Chemistry Reference
In-Depth Information
Figure 3.8 Dynamic viscosity versus frequency for (*) 2.5%
þ
2.5% (5b
þ
5c)
.
PVP, (A)
2.5% 5c
.
PVP, (
4
)5%5c
.
PVP, (
þ
) 2.5% 5b
.
PVP, and (
)5%5b
.
PVP. All networks are
10% by total weight in DMSO at 20 8C. Data taken from Loveless et al. (2005).
Similar effects are observed in the G
0
and for different mixtures of networks (includ-
ing those with three different cross-linkers). In all cases, the frequency onsets and
magnitudes of the transitions are anticipated by the behavior of networks with a
single cross-linker.
The dominance of the supramolecular dissociation kinetics, however, is only
evident above a certain concentration of cross-linker. Relaxation mechanisms
below the percolation threshold involve the diffusion of discrete aggregates, and
they are therefore independent of the kinetics of the cross-linking interactions. The
chemistry of the supramolecular interactions can be exploited in combination
with details of the percolation behavior to provide a useful mechanism for engineer-
ing dramatic and reversible, chemically induced changes in mechanical properties,
because slight shifts in binding equilibria are enough to move from one regime to
the other. In the case of pincer-based networks, for example, gel -sol transitions
are induced by a range of chemical signals, including competing ligands and acids,
some of which can be coupled to thermal processes (Loveless et al. 2007).
Because the low viscosity state is independent of the cross-linking kinetics and the
high viscosity state depends directly on those kinetics, the magnitude of the
change in properties is determined by the kinetics, and several orders of magnitude
(or more) changes in viscosity are readily demonstrated. Gel -gel transitions can be
similarly engineered by a strategy using mixtures of cross-linkers with different
dissociation rates.
The combination of chemically diverse cross-linkers in a single supramolecular
material is especially promising in this regard. Pollino and Weck (2005) have
explored combinations of interactions (hydrogen bonding, coordination, pseudoro-
taxanes) for side chain functionalization and cross-linking. The potential benefit of
this strategy is in establishing a system that is responsive to a variety of external
stimuli independently of each other. SPs containing two sets of cross-links defined
