The Rouse model is a well-established model for non-entangled polymer chains and also
serves as a fundamental model for entangled chains. The dynamic behaviour of this model
under strain-controlled conditions has been fully analysed in the literature. However,
despite the importance of the Rouse model, no analysis has been made so far of the
orientational anisotropy of the Rouse eigenmodes during the stress-controlled, creep and
recovery processes.
For completeness of the analysis of the model, the Rouse equation of motion is
solved to calculate this anisotropy for monodisperse chains and their binary blends
during the creep/recovery processes. The calculation is simple and straightforward,
but the result is intriguing in the sense that each Rouse eigenmode during these
processes has a distribution in the retardation times. This behaviour, reflecting the
interplay/correlation among the Rouse eigenmodes of different orders (and for
different chains in the blends) under the constant stress condition, is quite different
from the behaviour under rate-controlled flow (where each eigenmode exhibits
retardation/relaxation associated with a single characteristic time). Furthermore, the
calculation indicates that the Rouse chains exhibit affine deformation on sudden
imposition/removal of the stress and the magnitude of this deformation is inversely
proportional to the number of bond vectors per chain. In relation to these results, a
difference between the creep and relaxation properties is also discussed for chains
obeying multiple relaxation mechanisms (Rouse and reptation mechanisms).