Fold-back

Flow behaviour of polymer melts is still difficult to predict in detail. Here, we only mention two aspects. The viscosity of a polymer melt decreases with increasing shear rate. This phenomenon is called shear thinning [48]. Another particularity of the flow of non-Newtonian liquids is the appearance of stress nonnal to the shear direction [48]. This type of stress is responsible for the expansion of a polymer melt at the exit of a tube that it was forced tlirough. Shear thinning and nonnal stress are both due to the change of the chain confonnation under large shear. On the one hand, the compressed coil cross section leads to a smaller viscosity. On the other hand, when the stress is released, as for example at the exit of a tube, the coils fold back to their isotropic confonnation and, thus, give rise to the lateral expansion of the melt. 

The biologiccJ function of a protein or peptide is often intimately dependent upon the conformation(s) that the molecule can adopt. In contrast to most synthetic polymers where the individual molecules can adopt very different conformations, a protein usually exists in a single native state. These native states are found rmder conditions typically found in Uving cells (aqueous solvents near neutred pH at 20-40°C). Proteins can be unfolded (or denatured) using high-temperature, acidic or basic pH or certain non-aqueous solvents. However, this unfolding is often reversible cind so proteins can be folded back to their native structure in the laboratory. 

Polymer crystals form by the chain folding back and forth on itself, with crystal growth occurring by the deposition of successive layers of these folded chains at the crystal edge. The resulting crystal, therefore, takes on a platelike structure, the thickness of which corresponds to the distance between folds. 

The chain direction within the crystal is along the short dimension of the crystal, indicating that the molecule folds back and forth, fire hose fashion, with successive layers of folded molecules accounting for the lateral growth of the platelets. 

ColEl Regulation by RNA Hairpins. Rephcation of the E. coli plasmid ColEl is regulated by two short RNA molecules and a protein in a system that provides an example of the unique stmcmral elements accessible to RNA molecules. Multidimensional heteronuclear nmr spectroscopy has been used to characterize the complex formed between the two RNAs (25). Each of the RNA molecules fold back on the other to form a pair of hairpin... 

Current mode control has an inherent overcurrent protection. The highspeed current comparator provides pulse-to-pulse current limiting. This form of protection is a constant power form of overload protection (see Section 3.11). This form of protection folds back the current and voltage to maintain a constant power into the load. This may not be optimum for all products, especially where the typical failures slowly increase the failure current. Another form of overload protection can also be placed in the circuit. 

Andrew Keller (1925-1999) who in 1957 found that the polymer polyethylene, in unbranched form, could be crystallised from solution, and at once recognised that the length of the average polymer molecule was much greater than the observed crystal thickness. He concluded that the polymer chains must fold back upon themselves, and because others refused to accept this plain necessity, Keller unwittingly launched one of the most bitter battles in the history of materials science. This is further treated in Chapter 8, Section 8.4.2. 

With the first successful growth of a polymer single crystal in the 1950s it was found that the polymer chains are folded back and forth many times inside the crystal [161]. 

The single crystal of a polymer is a lamellar structure with a thin plateletlike form, and the chain runs perpendicular to the lamella. The crystal is thinner than the polymer chain length. The chain folds back and forth on the top and bottom surfaces. Since the fold costs extra energy, this folded chain crystal (FCC) should be metastable with respect to the thermodynamically more stable extended chain crystal (ECC) without folds. 

A helix-loop-helix motif is a DNA-binding motif, related to the leucine-zipper. A helix-loop-helix motif consists of a short a helix, connected by a loop to a second, longer a helix. The loop is flexible and allows one helix to fold back and pack against the other. The helix-loop-helix structure binds not only DNA but also the helix-loop-helix motif of a second helix-loop-helix protein forming either a homodimer or a heterodimer. 

RNA exists as a single strand, whereas DNA exists as a double-stranded helical molecule. However, given the proper complementary base sequence with opposite polarity, the single strand of RNA—as demonstrated in Figure 35-7—is capable of folding back on itself like a hairpin and thus acquiring double-stranded characteristics. 

Figure 1.34 (a) Reduced S/N ratio resulting from noise folding. If the Rf carrier frequency is placed outside the spectral width, then the noise lying beyond the carrier frequency can fold over, (b) Better S/N ratio is achieved by quadrature detection. The Rf carrier frequency in quadrature detection is placed in the center of the spectrum. Due to the reduced spectral width, noise cannot fold back on to the spectrum. 

Fig.1.6 Left the field of view has been chosen sufficiently large to coverthe whole object a correct image is obtained. Right the field of view is only half as wide the top and bottom portions of the object are folded back into the opposite side of the image. In a real NMR image, the assignment of position becomes ambiguous.

Polycarbonates exhibit no crystalline structure when used in common manufacturing processes. At the molecular level, there is some evidence of localized ordering along a chain. It is believed that the polycarbonate repeat units can fold back onto the chain in a structure that resembles the letter Z. These individual units do not associate on a large enough scale to create a regular crystalline material. Under specific conditions, such as forcing the polymer to cool very slowly, we can create small crystalline domains. Since these conditions are not met in commercial processes, it is safe to say that the polycarbonates that we encounter are universally amorphous. 

Aliased signals Signals that fall outside the spectral window (i.e., those that fail to meet the Nyquist condition). Such signals still appear in the spectrum but at the wrong frequency because they become folded back into the spectrum and are characterised by being out of phase with respect to the other signals. 

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