Page 60 - Plastics News June 2017

TECHNOLOGY

Oyster shells inspire new method to make superstrong,

flexible polymers

esearchers at Columbia Engineering have demonstrated in an amorphous polymer matrix (i.e. the polymer does
Rfor the first time a new technique that takes its inspiration not crystallize), to date no one has been able to tune
from the nacre of oyster shells, a composite material that has nanoparticle assembly in a crystalline polymer matrix.
extraordinary mechanical properties, including great strength One related approach relied on ice-templating. Using
and resilience. By changing the crystallization speed of a this technique, investigators have crystallized small
polymer initially well mixed with nanoparticles, the team molecules (predominantly water) to organize colloid
was able to control how the nanoparticles self-assemble particles, but, due to the intrinsic kinetics of these
into structures at three very different length scale regimes. processes, the particles are normally expelled into the
This multiscale ordering can make the base material almost microscale grain boundaries, and so researchers have
an order of magnitude not been able to order nanoparticles across the multiple
stiffer while still scales necessary to mimic nacre.
retaining the desired
deformability and Kumar's group, experts in tuning the structure and
lightweight behavior of therefore the properties of polymer nanocomposites,
the polymeric materials. found that, by mixing nanoparticles in a solution
of polymers(polyethylene oxide) and changing the
The study, led by Sanat crystallization speed by varying the degree of sub-
Kumar, Bykhovsky Professor of Chemical Engineering, is cooling (namely how far below the melting point the
published June 7 online in ACS Central Science. "Essentially, crystallization was conducted), they could control how
we have created a one-step method to build a composite the nanoparticles self-assembled into three different
material that is significantly stronger than its host material," scale regimes: nano, micro, and macro-meter. Each
says Kumar, an expert in polymer dynamics and self-assembly. nanoparticle was evenly swathed by the polymers and
"Our technique may improve the mechanical and potentially evenly spaced before the crystallization process began.
other physical properties of commercially relevant plastic The nanoparticles then assembled into sheets (10−100
materials, with applications in automobiles, protective nm) and the sheets into aggregates on the microscale
coatings, and food/beverage packaging, things we use every (1−10 μm) when the polymer was crystallized. "This
day. And, looking further ahead, we may also be able to controlled self-assembly is important because it
produce interesting electronic or optical properties of the improves the stiffness of the materials while keeping
nanocomposite materials, potentially enabling the fabrication them tough," says Kumar. "And the materials retain the
of new materials and functional devices that can be used low density of the pure semicrystalline polymerso that
in structural applications such as buildings, but with the we can keep the weight of a structural component low,
ability to monitor their health in situ." About 75 percent of a property that is critical to applications such as cars
commercially used polymers, including polyethylene used for and planes, where weight is a critical consideration.
packaging and polypropylene for bottles, are semicrystalline.
With our versatile approach, we can vary either the
"While achieving the spontaneous assembly of nanoparticles particle or the polymer to achieve some specific material
into a hierarchy of scales in a polymer host has been a behavior or device performance." Kumar's team plans
'holy grail' in nanoscience, until now there has been no next to examine the fundamentals that enables particles
established method to achieve this goal," says Dan Zhao, to move toward certain regions of the system, and to
Kumar's PhD student and first author on this paper. "We develop methods to speed up the kinetics of particle
addressed this challenge through the controlled, multiscale ordering, which currently takes a few days. They then
assembly of nanoparticles by leveraging the kinetics of plan to explore other application-driven polymer/
polymer crystallization." particle systems, such as polylactide/nanoparticle
While researchers focusing on polymer nanocomposites systems that can be engineered as next-generation
have achieved facile control of nanoparticle organization biodegradable and sustainable polymer nanocomposites.

Plastics News | June 2017 60