The results of thermoanalytical investigations of polyimides are considered. It is shown that combined application of methods of thermal analysis makes it possible not only to determine the thermal and structural characteristics of polyimide articles but also to observe the formation of polyimides, their thermal degradation and transformation into new structures under the influence of high temperatures.
A number of temperature and kinetic parameters of thermal degradation of polyimides over the temperature range from 20 to 1000° have been determined on the basis of thermogravimetric investigations of a polyimide sample obtained from pyromellitic dianhydride and diaminodiphenyl ether. It was shown that such kinetic parameters as the reaction order and the activation energy of thermal degradation are directly dependent on the conditions under which the thermal analysis is carried out. It was found that when the static and dynamic conditions of the environment of a polymer sample are varied, the thermochemical processes occurring in a polyimide over the temperature range investigated are drastically changed.
Thermogravimetry of polyimides based on dianhydrides of pyromellitic, diphenyl- and diphenyloxidetetracarboxylic acids and a series of diamines has been used to determine their temperature characteristics and calculate their activation energies. The dependence of the thermal stability of the polyimides on the structure of their diamine component was established. It was shown that the effect of the thermal stability of the polyimides progressively weakens with the increase in the rigidity of their chemical structure.