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Bar-Cohen : Biomimetics: Biologically Inspired Technologies DK3163_c012 Final Proof page 331 21.9.2005 11:55pm

Multifunctional Materials 331

O
O O O O
O O
O + N N
O 4 ∆
N
O O
O
Furan Maleimide cyclo adduct
Figure 12.26 The polymer consists of a multifuran molecule combined in stoichiometric ratio with a multi-
maleimide molecule.

the material recovered an average of 78% of the original fracture load, indicating that the material
could be repaired multiple times. It was noted that the crack usually propagated along the same
crack plane.
In addition, we have observed the healing mechanism in samples of this healable polymer. Our
aim was to arrest crack growth prior to complete fracture of the sample. In this way we could improve
the ability to match the severed interfaces back to their original location prior to heating. Due to the
inherent high mechanical strength of this material, a controlled cracking procedure was devised.
Samples were machined to 0.25 in. by 0.15 in. by 0.20 in. dimensions with a 0.08 in. diameter hole
penetrating through the middle. Two notches were cut into the hole to initiate the crack on opposing
sides of the hole. The samples were cooled in liquid nitrogen and immediately loaded in compression
in the direction of the machined notch. The applied load caused cracks to grow from the notches in a
controlled manner in the direction of the applied load. Cracked samples were then placed in a spring
device that applied compression normal to the crack faces so as to put the crack faces in contact. Heat
was then applied at various levels and durations. For samples treated for at least 6 h above 808C
under a nitrogen atmosphere with about 8 kPa of compression normal to the interface, the crack was
observed to disappear, indicating healing. In these cases, no visible scar remained, apart from the
initial starter notch. These tests were only qualitative in nature. However, it appeared that the crack
had been completely repaired and visually the material had been restored to its original state. Figure
12.27 shows representative photographs before and after the healing event.

Figure 12.27 Optical photographs taken at 20 magnification of representative healable polymer sample.
Diagonal view of sample with predrilled hole and starter notches is shown. (Left) Polymer sample after controlled
cracking. Note cracks have propagated from the starter notches to the left and right of the sample. (Right) Same
sample after healing for 6 h under nitrogen and about 8 kPa of compression normal to the crack face. Crack faces
have disappeared leaving only starter notches and predrilled hole visible.