STABILIZATION AND MODIFICATION OF CELLULOSE DIACETATE - Photochemical Reactions in Heterochain Polymers - page 53

only 1,5-2 times larger than is vacuum. This agrees with the data of [161] and shows, that
reaction of acetic acid formation in the absence of oxygen plays an important role at CA
photodestruction.
Evaluation of quantum yield of acetic acid formation at light intensity (λ=254 nm)
I=1·10 15 quant/cm 2 ·sec gives the value Ф=0,02 close to the value of quantum yield of ester
groups destruction (Ф=0,015), measured in similar conditions [162]. Close value of quantum
yield of acetic acid formation Ф=0,01 has been obtained in the case of polyvinyl acetate
photolysis at lower intensity - I=5,7·10 14 quant/cm 2 ·sec [163]. Data on effect of temperature
on the rate of acetic formation are given in Table 1. Activation energy of the reaction of acetic
acid formation at CA irradiation, calculated according to these results, is E φ =9,66 kJ/mole,
which is characteristic for photoprocesses.
Table 13. Effect of temperature on the rate of acetic acid formation at CA ageing in
vacuum in darkness (W T ) and under light action (W φ )
T, K W T ·10 6 mole/kg·sec W φ ·10 6 mole/kg·sec
253 0,01 0,46
298 0,001 - 0,01 1,15
366 3,90 2,12
W φ has been got by subtraction W T from the general rate of the process under light action.
Let's note, that acetic acid formation runs not only under the light action, but in darkness
too. Evaluation of activation energy of dark process gives the value E T = 42-55 kJ/mole.
Measurement of stationary rates of acetic acid formation in the wide range of light intensities
shows that dependence of the process rate on light intensity has complex character (Figure
2.1.). At large intensities (I>1·10 14 quant/cm 2 ·sec), W - I 1,5 ; at low intensities (I<1·10 13
quant/cm 2 ·sec), W - I 0,5 and only at intermediate values I=(0,1-1) · 10 14 quant/cm 2 ·sec - the
rate of the process is proportional to light intensity.
Presence of induction periods on kinetic curves of acetic acid accumulation (Figure 2.1.)
and existence of higher, than a unit, order of the reaction according to light intensity (Figure
2.2.) does not agree with the hypothesis on acetic acid formation in the primary process of
photolysis of ester bond.
Additional confirmation of this conclusion has been got during the experiments on
measurement of dependence of the rate of acetic acid formation on light intensity on one and
the same film. It has been found that if the film is being irradiated changing from lower
intensity to greater one, then W - I 1,5 (Figure 2.3, strait line 1). And if during film irradiation
intensity is successively being decreased, then W - I (Figure 2.3, strait line 2).
These data show that acetic acid is being formed at photolysis of intermediate product,
concentration of which depends on light intensity. Such intermediate product may be radicals,
photolysis of which leads to acetic acid formation. Scheme 2 illustrates this on the whole:
CA + hv → R*
P + hv → AcOH + R*
R* + R* → products
Scheme 2.