Viscoelastic constant

Failure can be considered as an actual rupture (stress-rupture) or an excessive creep deformation. Correlation of stress relaxation and creep data has been covered as well as a brief treatment of the equivalent elastic problem. The method of the equivalent elastic problem is of major assistance to designers of plastic products since, by knowing the elastic solution to a problem, the viscoelastic solution can be readily deduced by simply replacing elastic physical constants with viscoelastic constants. 

Nishinari, K. 1976. Longitudinal vibrations of high elastic gels as a method for determining viscoelastic constants. Jpn. J. Appl. Phys. 15 1263-1270. 

A major goal in the physics of polymer melts and concentrated solutions is to relate measurable viscoelastic constants, such as the zero shear viscosity, to molecular parameters, such as the dimensions of the polymer coil and the intermolecular friction constant. The results of investigations to this end on the viscosity were reviewed in 1955 (5). This review wiU be principaUy concerned with advances made since in both empirical correlation (Section 2) and theory of melt flow (Section 3). We shall avoid data confined to shear rates so high that the zero shear viscosity cannot be reliably obtained. The shear dq endent behavior would require an extensive review in itself. 

Equation 62 is very attractive for the study of adhesion between polymers and sohd surfaces, since it allows for the determination of the viscoelastic constants of the adhesive in the immediate vicinity of the contact. Unfortunately, the QCM does not work well with semi-infinite media when the viscosity, r], is larger than about 50 cP. The bandwidth in this case is too large. Most polymers exceed this limit. If, however, the contact area can be confined to a small spot in the center of the crystal the measurement becomes feasible [74]. Such a small contact area can, for instance, be estabhshed with a JKR tester [75]. The area of contact can be determined by optical microscopy. Of course, this kind of sample is laterally heterogeneous and the apphcabihty of simple models may be questioned. Experiment shows that the finite contact area can be reasonably well accounted for by modifying as ... 

In principle, viscoelastic constants can be extracted from the experimental data by fitting Eq. 70 (or any of the more complicated equations below) to the data. For a small film fhickness, certain approximations hold which make the derivation more transparent. If kfdf is much less than unity, the tangent in Eq. 70 can be Taylor-expanded to third order as tan(x) l/3x, resulting... 

Akagi T (1980) Study to specify viscoelastic constants. Report on Scientific Research Funds of Ministry of Education (in Japanese)... 

Certain viscoelastic constants can also be obtained by integration over the spectra. Thus for an uncross-linked polymer (viscoelastic liquid), by setting co = 0 in equation 25, we obtain for the steady-flow viscosity... 

The relations among the various types of linear viscoelastic functions are summarized in Fig. 3-9 with equation numbers identified. The equation numbers for calculation of viscoelastic constants are identified in Table 3-1. 

The practical significance of the terminal relaxation time r in several qualitative aspects of behavior has already been mentioned in connection with equation 7 for polymers of low molecular weight. The same considerations apply to polymers of high molecular weight, where t (or Ta, in the framework of the tube model) and the other two viscoelastic constants ijo and Jg which characterize the terminal zone are even more important in the processing and use of polymeric materials. Rough estimates of these quantities can sometimes be made from the equations in Section C3 above for practical purposes. 

The suppression of director fluctuations near the nematic-smectic A (N-A) transition because of divergence in the twist and bend elastic constants and the twist viscosity 7e [6.6] are now examined. Above the phase transition at T/vaj there are cybotactic smectic A clusters in the nematic phase, whose dimension is measured by a coherence length In fact, it is the coupling between the nematic director and the smectic order parameter that causes the viscoelastic constants to approach infinity at TnA Hence, A22J A33 oc while 7e oc Suppose there is interest in a frequency far below the high-frequency cutoffs such that A >> 1. In this limit,... 

The viscoelastic constants of the lyotropic nematic polymer poly-y-benzyl glutamate was measured by Taratuta et al. [70]. Santos and Durand [71 ] measured order and mi-... 

The characteristic frequency dependence is a direct result of the gapless Goldstone mode type nature of the director fluctuations. The constants C[ and C2 depend on the magnitude of the nematic order parameter, the viscoelastic constants, the molecular geometry of the spin positions and the orientation of the director with respect to the external magnetic field. 

E. Miraldi, L. Trossi, P. T. Valabrega, Generalized method for nematic liquid crystal viscoelastic constant determination by quasielastic light scattering, II Nuovo Cimento 1980, 60, 165-186. 

The complex viscoelastic constants of the polymer matrix are given by the following formula ... 

Fig. 1.6 Reduction of the viscoelastic constant ratio Xi/k of positive dielectric commercial LC mixtures developed from 1970 to the 1990s by Roche, BDH, and Merck. RO = Roche E-7 = BDH and ZLI = Merck yi = rotational viscosity,...

Zang, Y. H., Muller, R., Froelich, D. Influence of molecular weight distribution on viscoelastic constants of polymer melts in the terminal zone. New blending law and comparison with experimental data. Polymer (1987) 28, pp. 1577-1582... 

Contemporary applications of liquid crystals [1,2] exploit the unique properties of these materials arising from their anisotropic response to external fields and forces. For example, the anisotropy in the dielectric properties makes it possible to construct electro-optical displays, and the characteristic response time of such devices is determined by the anisotropic viscoelastic properties of the liquid crystal [3]. In turn, these viscoelastic properties are related to various kinds of flows and deformations of the material in question. The exact number and nature of viscoelastic constants required to characterise fully the properties of the phase are determined by careful consideration of both static and dynamic behaviour [4]. The specific focus of this Datareview is the description of experimental techniques for measuring the various types of viscosity coefficients allowed in nmiatic phases. 

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