Orientation effects viscoelastic materials

If a viscoelastic material is forced to flow from a large reservoir through a circular tube, the diameter of the extrudate is found to be larger than the tube diameter. Many researchers [61-65] have argued that that causes the die swell. They developed the following three points of view for the cause of the die swell polymer chain orientation within the capillary caused by the high shear field recovery of the elastic deformation and viscose heat effects. The most important concept is the recovery of the elastic deformation imposed in the capillary. 

The basis of the majority of specific liquid crystal electrooptical effects is found in the reorientation of the director (the axis of preferred orientation of the molecules) in the macroscopic volume of the material under the influence of an externally applied field or the fiow of the liquid. Anisotropy of the electrical properties of the medium (of the dielectric susceptibility and the electrical conductivity) is the origin for reorientation, whereas the dynamics of the process also depend on the viscoelastic properties and the initial orientation of the director of the mesophase relative to the field. The optical properties of the medium, its local optical anisotropy, are changed as a result of this reorientation of the director (either occurring locally or throughout the whole of the sample) and underlies all the known electrooptical effects. 

Tube models have been used to predict this material function for linear, monodisperse polymers, and a so-called standard molecular theory [159] gives the prediction shovm in Fig. 10.17. This theory takes into account reptation, chain-end fluctuations, and thermal constraint release, which contribute to linear viscoelasticity, as well as the three sources of nonlinearity, namely orientation, retraction after chain stretch and convective constraint release, which is not very important in extensional flows. At strain rates less than the reciprocal of the disengagement (or reptation) time, molecules have time to maintain their equilibrium state, and the Trouton ratio is one, i.e., % = 3 7o (zone I in Fig. 10.17). For rates larger than this, but smaller than the reciprocal of the Rouse time, the tubes reach their maximum orientation, but there is no stretch, and CCR has little effect, with the result that the stress is predicted to be constant so that the viscosity decreases with the inverse of the strain rate, as shown in zone II of Fig. 10.17. When the strain rate becomes comparable to the inverse of the Rouse time, chain stretch occurs, leading to an increase in the viscosity until maximum stretch is obtained, and the viscosity becomes constant again. Deviations from this prediction are described in Section 10.10.1, and possible reasons for them are presented in Chapter 11. 

Since fibers consist primarily of oriented crystallites, it is unfair to classify them as heterophase. However, the generalizations of time-temperature superposition that work so well with amorphous polymers do not apply to fibers. Fibers do exhibit viscoelasticity qualitatively like the amorphous polymers. It comes as a surprise to some that J. C. Maxwell, who is best known for his work in electricity and magnetism, should have contributed to the mathematics of viscoelasticity. The story goes that while using a silk thread as the restoring element in a charge-measuring device. Maxwell noticed that the material was not perfectly elastic and exhibited time-dependent effects. He noticed that the material was not perfectly elastic and showed time effects. The model that bears his name was propounded to correlate the real behavior of a fiber. 

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