Materials Science and Engineering Laboratory FY 2004 ... - NIST

Technical Highlights
Polymer Library Fabrication Techniques Using
Microfluidic Technology
Advanced material products incorporate highly
designed polymer molecules, yet the effect of
molecular parameters on end use properties
is not understood. From both a scientific and
an industrial perspective, there is a need for
simple and economic synthetic methods that
generate combinatorial polymer libraries that
systematically vary molecular mass, architecture,
and molecular composition. New methods at the
NIST Combinatorial Methods Center (NCMC)
enable the fabrication of polymer molecule
libraries that are compatible with high-throughput
measurement methods.
Kathryn L. Beers and Tao Wu
For a large number of specialty polymers, the
properties critical to their end use (e.g., products
ranging from personal care to nanotechnology
applications) depend upon molecular variables such as
chain sequence and composition, branching, and molecular
mass. Often times, as in the case of the bicontinuous
microstructures found in some block copolymers, the
target variable space is narrow and difficult to define
precisely. Tools exist to characterize these key properties
in a high-throughput manner; however, there are few
synthetic techniques that complement these measurement
methods without capital-intensive automation. Moreover,
in practice, current parallel polymer synthesis processes
generally involve specimen volumes that are much
larger than required for the screening tests, resulting
in unnecessary chemical waste and over-use of
expensive monomers.
As part of the polymer formulations project at
NIST, new synthetic platforms are being developed for
the fabrication of molecular gradient libraries that meet
these challenges. Our approach leverages microfluidic
technology to control and confine liquid chemical
environments on the microscale. Using this technology,
we have produced devices to achieve solution phase,
surface-grafted, and suspension polymerizations in a
manner that produces gradients in molecular properties
in a controlled and addressable manner.
Figure 1 shows the first example of our new synthesis
platform: a controlled radical polymerization (CPR)
chip. Fabrication of the CPR chip uses conventional
photolithographic techniques to pattern a thiolene-based
optical adhesive between two glass slides, thus creating
channel structures. Solutions of monomer, initiator, and
catalyst are introduced through input ports at one end of
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Figure 1: a) CRP chip for producing well-defined polymeric
materials tuned by flow rate and input stoichiometry; b) SEC data
for polymers produced at different flow rates.
the device and actively mixed with an enclosed spin bar.
Controlled polymerization in a main channel is achieved
through an atom transfer mechanism, where molecular
weight is tailored by the ratio of monomer to initiator and
the reaction time. By maintaining plug flow in the channel,
controlling the stoichiometry of the reagents (input),
and the flow rate of the solution through the channel, a
polymer product with a predictable, continuous gradient
in molecular weight and low polydispersity is produced.
We validated polymerization kinetics in the CRP chip
against published cases of synthesis carried out in
large-scale reaction flasks.
Whereas conventional techniques yield grams of a
single material, the CRP chip produces microgram-scale
material libraries that exhibit a systematic change in
molecular properties. Accordingly, this technique is
extremely powerful for optimizing materials with a
narrow target molecular parameter space, or for
developing new materials with expensive precursors,
such as proteins and other biomolecules — all with
minimal waste. Products of the CRP chip are neatly
matched to the scale and design of NCMC methods to:
1) prepare gradient thin films for solid materials property
measurements (e.g., modulus, adhesion,
microstructure); and
2) gauge solution properties (e.g., viscosity, stability)
through other fluidic devices being developed in the
Center (see Polymer Formulations project in
Advanced Manufacturing Processes).