Relaxation time, texture

There is a close relation between the proton spin lattice relaxation time (Tj) observed with pulsed NMR at room temperature (8, 9) and the microstructure of mesophase (8) transformed from the parent matrix of coal. That is, the longer the relaxation time is, the more sufficient the growth of mesophase from the matrix occurs, and shown in Table II. The parent materials, which give the fibrous-domain texture at the early stages of carbonization, have the longest relaxation time found so far, as described in the table. 

Whereas orientation and texture generated in flow take long periods of time (minutes) to relax, the stresses are found to relax in a matter of several seconds. In Figure 18 we have plotted shear stress versus time on cessation of flow. Here we see that the shear stress (it is plotted in Figure 18 as the reduced time dependent viscosity, n"/n ) relaxes faster with increasing y. However, by 7 = 10.0, it relaxes to zero in a time of less than 2 seconds. The normal stresses, which are plotted in figure 19 relax slower than the shear stresses which is what is observed for flexible chain polymers. The key point is that the time for the stresses to relax is much faster than would be expected based on a relaxation time determined from the flow curve or the time for relaxation of orientation. 

However Gleeson e.a. [60] note that molecular elasticity may play an indirect role, since below a threshold value of the Deborah number, bands do not form at all. For example, for a solution of PBG of molecular weight 186000 this critical shear rate was about 40 s , while for a solution of PBG of molecular weight of 86000 bands were formed at a shear rate of 0.25 s . For four solutions of widely varying viscosity and relaxation time, a critical value of De 0.01—0.04 was obtained. Thus molecular elastic effects during shear seem to be necessary to produce the texture that in turn drives band formation after shearing ceases. ... 

The baking process has been performed inside the NMR magnet and followed using the T2 relaxation time. The water mobility during storage of bread has been studied using the Ti and T2 relaxation curves. The texture parameters (elasticity and firmness) have correlated well with T2 curves of bread crumbs. 

The HPC lyotropic solutions are, regarding the ability to form band texture, among the most studied systems and thoroughly described in the literature [16, 18]. The formed periodic structures can be locked within the polymer after the solvent evaporatiOTi, if the evaporation time is shorter than the relaxation time t, and the topography can be tuned by control of the processing conditions [19]. In the case of the anisotropic aU-ceUulosic based composites this was attained due to the small thickness of the films (20-60 pm). 

The melt elasticity ratio is said to be important for the texture of the mixture and the shape of the immiscible domains. Furthermore, it is to be noted that the relaxation is time-dependent. A stable deformed droplet can only be obtained if the ratio of the characteristic relaxation time ratio of the dispersed and continuous phase is larger than one ... 

Series C compares LLDPE with different chemical structures, but transformed in the same processing conditions. Because of these changes in chemical structure, inducing for instance differences in the relaxation times, the quantitative levels of orientation are different from one polymer to another. Nevertheless, they all exhibit the same type of texture, with the same features as in series B for specimens cooled with a double-flux air ring (Fig. 15.28e) almost isotropic distribution of 6-axes, and c-axes preferentially in the film thickness. The amorphous chains, for their part, are isotropically located in the film plane (Fig. 15.28e). 

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