Rheology features

The kinetics and mechanisms of the C —> G transition in a concentrated solution of PS-fr-PI in the PS-selective solvent di-n-butyl phthalate was studied [137,149]. An epitaxially transformation of the shear-oriented C phase to G, as previously established in melts [13,50,150], was observed. For shallow quenches into G, the transition proceeds directly by a nucleation and growth process. For deeper quenches, a metastable intermediate structure appears, with scattering and rheological features consistent with the hexag-onally perforated layer (PL) state. The C -> G transition follows the same pathways, and at approximately the same rates, even when the initial C phase is not shear-oriented. 

The research carried out on these very simple potential forms has revealed that they capture many of the physical/rheological features present in real systems. By focusing on such potentials, which have one or two adjustable parameters one can gain insights into generic effects that span many examples of chemical system. These potentials are also, on occasions (e.g. the potential given in Equation (1)) amenable to simple analytic treatments. 

One of the unique rheological features of emulsions is that the apparent viscosity of the emulsion can drop below the viscosity of the continuous phase when the concentration of the dispersed phase is low, normally below 0.1 in volume fraction (194). When solids are added to the emulsion, the apparent viscosity can decrease even further and the volume fraction of the dispersed phase at which minimum viscosity occurs increases with increasing solids content. Figure 30 shows the apparent viscosity of water-and-sand-in-bitumen, pwsh, variation with the solid-free water volume fraction, j8w, for two shear rate values. The experimental data were provided by Yan (private communication), where the system consists of 52 pm sand particles treated with hexadecyltri-methylammonium bromide (HAB) and water droplets of a Sauter mean diameter of 9 pm dispersed in bitumen at 60 °C. The sand particle volume fraction on water-free basis is j8s = 0.193. The range of the water droplet volume fraction, on a solid-free basis, jfrw is between 0 and 0.4. It can be observed that a minimum viscosity is present at a solid-free water droplet volume fraction of about 0.1. For a lower solid concentration, Ps = 0.113, the minimum apparent viscosity is found at /3W = 0.05... 

As already stated, the required compacting pressure P is determined mainly by the rheological features of the mass and execution of the pressure consumer, in detail, through the angle in the pressure head and mouthpiece, the friction conditions, the degree of transformation etc. 

Heat transfer between phases is a strong dissipative factor that in principle can mask the rheological features in bubble dynamics. Nevertheless, even with account for heat dissipation the theoretical dependencies of R(t) are sensitive to rheological properties of solution. Typical results of air bubble dynamics simulations at a sudden pressure change in the solution with T p Tl are presented on the Figure 7.2.3,... 

More detailed information about rheological features in gas bubble dynamics in polymeric solutions can be received within linear approach to flie same problem that is valid for small pressure variations in the liquid. The equation deseribing gas bubble dynamics in a liquid with rheological equation [7.2.10] follows from [7.2.41], [7.2.10] and has the form ... 

In Chapter 17, we discuss rheological properties, in particular viscosity and elasticity, of colloidal systems. These properties are at the basis of quality characteristics such as strength, pliancy, fluidity, texture, and other mechanical properties of various materials and products. In addition to bulk rheology, rheological features of interfaces are discussed. Interfacial rheological behavior is crucial for the existence of deformable dispersed particles in emulsions and foams. Emulsions and foams, notably their formation and stabilization, are considered in more detail in Chapter 18. 

To see how these new rheological features appear, we calculate the number figff of elastically effective junctions in a unit volume from (8.31), and the number of elastically effective chains Veff from (8.32). 

Among the various microbial polysaccharides, approximately 20 are of industrial signihcance. As previously mentioned, the commercial worth of a polysaccharide is usually based on its rheological features specihcally its capacity to modify the flow properties of solutions. A preferred record of commercially significant polysaccharides, the microorganisms employed for their synthesis, and their applications are mentioned in the Table 2.2. A number of the significant characteristics of individual microbial polysaccharides are shortly explained hereunder. 

Quintana et al. (95) prepared two types of PECT copolyesters with 25 and 30% of CHDM units and containing small amounts of pen-taerithrytol branching agent. Then, they studied the rheological features and flow-induced crystallization of PECT copolymers by capillary extrusion experiments at 180 °C. They observed that copolymers with the lower content in CHDM were able to crystallize in these conditions whereas linear or branched PECT with 30 %-mole of CHDM units did not crystallize. 

As a conclusion to the aforementioned observations, it can be asserted that the rheological features of complex partially miscible polymer blends are only partially elucidated in terms of their thermodynamics, based on the behavior of immiscible blends and if their phase state is well known. 

S.I. Song, B.C. Kim, Characteristic rheological features of PVA solutions in water-containing solvents with different hydration states. Polymer, 45 2381-2386, 2004. 

We have carried out standard rheometric tests as done many times in the literature for entangled polymer solutions. These experiments include startup shear, large amplitude oscillatory shear (LAOS) and large step strain. In terms of the rheological features, we observed the same as others. For example, there is a stress overshoot in startup shear in the stress plateau region the apparent G can drop below G" at frequencies of the elastic plateau and amplitudes around and above 100% and relaxation modulus decreases in time after large step strains. 

Macroscopically, all rheological features of a material are described by the relationship between the stress and strain tensors. This relationship, referred to as the constitutive relationship, may differ from material to material because of a possible difference(s) in the molecular/stmctural origin of the stress in various materials. The origin of the stress is essential for understanding the macroscopic constitutive relationship(s) as well as for the rheological characterization of materials, which is the aim of this chapter. For some materials such as homoploymer systems and blends, the origin(s) of the stress is summarized below. 

One of the most prominent rheological features of entangled polymer liquids is the nonlinear (non-Newtonian) behavior... 

Despite the above uncertainties, the basic rheological features of entangled linear polymers appear to be captured by the current tube modd incorporating the mechanisms of reptation, CLF, thermal/convective CR, and the chain stretch with/with-out the interchain pressure effect. This model may serve as a basis for the rheological characterization of those polymers. 

For homopolymer chains having various stmctures, the rheological features are characterized by linear and nonlinear quantities summarized in the previous sections. The relationship between the chain stmcture and rheological featmes is understood to a considerable depth (though not perfectly) with the aid of the cmrent molecular model(s), as explained in those sections. On the basis of this relationship, the rheological properties may be utilized to estimate the molecular parameters of bulk polymers such as the molecular weight... 

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