Chemistry
Mechanisms of Photodegradation of Polymers Under Ultraviolet Exposure
Quick fact
UV radiation, particularly in the 300–400 nm range, can directly break carbon-carbon bonds in many polymers, but the more damaging process often involves oxygen: photooxidation, which creates radicals that trigger chain reactions, leading to loss of strength, yellowing, and embrittlement.
Why this is interesting
You've seen plastic chairs left outside become brittle and crack after years of sun. What is happening inside the material that makes it fail so dramatically?
Read the full explanation
Understanding Mechanisms of Photodegradation of Polymers Under Ultraviolet Exposure
Think of a polymer as a long chain of repeating units, like a string of beads. Each bead is a monomer. Under normal conditions, these chains slide past each other, giving the material flexibility and strength. When UV light hits the polymer, it can be absorbed by certain parts of the chains—the chromophores, which are groups that absorb light. If the photon has enough energy, it can break a covalent bond, snapping the chain into two shorter pieces. This is called chain scission. As more and more bonds break, the polymer becomes a jumble of shorter chains, losing its strength and becoming brittle. Additionally, broken chains can create reactive radicals, which can react with oxygen to form new chemical groups that absorb light, causing yellowing. Over time, the material cracks and crumbles.
A deeper explanation
The primary mechanism begins with absorption of UV photons by chromophores present in the polymer—either in the backbone or as impurities (e.g., residues from synthesis, additives, or oxidation products). This photoexcitation can lead to cleavage of the weakest bond. For example, carbonyl groups (C=O) are common chromophores; upon UV absorption, they can undergo Norrish type I (cleavage of the bond between carbonyl and alpha carbon) or Norrish type II (intramolecular hydrogen abstraction leading to scission) reactions. These produce free radicals, which are highly reactive. In the presence of oxygen, radicals rapidly form peroxy radicals (ROO•), which can abstract hydrogen from adjacent polymer chains, generating hydroperoxides (ROOH). Hydroperoxides are thermally and photochemically labile, breaking down into alkoxy (RO•) and hydroxyl (•OH) radicals, initiating a chain reaction. This auto-oxidation cycle accelerates degradation. In addition to chain scission, crosslinking—where two chains become covalently bonded—can occur, increasing stiffness and brittleness. Both processes alter the molecular weight distribution and mechanical properties. Understanding these mechanisms enables the design of UV stabilizers (such as UV absorbers that scavenge photons, or hindered amine light stabilizers that interrupt the radical cycle) to extend polymer lifetime.