Oxidation usually appears at the tertiary carbon atom existing in every repeat unit. A free radical is formed here, and then reacts further with oxygen, followed by chain scission to yield aldehydes and carboxylic acids.
In external applications, it shows up as a network of fine cracks and crazes that become deeper and more severe with time of exposure.
For external applications, UV-absorbing additives must be used in application which using in external. Carbon black also offers some defense from UV attack.
The polymer could oxidize at high temperatures, a common problem during molding operations. Anti-oxidants are normally added to avoid polymer degradation.