On the accuracy of SEC analyses of molecular weight distributions of polyethylenes

Molecular weights of National Bureau of Standards SRM 1476 polyethylene have been reported by six laboratories. The measured values are in remarkably good agreements and all show that Mw from SEC/LALLS analyses is significantly lower than the same average determined by LALLS on the whole polymer itself. This is shown to be due to the presence of high molecular weight species which become too diluted on passage through the SEC columns to be observed in the LALLS detector. The resulting error in Mw and higher averages may vary from slight to very serioius, depending on the molecular weight distribution of the particular polyethylene. A procedure is described to detect the presence of such high molecular weight species.