PHOTODEGRADATION OF SEMI-CRYSTALLINE POLYLACTIDE UNDER THE UV RADIATION: II. STAGES OF MATRIX TRANSFORMATION ACCORDING TO THE DSC METHOD
Journal: HERALD OF TECHNOLOGICAL UNIVERSITY
Rubrics: 1. CHEMISTRY
Authors:
1.
Institut biohimicheskoy fiziki im. N.M.Emanuelya RAN
2.
Institut biohimicheskoy fiziki im. N.M.Emanuelya RAN
3.
N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia
4.
N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, Moscow, Russia
5.
Institut biohimicheskoy fiziki im. N.M. Emanuelya RAN
6.
KNITU
Type:
Article
Pages:
from 16
to 22
Status:
Language:
Russian
Keywords:
HETEROPHASE POLYLACTIDE, THE PHASES PHOTO TRANSFORMATION FEATURES, PHOTO ANNEALING EFFECT
Abstract:
The structural changes occurring under UV irradiation in semi-crystalline poly-L-lactide (PL) films obtained from solution in chloroform are analysed using differential scanning calorimetry (DSC) data. When analysing the DSC data, it is taken into account that the process of UV photodegradation of PL proceeds under the conditions of interfacial mechanical stresses accumulated during the fabrication of PL films. Interfacial stresses loosen crystallites and transform them into traps for triplet-excited states arising during UV photoexcitation and migrating along the polymer groups C=O. At the first stage of the process, the photo scission of polymer C-C bonds occur in the nano cracks of mechanically loosened crystallites with the participation of triplet-excited C=OT groups according to the Norrish I mechanism. At the same time, the surfaces of the same internal cracks of imperfect crystallites act as rigid cages for the radicals arising in them, and the reactions of these radicals are limited only by the acts of recombination and disproportionation. These acts cause the relaxation of interfacial stresses and the appearance of the second stage of the process with radical-induced reactions already in the glassy phase and also according to the Norrish I mechanism. The presence of the photo annealing effect associated with the partial conversion of the photoexcitation energy of polymer C=O groups into heat and responsible for the intermediate (during UV irradiation) formation of secondary crystalline phase elements, which catalyze thermal crystallization during heating of PL in the DSC mode is noted. The fact established by us earlier that the photodegradation of semicrystalline PL begins not with the glassy phase, but with the photodisintegration of imperfect crystallites forming the primary crystalline phase, is confirmed. Kinetic analysis of the observed DSC thermograms showed that the crystallites of this primary phase begin to disintegrate during UV photolysis at the maximum rate and are continuously consumed during UV irradiation until they completely disappear.
The structural changes occurring under UV irradiation in semi-crystalline poly-L-lactide (PL) films obtained from solution in chloroform are analysed using differential scanning calorimetry (DSC) data. When analysing the DSC data, it is taken into account that the process of UV photodegradation of PL proceeds under the conditions of interfacial mechanical stresses accumulated during the fabrication of PL films. Interfacial stresses loosen crystallites and transform them into traps for triplet-excited states arising during UV photoexcitation and migrating along the polymer groups C=O. At the first stage of the process, the photo scission of polymer C-C bonds occur in the nano cracks of mechanically loosened crystallites with the participation of triplet-excited C=OT groups according to the Norrish I mechanism. At the same time, the surfaces of the same internal cracks of imperfect crystallites act as rigid cages for the radicals arising in them, and the reactions of these radicals are limited only by the acts of recombination and disproportionation. These acts cause the relaxation of interfacial stresses and the appearance of the second stage of the process with radical-induced reactions already in the glassy phase and also according to the Norrish I mechanism. The presence of the photo annealing effect associated with the partial conversion of the photoexcitation energy of polymer C=O groups into heat and responsible for the intermediate (during UV irradiation) formation of secondary crystalline phase elements, which catalyze thermal crystallization during heating of PL in the DSC mode is noted. The fact established by us earlier that the photodegradation of semicrystalline PL begins not with the glassy phase, but with the photodisintegration of imperfect crystallites forming the primary crystalline phase, is confirmed. Kinetic analysis of the observed DSC thermograms showed that the crystallites of this primary phase begin to disintegrate during UV photolysis at the maximum rate and are continuously consumed during UV irradiation until they completely disappear.
Keywords:
HETEROPHASE POLYLACTIDE, THE PHASES PHOTO TRANSFORMATION FEATURES, PHOTO ANNEALING EFFECT
HETEROPHASE POLYLACTIDE, THE PHASES PHOTO TRANSFORMATION FEATURES, PHOTO ANNEALING EFFECT
