Chemistry Reference
In-Depth Information
-C=C-, that is displaying formation of unsaturated groups [221]. They consider that light
−
С
−
O
−
absorption with λ below 300 nm is caused by groups C=O or
[158].
II
O
−
And those groups, which do not destruct themselves, may absorb light energy, causing
molecule destruction. Excitation in the region 340 nm relates to π*
Æ
n transition, and
fluorescence in the region 380-460 nm relates to S
1
(n- π*) state and interaction of C=O
groups with π - electrons of phenylene. Fluorescent properties of PETP appear in the
presence of abnormal units which are formed as a result of stilbene structure impurities taking
part in polycondensation and being present in monomer [222]. It is supposed that absorbed
energy may quickly migrate to other parts of molecule [10] in PETP as in other polymers.
Ester groups contained in PETP are subjected to the greatest transformations during
irradiation. In the work [221] it is shown that ester groups in PETP are exposed to the greatest
destruction under the action of ultra-violet irradiation as a result of which carboxyl and
hydroxyl groups are being formed. And this explains the increase of absorption bands
intensity in infrared spectra of PETP irradiated by ultraviolet light, these bands being related
to these groups.
Direction and rate of changes depend on wave length of incident light, intensity of
radiation and PETP structure. Investigations [115] on the study of PETP fluorescence at
excitation by light with wave length λ=340 nm show that excitation spectra depend on
relation of emission intensity at λ
max
370 and 390 nm and on film thickness.
According to the data, given in the works [223-224], it is seen that during PETP
irradiation by both polychromatic and monochromatic light with different wave length distant
ultra violet light up to 330 nm has the greatest destroying effect.
Arc carbon lamps, used in apparatuses of “Fedomer” type [225], xenon gaseons-disharge
lamps of high pressure, used in apparatuses of “Xenotest” type [226], tubular luminescent
erythritol lamps [227], mercury-quartz lamps [228] and other devices [229], used in some
countries while carrying out tests on light-fastnes of point, were proposed for light-fastness of
paint test.
Products of photodestruction are defined, as a rule, by the methods of EPR, infrared
spectroscopy, gas- liquid chromatography. Carbon oxide and dioxide, hydrogen, methane,
water, benzene, formaldehyde were identified by these methods.
Sharp change in supermolecular structure [230] is observed during artifical irradiation.
Great disagreement of results obtained while using different light sources may be
explained, on the one hand, by differences in spectral composition of radiation and, on the
other hand, by difference in temperature and humidity of the sample since PETP reacting with
water is subjected to destruction (hydrolysis takes place). The process of hydrolysis is
catalyzed by bases and acids. It is the presence of acids and bases, contaminating the
atmosphere that explains acceleration of PETP photodestruction while studying the effect of
lightning on the polymer [231, 232].
Direct photolysis or, as it is sometimes called photochemical destruction, leads to the
break of chemical bonds, absorbed by light, in macromolecule. Photochemical
transformations may be caused by radiation in the region 100-400 nm. This energy is quite
enough to break the bond C-C or C=O and also C-H [154]. Hence, in order for the destruction
to flow according to the mechanism of direct photolysis, PETP macromolecule must absorb
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