What distinguishes the viscosity of a polymer melt from that of Newtonian fluids?

The viscosity of a polymer melt is influenced by its molecular structure and how it flows under stress, which distinguishes it significantly from Newtonian fluids. The correct choice involves the behavior known as pseudoplasticity, where the viscosity of the polymer melt decreases with an increase in shear rate. This behavior occurs because as the shear applied to the polymer increases, its molecular chains begin to align and disentangle, allowing the material to flow more easily.

In contrast, Newtonian fluids have a constant viscosity that does not depend on the shear rate; they maintain the same flow behavior under varying applied stresses. This fundamental difference reflects the complex nature of polymer melts compared to simpler fluid systems.

The other options do not capture the notable characteristics of polymer melts accurately. For example, polymer viscosity is not constant; it varies with shear rate, which is why pseudoplasticity is a key feature. Additionally, while temperature does influence viscosity, it is not the only factor; shear rate plays a critical role as well. Lastly, pressure changes can indeed affect the viscosity of a polymer, but this is not a distinguishing factor of its viscosity compared to Newtonian fluids. Thus, the unique pseudoplasticity behavior is what sets the viscosity of polymer melts apart from that of Newtonian fluids