Chemistry Reference
In-Depth Information
stress-strain data for modular polymer 8, control polymer 9, and the regular PU that
does not contain any UPy units. On a stress-strain curve, the maximum stress at
break gives the tensile strength of the material whereas the whole area covered
under the curve reflects the total energy required to break the material, that is, the
toughness. The introduction of protected UPy units into the PU enhanced its
tensile strength and toughness. Even though the benzyl protection blocked the UPy
from dimerization to form loops, p-p stacking, dipole-dipole interactions, and
weak residual hydrogen bonding between the protected UPy units should increase
the tensile strength and toughness of protected polymer 9. For polymer 8 in which
UPy units can dimerize to form loops along the chains, the stress-strain curve
shows dramatic differences from the curve of the control polymer (9). It shows a
sigmoid stress-strain curve characteristic for elastic polymers. After a pseudo yield
region, the sample becomes much stiffer with a significant increase in modulus
and strength. Because of its high strength and toughness, the ultimate tensile strength
at break and the fracture toughness could not be obtained for polymer 8 because the
sample could not be broken even at the maximum load of the Mini-Instron for the
smallest sample specimen we could prepare. Nevertheless, quantitative comparisons
can be made at 950% strain for the three samples. At 950% strain, the tensile stresses
are 58.2, 17.4, and 6.0 MPa and the energy absorptions per unit volume are 191.5,
78.5, and 17.7 MPa for modular polymer 8, control polymer 9, and the PU
samples, respectively. This comparison clearly shows that modular polymer 8 is sig-
nificantly stronger and tougher than either control polymer 9 or the simple PU.
Polymer 8 is also very elastomeric, as evidenced by the high strain up to 900%
and the complete recovery to its original length in three consecutive extension-
retraction experiments. The huge hysteresis accompanied in one extension-retraction
cycle (inset, Fig. 10.4) further reveals the great energy dissipation capability of the
system, an important feature for high toughness. The bulk mechanical data correlate
well with our single chain force-extension observation, which successfully demon-
strates our biomimetic concept: the introduction of modular structures held by sacri-
ficial weak bonds into a polymer chain can successfully combine the three most
fundamental mechanical properties (high tensile strength, toughness, and elasticity)
into one polymer.
10.3.2. Synthesis and Single Molecule Nanomechanical Studies
of Peptidomimetic
b
-Sheet Modular Polymers
Although demonstrating our biomimetic concept, the first-generation UPy system had
a few limitations. The structure of the polymer had nonuniformity that arose from the
polydispersed poly(tetramethylene oxide) loop and the different enchainment of the
UPy units (head-head, head-tail, and tail-tail, Chart 10.2). The UPy units could
also randomly bind to each other within a chain or between different chains.
Finally, the binding strength of UPy is not tunable. For further exploration of
modular biomimetic materials with high strength and toughness, more uniform and
higher ordered polymer systems would be desirable. To address these issues, we
