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Name: Yana
Status: Other
Grade: Other
Location: N/A
Country: N/A
Date: June 2006


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)?



Replies:
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


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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