Radiation-Induced Radicals in Polyurea-Crosslinked Silica Aerogel

Free radicals are atoms or molecules with an odd number of electrons in an outer shell. Since electrons typically occur in pairs, this leaves one electron that is unpaired. In seek of another electron to pair with, free radicals react with and steal electrons from neighboring molecules, which then become free radicals themselves. This can start a chain reaction, cascading into large scale damage.

Ionizing radiation can tear through molecules, just as bullets can tear through things that we see. If free radicals can be detected, and seen to increase in a material upon radiation exposure, this can indicate molecular damage caused by radiation. While free radicals created in a material may not have immediate large-scale effects, they can eventually alter the material’s ability to function as intended. This will be important in assessing potential aerogel applications which involve radiation exposure.

Aerogels are gels in solid form which contain mostly air (instead of liquid); they can be thought of as an advanced form of styrofoam that can be much stronger, lightweight, and insulating. Aerogels can be made of many different materials, the most common being silica. Typical silica aerogels are very fragile. A modified version, known as polyurea-crosslinked silica aerogel (PCSA), may be more practical, in that it is good for applications requiring more strength and resistance to surroundings.

In this study, PCSA is exposed to several X-irradiation treatments and tested via electron spin resonance (ESR) technique, which has the unique ability to directly detect free radicals. For comparison, all treatments are equally duplicated with a material known to be very radiation-resistant (PEEK (polyether-ether ketone)), and one known to undergo large-scale free radical creation and degradation upon radiation exposure (UHMWPE (ultra-high molecular weight polyethylene). Results show significant quantities of free radicals produced in PCSA due to X-irradiation.