Rheological behaviour molecular weight distribution

Polyethylene is a man-made homopolymer. Its chemical synthesis is well understood. It is a random walk polymer with little secondary or tertiary structure. A batch can largely be characterised by its molecular weight distribution, and its rheology can be related to these parameters by developed rules of polymer behaviour. The action of specific chemicals as plasticisers can be used to modulate these bulk properties in a predictable way, allowing the nature and characterisation of its glass to fluid transition to be predicted. 

Under different deformation conditions, natural rubber (NR) may exhibit predominant viscous flow, elastic or viscoelastic behaviour. Thus, the time for the movement of the NR molecular chains, i.e. relaxation time, is vastly affected by those deformation rate and NR types. The variation of NR types such as smoked rubber sheet, rubber blocks such as skim block, STR 5L and STR 20, is another factor that influences rheological properties and processing of NR due to their different Mooney viscosity, molecular weight distribution and gel content. Types of NRs are based on the different production processes of NR in which rubber smoked sheets are produced from NR in the latex form whereas block types are produced from various types of NR sources such as from latex in the case of STR 5L, from rubber scrap in the case of STR 20 and from skim latex in the case of skim block. 

In recent years, it has been possible to demonstrate the correlation between the molecular weight distribution (M WD) of a polymer and its rheological behaviour. It is obviously necessary, therefore, to keep this parameter under control during the polymerisation process. By its very nature, the catalyst system used to manufacture ethylene-propylene elastomers will produce polymers with a fairly narrow MWD which can, however, be varied between certain limits. In commercial polymers, the M /M ratio, which is a good indication of the MWD, generally varies between 3 and 5. 

Rheological behaviour, viscosity and plasticity under given conditions are affected by the nature of the polymer, the average molecular weight, its distribution, and the molecular structure, branching, stereo-arrangement... 

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