Molecular weight and melt viscosity

Another very special feature of these polymers is the relationship between molecular weight and melt viscosity. For linear polymers, the increase in melt viscosity with molecular weight is linear with a transition to a 3.4 power law when the molecular weight reaches the critical mass for entanglements, M. For hyperbranched polymers, the increase in viscosity follows a different curve it is less pronounced and levels off at higher molecular weights [117] (Fig. 11). 

FIGURE 232 Dependence of polymer melt index (HLMI), molecular weight, and melt viscosity on reaction temperature. 

Processing of PP is usually performed at temperatures between 200 and 280°C, exceptionally even up to 300°C [2, 3]. With insufficient stabilization, molecular weight and melt viscosity of PP decrease while the melt flow increases [2]. 

The effects of shear rate (y) and polydispersity (the ratio Mw/Mn, where Mn denotes the number-average molecular weight) on melt viscosity have been reviewed by van Krevelen [7] from the viewpoint of fitting experimental data by empirical relationships, and by Kumar [5] from the viewpoint of fundamental theoretical treatments. It is clear from these reviews that the effects of y and polydispersity are inextricably intertwined and must be considered at the same... 

Anomalous relationship of melt viscosity to molecular weight. The melt viscosity of a block polymer is significantly higher than that of the corresponding random copolymer of the same molecular weight, and this deviation itself Increases with molecular weight of the blocks. 

Many different molecular weight grades of VP-based polymers, characterized by viscosity, are available commercially. The determination of viscosity is historically satisfactory for quality assurance purposes however, most physical properties of polymers are directly related to molecular weight. For example, the glass transition temperature and tensile strength of amorphous polymers are known to depend on molecular weight. The melt viscosity of polymers and the bulk viscosity of concentrated polymer solutions are also known to depend on molecular weight. 

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