Compression of polyelectrolyte brushes in a salt-free theta solvent |
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Authors: | M W Matsen |
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Institution: | School of Mathematical and Physical Sciences, University of Reading, Whiteknights, Reading, UK. m.w.matsen@reading.ac.uk |
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Abstract: | This paper examines the normal force between two opposing polyelectrolyte brushes and the interpenetration of their chains
that is responsible for sliding friction. It focuses on the special case of semi-dilute brushes in a salt-free theta solvent,
for which Zhulina and Borisov (J. Chem. Phys. 107, 5952 (1997)) have derived analytical predictions using the classical strong-stretching theory (SST). Interestingly, SST
predicts that the brushes contract as they are compressed together maintaining a polymer-free gap, which provides an explanation
for the ultra-low frictional forces observed in experiment. We examine the degree to which the SST predictions are affected
by chain fluctuations by employing self-consistent field theory (SCFT). While the normal force is relatively unaffected, fluctuations
are found to have a strong impact on brush interpenetration. Even still, the contraction of the brushes does significantly
prolong the onset of interpenetration, implying that a sizeable normal force can be achieved before the sliding friction becomes
significant. |
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