Polymers and UV Sensitivity

name         Yana
status       other
grade        other
location     N/A

Question -   Why polymers with aromatic groups inside their
chain (as polycarbonate or PET) are less sensitive to UV then
polymers with side aromatic groups (PS)?
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Yana,

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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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.

Vince Calder
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