Molecular Characterization of Polyolefins

The results of a round-robin test carried out by IUPAC Working Party IV. 2.2 on Molecular Characterization of Commercial Polymers with two grades of polypropylene with narrow (“controlled rheology” PP) and broad molar mass distribution (MMD) are summarized. The interlaboratory agreement for M w expressed by the relative standard deviation, σ(%), is ± 6 for PP with broad MMD (M w/M n = 5.7) and is better than for the controlled rheology grade [σ(%) = ± 11.7, M w/M n = 2.55]. This finding is explained by the influence of peak broadening in size exclusion chromatography (SEC). The measured MMDs are discussed with respect to the frequency dependence of the complex shear modulus and to differentiation between the respective grades by process control data from injection molding. A comparison of procedures for the determination of MMD and distribution of short-chain branching in polyethylene of medium and high density (PE-MD and PE-HD) with an alternative sequence of fraction is presented. The direct extraction procedure by Holtrup, which is fractionating according to molar mass followed by analytical temperature rising eluation fractionation (TREF), gives the most detailed results within a considerably shorter time than a Holtrup fractionation followed by preparative TREF; both procedures give a much more detailed result than fractionation by preparative TREF followed by SEC. The contour diagrams (CH3/1000C vs log M) of two PE-HD blow-molding grades with the same density values (ρ = 0.954) and melt flow index (MFI 190/2.16 = 0.20 dg/min) are interpreted with respect to their good but distinguishable environmental stress-cracking resistance.