ABSTRACT
For a given polymer, as explained earlier, the viscosity depends on processing variables such as the temperature. pressure, and the shear rate. However. the viscosity value changes as the chemical nature of the polymer changes. Thus, the shear viscosity of polyetbylene differs from that of nylon, even when the values of tbe three processing variables are kept unchanged. AdditionalJy. for a given polymer type. tbe viscosity is a Function of the molecular weight. the molecular weigbt distribution and the presence of any chain branching. ln the sections that follow. we systematically examine the influence of each of these variables. We present typical data, show how these data can be combined to give viscosity master curves, propose equations for data representation, and provide physical explanations for the recorded observations. Both model and commercial polymers are considered. As might be expected. data are most abundant for high-volume polymers such as polyetbylene, polypropylene, and polystyrene. Note that while the polyolefins are crystalline, polystyrene is amorphous, and the molecular weight of all three polymers, when manufactured commercially, is high enough that their rheological behavior is quite non-Newtonian. Also note that these three polymers are normally synthesized using chain growth (addition) polymerization, and they are reasonably stable at processing temperatures. As opposed to this,
other important polymers like polyesters and nylons are relatively Newtonian due to their lower molecular weight. Also. they are made by the process of step growth (condensation) polymerization; at processing temperatures. they are subject to hydrolysis or postcondensation, depending on the extent to which they are dried before being processed. Since most polymers tend to oxidize when kept at elevated temperatures for extended periods of time. it is normal to use an inert atmosphere of nitrogen or argon gas when making rheological measurements.
