Molecular basis stress relaxation

The isothermal curves of mechanical properties in Chap. 3 are actually master curves constructed on the basis of the principles described here. Note that the manipulations are formally similar to the superpositioning of isotherms for crystallization in Fig. 4.8b, except that the objective here is to connect rather than superimpose the segments. Figure 4.17 shows a set of stress relaxation moduli measured on polystyrene of molecular weight 1.83 X 10 . These moduli were measured over a relatively narrow range of readily accessible times and over the range of temperatures shown in Fig. 4.17. We shall leave as an assignment the construction of a master curve from these data (Problem 10). 

Trombitas, K., Wu, Y., and McNabb, M. (2003). Molecular basis of passive stress relaxation in human soleus fibers Assessment of role of immunoglobulin domain unfolding. Biophys. J. 85, 3142-3153. 

In this section we are going to examine such viscoelastic properties in some detail and we will start by examining in turn three important mechanical methods of measurement creep, stress relaxation, and dynamic mechanical analysis. This will lead us to interesting things like time-temperature equivalence and a discussion of the molecular basis of what we have referred to as relaxation behavior. 

Family of plant proteins essential for acid-induced cell wall loosening. See Cosgrove, D.J., Relaxation in a high-stress enviromnent the molecular basis of extensible cell walls and cell enlargement, Plant Cell 9, 1031-1041, 1997. 

McGrory and Tuminello [22 ] examined the possibility of using stress relaxation data in place of the storage modulus to determine MWD. Thus, in place of the plot of [G (molecular weight distribution, a plot of [G(t)/ G f... 

Dynamic mechanical techniques are important in the characterization of the rheological properties of polymers. In dynamic mechanical analysis, a small-amplitude oscillatory strain is applied to a sample, and the resulting dynamic stress is measured as a function of time. The dynamic mechanical technique allows the simultaneous measurement of both the elastic and the viscous components of the stress response. Typically, the temperature and deformation frequencies are changed in order to determine the mechanical relaxation spectrum of the system. The molecular basis of changes in the dynamic mechanical properties can be investigated using dynamic IR dichroism. 

On this basis it is possible to understand what will happen if an oriented crystalline film is heated to near Tm and shrinkage is prevented by external stresses. The stress in the amorphous regions will increase but, as shrinkage is prevented, they will partly relax owing to molecular slippage. Simultaneously the strength of the crystalline material will increase. 

It should be stressed that the wave-packet picture of photophysical relaxation and photochemical reaction dynamics described in this chapter is substantially different from the traditional concepts in this area. In contrast to the established picture of radiationless transitions in terms of interacting tiers of zero-order molecular eigenstates, the dynamics is rationalized in terms of local properties of PE surfaces such as slopes, barriers and surface intersections, a view which now becomes widely accepted in photochemistry. This picture is firmly based on ah initio electronic-structure theory, and the molecular relaxation d3mamics is described on the basis of quantum mechanics, replacing previously prevaUing kinetic models of electronic decay processes. Such a more detailed and rigorous description of elementary photochemical processes appears timely in view of the rich and specific information on ultrafast chemical processes which is provided by modern time-resolved spectroscopy. " ... 

All calculations have been performed taking fixed positions for the atoms in the structures, so that no relaxations are considered. The Siesta method, however, allows relaxations and molecular dynamics by calculating the forces on the atoms and the stress tensor from the Hellman-Feynman theorem with Pulay corrections The computational cost would be much higher than in the frozen geometry calculations, but the calculations for complex magnetic systems are cheaper than with other ab initio methods (KKR, FLAPW, LMTO). Apart from the use of pseudopotentials and a minimal basis set, the supercell construction does not require the inclusion of empty spheres as it is the case for the LMTO-ASA method. Besides, going from 2D to 3D supported clusters of similar size does not increase the computational cost like in the KKR-GF method where Green-Functions have to be computed. 

A molecular theory of the relaxation properties of filled elastomers has been developed by Sato 138) on the basis of the statistical concept of rubber-like elasticity. He has derived expressions for the estimation of Young s modulus stresses and mechanical losses of filled polymers. 

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