Polypyromellitimide aromatic

Uses. Pyromellitic dianhydride imparts heat stabUity in applications where it is used. Its relatively high price limits its use to these applications. The principal commercial use is as a raw material for polyimide resins (see POLYIMIDES). These polypyromellitimides are condensation polymers of the dianhydride and aromatic diamines such as 4, -oxydianifine ... 

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. 

The effect of aromatic diamine residues on the heat resistance of polyimides is less clear cut. It was only by comparing thermomechanical curves of polypyromellitimides obtained from 3,3-diamino-4,4 -dichlorodiphenylmethane, and l,l-dichloro-2,2-di(3-amino-4-chlorophenyl)-ethylene that the system containing the 1,1-dichloroethylene group was found to be less heat resistant. With polyimides prepared from other dianhydrides this rule holds true as well but is less distinct. 

Hie polyimides made from aromatic diamines are more stable but the decreased flexibility of an aromatic ring raises the glass transition temperature (Tg) of the polymer quite significantly, and no Tg can be measured for, say, the polyimide based on phenylene diamine. Thus flexibilizing units must be introduced, but these tend to raise the price of the diamine, and decrease the temperature stability. A compromise must be reached, and the best known polypyromellitimide is Kapton/Vespel made by du Pont this is based on oxydianiline. 

Example 2.3 Polyimides have been prepared from aromatic anhydrides and aliphatic diamines by melt fusion of salt from the diamine and tetracid (dianhydride). Ahphatic polypyromellitimide derived from straight-chain aliphatic diamines containing more than nine carbon atoms gave thin, flexible films, whereas those from shorter chain aliphatic diamines allowed the preparation of only thick, brittle moldings. Explain this observation. 

The aromatic polyimide (PI) of Fig. 3.49 is a relatively stable polymer, such as a polypyromellitimide, that is usable for high-temperature applications such as needed for insulation. It finally decomposes to small fragments that carry away most H-atoms and leave highly cross-linked, high-carbon chars. 

Polyimides (Pis) represent a broad class of high-Jg polymers derived from the polycondensation of an aromatic dianhydride and diamine. The most widely investigated polyimide is polypyromellitimide or poly(4,4 -oxydipheny-lenepyromellitimide) (Kapton ) whose repeat tmit structure is given below... 

Bruck, S. D., Thermal degradation of aromatic polypyromellitimide in air and vacuum. III. Pyrolytic conversion into a semiconductor. Polymer, 6, 319-332 (1965). 

The basic chemistry of aromafic polyimides was first disclosed in patents filed by Du Pont de Nemours [5] for polypyromellitimides and later on by Edwards [6, 7] for polyimides prepared with various aromatic dianhydrides and diamines. They compose... 

The Raman spectrum of the polypyromellitimide also produces the two peaks assigned to the carbonyl groups at 1789 and 1722 cm , with an intensity ratio opposite to that observed in the IR spectrum. Also, one is cautioned that the 1722 cm band is very weak, and may be overlooked (see Reference Spectrum 42). Bands at 1613 and 1600 cm , and at 1514 cm for the aromatic rings, are better defined than in the IR spectrum. 

The mechanical and electrical properties of a polypyromellitimide (see Chart 5.4) are reported to remain satisfactory in the presence of air up to absorbed doses of 10 MGy [75], although most aromatic polymers proved to be radiation-resistant only in the absence of O2. For example, the mechanical properties of polystyrene were not appreciably deteriorated when the polymer was exposed in the absence of O2 to absorbed doses up to 10 MGy, but they were decreased to 50% of their initial value at an absorbed dose of only about 0.80 MGy [76]. 

---