*4.1.1 Thermal (oxidative) degradation*

As repeatedly mentioned in this chapter, PIs and PI-based materials display outstanding thermal stability, both in air or inert atmosphere. Degradation starts at very high temperatures, usually above 350°C for aromatic PIs, depending on the polymer's structure. Naturally, decomposition follows at much higher temperature regimes.

The thermal degradation of the widely used Kapton® film mainly generates CO2 from various sources (initial hydrolysis of the imidic units, followed by decarboxylation of the resulting acidic group), CO, and other volatiles from further transformations of imidic and aromatic moieties (a total of five degradation steps with variable activation energies) [141–144].

Several studies investigated the exact origin and nature of the volatiles resulting from the thermally induced degradation of common thermoplastic PIs: CO2, HCN, NH3, N2, H2O, CH4, and HCs. The presence of F- or S-rich structural motifs determines other, usually toxic volatiles. The decomposition process follows several pathways: depolymerization, pyrolytic reformation, successive homolytic and hydrolytic cleavages, hydrogen ablation, progressive molecular rearrangements, and loss of organic functionality through radical scission [140, 145, 146]. In every pathway, CO2 remains the main degradation product, while the nature of the other volatiles is dictated by the chemical structure of the polymer.

The resulting carbon-rich materials display improved compatibility between the organic (PI) and inorganic (carbon) parts which are mixed at the molecular level and enable several features that can be tuned by thermal treatment. Such materials can find various applications as fillers for gas purification membranes, dielectric composite films, coatings for electronic devices, and others.

Naturally, the degradation of cross-linked PIs has a more puzzling nature due to the three-dimensional architecture of the materials and requires different temperature and activation energy ranges [8]. Nevertheless, the pyrolysis of such thermosets can be used to form new reticulated structures. For example, a PI bearing crosslinks of anhydride nature formed new reticulation points during thermal treatment at 430°C, which finally resulted in microporous membranes with potential gas transport properties [147].

In the presence of oxygen, the mechanisms of thermal polymer degradation become even more complex due to the formation of highly reactive peroxide macroradicals which enable a cascade of degradation reactions [148–150].

### *4.1.2 Hydrolytic degradation*

Several experimental and computational studies were also dedicated to the chemical (especially hydrolytic) degradation or aging of common or less conventional PIs and model compounds [143, 151–153]. Two main mechanisms were evidenced: hydrolytic degradation and water-induced plasticization, both of them

## *New High-Performance Materials: Bio-Based, Eco-Friendly Polyimides DOI: http://dx.doi.org/10.5772/intechopen.93340*

being thermally activated and severely time-dependent (long exposure times [month scale] are usually involved).

In its early stages, plasticization can be cautiously reversed with no significant impact upon thermal or mechanical features. Further on, water induces irreversible hydrothermal defects like blistering and/or delamination [153]. The process is stronger when high temperatures or pressures are involved [152, 154].

The mechanism is usually based on the hydrolysis-activated chain scission of PI macromolecules through the attack of water molecules on carbonyl groups or other humidity labile structural motifs [155, 156].
