Polyethylene polydispersity ratios

It may be shown that M > M. The two are equal only for a monodisperse material, in which all molecules are the same sise. The ratio MI /MI is known as the polydispersity index and is a measure of the breadth of the molecular weight distribution. Values range from about 1.02 for carefully fractionated samples or certain polymers produced by anionic polymerization, to 20 or more for some commercial polyethylenes. 

The Mw/Mn ratio is usually equal to 5-10 for polyethylene [49,64,66,67, 123,244-247], A much lower polydispersity is displayed by polymers obtained in polymerisation with homogeneous metallocene catalysts the Mw/Mn ratio usually does not significantly exceed a value of 2 [22,95,101,112,138,140], By polymerising propylene with soluble vanadium-based Ziegler-Natta catalysts at low temperature, a very narrow molecular weight distribution of the polypropylene has been found (the Mw/Mn ratio usually reaches values of 1.15-1.25) and a linear increase in its Mn with time has been observed, indicating a noticeable living character of the polymerisation [75,76,241],... 

Single-site ethylene polymerization catalysts based on titanium or zirconium as the active center are primarily used in the low-pressure manufacture of polyethylene with a relatively very narrow molecular weight distribution to include high-density polyethylene (HDPE), linear low-density polyethylene (LLDPE) and very low density LLDPE to include amorphous material. Table 4.1 summarizes a few examples of end-use applications for this type of polyethylene in terms of the density, molecular weight distribution and molecular weight for various applications. Note that in Table 4.1 a Melt Flow Ratio (MFR) value of 16 approximately corresponds to an polydispersity value of 2.0, which is indicative of a... 

Most LLDPE resins manufactured have a relatively narrow molecular weight distribution as indicated by Melt Flow Ratio values (MFR = FI/MI) of 16-30 or polydispersity values of 2-4. Films produced from this type of polyethylene have higher dart impact and tear properties as shown in the Table 6.5. However, LLDPE resin with a MWD broader than LDPE is used in geomembrane applications such as landfill covers and caps and other heavy-gauge film applications. 

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