Chemistry Reference
In-Depth Information
both within Pd complexes 4a-b and the slower, more strongly coordinating Pt
complexes 4c-d (Yount et al. 2005a, 2005b). As with the linear SPs, the independent
control of kinetics is particularly significant. Cross-linkers 4a and 4b are effectively
identical structural components within the network, and so their similar thermodyn-
amics (K
eq
¼
30 M
21
for 1a
.
2a and 1b
.
2a) ensure that the extent and nature of
crosslinking is essentially the same in the two samples [or between Pt(II) pincer mol-
ecules 1c-d; K
eq
¼ 8
10
3
M
21
for 4c
.
2a and 4
10
3
M
21
for 1d
.
2a].
The addition of 2% (by functional group) 4b to a 100 mg mL
21
DMSO solution
of PVP gives rise to a clear, thick, deep yellow solution whose viscosity is
2000
times greater than that of PVP alone (33 vs. 0.016 Pa s). The viscosity does not
increase upon the addition of the same quantity of monomeric 1b, and the viscosity
of 4b
.
PVP reverts to that of a free-flowing solution with the addition of the stronger
ligand 4b, which competes the metal away from the PVP side groups. The increased
viscosity of 4b
.
PVP networks is therefore attributed to some combination of the two
broad mechanistic possibilities brought about by interchain cross-linking: the motion
of equilibrium structures that are effectively intact on the timescale of the viscous
response versus rearrangement within the transient network via the dissocation
dynamics of the individual cross-links.
A comparison with cross-linker 4a proves the underlying dynamics are controlled
by metal-ligand dissociation. Ligand exchange kinetics for 4a are substantially faster
than for 4b but the association thermodynamics are very similar, and the effect of those
kinetics is dramatic. At 5% cross-linker, the dynamic viscosity of 100 mg mL
21
4a
.
PVP is only 6.7 Pa s, a factor of 80 less than that of the isostructural network 4b
.
PVP. Although the association constants are not identical, the effect of the thermodyn-
amics would be to increase the viscosity of 4a
.
PVP relative to 4b
.
PVP, the opposite
direction of that observed. The kinetics dominate even the extent of cross-linking; 5%
4a
.
PVP is less viscous by a factor of 5 than is 2% 4b
.
PVP.
The frequency-dependent storage (G
0
) and loss moduli (G
00
) for multiple net-
works of either 4
.
PVP or the related 5
.
PVP are reduced to a single master plot
when scaled by the corresponding ligand exchange rates, which are measured on
model systems (data for 5
.
PVP are provided in Fig. 3.7). These scaled plots are
similar to linear free energy relationships, in which rate or equilibrium constants
have been replaced by material properties. The dynamics at the cross-links
determine the dynamic mechanical response of the materials, and subsequent exper-
iments show that the mechanism of ligand/cross-link exchange in the network is the
same mechanism observed in model systems: solvent-assisted exchange, in which a
solvent molecule of DMSO first displaces the bound pyridine; subsequently, a new
pyridine then displaces the DMSO. The solvent-assisted pathway requires that stress
relaxation (flow) occur while the crosslink is dissociated from the network.
Mechanical properties are determined by the dissociation of the cross-linkers from
the network, but it is the rate of dissociation rather than the fraction of time in
the dissociated state (equal in 4a
.
PVP and 4b
.
PVP) that governs the properties
(Yount et al. 2005a).
Experiments on SP networks formed from multiple types of cross-linkers show
that
the response to an applied stress occurs through sequential,
individual
