The term “polymeric liquids” is used to describe both solutions of polymers and molten polymers. All polymeric liquids exhibit non-Newtonian flow behavior, including a shear stress-dependent viscosity and elasticity. However, concentrated solutions and melts of polymers whose molecular weights exceed a certain critical value (the “critical molecular weight for entanglement,” Mc) exhibit quite remarkable deviations from classical fluid behavior, especially marked elasticity. Among the remarkable rheological phenomena exhibited by these materials are elastic recoil and flow-induced an-isotropy. Indeed, in certain situations, such materials can exhibit elastic effects that are almost indistinguishable from those exhibited by cross-linked rubbers. This behavior is important, because most commercial “thermoplastics,” such as polyethylene and polystyrene, have high molecular weights (M > Mc) and are processed in the molten state.

A given generic polymer, polyethylene for example, can exhibit a wide range of properties depending on the molecular weight distribution. Another important aspect of molecular structure is branching. For many monomers (the molecular building blocks that make a polymer molecule), two types of polymer structure are possible, linear and branched. For example, ethylene can be polymerized in two ways to form either linear polyethylene or branched polyethylene. Branching enhances the non-Newtonian and elastic aspects of the melt flow behavior. Yet another possible aspect of polymer molecular structure is the presence of a comonomer.