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42996-AC7
Semidilute Polymer Solutions under Shear

Grzegorz Szamel, Colorado State University

Initially, our research interests were focused on the understanding of the

relative importance of the intra- and interchain contributions

to the steady state static structure factor of a semidilute polymer

solutions.

To investigate the role of intra- and interchain contributions to

the steady state structure factor we performed a series of Brownian

Dynamics simulations of a bead-spring polymer model (we used a model with

FENE bonds and Lennard-Jones non-bonded interactions that was previously

investigated by MacDowell et al., J. Chem. Phys. 113, 419 (2000)). We

found that the contributions to structure factor from intra- and

interchain correlations, which cancel each other almost completely for an

equilibrium semidilute solution, are modified in different ways by the

shear flow. Incomplete cancellation of these contributions leads to

anisotropic patterns that resemble those observed in light scattering

experiments on sheared semidilute solutions (Wu et al., Phys. Rev. Lett.

66, 2408 (1991)). For small wave vectors the structure factor change is

dominated by the interchain contribution. We also monitored the

distortion of the pair correlation function and showed that for small

distances it is dominated by the intrachain contribution. Finally, we

investigated non-linear shear viscosity and found that, like the

short-distance part of the distortion of the pair correlation function,

it is predominantly of intrachain origin.

Recently, we have been interested in the origin of the anisotropic

scattering patters. To this end we have investigated the single-chain

dynamics in a sheared semidilute polymer solution: we have focused on the

relaxation of the end-to-end vector correlation function, the Rouse modes

and the radius of gyration tensor correlation function. In equilibrium and

for small shear rates these quantities show double exponential relaxation.

With increasing shear rate they exhibit oscillatory relaxation, which

hints at the tumbling motion of the chain. We should note that

such a tumbling motion has been previously found in experimental

(Smith et al., Science 283, 1723 (1999) and simulational (Hur et al.,

J. Rheol. 44, 713 (2000)) studies of dilute polymer solutions. Depending

on the nature of the single-chain dynamics, we have characterized shear

rates as small, moderate and high. In the high shear rate regime, the

frequency of the oscillations of the end-to-end vector correlation

function shows a power law dependence on the shear rate. A real time

analysis of the instantaneous values of the radius of gyration tensor,

the end-to-end distance, and the normal stress sheds light on the

correlation between fluctuations of these quantities.

A detailed investigation of the wavevector dependent relaxation of the

collective density fluctuations in our sheared semidilute solution is

in progress. It will be also interesting to study the total stress

relaxation and its coupling to the collective density fluctuations. Also

in progress is a study of the chain length dependence of the anisotropic

scattering patterns.

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