Problems of the molecular weight determination in polyethylene
By means of the gel permeation chromatography (GPC), viscometry and FTIR spectroscopy the molecular weights (M) of polyethylenes (PE) were established. PE was synthetized on a new, uperactive MgCl2(THF)2/TiCl4/Al(C2H5)2Cl catalyst. Polyethylene obtained on the basis of "classic" TiCl4/Al(C2H5)2Cl catalyst was examined for comparison. It was stated that PE obtained on Mg-Ti-Al catalyst posseses very high M of 106 and more. In comparison to PE prepared on a "classic" catalyst, the new PE is much less branched, of lower polydispersity, higher crystallinity and better mechanical strength properties. In establishing M of PE by the GPC method the possibility of using universal calibration in the range of molecular weights to several hundred thousands with narrow polystyrene standards was confirmed. For PE with M > 106 the GPC results seem to be underrated. The results of research suggest the necessity of improving the GPC calibration method in the range of extremely large M, which should lead to the explanation of the discrepancies observed among M values determined by different methods. FTIR method can be used only for approximate evaluation of M of PE on the condition that calibration samples of PE with known M were obtained on the same catalytic system.
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