Listening to the leaves: Adding bioinspired veins to foamed polymers


Date:
January 12, 2022
Source:
Beckman Institute for Advanced Science and Technology
Summary:
Vascular systems found in trees transport vital nutrients from root,
to branch, to leaf. In a new study, researchers have developed
a chemical process to mimic this arboreal architecture in foamed
polymers, enabling directional fluid transport and adding structure
throughout the material.



FULL STORY ==========================================================================
Many lessons learned in life are learned from trees. Stand firm. Good
things take time. Bend, don't break. But metaphors aside, our stately
arboreal neighbors offer a wealth of scientific wisdom -- and we have
a lot to learn.


========================================================================== Simply by existing, trees are nature's first materials scientists. Like
many plants, they have vascular systems, networks of tube-like channels
that transport water and other vital nutrients from root, to branch,
to leaf.

A research team at the Beckman Institute for Advanced Science and
Technology developed a chemical process to create foamed polymers with
vascular systems of their own, controlling the direction and alignment
of the hollow channels to provide structural support and efficiently
move fluids through the material.

Their work, "Anisotropic foams via frontal polymerization," was published
in Advanced Materials.

Structure made simple Polymeric foams are efficient thermal
insulators with applications from packaging to refrigeration to home insulation. Hollow channels are often formed during the polymerization
process, but existing methods to fine-tune their structure -- or turn
them into something resembling a working vascular system - - relied on
complex techniques and instruments. Led by Diego Alzate-Sanchez, this
team sought to design a simpler method.



==========================================================================
"In our research group, we observed these vein-like structures appearing
in the polymers. But while some scientists just saw the channels as
empty voids that weaken the polymer, we saw them as a chance to create something productive," said Alzate-Sanchez, a postdoctoral research
associate at the Beckman Institute.

For this University of Illinois team, the naturally occurring channels
were not cause for alarm, but a source of scientific inspiration --
or rather, bioinspiration.

Listening to the leaves Looking to the oaks and maples dotting the
Urbana campus, the researchers sought to equip polymeric foam with a
vascular system that mimicked the structure found in trees. Organizing
the channeled system in a parallel structure enables the transport of
fluids in a single, predetermined direction.

"Think about a tree trunk," said Jeffrey Moore, the director of the
Beckman Institute and the PI on this study. "The water needs to travel
in the right direction, from the roots to the leaves. It needs to get
from Point A to Point B in the most direct way possible; not to Point
C or to somewhere else entirely." Because movement in one direction is
favored over movement in another, this structure is known as anisotropic,
or unequal. Imagine adjacent lanes of traffic on a northbound highway; traveling east or west is much more challenging than going with the
flow. Previously, most vascular systems embedded in foam materials
followed an isotropic structure, with the channels moving equally in all directions. If anisotropy is a highway, isotropy is an arena of bumper
cars weaving through one another in meandering, multidirectional paths.



==========================================================================
More than just fluids For a materials scientist, a one-way vascular
highway enables unique opportunities to conduct more than just water.

In this study, Alzate-Sanchez and his team demonstrated the channels'
use for transporting fluids through the polymers in a predetermined
direction; looking ahead, the ability to manufacture a directional flow
could involve various forms of energy.

"Materials with anisotropic properties are important. For example,
anisotropic thermal insulators can conduct heat in one direction and
block it in the opposite direction. The same is true for electricity,
light, or even sound.

Depending on how you align the foam, sound can go in one direction,
but it will be blocked in the other direction," Alzate-Sanchez said.

Getting reactive To determine a way to control the cellular structure
of foamed materials -- and in particular, force anisotropy -- the
team analyzed each component of the chemical reaction used to create
the polymer.

The reaction begins by combining a monomer called dicyclopentadiene, or
DCPD; a catalyst; and a blowing agent to help give the final product its foam-like consistency. This mixture, referred to as the resin, is poured
into a test tube. Heating the test tube triggers frontal polymerization,
a reaction that cures -- or hardens -- the resin into a foamed cellular
solid. The final product is poly-DCPD, the original monomer DCPD having
been polymerized.

Three of the reaction's ingredients were under scrutiny: the type
of blowing agent used; the concentration of the blowing agent;
and the gelation time of the resin. Gelation is caused by background polymerization, and refers to the delay time before frontal polymerization
is triggered, when the room- temperature resin gradually assumes a soft, gel-like consistency in the test tube.

The researchers discovered that the resin's viscosity -- or its
flowability, a direct result of its softening during the gelation period
-- is the strongest indicator of anisotropy in the final product. In
other words, increasing or decreasing gelation time enables direct
control over the foam's cellular structure.

"This work provides a fast and efficient way to make directional vascular structures from simple components and processes," Moore said.

The team's full factorial experimental design involved methodically
testing 100 different combinations of blowing agent, concentration,
and gelation time, and measuring the levels of anisotropy, hardness,
and degree of porousness achieved with each variation.

A collaborative effort Each foam sample was analyzed with X-ray
micro-computed tomography imaging. The novel pairing of polymeric foam
with micro-CT imaging -- a technology typically reserved for analyzing
hard materials -- was a uniquely collaborative venture involving
coauthor Mariana Kersh, an associate professor of mechanical science
and engineering.

"What Beckman does well is to encourage a culture in which we recognize
that we have much to learn from each other, even if our applications are different," Kersh said. "This exchange and willingness to learn about
something other than your core area meant that the idea that our tools
in bone could be used to characterize the porosity in foams suddenly
seemed obvious and intuitive." In addition to Alzate-Sanchez, Moore,
and Kersh, coauthors on this study include graduate research assistant
Morgan Cencer, recent materials science and engineering grad Michael
Rogalski, and Nancy Sottos, the Maybelle Leland Swanlund Endowed Chair
of Materials Science and Engineering at UIUC.

========================================================================== Story Source: Materials provided by Beckman_Institute_for_Advanced_Science_and_Technology.

Original written by Jenna Kurtzweil. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Diego M. Alzate‐Sanchez, Morgan M. Cencer, Michael Rogalski,
Mariana Kersh, Nancy Sottos, Jeffrey S. Moore. Anisotropic Foams
Via Frontal Polymerization. Advanced Materials, 2021; 2105821 DOI:
10.1002/ adma.202105821 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220112145126.htm
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