Biomedical Engineering Reference
In-Depth Information
is applied, before cooling below
to freeze-in the microstructure
with field still in place (experimental methods are further discussed
in Section 2.4.4.2). In this case, the minority PLA domains were
selectively etched away after alignment (see Section 2.3.1) in order
to provide imaging contrast for scanning electron microscopy
(SEM). A well-ordered array of hexagonally packed pores is visible
at the surface, suggesting that the PLA cylinders are vertically
aligned. The cross-sectional view confirms that the cylinders span
the full film thickness. In contrast, a film thermally annealed without
an electric field adopts a
T
g
-lying cylinder morphlogy throughout
the film as a result of the energetic preference of the PFS phase to
contact the substrate (Fig. 2.7b,d).
C
2.4 POROUS BLOCK COPOLYMER FILM TEMPLATES
The key to electrochemical replication of block copolymer film
morphologies lies in producing a porous template coating on a
suitable conducting substrate. If an electrolyte, containing precursor
ionic species, is able to reach this underlying working electrode,
electrochemical synthesis of new material can initiate here and
grow upward through the pore structure. In essence, this requires
that two processing stages can be successfully completed:
I. Alignment of the copolymer microphase such that the
minority phase forms continuous pathways from the
surface to the substrate
II. Complete selective removal of the minority component
without compromising the majority phase
Hereafter, electrochemical growth of new material in the pores of the
film is reliant on optimizing many of the same parameters demanded
by other nanoporous electrode structures, such as anodized alumina
and track-etched membranes [45].
Stage I must be addressed using one or more of the alignment
methods discussed in Section 2.3.1. The most general approach
to achieving stage II is to work with selectively sacrificial block
copolymers [46, 47], in which one component is susceptible to
selective etching (potentially a wet chemical, photochemical, or gas
phase reaction). The first nanoporous polymer materials made in
 
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