Biomedical Engineering Reference
In-Depth Information
composites with desirable intrinsic properties. This is rather more
difficult than it sounds; despite over three decades of development,
there remain surprisingly few examples of functioning device
applications that truly exploit ordered BCP assemblies. Broadly
speaking, there are three approaches to overcome this challenge:
I
Direct synthesis
: a BCP made by chemically tethering two or
more electronically functional polymers.
II
Sacrifical templating
: a BCP in which one component can
be selectively removed after phase separation to leave
a mesoporous matrix of the second. This material acts as a
template to be subsequently filled with a chosen functional
material. The template can be removed later to leave a
freestanding replicate of the BCP morphology.
: a blend of the BCP with chemical
precursors of functional materials, relying on microphase
separation as an in situ structure director.
While route I is perhaps the most intuitive and direct approach,
these copolymers are generally rather difficult to synthesize, which
limits the available choice of composite materials to synthentically
compatible copolymers. Route II, on the other hand, is more within
the grasp of today's materials and allows us to concentrate efforts
to control self-assembly on relatively well-understood model
copolymers, while significantly expanding the selection of functional
materials that can be synthesized in the template. Electrochemical
deposition, already well developed in the replication of other nano
and microscale porous systems such as anodized alumina and track-
etched membranes, is ideally suited to reproducing the extremely
small and high-aspect-ratio pores typical of BCP structures.
Figure 2.1 outlines the concept of sacrificial copolymer templating.
Electrochemical replication of porous copolymer templates was
first demonstrated by Thurn-Albrecht
III
Structure directing agent
in 2000. They produced
a hexagonally ordered array of 10 nm diameter cobalt nanowires,
standing vertically on a substrate some 25 nm apart with a density in
excess of 1.9 × 10
et al.
. This work not only demonstrated the great
potential of porous BCP templates but also highlighted the main
practical difficulties of this method and how they can be overcome.
First, the BCP must be selectively degradable, that is, one block
11
cm
-2
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