**8. References**


<sup>\*</sup> Corresponding Author


limiting long wavelength approximation employed.

and John W. Hanneken

*Physics Department, University of Memphis, Memphis, USA* 

**7. Summary** 

macroscopic point of view.

**Author details** 

**8. References** 

Hermann.

Corresponding Author

 \*

B.N. Narahari Achar\*

fractional calculus leads to the propagation of viscoelastic waves and the details can be found in Mainardi[9]. The approach developed in the present work is based on lattice dynamics and hence is a dynamical model. It is microscopic in approach and hence macroscopic properties are described in the so called long wave limit. Further deformation properties which are 'static' in nature are described in the zero frequency limit. As has been shown above the microscopic approach leads to the macroscopic properties in the appropriate limit. It has been noted [26] that the presence of spatial inhomogeneity in polymeric materials gives rise to fractional differential operators in time in the relevant evolution equations, while temporal in-homogeneity leads to fractional differential operators in space. Since only fractional order operation in time has been considered in the present work, there is an implicit in-homogeneity in space which results in a tendency of the particles to cluster and move in a collective fashion. Such collective motion can be considered as elastic waves of very large wavelength, much larger than the scale of the in-homogeneity. This provides a sort of justification for the

A microscopic approach based in lattice dynamics, but from the fractional calculus point of view has been developed for the theory of viscoelasticity. The system considered is a linear chain of fractional oscillators subject to a sinusoidal forcing at one end. For the system starting from a quiescent state, the response to sinusoidal forcing consists of a transient and a steady state part. The decay of the transient is characterized by a distribution of relaxation times and the expression for the relaxation spectrum has been obtained. The steady state is essentially a attenuated sinusoidal wave, whose phase velocity and attenuation have been studied and an expression for the specific dissipation function has been obtained. The results obtained are consistent with the results obtained by Mainardi and others from a

[1] Gross B (1953) Mathematical Structure of the Theories of Viscoelasticity Paris:

[2] Bland D R (1960) The Linear Theory of Viscoelasticity Oxford: Pergamon. [3] Christensen R M Theory of Viscoelasticity New York: Academic Press.

	- [22] Erdelyi A (ed.) 1955 *Table of Integral Transforms* Vol . I (New York: McGraw Hill)
	- [23] Schneider W R and Wyss W 1989 Fractional diffusion and wave equations. *J. Math Phys*. 30 134-144

**Chapter 4** 

© 2012 Wang, licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use,

© 2012 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution,

distribution, and reproduction in any medium, provided the original work is properly cited.

and reproduction in any medium, provided the original work is properly cited.

**Die Swell of Complex Polymeric Systems** 

Die swell is a common phenomenon in polymer extrusion. When a viscoelastic fluid flows out of a die, the extrudate diameter is usually greater than the channel size. This is called die-swell, extrudate swell or the Barus effect. The degree of extrudate swell is usually expressed by the die-swell ratio (B) of extrudate diameter versus die diameter. A better understanding of such flow behavior will be beneficial for the optimization of processing parameters and the design of extrusion equipment, both of which affect product quality and

Innumerable valuable experimental data of die swell have been published, each focusing on different aspects affecting extrudate swell. There have been various interpretations of the extrusion swell of viscoelastic fluids from the macroscopic view of polymer rheology, such as a normal stress effect, an elastic energy effect, an entropy enlargement effect, an orientation effect and a memory effect. In fact, these interpretations are all related to each other[1,2]. It is generally believed that die swell is an important characteristic of the fluid elasticity during flow. The most common technique used to study rheological properties of polymer melts is capillary rheometry. In a capillary, polymer flows from the reservoir through a die and finally swells out of the exit. Under the action of extension, shear and compression, some molecular chains become disentangled, uncoiled or oriented in the convergent region resulting in an entry effect. During die flow, the resultant stress and strain cannot be relaxed completely. Simultaneously, some chains continue to be sheared and elongated during extrusion. On emerging from the die exit, the molecules are relaxed in elastic deformation by reentanglement and recoiling. The extrudate tends to contract in the flow direction and to grow in the normal direction, leading to extrudate swell[3]. As it does inside the capillary, the swelling evolves with time outside the capillary to reach a maximum at a certain extrusion distance. Graessley et al.[4] have reported that die swell occurs in two steps: (1) a very rapid swelling with a relatively large swell ratio very close to the die exit, which is known as running die swell; (2) a subsequent slow expansion to give

Additional information is available at the end of the chapter

Kejian Wang

http://dx.doi.org/10.5772/50137

**1. Introduction** 

production cost.

