Aromatic polyimides thermal stability

More recently, St. Clair and co-workers176) reported the use of aromatic amine terminated polydimethylsiloxane oligomers of varying molecular weights in an effort to optimize the properties of LARC-13 polyimides. They observed the formation of two phase morphologies with low (—119 to —113 °C) and high (293 to 318 °C) temperature Tg s due to siloxane and polyimide phases respectively. The copolymers were reported to have improved adhesive strengths and better thermal stabilities due to the incorporation of siloxanes. 

When Tg[(CH2)3NH2]8 reacts with aromatic dianhydrides, it forms hyper-branched polyimide nanocomposite networks having a high thermal stability (up to 500 °C) as shown recently by Se kin et al. (Figure 41). 

Aromatic polyimides are well known for their unusual array of favorable physical properties, including excellent thermal stability and excimer-laser processing characteristics. The polyimide structure possesses lower-energy transitions such as n —> n, n —> o, n —> n, and a — n (in order of increasing energy71). However, the w — n and o —> n transitions are forbidden by symmetry rules and related absorptions are significantly weaker than those for... 

Aromatic polyimides have excellent thermal stability in addition to their good electrical properties, light weight, flexibility, and easy processability. The first aromatic polyimide film (Kapton, produced by DuPont) was commercialized in the 1960s and has been developed for various aerospace applications. The structure of a typical polyimide PMDA/ODA prepared from pyromellitic dianhydride (PMDA) and 4,4 -oxydianiline (ODA), which has the same structure as Kapton, is shown in (1). Aromatic polyimides have excellent thermal stability because they consist of aromatic and imide rings. 

For thermal stability polypyromellitimides are somewhat inferior to polyimides based on the anhydrides of other aromatic tetracarboxylic acids. This is apparently due to the higher chlorine content per unit polymer weight in the former. 

Figure 14.12 Shape of thermal stability ceiling curves for a commodity polymer of the PVC-PP type (com) and a thermostable polymer of the aromatic polyimide type (ther).

Fully imidized soluble polyimides have ben prepared using monomers derived from diphenylindane and aromatic dianhydrides. Technical polymers (XU218, for instance), prepared from 1,1,3-trimethyl-diaminophenylindane and benzophenone-tetracarboxylic acid dianhydride, have been marketed over the last decade. Despite the partially aliphatic nature of polyimides containing the indane group, they show considerable retention of the thermal stability, with Tg values over 300 °C [107-110]. 

Because of their toughness, flexibility and remarkable thermal stability, linear all-aromatic polyimides are excellent candidate film and coating materials for advanced electronic circuitry and wire coating applications. In past years, however, the inherent insolubility (1-2) of these polymers has somewhat limited their usefulness for electronic applications. 

Cyclotrimerization of polyfunctional aryl acetylenes offers a unique route to a class of highly aromatic polymers of potential value to the micro-electronics industry. These polymers have high thermal stability and improved melt planarization as well as decreased water absorption and dielectric constant, relative to polyimides. Copolymerization of two or more monomers is often necessary to achieve the proper combination of polymer properties. Use of this type of condensation polymerization reaction with monomers of different reactivity can lead to a heterogeneous polymer. Accordingly, the relative rates of cyclotrimerization of six para-substituted aryl acetylenes were determined. These relative rates were found to closely follow both the Hammett values and the spectroscopic constants A h and AfiCp for the para substituents. With this information, production of such heterogeneous materials can be either avoided or controlled. 

Aromatic polyimides have found extensive use in electronic packaging due to their high thermal stability, low dielectric constant, and high electrical resistivity. Polyimides have been used as passivation coatings, (1) interlayer dielectrics, (2) die attach adhesives, (3) flexible circuitry substrates, (4) and more recently as the interlevel dielectric in high speed IC interconnections. (5) High speed applications require materials with a combination of low dielectric constant, flat dielectric response versus frequency and low water absorption. 

Firstly, this is a polyimide with an aromatic backbone, and, therefore, possesses high thermal stability and resistance to many chemicals. The thermal stability was confirmed by TG (see Figure 2). 

Aromatic polyimides possess outstanding thermal stability as well as being unusually high melting, intractable and insoluble (1). Polyimides are prepared either by polyamide salt techniques, by condensation of dianhydrides with diisocyanates (2) or by reaction of an aromatic diamine with a dianhydride to give a poly(amic acid) followed by dehydration to give the polyimide. The polyimides from a variety of diamines have been reported and the dianhydride unit has been varied widely (1). 

Aromatic polyimides are well recognized to have excellent chemical stability, so they have found applications in extremely harsh and corrosive environments. For example, they are used as insulators in nuclear facilities and as thermal blankets on spacecraft where the materials may be exposed to high-energy radiation in the presence of oxygen. The best known of the commercial polyimides is Kapton, which is marketed by Du Pont. The chemical structure of Kapton is shown in Figure 1, along with that of Ultem, another commercial polyimide marketed by General Electric. 

Aromatic polyimides and polyamides exhibit outstanding thermal stability and strength, two properties responsible for the industrial importance of these materials. The macroscopic properties are controlled to a large extent by the state of aggregation of the macromolecules and their relative orientation. Photophysical measurements, aimed at detecting polarization properties and the formation of excited-state charge transfer (CT) states have proven extremely useful tools to elucidate the microstructure of these materials. 

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