Chemistry Reference
In-Depth Information
3.6.
L
IGHT
-S
TABILIZATION OF
P
OLYESTERS BY
H
EXAAZOCYCLANES
Energy, which molecule gets at photon absorption, may be consumed in photophysical
and photochemical transformation.
Groups of atoms with double bonds absorb light with λ>250 in organic compounds, that
do not contain other atoms except C, H, O, N. Energy levels, conforming these wave lengths,
correspond to excitation energies of π-electrons with double bonds (π → π* transition). Large
wave lengths of incident light are sufficient for excitation of undivided electrons of
heteroatoms, taking part in formation of double bonds with carbon atom (n → π* transition).
First, valence bonds of macromolecule break under irradiation effect and radicals are
being formed, which further undergo “dark” elementary reaction. For hemolytic break of the
bond it is necessary for photon to possess definite minimum of energy, which depends on the
wave length of photon.
In fact, energy, necessary for primary formation of radicals at polymer photolysis, is
greater than the energy of bond dissociation, that is explained by the presence of cage effect
in polymers [8].
Physical meaning of cage effect is in the fact, that a pair of free radicals, being formed at
homolytical bond break, in condensed phase is surrounded by the “wall” of other molecules.
Possibility, that two such radicals will recombinate before they will be able to disperse, is
rather great. That is why additional energy is needed, and radicals, possessing this energy will
be able to leave molecular cage. Besides particles, being in such cage, should be, in a definite
way, oriented in respect to each other.
Restriction of molecular mobility is observed in solid matrices. Stabilization of not only
free radicals, but instable molecular intermediate products may be the result of it.
Polyethyleneterephthalate (PETP) is resistant to ultra-violet irradiation, but at long
sunlight action destruction becomes visible, especially during irradiation by the light with
wave length from 300 to 330 nm [47]. Polyester does not destruct at photon energy lower
than 3 e.v. (electron volt) [48].
Polyester fibre displays high stability at its protection from a part of spectrum, that causes
its photochemical destruction.
In solid polymers, recombination of radicals is diffusive-controlling process (regardless
of whether it is controlled by physical diffusion or chemical relay). High rate of diffusion
leads to more uniform distribution of stabilizer in polymer mass and its fast migration to
material surface, if necessary.
Probability of photooxidation is different in different points of polymer. First, particles,
situated in the points, where quantum yield is higher undergo a reaction. That is why quantum
yield gradually decreases during the reaction.
During photooxidation, limiting stage of the reaction P*+ O
2
→ PO
2
is microdiffusive
stage, that is translational transport of dissolved oxygen before its meeting with macroradicals
localized in lattice. Following kinetic stage - addition of oxygen to macroradical in the cage -
is very fast.
During polymer oxidation, concentration of peroxide macroradicals and hydroperoxide
groups should be the function of distance from polymer surface, even in that case when
oxygen diffusion does not limit oxidation rate. For the same reason constant of the rate of
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