Chemistry Reference
In-Depth Information
extent absorbs ultra-violet rays, protecting deep layers of the fibre and inhibits the process of
their photooxidative destruction [240].
O
C
O
C O CH CH 0
*
O
C
O H
hv
C
CH
O
2
2
O CH
2
O
C
O
C O CH CH 0
O
C
OH
C=O
+
CH=CH=O
2
2
2
Many reactions of photosensibilized oxidation run with oxygen, taking part in it, and
being in electron-excited singlet state. Singlet oxygen may form products of oxidation with
the yield being close to theoretical [241].
When it was proved that singlet oxygen takes part in reactions of photosensibilized
oxidation Trossol and Winslow have suggested new mechanism of photooxidative destruction
of polyesters including 4 stages [242]:
(1)
light absorption by carbonyl groups;
(2)
photodecomposition in the reaction of the II type according to Norrish, proceeding
with the participation of excited state n
Æ
π* carbonyl groups;
(3)
formation of singlet oxygen in reactions of suppression of triplet state n
Æ
π*
carbonyl groups;
(4)
reaction of singlet oxygen with vinyl groups formed according to reaction of the II
type according to Norrish.
This mechanism includes only initial stages of reaction. Reactions following the
formation of peroxide compounds are supposed to be the same as in more widespread kinetic
schemes describing the course of radical-chain processes. Hence, one may come to a
conclusion that during polyesters oxidation there proceed two processes at one and the same
time:
(1)
leading to the formation of peroxide compounds (radical-chain);
(2)
proceeding with singlet oxygen participation.
Studying mechanism of photo- and photooxidative destruction Ibishkovitch and others
[243] have shown that during PETP photodestruction intrinsic viscosity decreases, carboxyl
groups contain increases, sample density rises, optical density increases at 373 and 632 cm”
1
Empirical equations, describing kinetics of the change of PETP characteristics have been
found in the same work:
η=o,741
•
t
0,1
; [-COOH]=0,05
•
t
0,8
; P=1400+42α
•
10093t; D=3+0,263t
0,263
Change of molecular weight distribution of the polymer [244] occurs at the increase of
the sample optical density and exposure dose, that is why corresponding corrections to the
molecular-weight distribution [245] should be introduced into the results of measurements. In
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