A sample of a co-continuous polymer composite material produced in the lab by a team including MIT postdoctoral researcher Lifeng Wang. Device in background is used to test the strength of the material. Photo: Melanie Gonick |
A
team of researchers at MIT has found a way to make complex composite materials
whose attributes can be fine-tuned to give various desirable combinations of
properties such as stiffness, strength, resistance to impacts, and energy
dissipation.
The
key feature of the new composites is a “co-continuous” structure of two
different materials with very different properties, creating a material
combining aspects of both. The co-continuous structure means that the two
interleaved materials each form a kind of three-dimensional lattice whose
pieces are fully connected to each other from side to side, front to back, and
top to bottom.
The
research—by postdoc Lifeng Wang, who worked with undergraduate Jacky Lau and
professors Mary Boyce and Edwin Thomas—was published in Advanced Materials.
The
initial objective of the research was to “try to design a material that can
absorb energy under extreme loading situations,” Wang explains. Such a material
could be used as shielding for trucks or aircraft, he says: “It could be
lightweight and efficient, flexible, not just a solid mantle” like most
present-day armor.
In
most conventional materials, once cracks start to form they tend to propagate
through the material, Wang says. But in the new co-continuous materials, crack
propagation is limited within the microstructure, he says, making them highly “damage tolerant” even when subjected to many crack-producing events.
Some
existing composite materials, such as carbon-carbon composites that use fibers
embedded in another material, can have great strength in the direction parallel
to the fibers, but not much strength in other directions. Because of the
continuous 3D structure of the new composites, their strength is nearly equal
in all dimensions, Wang says.
Thomas,
the Morris Cohen Professor of Materials Science and Engineering and head of
MIT’s Department of Materials Science and Engineering, says that in most
existing composite materials, the fibers form disordered mass with “zero continuity,”
while the other material—typically a resin that fills the space and then
hardens—is continuous and connected in three dimensions. The material that
forms the continuous structure “tends to dominate the properties” of the
composite, he says. “But when both materials are continuous, you can get
benefits that are surprisingly synergistic, not just additive.”
In
their experiments, the MIT researchers combined two polymer materials with
quite different properties: one that is glass-like, strong but brittle, and
another that is rubber-like, not so strong, but tough and resilient. The
result, Thomas says, was a material “that is stiff, strong and tough.”
In
the quest for new materials with specific combinations of properties, Thomas
says, “we’ve pretty much exhausted the natural homogeneous materials,” but the
new fabrication techniques developed in this research can “take to another
level” the material development process.
The
researchers designed the new materials through computer simulations, then made
samples that were tested under laboratory conditions. The simulations and the
experimental data “agree nicely,” Thomas says. While this initial research
focused on tuning the material’s mechanical properties, the same principles
could be applied to controlling a material’s electrical, thermal, optical or
other properties, the researchers say.
The
process could even be used to make materials with “tunable”
properties: for example, to allow certain frequencies of phonons to pass
through while blocking others, with the selection of frequencies tuned through
changes in mechanical pressure. It could also be used to make materials with
shape-memory properties, which could be compressed and then spring back to a
specific form.
The
next step in the research, Thomas says, is to make co-continuous composites out
of pairs of materials whose properties are even more drastically different than
those used in the initial experiments, such as metal with ceramic, or polymer
with metal. Such composites could be very different from any materials made
before, he says.
