Solid-like properties

Because the shear stress is always zero at the centerline in pipe flow and increases linearly with distance from the center toward the wall [Eq. (6-4)], there will be a finite distance from the center over which the stress is always less than the yield stress. In this region, the material has solid-like properties and does not yield but moves as a rigid plug. The radius of this plug (r0) is, from Eq. (6-4),... 

Figure 14 Schematic representation of the microphase separation of block copolymers. The left graph shows atomic-scale details of the phase separation at intermediate temperatures, and the right graph shows a lamellar phase formed by block copolymers at low temperatures. The block copolymers have solid-like properties normal to the lamellae, because of a well-defined periodicity. In the other two directions, the system is isotropic and has fluid-like characteristics. From reference 54.

Semisolids dosage forms, as a class, are plastic in behavior, i.e., they retain their shape until acted upon by an outside force, in which case they deform and the deformations are permanent. The common denominator to all semisolid systems which gives them their special rheological character is that they all have a permanent three-dimensional structure. This structure is sufficient when undisturbed to impart solid-like properties but which is easily broken down and realigned under some strain or applied force (4). The semisolid systems used pharmaceutically include... 

After whipping whippable emulsions obtain more solid-like properties. This means that ordinary viscometry measurements are not useful. 

The solid-like properties may be measured by non-destructive dynamic rheology analysis or by destructive methods using a Penetrometer, Jelly Tester, Instron instruments, or other types of texture analyzers. The latter methods are the most useful due to their simplicity and speed. Texture analysis of whippable emulsion must always be compared with the amount of air incorporated into the foam, which is known as percentage overrun and is calculated as follows ... 

Polymers are viscoelastic materials, meaning they exhibit both liquid-hke properties (visco) and solid-like properties (elastic). Whether a material behaves more as a viscous or more as an elastic material depends upon the temperature, the particular polymer and its prior treatment, polymer structure, and the time scale of deformation. The particular property demonstrated by a polymer under given conditions allows polymers to act as solid or viscous liquids, as plastics, elastomers, or fibers, and so on. 

The tangent of the phase angle (tan 5) offers a better indicator of structural integrity than either the G or G" measurements do individually. The phase angle (8) or its tangent (tan 8) are an indicator of the system s structural behavior or integrity because it indicates whether the system behaves predominantly as a solid, liquid, or viscoelastic material. The greater the tan 8, the more liquid-like the samples will behave, and conversely, low values indicate more solid-like properties. A tan 8 value of 1 is indicative of a viscoelastic material. 

In the b orientation in Fig. 10-28b, uniform shear tends to rotate the layers and change their spacing. Since the layer spacing is a solid-like property of the smectic phase, shearing in this orientation should produce a solid-like material response, at least for shearing stresses... 

Experience has shown structured fluids to be more difficult to manufacture, due to the complexity of their rheological profiles. In addition to elasticity, dilatancy, and rheopexy, certain structured fluid compositions may exhibit solid-like properties in the quiescent state and other flow anomalies under specific flow conditions. For emulsions and solid particulate dispersions, near the maximum packing volume fraction of the dispersed phase, for example, yield stresses may be excessive, severely limiting or prohibiting flow under gravity, demanding special consideration in nearly all unit operations. Such fluids pose problems in... 

Experimental data Experimentally, there are three concentration regions of emulsion flow (i) dilute for (() < 0.3, characterized by nearly Newtonian behavior semi-concentrated at 0.3 < (() < with mainly pseudoplastic character and concentrated at ( ) < (()< 1.0, showing solid-like properties with modulus and yield. 

Gels are solid materials that have flexible properties but do not actually flow in the same way that liquids do. They are made from a crosslinked bonding network of atoms, which actually contains a majority of liquid-like molecules interspersed by weight, but it still behaves as a solid. The crosslinked network within the gel gives it its solid-like properties, while the fluid component gives the gel its stickiness. 

The term false body has been introduced to describe the thixotropic behaviour of viscoplastic materials. Although the thixotropy is associated with the build-up of structure at rest and breakdown of structure under shear, viscoplastic materials do not lose their solid-like properties completely and can still exhibit a yield stress, though this is usually less than the original value of the virgin sample which is regained (if at all) only after a long recovery period. 

The transition temperature for adsorbed (presumably via multipoint attachment) poly(NIPAAM) molecules is lower than that in bulk solution, and the properties of the layer of collapsed macromolecules formed above the transition temperature depend strongly on the speed by which the temperature increases. At a low speed of temperature increase, the liquid-like polymer layer is formed, whereas at high speeds, the polymer layer has more solid-like properties (55). When cooling, the collapsed polymer molecules return to the initial loopy adsorbed conformation via transitional extended conformation. The relaxation process for the extended-to-loopy adsorbed conformational transition occurs slowly and depends on the temperature obeying an Arrhenius law. Kinetic constraints, it is proposed, play an important role in this transition (56). 

When the shift factors for the relaxation times are plotted as in Figure 13, an important type of behavior is seen. Here it is observed that the shift factors increase dramatically as the temperature decreases. In fact, if one extrapolates the behavior, there is a singularity point that is reached that is about 50 K below the nominal glass temperature determined from other test methods (such as dsc). This rapid increase of the material shift factor also corresponds (eq. 25) to a rapid increase in the relaxation times as the temperature is decreased. Hence, the glass temperature is approached because the mobility in the polymer becomes so slow that, in normal experimental time scales, the molecules cannot respond to the mechanical solicitation and the material exhibits nearly solid-like properties [imagine the glassy poly(methyl methacrylate) (PMMA) that makes... 

Network properties at different scales emerge from the above results. Some constraint points at a semi-local scale (say, cross-link junctions) remains in average fixed, which reflects a solid-like property at this scale. This results in long-time coherence in the local average anisotropy directions. 

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