Extensional flow viscosity measurements

Extensiometer n. A rheometer for measuring the extensional flow properties of molten polymers. In one early form, the Cogswell rheometer, useful at tensile viscosities over lO Pa/s, unidirectional tensile force was exerted on a polymer rod by a dead-weight acting through a cam and pulley. As the cam rotated, the moment arm exerted 

In extensional flow, the morphological rearrangements lead to high extensional-to-shear viscosity ratios (both measured at the same < ) and the deformation rate), and to yield stresses that are larger than in shear. The effects increase with concentration and anisometry of suspended particles [10]. 

A. E. Everage and R. L. Ballman, The extensional flow capillary as a new method for extensional viscosity measurement. Nature 273, 213-215 (1978). 

The material function of prime importance in extensional flow is the extensional viscosity whidi is basically a measure of the resistance of the material to flow when stress is applied to extend it. 

In extensional flow, the diagonal components of are non-zero (i.e. T,y = 0 for i j). In the case of uniaxial extension, Th is the primary stress that can be measured, while T22 and T33 are generally equal to the pressiure of the environment. Thus, the uniaxial extensional viscosity rj is defined by. 

Fig. 13.26. Uniaxial extensional flow generated between a fixed plate and a plate moving at desired velocity (v), while the entire geometry is held in a constant-temperature bath. Measurement of the sample dimension and the required tension force allows calculation of the elongational viscosity.

A commercial instrument for extensional viscosity measurements is currently offered by the Thermo Electron Corporation [40], The device uses capillary breakup techniques and is called the Haake CaBER . Vilastic Scientific, Inc. also offers an orifice attachment to their oscillatory rheometer for extensional viscosity determinations [41,42], The principle of operation of the rheometer is oscillatory tube flow [43,44], Dynamic mechanical properties can be determined 

The melt viscosity of LCPs is sensitive to thermal and mechanical histories. Quite often, instrumental influences are important in the value of viscosity measured. For example, the viscosity of HBA/HNA copolyesters are dependent on the die diameter in capillary flow (59). LCP melts or solutions are very efficiently oriented in extensional flows, and as a result, the influence of the extensional stresses at the entrance to a capillary influence the shear flow in the capillary to a much greater extent than is usually found with non-LC polymers. 

By contrast, quite different results have been obtained with dilute polymer solutions. Here the extensional viscosity may be as much as thousand times the shear viscosity. Measurement of extensional viscosity of such mobile liquids is far more difficult than shear viscosity, or even impossible. According to Barnes et al. (General references, 1993) "The most that one can hope for is to generate flow which is dominated by extension and then to address the problem of how best to interpret the data in terms of material functions that are Theologically meaningful". An example of the difficulties that arise with the measurement of extensional viscosity is shown In Fig. 16.21 for a Round Robin test 

At low shear rates, polymeric liquid properties are characterized by two constitutive parameters zero shear rate viscosity t]o and recoverable shear compliance Jq, which indicates fluid elasticity. At higher shear strain rates, rheological behavior is measured with a viscometer. Extensional strain viscosity, associated with extensional flow, occurs with film extrusion. 

The important rheological properties which need to be studied and measured in order to be able to characterize suspensions are the same as those which have already been indicated in Chapter 2. However, only the viscous flow behavior in shear and extensional flow will be discussed in this chapter. In particular, shear viscosity will be dealt with in sufficient detail because of the wealth of information that exists on it. 

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