STABILIZATION AND MODIFICATION OF CELLULOSE DIACETATE - Photochemical Reactions in Heterochain Polymers - page 65

On the other hand, inhibiting effect of nitroxylic radical at irradiation is not observed
owing to its strong action as photosensitizer. Photosensitization is absent after irradiation in
dark reaction and nitroxylic radical displays its inhibiting action. That is why oxidation after
irradiation (post-effect) is not observed (Figure 2.7, curve 2).
It should be noted that HAC - XLIX does not suppress CDA oxidation in spite of the fact
that concentration of HAC - XLIX is larger than that of nitroxylic radical by 1,6 times.
Equation (1) was used to define kinetic parameters of oxidation. It has been found that
the value K n RH/K o =0,9·10 -5 mole/kg for CDA in the presence of HAC - XLIX is close to the
value K n RH/K r =1,0·10 -5 mole/kg for unstabilized CDA.
So, we may come to a conclusion that HAC - XLIX is not an inhibitor of radical-chain
photooxidation of CDA.
If the additive is not an inhibitor and acts only as ultraviolet absorber and sensitizer, then
dependence of quantum yield of photooxidation on light intensity at strong light absorption
[168] should satisfy the equation (6):

Фо 2 = αФ i + K n [RH]/K 0 ·√Ф i ·10 -3 ·1/√I 0 εС (6)

where: Ф i - quantum yield of photoinitiation of the dye caused by photosensitization; α -
quantity of oxygen molecules absorbed during non-chain photooxidation per 1 formed free
radical; ε 1 C - extinction coefficient and dye concentration.
Figure 2.8 shows that experimental dependence of Фo 2 on light intensity I o at CDA
photooxidation in the presence of HAC - XLIX agrees with theoretical ones according to the
equation (6).
Calculations according to the equation (6) show that the value Ф i achieves 0,004. Hence,
it follows that HAC - XLIX, introduced into CDA, is not an inhibitor. It acts as ultraviolet
absorber and photosensitizer oxidation according to radical mechanism.
Obtained result allows to explain dependence of efficiency on concentration of HAC -
XLIX (Figure 2.9). Photosensitizing is absent and HAC acts only as ultraviolet absorber at
small concentrations of CC (0-1,5%), where the rate of photoinitiation, caused by HAC, is
much lower than the rate of photoinitiation, caused by pure CDA. Light stability of polymer
increases by i φ times over this range of concentration A=1. Photosensitizing becomes visible
(A<1) at relatively higher concentration of HAC.
As it follows from above-mentioned conclusions, decrease of photosensitizing action of
HAC has to improve its light-protective effect.
All investigated compounds have C=N group. Photochemical reactions of C=N - group
are similar to carbonyl group [178]. That is why breaking of hydrogen atom from substrate by
excited state of C=N - group should be primary photochemical reaction of stabilizer. In the
case of HAC - LVI and HAC - LVIII additional way of sensibilization, caused by rhodamine
fragment, introduced into HAC structure, may take place. But value A for HAC - LVI
(A=0,072) and HAC - LVIII (A=0,111) is of the same order, as for other HAC. Hence,
rhodamine fragment does not probably influence photosensitization. It should be noted that
intramolecular transfer of hydroxyl proton is possible in excited state of HAC - LVIII
through six-membered cycle.
Intramolecular transfer of proton suppresses reaction of photoinitiation [10] and should
lead to the greatest value of A for HAC - LVIII. Experimental data do not agree with this
one, as value A=0,111 for HAC - LVIII is less, than value A=0,26 for HAC - L.