Simulations of fast shear flows of PS oligomers confirm monomeric friction reduction in fast elongational flows of monodisperse PS melts as indicated by rheooptical data

It is known that polystyrene melts behave anomalously in fast elongational flows insofar as the steady-state elongational viscosity keeps decreasing with increasing stretching rate ε, without showing the typical upturn at ετ R ≈ 1, with τ R the Rouse time. The authors have recently suggested that such an anomalous behavior might be due to a decrease of the monomeric friction coefficient brought about by alignment of the Kuhn segments of the polymer to the stretching direction. Here we perform a quantitative analysis of such a possibility by first determining from existing stress-optical data how such a reduction should correlate to the order parameter of the Kuhn segments and then by performing nonequilibrium molecular dynamics (NEMD) simulations over a sequence of styrene oligomers. We have used NEMD not only to obtain diffusion coefficients of those oligomers but also, for what seems to be the first time, friction coefficients. Indeed, it is well-known that the Einstein relationship linking friction to diffusion does not hold true far from equilibrium. The friction coefficients so obtained correlate to the order parameter of the monomers in much the same way as in the polymeric case, and by increasing the length (or mass) of the oligomer, they appear to approach a similar characteristic curve. © 2012 American Chemical Society.