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| 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. |
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| Kathryn L. Beers and Tao
Wu |
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| 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. |
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| 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. |
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| 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
a channel structures. Solutions of monomer, initiator, and catalyst
are introduced through input ports at one end of 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, 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. |
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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. |
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| 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). |
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| 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). |
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| Figure 2: (a) Schematic of microchannel confined surface
initiated polymerization (µSIP) used to produce surface grafted
polymer gradients. (b) Image of a grafted polymer molecular mass gradient
and a patterned substrate prepared using µSIP. |
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| Another of our microfluidic devices exploits plug flow to achieve
molecular gradients of surface grafted polymers. Microchannel confined
surface intitiated polymerization (µSIP; Fig. 2) employs a shallow
channel (= 300 µm deep), formed through a patterned polydimethylsiloxane
(PDMS) stamp, to confine a solution of monomer and catalyst over an
initiator-functionalized silicon substrate. The result is a polymer
grafted surface (brush) with ge-ometry determined by the channel design
and a gradient in molecular properties determined by the solution
flow rate . |
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| Several key features of mSIP illustrate its utility for combinatorial
library fabrication. (1) The surface in contact with the PDMS stamp
retains initiating capacity after the stamp has been removed, as do
grafted polymers synthesized through an Atomic Transfer Radical Polymerization
(ATRP) route. Accordingly, complex graft copolymer libraries can be
built through sequential iterations of µSIP. (2) Utilizing multiple
channels, it is possible to pattern the same surface with multiple
brush configurations, as flow and stoichiometry conditions can be
varied from channel to channel. (3) Confined gradients formed inside
microchannels enable fabrication of grafted libraries of both statistical
copolymers and gradient (tapered) block copolymers |
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| As a result, µSIP represents a significant improvement over
existing techniques for grafting polymers from surfaces. Moreover,
it enables fabrication of combinato-rial substrates that could play
a critical role in nanomaterials development, since many routes for
nano-fabrication (e.g., self assembly) are extremely sensitive to
substrate chemistry. Moreover, we envision µSIP to be a useful
tool for nanometrology. For example, we intend to employ these techniques
toward the fabrication of micropatterned substrates useful for the
calibration of new scanned probe microscopy (SPM) methods (see Combinatorial
Gradient Reference Specimens for Advanced Scanned Probe Microscopy
project under Nanometrology). |
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| Our third device example concerns the preparation of polymer colloids
and droplets via suspension polymerization in microfluidic channels
(Fig. 3). Indeed, these synthetic routes represent the most important
methods used by industry. In this respect, our thiolene based devices
represent a major advance since they enable the creation of organic
(hydrophobic) droplets in a hydrophilic (e.g., water) matrix. This
is a key requirement for reproducing necessary conditions for colloidal
and suspension polymerization routes in microchannels, a major (unmet)
challenge for channels fabricated from PDMS. Several preliminary devices
have been designed to prepare and polymerize oil droplets in an aqueous
continuous phase (Fig. 3). In current work, we aim to polymerize organic
phase droplets to form gradients of polymer microbeads. |
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| Figure 3: A device that produces, polymerizes and characterizes
polymer colloids is one of the projects future directions. This device
shows an important NIST-developed milestone, two-component toluene
droplets suspended in an aqueous continuous phase. |
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| Each of the three synthetic methods described above has unique advantages,
including small specimen volumes, the ability to make molecular gradients,
and the type of polymers (solution, block, graft, colloidal) they
produce. These methods are designed to interface with existing NCMC
methods, including solution blending, rheological and interfacial
tension measurements, and gradient thin film deposition high throughput
solid characterization and microstructure analysis. As a part of the
combinatorial and high throughput toolset at NIST, the ability to
prepare molecular gradients in a variety of forms is a fast, accurate,
and inexpensive resource to prepare many important polymer libraries. |
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| For More Information on this Topic |
| K. L. Beers, T. Wu, C. Xu, Z. T. Cygan, Y. Mei, M. J. Fasolka and
E. J. Amis (Polymers Division, NIST) |
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