Determination of long chain branching in PE samples by GPC-MALS and GPC-VIS: Comparison and uncertainties

Long-chain branching (LCB) is a well-known structural phenomenon in polyethylene whose characterization is of main interest because it affects a lot of properties of these polymers. Quantification of LCB can be carried out by GPC-VIS (gel permeation chromatography with a viscometer detector to determine the intrinsic viscosity) and by GPC-MALS (GPC with multi-angle light scattering detector to determine the radius of gyration) by calculating branching indexes with a linear reference sample and finally the LCB (λ) distribution. Usually, both techniques are used to determine the value of λ and its distribution, but taking into account that its value can be low there is fundamental establish the range of application in function of the uncertainties on the results.In this work, a numerical method, previously developed for the reduction of GPC-MALS data, has been used with GPC-VIS data and extended to determine branching indexes and LCB. Final results obtained by GPC-MALS and GPC-VIS were compared with special focus to ranges of application and reproducibility of the results. Branching indexes allowed evaluating the structure factor in function of molecular weight and a qualitative dependence with polydispersity index was found.Distribution of LCB with molecular weight has been determined and reproducibility checked in order to establish the limitations of each technique and the corresponding range of application.

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► Branching index are calculated by GPC-MALS and GPC-VIS.
► Determined structure factor present a clear systematic trend with molecular weight.
► Change of structure factor appears to be related to polydispersity index.
► Estimation of uncertainties allow to establish ranges of confident application.