymers.
  1. Polyimides are mainly synthesized from dianhydrides and diamines, which are relatively easy to synthesise compared to the monomers of many other heterocyclic polymers such as polybenzimidazoles, polybenzazoles, polybenzothiazoles, polyquinolines and polyquinolines, which are widely available.
  2. Polyimides can be obtained by low-temperature condensation of dianhydride and diamine in polar solvents, such as DMF, DMAc, NMP or T HF/methanol, to obtain soluble polyamide acids. Polyimide solutions or powders are obtained. Dianhydride and diamine can also be condensed by heating in a high boiling point solvent, such as a phenolic solvent, to obtain polyimides in one step. In addition, polyimides can also be obtained by reacting the dianhydride ester of a tetra acid with a diamine. Polyimides - chemistry, structure-property relationships, and materials Polyimides can also be obtained from polyamide acids by first converting them into polyimides and then thermally converting them into polyimides. The former method, known as polymerization of monomeric reactants (PMR), allows for low-viscosity, high-solids solutions with a window of low melt viscosity during processing, and is particularly suitable for the manufacture of composites. The latter increases the solubility of the polymer and does not give off low molecular compounds during the conversion process.
  3. As long as the dianhydride (or tetra acid) and diamine are of acceptable purity, sufficiently high relative molecular masses can easily be obtained, irrespective of the method of polycondensation, and the addition of a unitary anhydride or unitary amine allows for easy regulation of the relative molecular mass.
  4. Low relative molecular mass polyimides containing anhydride (or ortho-dioic acid) end groups and ammonia end groups can react with each other when heated under a vacuum, allowing the relative molecular mass to continue to increase.
  5. Reactive oligomers can easily be formed by introducing reactive groups at the chain ends or on the chains, resulting in thermosetting polyimides.
  6. Esterification or salt formation using carboxyl groups in polyamide acids and the introduction of photosensitive groups or long-chain alkyl groups to obtain amphiphilic polymers, which can be used to obtain photoresists, hybrid materials, or for the preparation of LB films.
  7. Inorganic salts are not generally produced in the synthesis of polyimides and the prepolymer or polyimide solution can be used directly for coating or spinning without the need for laborious washing to remove inorganic salt by-products that degrade performance, which is particularly advantageous for the preparation of insulating materials.
  8. Dianhydrides and diamines as monomers sublimate readily under high vacuum and at appropriate temperatures, so that vapor phase deposition can be used to form polyimide films on workpieces, particularly devices with uneven surfaces or sharp

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