Molecular weight distribution Subject

Low Temperature Brittleness. Brittleness temperature is the temperature at which polyethylene becomes sufficiently brittle to break when subjected to a sudden blow. Because some polyethylene end products are used under particularly cold climates, they must be made of a polymer that has good impact resistance at low temperatures namely, polymers with high viscosity, lower density, and narrow molecular weight distribution. ASTM D746 is used for this test. 

The narrow molecular weight distribution means that the melts are more Newtonian (see Section 8.2.5) and therefore have a higher melt viscosity at high shear rates than a more pseudoplastic material of similar molecular dimensions. In turn this may require more powerful extruders. They are also more subject to melt irregularities such as sharkskin and melt fracture. This is one of the factors that has led to current interest in metallocene-polymerised polypropylenes with a bimodal molecular weight distribution. 

In PLP the sample is subjected to a series of short (<30 ns) laser pulses at intervals t. Analysis of the molecular weight distribution gives the length of chain formed between successive pulses (v) and this yields a value for kp (eq. 13). 

There are several important reasons for wanting to know molecular weights in polymer science. From the viewpoint of inorganic polymers, the main uses are for the interpretation of molecular-weight dependent properties, and for the elucidation of polymerization mechanisms. The latter involves characterization of the molecular weight distribution, which is the subject of the following section. 

In Figure 6 the GPC traces for the four fractions and for the residue from each of the three cyclohexane fractional destractions are overlaid. The trend to higher molecular weight distributions as the fractionation proceeded is evident, as well as the reproducibility of the fractionation process. Additional characterization of similar samples produced in the FDU is the subject of another paper (10). 

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