*4.1.3 Radiation degradation*

Several studies on this issue were performed due to the employment of Kapton® and other commercial PIs in aircraft and aerospace applications. Kapton® in particular is renowned for its outstanding resilience to ionizing irradiation, as compared to other widely used PIs.

The photodegradation of PIs results in macro- and micro-cracks coming from the scission of macromolecular backbones and low molecular mass volatiles such as CO and CO2 (and CHF3 in the case of 6FDA-containing PIs). The resulting oligomeric or polymeric chains display phenol, amine, isocyanate, and carboxylic acid terminal groups [157, 158]. Most UV irradiation experiments showed biexponential degradation kinetics [159].

In the case of beam-induced degradation processes, CO and CO2 are the major volatile fragments, with an additional significant contribution of short hydrocarbons like CxHy [160–162]. The degradation requires heavy energetic conditions and can be inhibited by the addition of various fillers [157]. Additionally, the process is accelerated by air exposure, which quickly determines a significant loss of radicals.

When the aging experiments mimicked low earth orbit, space-flight conditions (temperatures between 10 and 300 K, ultra-high vacuum [10–11 Torr], and high concentration of electronically excited atoms [108 atoms•cm<sup>3</sup> of atomic O]), the degradation proceeded through chain scission and generated H2, CO, and CO2 [163, 164]. Similar experiments mimicking geostationary orbit conditions (involving high energy [90 keV] electrons) showed a board range of radiation-induced damage (breakage of chemical bonds and formation of new ones) and a strong effect on optical (lower transmittance) and charge transport (higher conductivity) features [165].

Most ion- or radiation-induced degradation studies have been performed at room temperature, and very little is known about degradation effects determined by irradiation experiments performed at extreme (very low [cryogenic] or high) temperatures [160].

### *4.1.4 Biodegradation*

PIs are generally known to be biostable and bioinert, a very useful trait when it comes to biomedical applications (implantable devices, especially) [8]. However, the biodegradation of some PI-based materials would prove an advantageous feature in terms of recyclability and environmental impact. However, this requires specially designed building blocks and very strict control of the overall synthetic procedure.

One successful example in this regard is the two-stage synthesis of a biodegradable PI starting from an aliphatic amine derived from poly(propylene fumarate) and two commercial aromatic dianhydrides [166].

Both the PAA precursor and the cyclized PI proved biodegradable in a buffer solution, with a weight loss between 20 and 40% after 60 days of exposure, depending on the amount of amine used.

This represents a good starting point for a new research topic in the field of bioPIs. Similar studies are currently considering some of the partially or completely bio-based PIs described in the second section of this chapter.
