engleski

Comparative study of complex mechanisms of various polystyrene degradations and oxidations

Comparative investigations of the polymeric residues remained after degrading polystyrene by heat or by UV irradation reveal numerous profound changes of the structural and molecular parameters of the polymer when subjecting it to either of these main types of degradation. The polymeric residues of the commecial and of the prepared polystyrene samples were investigated in view of the following characteristics: discolouration, IR, UV and 13C NMR spectra, DSC-analysis, GPC-elution curves and X-ray patterns. The results show pronounced heterogeneity of the degradative reactions occuring in the framework of the termal and pgotochemical degradation of polystyrene, including the existence of crosslinking processes and producing thus very inhomogeneous polymeric material. The presumption of the existence of different mechanism paths occuring in various of polystyrene structural fractions was confirmed and further promoted, especially from the stanpoint that the lower-molecular fractions of the polymer should behave more sensitively against the degradative attack.Many-sided meanings of the informations obtained by analysing degraded polystyrene samples allow to postulate the present degradative mechanism by comparing among the various types of degradative processes. The gained informations about the locations and ranges of absorption band in UV, IR and 13C NMR spectra, as well as the complementary evidence from other determinations enabled the claim that polystyrene thermo-oxidative degradation woul preferentially take place in the way to produce acetophenone structures near the chain ends. In the initial degradation periods this would be related whit substantial lowering of the molecural mass. The photo-oxidative degradation mechanism cause the build-up ketone and aldehyde structures and ring-opening reactions concurent with chain crosslinking reactions. Comparing long-wave and short-wave UV irradiations of polystyrene does not only show the existenc of inequal induction times, but additionally the presence of two sets of similar reactions leading to the production of many oxidized structures. The investigations would show that oxidative polystyrene degradation is a very complex phenomenon in which the prefered courses of chemical reactions are in the relations to the initial reaction steps, namely they do depend on structures formed in the early stages of the thermal and photo-degradative processes.

polystyrene ; degradations ; oxidations

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