Elongational Rheometry

The various experimental techniques may broadly be classified, in accordance with the degree of control of the elongational deformation, into two types (1) controlled flow, and (2) uncontrolled flow. The former refers to an experiment where the sample is subjected to a constant elongation rate, and the latter to a non-constant elongation rate. For the purpose of screening constitutive equations for viscoelastic fluids, the controlled experiment, which generates a constant elongation rate, is preferred because 

To overcome some of the difficulties enumerated above, an alternative experimental technique was suggested (Meissner 1972), in which two sets of gripping wheels are used, instead of end loading. In this way, the specimen is stretched between the distance L fixed in space and e is determined by Eq. (5.95). 

In this type of apparatus, the tensile stress is measured through the deflection of a spring associated with one of the rotating wheels. In determining with the methods described in this section, one may have to wait a long time for the stress to build up to the level where a steady-state (in both the Lagrangian and Eulerian senses) is attained because the specimen used is strain free before the test begins. 

Note that capillary/slit rheometry is also very useful for determining the shear flow properties of heterogeneous polymer systems, including immiscible polymer blends which are discussed in Chapter 11, highly-filled molten polymers, which are discussed in Chapter 12, and molten polymers with solubilized gaseous component, which are 

LID Ratio Toted Pressure Drop Ap (MPa) Apparent Shear Rate Exit Pressure (kPa) 

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It is clear from the apparatus description in Fig. 2.49 that carrying out tests to measure extensional rheometry is a very difficult task. One of the major problems arises because of the fact that, unlike shear tests, it is not possible to achieve steady state condition with elongational rheometry tests. This is simply because the cross-sectional area of the test... 

Han, T., Yarin, A. L., Reneker, D. H. Viscoelastic electrospun jets initial stresses and elongational rheometry. Polymer 49, 1651-1658 (2008). 

J. Meissner, Experimental aspects in polymer melt elongational rheometry, Chem. Eng. Commun. 33, 159-180 (1985). 

IR dichroism has also been particularly helpful in this regard. Of predominant interest is the orientation factor S=( 1/2)(3—1) (see Chapter 8), which can be obtained experimentally from the ratio of absorbances of a chosen peak parallel and perpendicular to the direction in which an elastomer is stretched [5,249]. One representation of such results is the effect of network chain length on the reduced orientation factor [S]=S/(72—2 1), where X is the elongation. A comparison is made among typical theoretical results in which the affine model assumes the chain dimensions to change linearly with the imposed macroscopic strain, and the phantom model allows for junction fluctuations that make the relationship nonlinear. The experimental results were found to be close to the phantom relationship. Combined techniques, such as Fourier-transform infrared (FTIR) spectroscopy combined with rheometry (see Chapter 8), are also of increasing interest [250]. 

The theory of sqeezing flow rheometry assumes that the sample is nonelastic. Tests on viscoelastic samples should therefore be carried out at low strain rates, to minimize elastic response, and results should be reported as apparent elongational viscosity. 

Rohm (1993a) obtained agreement between values of the elongational viscosity of butter measured using creep testing and those obtained using lubricated squeezing flow rheometry, as expected from theory. 

Kotaka T, Kojima A and Okamoto M (1997) Elongational flow opto-rheometry for polymer melts - 1. Construction of an elongational flow opto-rheometer and some preliminary results. Rheol Acta 36 646-56. 

The rheological behaviour of the two polymers was determined using classical techniques of rheometry, already described in Chapter II. 1 (rotational and capillary rheometers for shear viscosity and first normal stress difference measurements CogsweU method for the elongational viscosity). 

The extensional viscosity functions of emulsions were characterized using a Capillary Breakup Extensional Rheometry (CaBER). The apparent extensional viscosities ) of SE-la emulsions as a function of strain rate e are given in Fig. 23.5 for the different mean emulsion drop sizes of 2,4, and 10 pm. From this, an extensional viscosity characteristics for the emulsion with a drop size of 10 pm was derived close to constant and rather low (ca. 0.2 Pas, Newtonian-like). For emulsion drop sizes of 2 and 4 pm, pronounced extensional thinning was observed in the low elongation rate domain, whereas some pronounced extensional thickening behavior was monitored in the higher elongation rate domain. 

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