STABILIZATION AND MODIFICATION OF CELLULOSE DIACETATE - Photochemical Reactions in Heterochain Polymers - page 49

Chapter 2
S TABILIZATION AND M ODIFICATION OF
C ELLULOSE D IACETATE
2.1. Modern State of Investigations of Photochemical Destruction of CDA
Cellulose and its derivatives - cellulose acetate - are renewed polymers, that, together
with the whole complex of valuable and indispensable properties, defines continuous growth
of their production.
Acetate fibres differ from cellulose fibres in light and thermooxidative stability, as the
presence of ester groups decreases stability of molecular structure, owing to which destructive
processes begin at much lower temperatures and weak energy effects.
Since macromolecules of cellulose acetate are constructed on the basis of cellulose then
mechanism of photodestruction of these polymers may be considered as general.
Many summarizing works [141-147], published from 1962 to 2000, and are devoted to
the questions of photochemistry. In this survey there are works, which are not included into
above-mentioned literature surveys, and publications of the last years.
The most important energetic factor, which photodestruction of cellulose and its
derivatives depend on, is intensity of irradiation and wave length. Destruction of cellulose and
its derivatives under atmospheric conditions, proceeding as a result of photochemical
reaction, on the whole takes place under the action of ultraviolet rays with λ=200-360 nm.
Since cellulose contains three types of chromophore groups - hydroxyl, acetate and
semiacetate and also aldehyde - then it is considered that light absorption in the region of
250-300 nm is caused just by them. At the same time some authors, bringing the possibility of
light absorption by acetal chromophore in question [148], have put forward the supposition
[149] that photochemically active centres in cellulose materials, containing carboxyl and
hydroxyl groups, may be molecular complexes between these groups, connected by the
system of hydrogen bounds with definite energy of interaction. Disproportion of
intermolecular bonds, providing fixation of excited state in cellulose matrix takes place in
such complexes at their excitation. Thus, there are many different hypotheses, often
contradicting each other, about the effect of chromophore groups on light absorption by
cellulose.
There are many data about the nature of free-radical particles, being formed at irradiation
of cellulose by ultraviolet light. Since, being formed products of phototransformation are