STABILIZATION AND MODIFICATION OF CELLULOSE DIACETATE - Photochemical Reactions in Heterochain Polymers - page 102

Presence of conjugated π - bonds in the dye structure facilitates uniform distribution of π
- cloud of unpaired electrons, that is why singlet does not change at excitation. Kinetic data
of radicals accumulation in irradiated dyed PVA-films are given in Figure 2.40. From this
figure it is seen that dyes LIX and LXI retard the rate of radicals formation in PVA to a
greater extent than dyes LX and LXI. Such effect of increasing photo- and thermal stability of
dyed PVA-films may be classified as partial structurization of polymer during thermal
treatment and ultra-violet.
Presence of the processes of structurization in undyed film during the first period of
irradiation may be observed while investigating viscosity of irradiated PVA-films, that proves
some increase in intrinsic viscosity (see Figure 2.41). Such increase in intrinsic viscosity is
not observed in dyed films. Probably PVA structurization after ultra-violet irradiation takes
place by recombination radicals of polymer adjacent chains. The rate of radicals accumulation
in dyed PVA-films is less (Figure 2.41) and at the same time probability of structurization by
recombination PVA macroradicals decreases.
Figure 2.40. Kinetic of free radicals formation at ultra-violet irradiation: 1 - PVA covalently linked
with LIX; 2 - PVA covalently dyed by LXI; 3 - PVA, dyed by LX; 4 - PVA, dyed by LXII; 5 -
undyed PVA.