ФОТОДЕГРАДАЦИЯ ПОЛУКРИСТАЛЛИЧЕСКОГО ПОЛИЛАКТИДА ПОД ДЕЙСТВИЕМ УФ РАДИАЦИИ. I. ГЕТЕРОФАЗНЫЙ МЕХАНИЗМ РАЦЕМИЗАЦИИ МАКРОМОЛЕКУЛ

Abstract

The regularities of UV photodegradation of semi-crystalline films of poly-L-lactide (PLA) formed from solution in chloroform have been analyzed on the basis of IR spectroscopy and gel permeation chromatography. When analysing the experimental results, we took into account the fact that during the preparation of semi-crystalline films of poly-L-lactide, mechanical stresses accumulate in them. The presence of the latter is the reason why the acts of photodegradation proceeding according to the Norrish I mechanism prevail at the initial stage of the process in imperfect elements of the crystalline phase, in which enantiomerisation and racemisation of PLA macromolecules occur simultaneously with the rupture of polymer chains into radicals. In this work, the role of such important factors as the presence of interfacial stresses stabilising in films at the stage of their formation and the role of triplet energy migration through the system of carbonyl groups are considered for the first time. The imperfectly structured primary elements of the crystalline phase formed in the matrix of solid PLA represent ‘traps’ of triplet energy of photoexcitation migrating along the matrix C=O-groups. In such traps, C-C-bonds are broken on the inner walls of nanoscale slits of imperfect crystallites, and under such harsh conditions, either recombination with restoration of molecular chains or disproportionation of end radicals with fixation of chain fragmentation acts are realised. Acts of recombination of radicals with free valence on carbon atoms are preceded with a small probability by acts of enantiomerization of chiral carbon from the L state into the D state. These acts take place in the middle segments of polymer chains, resulting in the appearance of interchain bonding sites in the form of racemates. During the continuing photoprocess, the acts of macromolecule rupture propagate to the glassy phase of PLA. The observed peculiarities of photodegradation are explained by the heterophase mechanism of transfer of triplet excitation energy by carbonyl C=O-groups of the polymer from the glassy to the crystalline phase, in which C=O-groups not only accept the triplet energy migrating through the matrix, but also transform it into the energy of polymer chain scission.