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contains one biomimetic module. For practical applications, it would be advan-
tageous to use a relatively small amount of biomimetic modules to achieve
maximum property enhancement. With these considerations in mind, we recently
developed 3-D network polymers having biomimetic modules as cross-linkers
(Kushner et al. 2007). As explained in more detail in the following paragraph, the
network structure makes it less critical for chains to orient in any specific direction
because the stress from any direction should be transferred to the cross-linking junc-
tion points. In addition, only a relatively small amount of modular cross-linker is
needed to enhance the network properties.
Although many engineering approaches and chemical modifications (Mark et al.
1994) have been developed to improve the mechanical properties of elastomers, it
remains a challenge to design ideal elastomers that simultaneously possess high
modulus, high tensile strength, and high extensibility. Typically rigid elastomers
tend to fail after only a short extension. Although flexible elastomers are more
extensible, they usually have low moduli and exhibit shallow stress response. We
envisioned that the incorporation of reversibly unfolding modular cross-linkers
into a network should result in elastomers with combined high moduli, toughness,
and elasticity. As illustrated in Figure 10.7, a stress applied from any direction
will be ultimately transferred across the individual network junctions, where bio-
mimetic modules can be reversibly unfolded (Kushner et al. 2007). Because it
requires significant forces to unfold the modules held by strong multiple hydrogen
bonds, further extension can be gained without sacrificing the strength. Interesting
work has been reported on using interchain hydrogen bonding to improve polymer
physical properties (Stadler and Burgert 1986; de Lucca Freitas and Stadler 1987;
Mueller et al. 1995; Yamauchi et al. 2003; Elkins et al. 2005; Park et al. 2005).
However, most of these studies are focused on the investigation of solution and
melt rheological properties. To the best of our knowledge, there is no report in
the literature on using molecularly engineered cross-linkers to enhance elastomer
mechanical properties.
Based on our and other's single molecule nanomechanical studies (Guan et al.
2004; Roland and Guan 2004; Zou et al. 2005), the UPy module still has significantly
higher mechanical stability than other peptidomimetic b-sheets that we investigated.
This could be partially attributed to the compact structure of UPy that favors high
Figure 10.7 Introducing biomimetic modular cross-linkers to enhance elastomer mechanical
properties. Adapted from Kushner et al. (2007). Copyright 2007 American Chemical Society.
