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

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.
Another of our microfluidic devices exploits
plug flow to achieve molecular gradients of surface
grafted polymers. Microchannel confined surface
initiated polymerization (µSIP; Figure 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 geometry determined
by the channel design and a gradient in molecular
properties determined by the solution flow rate.
Several key features of µSIP 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.
As a result, µSIP represents a significant
improvement over existing techniques for grafting
polymers from surfaces. Moreover, it enables
fabrication of combinatorial substrates that could play
a critical role in nanomaterials development, since
many routes for nanofabrication (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
Technical Highlights
(see Combinatorial Gradient Reference Specimens
for Advanced Scanned Probe Microscopy project
under Nanometrology).
Our third device example concerns the preparation
of polymer colloids and droplets via suspension
polymerization in microfluidic channels (Figure 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 (Figure 3).
In current work, we aim to polymerize organic phase
droplets to form gradients of polymer microbeads.
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.
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.
For More Information on this Topic
K.L. Beers, T. Wu, C. Xu, Z.T. Cygan, Y. Mei,
M.J. Fasolka, E.J. Amis (Polymers Division, NIST)
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