Molecular weight distribution melt viscosity measurements

APHA color (269) is usually one of the specifications of PTMEG, sometimes viscosity is another (270). Melt viscosity at 40°C is often used as a rough measure of the molecular weight distribution within a narrow molecular weight range. Sometimes an empirical molecular weight ratio,... 

Studies of melt flow properties of polypropylene indicate that it is more non-Newtonian than polyethylene in that the apparent viscosity declines more rapidly with increase in shear rate. The melt viscosity is also more sensitive to temperature. Van der Wegt has shown that if the log (apparent viscosity) is plotted against log (shear stress) for a number of polypropylene grades differing in molecular weight, molecular weight distribution and measured at different temperatures the curves obtained have practically the same shape and differ only in position. 

It is well known that LCB has a pronounced effect on the flow behavior of polymers under shear and extensional flow. Increasing LCB will increase elasticity and the shear rate sensitivity of the melt viscosity ( ). Environmental stress cracking and low-temperature brittleness can be strongly influenced by the LCB. Thus, the ability to measure long chain branching and its molecular weight distribution is critical in order to tailor product performance. 

Quantitizing the effect of these parameters is difficult because of the many different possibilities in the shape fiinction of the base resin molecular weight distribution curve. When the distribution curve is monomodal and log normal, the breadth of the molecular weight distribution is a function of the slope of a line formed by measurement of melt flow at increasing load or shear rate. Extrapolation of this slope defines a shear rate for measuring a base resin viscosity, which correlates with Mooney viscosity of the chlorinated product. 

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