1.Introduction

Polymers composed of long molecular chains have unique viscoelastic properties, which combine the characteristics of elastic solids and Newtonain fluids[1].
Viscoelasticity makes the material s response to stress-strain behavior time dependent and their deformation partially reversible. Polymer melt considers as a non-Newtonian fluid. This behavior can be attributed to its molecular structure. Polymers consist of long molecules that entangle with each other, forming several flexible, reversible "joints". These enable different conformations of the molecules by a rotation along the backbone and cause the elastic behavior of polymer melts. The chains can also move with respect to each other by a crawling kind of movement called reptation. These rotation and reptation occurring above the glass transition temperature of the polymer are called Brownian motions, and they tend to return the molecules towards the equilibrium, i.e., to the energetically most preferable state, after being oriented by applying deforming stress. This will not however, occur immediately after removing the stress within a certain relaxation time, dependent on the molecular characteristics of the polymer[2].
In the dynamic mechanical thermal analysis(DMTA), the mechanical response of a viscoelastic material is studied as a function of temperature and time, while it is subjected to a sinusoidal strain[3]. For viscoelastic materials, DMTA and rheological testscan be used to:
1- Determine the viscoelastic parameters (G*,G ,G", tan ?, ?*,?o) as a function of temperature, frequency or time. G* is