Collapse of polymer brushes grafted onto planar ... - Wageningen UR

COIL-SIZE OSCILLATORY PACKING IN POLYMER SOLUTIONS NEAR A SURFACE
J. van der Gucht, N.A.M. Besseling, J. van Male, M.A. Cohen Stuart
Laboratory of Physical Chemistry and Colloid Science, Wageningen University,
Dreijenplein 6, 6703 HB Wageningen, The Netherlands
email: jasper@fenk.wau.nl
ABSTRACT
Depletion of polymers at a surface has been studied extensively before. Theories predict that the segment
concentration profile decreases monotonously with decreasing distance to the surface 1,2) . Furthermore it is
generally accepted that the only relevant lengthscale above the overlap concentration is the “blob size”. The
coil size is believed to be irrelevant in this regime 1,2) . We have performed detailed calculations of the density
profile of nonadsorbing polymers near a surface, using the theory developed by Scheutjens and Fleer 1) . Our
results indicate that there is a damped oscillatory contribution to the density profile. Both the period of the
oscillations and the decay length are proportional to the size of the individual coils and are independent of the
polymer concentration, also above the overlap concentration. This indicates that the size of the individual
coils is still a relevant lengthscale above the overlap concentration. The oscillations are associated with a
liquid-like layering of polymer coils near the surface. In dilute solutions no oscillations are observed, because
the decay length of the oscillations is smaller than the depletion correlation length. This is similar to the
behaviour of simple fluids, where the Fisher-Widom line marks the transition from monotonic to oscillatory
decay of the density correlation function 3) . The Fisher-Widom line in polymer solutions is proportional to the
overlap concentration. On the oscillatory side of the Fisher-Widom line the interaction energy between two
plates immersed in a solution of nonadsorbing polymers is an oscillatory function of the separation distance.
The size of the oscillations is too small to be detected experimentally, but the effect is expected to be
stronger in for example branched polymer solutions. As indicated by Evans et al. the oscillations might be of
importance in, for example, wetting phenomena 4) .
References
1. Fleer, G.J., Cohen Stuart, M.A., Scheutjens, J.M.H.M.; Cosgrove, T., Vincent, B. Polymers at Interfaces.
Chapman and Hall: London, 1993.
2. De Gennes, P.G., Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, London.
3. Fisher, M.E., and Widom, B., 1969. J. Chem. Phys. 50, 3756.
4. Evans, R., Leote de Carvalho, R.J.F., Henderson, J.R. and Hoyle, D.C., 1993. J. Chem. Phys., 100, 591.