Chemistry Reference
In-Depth Information
Figure 10.8 Stress-strain curves for 6% crosslinked poly(n-butyl acrylate) elastomer for the
sample and control specimen (strain rate ¼ 100 mm/min, room temperature). Adapted from
Kushner et al. (2007). Copyright 2007 American Chemical Society.
control polymer (Kushner et al. 2007). Furthermore, for the PEG control samples, the
increase of the modulus at higher cross-linker levels trades off with the maximum
elongation. This is typical for regular thermoset elastomers: higher cross-linking
density results in more rigid and less elastic rubbers (Mark et al. 1994; Treloar
1975). In contrast, in our UPy samples we observed a consistent increase in the
modulus and tensile strength without sacrificing the extensibility. We attribute this
to the unique structure of our modular cross-linker. Although the modulus is
increased at higher cross-linking density, the reversibly folding cross-linkers can
act as energy dissipating units to prevent fracture formation. To extend the control
polymer (an entropic elastomer) by 100%, the free energy change (DG) is roughly
-TDS, which is estimated to be
0.6 kcal/mol (Heimenz 1984). In contrast, it
takes
11 kcal/mol to fully unfold the UPy folded module (based on
K
d
¼
10
28
M in toluene). The enhancement of the energy dissipation by the
modular cross-linker was also supported by the much larger loss moduli measured
by dynamic mechanical analysis for the real samples as compared to the controls.
This work introduces a novel biomimetic concept to enhance rubber properties
through design of molecularly engineered cross-linkers. Further studies are currently
ongoing in our laboratory to extend this concept to other network systems and to
probe the mechanisms for property enhancement.
10.4. CONCLUSION AND PERSPECTIVE
Nature has evolved many excellent materials, including both organic and polymeric
materials such as silks (Oroudjev et al. 2002), adhesion proteins (Law et al. 2003),
bioadhesives (Lin et al. 2007; Waite et al. 2005; Zhao et al. 2006), and connective
