Stretched relaxation

K. J. Gaffney, Paul H. Davis, I. R. Piletic, N. E. Levinger, and M. D. Fayer, Hydrogen bond dissociation and reformation in methanol oligomers following hydroxyl stretch relaxation. J. Phys. Chem. A 106, 12012 12023 (2002). 

Tj stretch relaxation time of a pom-pom cross-bar t(s) relaxation time of a tube segment with arc co-ordinate s... 

Finally, it is worthy of remark that, though the comparison between the timescales leads to an almost perfect agreement between the predictions of the Allegra and Ganazzoli model and the collective and self-motion results, it is evident that clear differences appear when comparing the spectral shapes of the respective functions. The model delivers close to exponential decays for both correlators while experimentally one observes significantly stretched relaxation function (j0=O.5). 

Fig. 53. Computed decay of the dynamic scattering function for a rather stiff dumbbell according to Ref.215). The rotatory diffusion coefficient is 0, qL = 2, a = L/50, r = 0/2500. L = length of the dumbbell, a2 = mean square amplitude of the bond stretching, r = a2/4 D stretching relaxation time321 ...

Briefly returning to the Coulomb force theme of Section II, although not presented in Ref. 57, subsequent (unpublished) results have shown that Coulombic forces are dominant in the OH stretch relaxation. Figure 4 displays the results for the bend power spectrum for two different frequency ranges, including the contribution of Coulomb and non-Coulomb forces. From Fig. 4a, Coulomb forces clearly dominate over the full range of interest a similar behavior has also been found for the stretchings spectra (not shown).1 ... 

A more refined and realistic treatment takes account of multiple, partly superimposed relaxation processes (stretched relaxation) by including a distribution parameter P in the relaxation function, where 0 < p < 1 ... 

Figure 7. The coil-stretch relaxation time (t) as a function of solution viscosity.

The low temperature y relaxation has been examined by Buckley, Gray and McCrum in Rigidex 9 samples both uniaxially drawn and annealed and drawn at constant width to give bia.xial orientation. In each case the modulus parallel to the stretch relaxed more than that in the perpendicular direction. This was interpreted in terms of a series Takayanagi model as evidence for deformation of amorphous material in an inclined lamellar structure. The work by Stachurski and Ward already discussed did not throw any light on the nature of the y transition. 

Stretching/relaxing provides the energy stimulus that transforms into a chemical energy output for remodeling an arterial wall in order to better sustain the pulsing forces. 

Figure 9.21. Cellular mechanochemical transduction makes possible tissue restoration by elastic protein materials containing cell attachment sequences that function as temporary functional scaffoldings. The attached cells become stretched as the matrix is stretched. The forces and frequencies of the stretch/relaxation cycles provide the chemical signal to the nucleus to turn on the genes for production of an extracellular matrix sufficient to...

In an ideal or perfect elastomer the energy repeatedly invested in extension is repeatedly and completely recovered during relaxation. Ideality increases as the elastic force results from a decrease in entropy upon extension, because this occurs without stressing bonds to the breaking point. Elastin models and elastin itself in water provide examples of such entropic elastomers with about 90% of the elastic force being entropic, that is, the /e//ratio of Equation (4) is about 0.1. This is essential to human life expectancy, because the half-life of elastin in the mammalian elastic fiber is on the order of 70 years. This means that the elastic fibers of the aortic arch and thoracic aorta, where there is twice as much elastin as collagen, will have survived some billion demanding stretch-relaxation cycles by the start of the seventh decade of life. This represents an ultimate in ideal elasticity. 

Fig. 11.7c ), and the strain is recovered only in part during the successive relaxing step (curve b of Fig. 11.7c) (De Rosa et al. 2005 De Rosa and Auriemma 2007, 2011). However, after the first mechanical cycle presenting large hysteresis, the sample exhibits elastic recovery in the successive cyclic stretching-relaxation steps (curves c, d of Fig. 11.7c ) (De Rosa et al. 2005 De Rosa and Auriemma 2007).

Hatfaludy, S., J. Shansky, and H. H. Vandenburgh. 1989. Metabolic alterations induced in cultured skeletal muscle by stretch-relaxation activity. Am J Physiol 256(1 Pt 1) C175-81. 

Vandenbiugh, H. H., S. Hatfaludy, P. Karlisch, and J. Shansky. 1989. Skeletal muscle growth is stimulated by intermittent stretch-relaxation in tissue culture. Am J Physiol 256(3 Pt l) C674-82. 

Similar values of G and can be obtained by fits of linear viscoelastic data [49]. Once G and Zj have been assigned, the value of the stretch relaxation time can be obtained from Zj i, since both of these time constants are controlled by the friction coefficient f. Since Tj i depends on the number of entanglements Zj to the second power, and Zj depends on Z to a higher power, 3.4, the values of and z j must converge as Z decreases. Pattamaprom and Larson [36] assume that the two time constants become equal at the critical molecular weight for entanglement Mq, which is aroimd twice M. With this assumption, one readily finds that... 

In tube theories, strain hardening is produced by chain stretch, which is discussed in Section 11.3.2. For linear polymers in extensional flow, chain stretch requires that the strain rate exceed the inverse Rouse stretch relaxation time given by Eq. 11.1, i.e., / 5 k T) =... 

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