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structure of polymer. Besides, comparison of DTA of initial and modified PETP - fibres
points to the shift of the maximum of melting peak into the region of lower temperatures.
Hence, in modified fibres crystals have mainly morphological form II (extended polymer
chains, combined into joints of crystallites), while in initial PETP crystals mainly have
morphological form I (folded structure). That is why endothermic effect at the temperature
269 ºC is observed for initial PETP, explained by melting of plane folded crystallites
(morphological form I). In modified PETP - fibre this effect is observed at temperature 245-
263 ºC, it is explained by melting crystallite joints (morphological form II).
Intensity of endothermic peak, corresponding to melting region is qualitative criterion of
polymer thermal stability [300]. In our case it is seen that intensity of melting peak of
modified and unmodified polymer are the same. Hence, introduction of hexaazocyclanes at
any rate, does not deteriorate thermal stability of PETP - fibres. Only thermal effects,
connected with crystallite melting, are registered in thermograms during heating of PETP
samples. On DTA curves of modified PETP - fibres it is seen that the range of melting peak
in them is larger than in initial PETP.
Proceeding from above-mentioned one may assume that PETP - fibres modified by
hexaazocyclanes possess greater degree of crystallinity. This proves the conclusion made
earlier that hexaazocyclanes molecules being introduced become additional centers of
crystallization, thereby increasing degree of crystallinity of modified PETP - fibre.
In addition to this, hexaazocyclanes additives cause decrease of melting temperature
(Figure 3.11 - 3.13), that is hexaazocyclanes have plasticizing effect on PETP.
Kinetics of radical chain process of polymer thermal destruction includes stages of
initiation, growth of reaction chain, chain transmission, its break. Reaction of chain
transmission occurs mainly at the expence of hydrogen break from polymer chain.
PETP is rather stable at relatively low temperature regarding oxygen. However, oxidation
process runs at considerable rate at the temperatures above 220-250 ºC.
Oxidation of polymer is accompanied by the change of their structure - physical
properties - crystallinity, molecular mobility, strength and so on. Orientation of crystalline
regions is disturbed in stressed samples; the number of crystals with definite space orientation
of crystal lattice axes decreases, the form of the curve of crystal distribution axes in respect to
orientation axis changes. Change for the worse of crystals orientation is explained by stress
relief, occurring in polymer at the expense of oxidative destruction of macromolecules in
amorphous intercrystalline region [301].
Only a few works are devoted to the investigation of the mechanism of thermooxidative
destruction process. This is explained by the fact that PETP visible oxidation begins at
relatively high temperature (above 250 ºC), when together with oxidative processes there are
processes of purely thermal destruction. In the presence of oxygen it should be expected that
the process of destruction will run according to radical - chain mechanism with initiating
along the bond C-H at methylene group:
O
O
RH
RH
⎯
⎯→
R
•
+
HOO
•
⎯
⎯→
ROO
•
⎯
⎯→
ROOH
2
2
Oxidation of solid polymers is radical - chain process with marked branching and square
break of kinetic chains, the main branching product being hydroperoxide. Fast - decomposing
hydroperoxide is localized in amorphous phase, being more stable in crystals [302].
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