Theory of Thermal Stability

From a practical point of view, thermal stability of a polymer can be defined as the temperature, or temperature range, that the material can withstand and still retain useful properties, in a given application, for a stated period of time. Examples of conditions used to define thermal stability requirements are 260°C for approximately 1000 hours, 535°C for one hour. 

The strength of chemical bonds imposes an upper limit on the vibrational energy that a molecule may possess without bond rupture. Since heat increases the vibrational energy, the heat stability of a polymer is related to the bond dissociation energies. Hence, a polymer having predominantly weak bonds between atoms should not be used for high-temperature applications. 

Additional thermal stability of polymers may arise from secondary valence forces. These are due to dipole-dipole interactions (8kcal/mol) and hydrogen bonding (6-10 kcal/mol). The latter effect is particularly important in strongly polar polymers such as polyamide and polyurethanes. These forces have pronounced effects on melting points and glass transition temperatures of the polymers. 

High thermal stability can be achieved by making full use of resonance energy and resonance stabilization of aromatic and heterocyclic structures such as in polyphenylene and polyphenylquinoxalines. 

With these factors in mind, the following considerations apply to the selection of structures for attaining thermally stable polymeric materials  

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