Chemistry Reference
In-Depth Information
In addition to structural instability of some polymer molecules in the ultraviolet light, degradation
may also be accelerated by chromophores that can form from oxidation during preparation or
processing. These compounds can act as excited donors and transfer the energy to the polymers
that may act as acceptors. An intramolecular energy transfer may, actually, occur within the same
polymer molecule. This can take place between an excited chromophore that is present in one
segment of the chain and an acceptor at another segment. Also, chromophores, like carbonyl groups,
can undergo Norrish Type I or a Norrish Type II reaction.
Several studies were carried out on
[
539
,
540
]. The primary process of
photo-degradation, however, is still being elucidated. Direct irradiation of pure low-density polyeth-
ylene with ultraviolet light from a mercury lamp results in formation of free-radicals that were
identified by ESR. They are alkyl radicals of the type: ~CH
2
-CH
2
•
and ~CH
2
-CH
•
-CH
2
~[
539
,
540
].
The true absorption bands of polyethylene, however, are located at wavelengths shorter than 200 nm.
It is difficult, therefore, to accept that the radiation from the lamp caused chain scissions, because the
output of the lamp is much longer in wavelength. It is suspected, therefore, that the degradation is a
result of photo-oxidation. This is discussed in the next section.
A similar photo-degradation process is believed to take place in polypropylene [
541
]. The
formation of free-radicals is ascribed to presence of oxidized molecules that form during processing.
The oxidation products are carbonyl compounds and hydroperoxides [
542
]. The photolysis of the
carbonyl derivatives is as follows [
541
]:
photolysis of polyethylene
O
hν
CO
CH
2
+
CH
2
+
And the photolysis of chains carrying hydroperoxide group can be shown in this manner [
541
]:
hν
+
OH
COOH
O
hν
CH
2
+
O
+
CH
3
Photo-degradation of 1,2-polybutadiene was studied on a film [
543
]. Among the degradation products
were found hydrogen, compounds with methyl groups, vinyl groups, and cross-linked material.
The following mechanism of photo-initiation of the degradation reaction was proposed [
543
]:
hν
210 nm
+
H
CH
2
+
+
