Chemistry Reference
In-Depth Information
In the case of dense, surface-grafted polymer brush thin films, cross-links are
introduced by the simple addition of solutions containing bis(Pd
II
-pincer) compounds
4a or 4b to grafted PVP brushes (Loveless et al. 2006). Because the association con-
stants for pyridine coordination are similar, the uptake of 4a and 4b from equimolar
solutions into the PVP brushes (at constant grafting density and molecular weight) is
effectively equivalent, producing samples with comparable structure (number and
placement of cross-links).
AFM studies on the thin (
50 nm) brush layers demonstrate that the dynamics of
supramolecular cross-links contribute to the friction of the soft brush surfaces
(Fig. 3.11). When the faster 4a cross-linker is added, both the absolute friction
values and the coefficient of friction (COF) drop to
30% of those of the
uncross-linked PVP control. When the slower cross-linker 4b is added, however,
the absolute friction value and the COF increase dramatically: both the COF
and the absolute friction values for PVP
.
4b are more than twice that of PVP
alone. The absolute lateral force measured is proportional to the normal force
applied; this is consistent with the model that the more the tip is pressed into the
brush layer the more force it takes to drag it laterally through the brushes.
The combined results demonstrate the complexity of the system. Cross-linking
must include kinetic contributions to the lateral resistance that are similar to those
observed in the networks, but a combination of structural and dynamic factors is
likely responsible for the significant but opposite effects from kinetically dissimilar
cross-links. Stimulus-responsive polymer brush layers hold great potential (Minko
et al. 2000; Motornov et al. 2003; Granville et al. 2004; Kaholek et al. 2004a,
Figure 3.11 Poly(4-vinylpyridine) brushes subsequently exposed to metallic cross-linkers.
The addition of 4a decreases the lateral friction, but the addition of 4b increases the lateral
friction.
