Polymer rheology Newtonian viscosity

PTT exhibits melt rheological behavior similar to that of PET. At low shear rates the melt is nearly Newtonian. It shear-thins when the shear rate is >1000s 1 (Figure 11.10) [68], At the melt processing temperatures of PET, 290°C, and of PTT, 260°C, both polymers have similar viscosities of about 200Pas. However, PTT has a lower non-Newtonian index than PET at high shear rates. The flow behavior can be modeled by the Bueche equation, as follows ... 

Polymer rheology can respond nonllnearly to shear rates, as shown in Fig. 3.4. As discussed above, a Newtonian material has a linear relationship between shear stress and shear rate, and the slope of the response Is the shear viscosity. Many polymers at very low shear rates approach a Newtonian response. As the shear rate is increased most commercial polymers have a decrease in the rate of stress increase. That is, the extension of the shear stress function tends to have a lower local slope as the shear rate is increased. This Is an example of a pseudoplastic material, also known as a shear-thinning material. Pseudoplastic materials show a decrease in shear viscosity as the shear rate increases. Dilatant materials Increase in shear viscosity as the shear rate increases. Finally, a Bingham plastic requires an initial shear stress, to, before it will flow, and then it reacts to shear rate in the same manner as a Newtonian polymer. It thus appears as an elastic material until it begins to flow and then responds like a viscous fluid. All of these viscous responses may be observed when dealing with commercial and experimental polymers. 

A modified version of the free-volume theory is used to calculate the viscoelastic scaling factor or the Newtonian viscosity reduction where the fractional free volumes of pure polymer and polymer-SCF mixtures are determined from thermodynamic data and equation-of-state models. The significance of the combined EOS and free-volume theory is that the viscoelastic scaling factor can be predicted accurately without requiring any mixture rheological data. 

Rheological properties of the polymer melt non-Newtonian viscosity as a function of shear rate and temperature and/or viscoelastic material characteristics. 

In other words, apparent viscosity (as well as other apparent values in polymer rheology, snch as apparent shear rate and apparent shear stress) is a value calculated assuming Newtonian behavior and considering all pressure drops within the capillary (when using a capillary rheometer). A nonlinearity between shear rate and shear stress is typically observed for polymer melts. The fluid may behave like Newtonian at a very low shear rates to give a limiting viscosity iJq. 

There is a mounting evidence that PDB is not a rule for miscible polymer blends. Depending on the system and method of preparation, polymer blends can show either a positive deviation, negative deviation, or additivity. Note that miscibility in polymeric systems requires strong specific interactions, which in turn affect the free volume, thus the rheological behavior. It has been demonstrated that Newtonian viscosity can be described by the relation [Utracki, 1983 1985 1986] ... 

Accessible precursor architectures (building imits, dimensionality of the polymer molecule) are dictated by the methods of the chemical synthesis of the monomer units and the associated polymerization reactions. The type of shape-forming procedure used (fiber-extrusion firom solution or melt, spincoating, etc.) engenders constraints on what is considered useful polymer rheology. Especially in the case of fiber-drawing or -extrusion the precursor should exhibit thixotropic or non-Newtonian viscoelastic behavior. The viscosity should be sufficiently high at zero shear such that once formed, the material will retain its new shape. 

On the other hand, in a non-Newtonian fluid, the viscosity depends on the shear rate. Besides showing very high non-Newtonian viscosities, polymers exhibit a complex viscoelastic flow behavior, that is, their flow exhibits memory , as it includes an elastic component in addition to the purely viscous flow. Rheological properties are those that define the flow behavior, such as the viscosity and the melt elasticity, and they determine how easy or difficult is to process these materials, as well as the performance of the polymer in some applications. The rheology of the polymers and its effect on the processing of these materials are studied in Chapters 22 and 23. 

Low molecular weight PS was mixed vdth poly(methyl phenyl siloxane), PMPS, to form an immiscible blend with an upper critical solution temperature (UCST) [199]. The viscoelastic properties were studied by dynamic and steady-state shearing the neat polymers showed Newtonian behavior. Within the miscible region the blend viscosity followed the Mertsch and Wolf equation, Eq. (2.35), but with the parameter /fit = calculated from Bondi s tables. The phase separation created a rheolog-ically complex behavior. 

Rheological models have also been developed to describe fluid behavior over the shear rate range which include Newtonian behavior at low and high shear rates. The Carreau Model Pi has been found to fit polymer data satisfactorily. Equation 2 is the Carreau Model A. In Equation 2, i is the Newtonian viscosity in the low shear region, x is the Newtonian viscosity in the high shear regions, and i is the shear rate. The parameter n is the power-law exponent and Tj- is a characteristic time constant. All parameters are determined by fitting experimental data. 

Solution Rheology. Solutions of polyacrylamides tend to behave as pseudoplastic fluids in viscometric flows. Dilute solutions are Newtonian (viscosity is independent of shear rate) at low shear rates and transition to pseudoplastic, shear thinning behavior above a critical value of the shear rate. This critical shear rate decreases with the polymer molecular weight, polymer concentration, and the thermodynamic quality of the solvent. A second Newtonian plateau at high shear rates is not readily seen, probably because of mechanical degradation of the chains... 

The flow properties of colloidal dispersions are of great importance in handling dispersions in various practical situations. Another important aspect is that the viscosity and other rheological parameters can be used to determine the size, shape and interaction potentials of the dispersed particles. For most simple liquids, the viscosity is independent of the shear force. In this case, the liquid is then called Newtonian. For more complicated systems, when the liquid contains colloidal particles and/or polymers, the (apparent) viscosity depends on the shear force, except at very low concentrations of dispersed material. The fluid is then designated non-Newtonian. In this section, only systems containing particles are discussed, without the complicating influence of macromolecules. 

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