Application of stress on materials increases the energy of the system. After removal of stress,\ud macromolecules comprising the material shift towards equilibrium to minimize the total\ud energy of the system. This process occurs through molecular rearrangements or “relaxation”\ud during which macromolecules attain conformations of a lower energetic state. The time,\ud however, that is required for these rearrangements can be short or long depending on the\ud interactions between the macromolecular species that consist the material. When\ud rearrangements occur faster than the time of observation (experimental timescale) then\ud molecular motion is observed (flow) and the material is regarded as viscous