Polymer Brushes at moderate and high grafting densities:
Effects of chain stretching and end-groups
Jens-Uwe Sommera , Gui-Li Hea,b and
Holger Merlitza,b
a Leibniz-Institute of Polymer Research Dresden, Hohe
Straße 6, D-01069 Dresden, Germany
b Department of Physics and ITPA, Xiamen University, Xiamen
361005, P.R. China
Polymer brushes obtained by end-grafting of linear chains are
promising systems for various applications ranging from colloid
stabilization over switchable surfaces to biocompatible surfaces.
Recent experimental techniques allow high grafting densities
implying strong stretching of the individual chains1. We have
investigated static and dynamic properties of polymer brushes at
moderate and high grafting densities using molecular dynamical
techniques.
Severe deviations from the predictions of
self-consistent field theories can be related to the finite
extensibility of chains which plays an important role for higher
grafting densities2. In accordance with previous simulations we
have shown that the distribution of chain ends is strongly peaked
at the brush surface and that the overall density profile
displays a steep decrease in the surface region. Thus,
conformational states of chains are strongly influenced by the
surface properties of the polymer brush. As a result small
modifications of individual chain properties lead to large
effects: Reducing the size of end-monomers leads to a collapse of
modified chains, while a corresponding inflation of end-monomers
gives rise to considerable stretching3. The figure shows a
snapshot of a polymer brush with inflated monomers. It can be
clearly observed that inflated end-monomers are "swimming" on top
of the brush (high stretching). A similar effect can be achieved
by a slight modification of the chain lengths (adding or removing
individual monomers). Our results indicate that dense brushes
with a stretching ratio larger then about 0.6 lead to a surface
dominated behavior which turns out to be highly selective with
respect to chain size and types of end-monomers.
Figure: Snapshot of a polymer brush containing chains with
inflated end-monomers. The end-monomers are two times larger
(diameter) as compared to the other monomers. The larger
end-monomers are "swimming" on top of the brush causing the
corresponding chains to be more stretched3
References
C. Devaux, F. Cousin, E. Beyou, and J.- P. Chapel: Low
swelling capacity of highly stretched polysterene brushes.
Macromolecules, 38, 4296-4300 (2005).
Gui-Li He, Holger Merlitz, Jens-Uwe Sommer, and Chen-Xu Wu:
Static and Dynamic Properties of Polymer Brushes at Moderate and
High Grafting Densities: A Molecular Dynamics Study,
Macromolecules 40, 6721 (2007).
Holger Merlitz, Gui-Li He, Chen-Xu Wu and J.-U. Sommer:
Surface instabilities of monodisperse and densely grafted polymer
brushes, Macromolecules in press (2008)
Severe deviations from the predictions of
self-consistent field theories can be related to the finite
extensibility of chains which plays an important role for higher
grafting densities2. In accordance with previous simulations we
have shown that the distribution of chain ends is strongly peaked
at the brush surface and that the overall density profile
displays a steep decrease in the surface region. Thus,
conformational states of chains are strongly influenced by the
surface properties of the polymer brush. As a result small
modifications of individual chain properties lead to large
effects: Reducing the size of end-monomers leads to a collapse of
modified chains, while a corresponding inflation of end-monomers
gives rise to considerable stretching3. The figure shows a
snapshot of a polymer brush with inflated monomers. It can be
clearly observed that inflated end-monomers are "swimming" on top
of the brush (high stretching). A similar effect can be achieved
by a slight modification of the chain lengths (adding or removing
individual monomers). Our results indicate that dense brushes
with a stretching ratio larger then about 0.6 lead to a surface
dominated behavior which turns out to be highly selective with
respect to chain size and types of end-monomers.
