Chemistry Reference
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polymer 9 that does not have the modular structure, we only observed a single peak
that is characteristic for the entropic extension of a random coil chain (Fig. 10.3b). In
contrast, modular polymer 8 showed distinct sawtooth patterns in the force-extension
curve (Fig. 10.3a, c), similar to those observed in titin and other modular biopolymers
(Kellermayer et al. 1997; Rief, Gautel, et al. 1997; Marszalek et al. 1999; Li et al.
2000). The big contrast for the force-extension curves observed for polymers 8 and
9 strongly suggests the sequential unfolding of the UPy dimers along the modular
polymer 8 chain. As the polymer chain was stretched, the force rose gradually
until one UPy dimer could not sustain the force and broke instantaneously, adding
additional length to the polymer chain. Therefore, the peak force corresponds to
the force required to break a UPy dimer at a specific pulling rate and the spacing
between the peaks represents the gain in length during the unfolding. The curves
can be fit nicely with a wormlike chain (WLC) model, which predicts the relationship
between the extension of a single polymer chain and the entropic restoring force gen-
erated (Rief, Gautel, et al. 1997; Ortiz and Hadziioannou 1999). This further confirms
that the observed curves are from stretching individual polymer chains.
The mechanical properties of the polymers were tested in bulk as well to correlate
with the single chain nanomechanical studies. Polymer samples were cast into thin
films and subjected to stress-strain analysis. Figure 10.4 shows the comparison of
Figure 10.4 Stress-strain curves for control and modular polymers. The curve in the bottom
(- - -) is for the polyurethane (PU) made from poly(tetramethylene glycol) and HDI without
any UPy units. The curve in the middle (
....
) is control polymer 9 containing the protected UPy
units. The curve at the top (-
.
-) is for modular polymer 8 containing UPy dimerized loops.
Modular polymer 8 could not be broken by the Mini-Instron at its maximum load.
A stretching-retraction cycle for polymer 8 is shown in the inset, which shows a huge hyster-
esis, indicating great energy dissipation during the cycle. Adapted from Guan et al. (2004).
Copyright 2004 American Chemical Society.
