A recently proposed self-consistent reptation model - alreadysuccessful at describing highly nonlinear shearing flows of manytypes using no adjustable parameters - is used here to interpretthe linear viscoelasticity of the same entangled polystyrenesolution. Using standard techniques, a relaxation spectrum isfound for the entangled solution in terms of a spectrum ofrelaxation times from both small-amplitude oscillatory shear flowand from stress relaxation following small step shear strain flow.The spectrum exhibits a plateau at moderate time constants, and adownturn at large time constants. The classical reptation picture,however, exhibits the opposite trend. Using the newly proposedmodel, we can switch on (or off) dynamics not included in theclassical Doi-Edwards model: chain-length fluctuations andconstraint release. We find that chain-length fluctuations areimportant to describe the plateau, and that incorporation ofconstraint release leads to the observed downturn. Constraintrelease has already been shown to be important in nonlinear flowsand for polydisperse systems, but is clearly shown here to benecessary to describe the linear viscoelasticity of monodispersesystems.
|Publication status||Published - 2000|