Extensional rheology

Very few studies have been performed investigating the effect of branching on the extensional rheological properties of polystyrenes. Such investigations can be valuable because many of the fabrication operations associated with commercial applications of polystyrene include operations in which the polystyrene melt undergoes an extensional deformation. Some examples are extruded foam sheet, blown film, oriented (tentered) sheet, and thermoforming. The types of deformations associated with these processing operations are best described as 

Typically, it is desirable for a polymer to have a certain level of melt strength so that it processes well in these types of fabrication operations. Ideally, one would like to characterize the melt strength of a resin using a test method that best captures the type of deformation experienced by a polymer in a commercial fabrication operation. Owing to the complex flows associated with such operations, the approach that is most commonly used is to characterize the melt strength of a polymer in a uniform flow field and then infer from this information its performance in a real fabrication operation. 

Watanabe, Y. Ishigaki, H. Okada, H. Suyama, S. Chem. Lett. 1993, 1089. 

Principles of Polymer Chemistry, Ithaca University Press, Ithaca, NY, 1953. 

Edwards, S. Theory of Polymer Dynamics, Oxford Press, Oxford, 1986. 

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Anklam et al. [91] have attempted to measure the extensional rheological properties of w/o emulsions and HIPEs, using a nozzle-type viscometer. However, the results showed a dependence on the nozzle size used, and long relaxation times. Experiments on other non-Newtonian fluids indicated that it was not possible to obtain reliable results with this kind of instrument. 

These are early days in the development of extensional rheometers for foods and in our understanding of the extensional rheology of foodstuffs. The lack of commercial availability of extensional rheometers could be stalling the application of extensional rheology in characterizing products and processing operations in the food industry. For industrially important food products, product specific rheometers may need to be developed. 

The comparatively less elastic nature of many food products makes the analyses of extensional rheology data easier than has been possible with polymeric fluids. This relative ease of data interpretation provides an opportunity both for learning the extensional behavior of materials and for effective application of extensional rheometry in the food industry. 

J. Plucinski, R.K. Gupta and S. Chakrabarti, Shear and Extensional Rheology of Mayonnaises, presented at the Second International Conference on Extensional and Shear Flow of Polymer Fluids from the Solution to the Melt, St. Andrews, Scotland, June 19-22, 1994. 

Dobraszczyk, B.J., Smewing, J., Albertini, M., Maesmans, G., and Schofield, J.D. (2003). Extensional rheology and stability of gas cell walls in bread doughs at elevated tempera-mres in relation to breadmaking performance. Cereal Chem. 80,218-224. 

The maximum strain rate (e < Is1) for either extensional rheometer is often very slow compared with those of fabrication. Fortunately, time-temperature superposition approaches work well for SAN copolymers, and permit the elevation of the reduced strain rates kaj to those comparable to fabrication. Typical extensional rheology data for a SAN copolymer (h>an = 0.264, Mw = 7 kg/mol,Mw/Mn = 2.8) are illustrated in Figure 13.5 after time-temperature superposition to a reference temperature of 170°C [63]. The tensile stress growth coefficient rj (k, t) was measured at discrete times t during the startup of uniaxial extensional flow. Data points are marked with individual symbols (o) and terminate at the tensile break point at longest time t. Isothermal data points are connected by solid curves. Data were collected at selected k between 0.0167 and 0.0840 s-1 and at temperatures between 130 and 180 °C. Also illustrated in Figure 13.5 (dashed line) is a shear flow curve from a dynamic experiment displayed in a special format (3 versus or1) as suggested by Trouton [64]. The superposition of the low-strain rate data from two types (shear and extensional flow) of rheometers is an important validation of the reliability of both data sets. 

F.N.Cogswell, Measuring the extensional rheology of polymer melts, Trans. Soc. Rheol. 16(1972), 383-403. 

Chellamuthu, M., and Rothstein, J. R, Distinguishing between Unear and branched wormlike micelle solutions using extensional rheology measurements, J. RheoL, 52, 865-884 (2008). 

Measurement of biaxial extensional rheological properties using bubble inflation and stability of bubble expansion in bread doughs. In Bubbles in food, ed. G. M. Gampbell, G. Webb, and S. S. Pandiella, 173-182. St. Paul, MN AACG International. 

Dobraszczyk, B. J., J. Smewing, M. Albertini, G. Maesmans, and J. D. Schofield. 2003. Extensional rheology and stability of gas walls in bread doughs at elevated temperatures in relation to bread-making performance. Cereal Chemistry 80 218-224. 

In addition, there is a lack of a clear in-depth understanding of the nonlinear interaction between the shear and extensional rheology of the fluids with the generation of viscoelastic... 

Rheological properties of the same polymeric solution measured in shear and elongational flows can be very different as discussed in earlier section. However, a cursory examination of current textbooks on rheology (e.g., Bird et al. [2], Ferry [3], and Tanner [14]) shows that shear rheology dominates and research in extensional rheology is comparatively much more recent. The measurements of the shear properties of polymeric fluids are well established and a number of rheometers are available for both melts and dilute polymeric solutions. Lately, more efforts have been directed in measuring the extensional properties of fluids [8, 9,15-19]. 

Bourbon AI, Pinheiro AC, Ribeiro C, Miranda C, Maia JM.Teixeira JA, et al. Characterization of galactomannans extracted from seeds of Gleditsia triacanthos and Sophora japonica through shear and extensional rheology comparison with guar gum and locust bean gum. Food Hydrocoil 2010 24(2—3) 184-92. 

It has been suggested that this type of instability may be related to the extensional rheological properties of the polymers used in the coextruded structure (28). 

Fig. 24. Extensional rheology data for LDPE, LLDPE, and mLLDPE. To convert Pa-s to P, multiply by 10.

Rothstein IP (2003) Transient extensional rheology of wormlike micelle solutions. J Rheol 47(5) 1227-1247... 

McKinley, G. H., P. Pakdel, and A. Oztekin, J. Non-Newtonian Fluid Mech., 67,19 (1996). McKinley, G. H., Extensional Rheology and Flow Instabilities in Elastic Polymer Solutions, in M. J. Adams, R. A. Mashelkar, J. R. A. Pearson, and A. R. Rennie, Eds., Dynamics of Complex Fluids, Imperial College Press and the Royal Society, London, 1998,... 

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