Introduction

This chapter deals with viscoelastic behavior in the liquid state, particular emphasis being placed upon those aspects associated with the flow properties of polymer melts and concentrated solutions. The time-dependent response of polymers in the glassy state and near the glass transition, one variety of viscoelasticity, was discussed in Chapter 2. The concern in this chapter is the response at long times and for temperatures well above the glass transition. The elastic behavior of polymer networks well above the glass transition was discussed in Chapter 1. The conditions here are similar, and elastic effects may be very important in polymeric liquids, but steady-state flow can now also occur because the chains are not linked together to form a network. All the molecules have finite sizes, and, for flexible-chain polymers, the materials of interest in this chapter, the molecules have random-coil conformations at equilibrium (see Chapters 1 and 7).

The discussion in this chapter covers linear viscoelasticity [1], a primary means of rheological characterization for polymer liquids, and simple shear flow under steady-state conditions [2, 3], a relatively well-understood bridge into nonlinear viscoelastic behavior. The effects of large-scale chain structure – molecular weight, molecular weight distribution, and long-chain branching – will be discussed, and some theoretical ideas about molecular aspects will be described. The general mathematical framework of the subject [4, 5], and applications to the solution of practical flow problems [6–8] are more advanced topics and will not be discussed here.