Anisotropy of segments

The detection of stress distributions in polymer materials underlines the unique contrast features available through NMR imaging. Stress distributions were first mapped by T2-parameter images and subsequent calibration to stress images [153]. T2 depends on strain, because strain effects the timescale and anisotropy of segmental dynamics in the intercross-link chains (cf. Section... 

The anisotropy of segmental motion exhibited in Fig. 19 may arise, as noted above, either from the intramolecular or from the intermoleoilar ccmstraint to the rotational motion. The anisotropy d orioitational condadon decay was indeed noted already by Weber and Helfand [47] in their Brownian dynamics simulation of polyethylene of infinite chain length. Their orioitational time-correlation function of the chord vector ( = 0°) decayed much more slowly than those of either the bisector vector ( = 0°, = 0°) or the out-of-plane vector ( = 0°, = 90°). What they modeled was a phantom chain having no... 

Typically, a segment of a polymer chain will possess an anisotropic polarizability with reference to the local chain axis. So a polymeric fiuid with anisotropy of segment orientations will itself be optically anisotropic, causing (in the nonabsorbing case) polarized light passing through the fiuid to suffer a rotation of its plane of polarization. In other words, the tensorial index of refraction will be nontrivial. It is usually assumed to be proportional to the orientational tensor defined as... 

The optical properties of random copolymers of PM-16 and PCMA [78], which exhibit strong nonlinearity of the dependence of the optical shear coefficient [n]/[Ti] and the calculated values of the optical anisotropy of segment ttj - 02 of the copolymer on its molar composition (Figs. 3.9 and 3.10), are direct experimental confirmation of the intramolecular interactions responsible for the existence of liquid-crystalline ordering on the molecular level. 

The effect of the molecular weight on dependence t (c) for solutions of PBA and PPTA is shown in Fig. 9.21. The value of c, corresponding to the viscosity maximum T, is inversely proportional to the degree of geometric anisotropy of segment (x), and for maximally rigid macromolecules, to the value of M [13]. This made it possible to estimate the M of the first samples of PBA with the critical transition concentration, using the relation v - 10/x. 

Anisotropy of Segments and Monomer Units of Polymer Molecules... 

ANISOTROPY OF SEGMENTS AND MONOMER UNITS OF POLYMER MOLECULES... 

The observed apparent decrease in activation volume (increase in slope) with strain level in the Eyring plots can be explained by a variant of the previous model, proposed recently by Buckley [12], which recognises intrinsic anisotropy of segment diffusion because of local correlation of molecular segments. Arm (B) of the model must then be replaced by an aggregate of anisotropic flow units. The quasi-linear relation between the viscous part of the rate of deformation D and bond stretch deviatoric stress s of the previous model for PET [6], D" = Os , where 0 is a scalar function of the stress invariants then takes a new form for each unit, D = 0s , where 0 becomes a fourth rank fluidity tensor. 

The intercept, 1/Po, is called the anisotropy of the molecule and is an indication of the nonrotational depolarization of the molecule. This intrinsic depolarization is due to the segmental motion of the fluorophores within the molecule the depolarization due to energy transfer and the angular difference in transition dipole moments of the absorbing and emitting states. 

The overall tumbling of a protein molecule in solution is the dominant source of NH-bond reorientations with respect to the laboratory frame, and hence is the major contribution to 15N relaxation. Adequate treatment of this motion and its separation from the local motion is therefore critical for accurate analysis of protein dynamics in solution [46]. This task is not trivial because (i) the overall and internal dynamics could be coupled (e. g. in the presence of significant segmental motion), and (ii) the anisotropy of the overall rotational diffusion, reflecting the shape of the molecule, which in general case deviates from a perfect sphere, significantly complicates the analysis. Here we assume that the overall and local motions are independent of each other, and thus we will focus on the effect of the rotational overall anisotropy. 

Anisotropy of phosphorescence then becomes a powerful tool to study the overall rotation of large biological macromolecules and to study segmental motions which occur in these structures. 

In addition, the optical anisotropy of the statistical segment Aa calculated [50, 51] from the stress-optical coefficient C,... 

Steady-state measurements of the fluorescence anisotropy of fluorescein derivatives form the basis of a sensitive analytical technique also used to detect and quantitate proteins [36], steroids [37-39], therapeutic drugs, and narcotics [40-42], In a different approach, the anisotropy of the fluorescein conjugate is measured as a function of the medium viscosity to determine the segmental mobility of the chains that cover the binding site [43-45],... 

Fig. 1 Anisotropy of the principal elastic interactions in a C-C chain. (1) Bond stretching (2) Bond angle bending (3) Bond rotation (4) Inter-segmental attraction...

Abstract The discussion of relaxation and diffusion of macromolecules in very concentrated solutions and melts of polymers showed that the basic equations of macromolecular dynamics reflect the linear behaviour of a macromolecule among the other macromolecules, so that one can proceed further. Considering the non-linear effects of viscoelasticity, one have to take into account the local anisotropy of mobility of every particle of the chains, introduced in the basic dynamic equations of a macromolecule in Chapter 3, and induced anisotropy of the surrounding, which will be introduced in this chapter. In the spirit of mesoscopic theory we assume that the anisotropy is connected with the averaged orientation of segments of macromolecules, so that the equation of dynamics of the macromolecule retains its form. Eventually, the non-linear relaxation equations for two sets of internal variables are formulated. The first set of variables describes the form of the macromolecular coil - the conformational variables, the second one describes the internal stresses connected mainly with the orientation of segments. 

Abstract Macromolecular coils are deformed in flow, while optically anisotropic parts (and segments) of the macromolecules are oriented by flow, so that polymers and their solutions become optically anisotropic. This is true for a macromolecule whether it is in a viscous liquid or is surrounded by other chains. The optical anisotropy of a system appears to be directly connected with the mean orientation of segments and, thus, it provides the most direct observation of the relaxation of the segments, both in dilute and in concentrated solutions of polymers. The results of the theory for dilute solutions provide an instrument for the investigation of the structure and properties of a macromolecule. In application to very concentrated solutions, the optical anisotropy provides the important means for the investigation of slow relaxation processes. The evidence can be decisive for understanding the mechanism of the relaxation. 

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