Polymers and UV Sensitivity
Date: June 2006
Why polymers with aromatic groups inside their
chain (as polycarbonate or PET) are less sensitive to UV then
polymers with side aromatic groups (PS)?
The UV sensitivity of a polymer depends on many factors than the
nominal chemical formula. Commercial polymers are most often
"formulated" with any number of additives to achieve a desired
balance of properties. So your statement may not be universally
true. Neglecting that caveat, the UV degradation process involves
the absorption of UV radiation (usually by the aromatic group) to
form a free radical that then initiates a series of different
reactions -- hydrogen abstraction, chain scission, ..., and so on.
An appended aromatic group is more mobile than an aromatic group in
the primary polymer chain, so it has more freedom to initiate these
degradation reactions. The aromatic group in the main chain is "tied
down" so the degradation reactions "tend" to be slower.
One of the reasons is that aromatic groups within the main chain
(such as those in PET) may be conjugated through carbonyl groups or
heteroatoms. Such combination of aromaticity, conjugation and
heteroatoms tends to lower the light absorption onset and the lambda
max (the wavelengths that the polymer can absorb). Moreover, this
combination also allows for multiple quantum levels of absorption so
that many different wavelengths can be absorbed.
Polymers with aromatic groups in the side chain (such as
polystyrene) are not conjugated since the aromatic group is
separated by the single bonds of the main chain.
However, polymers with side chains that have conjugation and
heteroatoms within the side chain are UV sensitive.
Greg (Roberto Gregorius)
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Update: June 2012