The flow properties of polymer melts

We consider three types of flow elongational flow, simple shearing flow, and first, but quite briefly, the behaviour of melts under hydrostatic pressure. This latter behaviour is particularly significant in injection moulding in which liquid flows may be generated by hydrostatic pressures 10 atmospheres (1 atmosphere =10 Pa). 

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In the subsequent 20 years (1960-80), the main principles of modern polymer physics were developed. These include the Edwards model of the polymer chain and its confining tube (Chapters 7 and 9), the modern view of semidilute solutions established by des Cloizeaux and de Gennes (Chapter 5), and the reptation theory of chain diffusion developed by de Gennes (Chapter 9) that led to the Doi-Edwards theory for the flow properties of polymer melts. 

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 other two examples deal with the flow properties of polymer melts as they are encountered under ordinary processing conditions. Figure 7.2 presents results of measurements of the viscosity of a melt of polyethylene, obtained at steady state for simple shear flows under variation of the shear rate. Data were collected for a series of different temperatures. At low strain rates one finds a constant value for the viscosity coefficient, i.e. a strict proportionality between shear stress and shear rate, but then a decrease sets in. This deviation from linearity is commonly found in polymers and begins even at moderate strain rates. As one observes a decrease in the viscosity, i.e. the ratio between shear stress and shear rate, the effect is usually called shear thinning . 

When the flow properties of polymer melts and solutions can be measured over a wide enough range of shearing, the logarithmic flow curves appear as in Figure 14.5. It is generally observed that ... 

The other two examples deal with the flow properties of polymer melts as they are encoimtered under ordinary processing conditions. Figure 9.2 presents results of measurements of the viscosity of a melt of polyethylene, obtained at steady state for simple shear flows imder variation of the shear rate. Data... 

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