Chemistry Reference
In-Depth Information
Figure 7.14 (a) Chemical structure of a phosphine-platinum mechanoresponsive MSP
prepared by Paulusse et al. (2006, 2007) and (b) schematic representation of gels prepared
by a similar polymer cross-linked with Ir
I
or Rh
I
salts.
different MSP gels show very different mechanical properties with the Ir
I
gels exhi-
biting a higher modulus than the Rh
I
gels. This is attributed to the different binding
kinetics of these two metal ions. The exchange rate of Ir
I
phosphinite complexes
(
10 days) is much slower than that of the Rh
I
phosphinite complexes (ΒΌ 26 min),
further illustrating the importance of kinetic considerations when designing
dynamic systems and again highlighting the tunability inherent in MSPs.
Multistimuli/multiresponsive gels (Beck and Rowan 2003; Weng et al. 2006)
were prepared by adding different metal ions to a ditopic monomer (Fig. 7.15, 24).
These materials form acetonitrile organogels that exhibit thermo-, chemo-, and
mechanoresponses. Optical microscopy studies suggest that the material is composed
of spherulitic particles, which aggregate to yield sample-spanning phases responsible
for gelation. When subjected to increasing shear stress, the gels exhibit a yield point
followed by a shear-thinning region. The study examined how different metal salts
[e.g., Zn(ClO
4
)
2
and Zn(ClO
4
)
2
with 2% La(ClO
4
)
3
] could be used to tailor the
nature of the self-assembly and macroscopic properties of the gel, probing the
effect on the gel of metal salts with different coordination abilities. Although Zn
II
ions bind two Mebip ligands, the larger lanthanide ion present in La(ClO
4
)
3
can
bind up to three. The particles in the Zn
II
only gel are almost completely birefringent,
suggesting that they are crystalline (Fig. 7.15a). However, in the Zn
II
/La
III
gel a bire-
fringent core of the particle is surrounded by a large diffuse amorphous halo
(Fig. 7.15b). It is speculated that the presence of the 3:1 Mebip/La
III
complexes in
