Polymer rheology viscosity

Most properties of linear polymers are controlled by two different factors. The chemical constitution of tire monomers detennines tire interaction strengtli between tire chains, tire interactions of tire polymer witli host molecules or witli interfaces. The monomer stmcture also detennines tire possible local confonnations of tire polymer chain. This relationship between the molecular stmcture and any interaction witli surrounding molecules is similar to tliat found for low-molecular-weight compounds. The second important parameter tliat controls polymer properties is tire molecular weight. Contrary to tire situation for low-molecular-weight compounds, it plays a fimdamental role in polymer behaviour. It detennines tire slow-mode dynamics and tire viscosity of polymers in solutions and in tire melt. These properties are of utmost importance in polymer rheology and condition tlieir processability. The mechanical properties, solubility and miscibility of different polymers also depend on tlieir molecular weights. 

Viscosity measurement or rheology. Viscosity is an important property of many fluids, particularly consumer products. Pancake syrup that does not appear thick or a runny shampoo has little consumer appeal. Viscosity is also an important measurement for liquid polymers. It is a measure of polymer chain length and branching. On the other hand, viscosity does not have the sensitivity to detect small amounts of impurities in pure substance. 

Reduction in the apparent bulk viscosity due to a change in polymer rheology. It is well known that ultrasound can lead, via degradation, to a reduction in polymer solution viscosity. Although Fairbrother did not investigate whether degradation of the polymer, and subsequent reduction in R.M.M. and hence viscosity had occurred, it seems reasonable to assume that the polymer melt with an initial viscosity of 30000-100000 poise would certainly have resisted cavitation and thus degradation. 

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. 

An important difference between the PS-gas systems (Kwag et al., 1999) and the PDMS-C02 system (Gerhardt et al., 1997) is that the viscosity measurements of the PS-gas systems are conducted at temperatures within 75 °C of T of PS, whereas the PDMS-C02 measurements were performed nearly 200 °C above Tg of PDMS. The difference between these two thermal regimes leads to several differences in the observed rheological behavior. The viscosity reductions relative to the pure polymer are much greater for PS-gas systems than for PDMS-C02 systems at similar dissolved gas compositions, and the dependence of ac on temperature is much more pronounced for the PS-gas systems. These trends are consistent with the observations of Gerhardt et al. (1997, 1998) that the effect of dissolved gas on polymer melt viscosity occurs primarily through a free-volume mechanism. 

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. 

Zero-shear-rate viscosity Infinite-shear-rate viscosity High shear relative viscosity Intrinsic viscosity, polymer rheology,... 

In polymer rheology, the results for viscosity are usually expressed in terms of the intrinsic viscosity [rj] defined by the relation... 

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. 

These conclusions are not surprising. A novel point, however, is the important adhesional role of roughness of the solid in wear, in addition to its obvious role in plowing wear. Another point is the source of the local heating which leads to enhanced wear the work that is done in drawing the craze fibrils of the polymer. Extensional viscosity is an importnat rheological property that... 

The purpose of this paper is to describe structure-property correlations for relating polymer solution viscosity (n) to fundamental molecular (e.g. polymer MW, MWD) and rheological (e.g. shear rate, normal stresses) and porous media flow(5,10) parameters. After discussing these topics, a listing of potential EOR polymer candidates is given. 

PART IV RHEOLOGY OF ASSOCIATING POLYMERS, 650 Viscosity of Associating Polymers in the Dilute Regime, 652 Viscosity of Associating Polymers in the Semi-Dilute Regime, 656... 

Polymer melt viscosity is the most important rheological property, which is dominant in sintering. Increased viscosity lowers the mobility of the melt, resulting in the reduction of sintering rate, which increases the cycle time of the process. 

Figure 24 Shear-dependent viscosity of polyacrylamide solutions. (From KC Tam and C Tiu, Water-soluble polymers (rheological properties) in Polymeric Materials Encyclopedia, JC Salamone, ed. Boca Raton, FL CRC Press, 1996, p. 8655.)...

VII. RHEOLOGY VISCOSITY ENHANCEMENT AND GELATION A. Unmodified Polymers... 

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