Biomedical Engineering Reference
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spontaneous standing morphologies (
) spanning the entire
film with judicial choice of film thickness and weak surface fields
[16, 17]. While simulations and theory predict that
C
⊥
could be
formed in much thicker films [15], in practice, the highest aspect ratio
of a film-spanning, equilibrium
C
⊥
microphase in a pure copolymer
on a neutral planar surface is around two. This can be increased by
incorporating a small amount of additional minority component
homopolymer, which segregates in the blend to the center of the
cylinders. Spanning vertical cylinders with aspect ratios of up to ~10
have been achieved in this way using films of PS-
C
⊥
b
-PMMA blended
with PMMA homopolymer [18].
2.3.1 Alignment of the Microphase
As described in Section 2.3, a film-spanning pore structure in films
of the order of one domain spacing thick can be achieved using
neutral substrate coatings and choice of film thickness. This is
often all that is required when making lithographic etching masks
or patterning arrays of dots or line arrays. However, when the
microphase must be aligned throughout a much thicker film,
additional external alignment fields are required. Some applications,
such as addressable memory structures, also demand long-range in-
plane order and/or alignment. A great deal of research effort has
been dedicated to developing generalized approaches to microphase
alignment, and a number of comprehensive reviews exist [8, 7].
Alignment and optimization of long-range
order
comprises a fascinating field of research in itself. Topographically
(graphioepitaxy) [19, 20] and chemically (heteroepitaxy) [21, 22]
prepatterned substrates exploit surface fields and in-plane
confinement to guide and order standing cylinders, spheres, or lying
nanowire structures. Similar confinement-alignment effects can be
achieved by patterning the copolymer film itself using, for example,
nanoimprint lithography (NIL) [17]. Long-range order and alignment
can be encouraged using zone casting [23] or temperature gradients
[24], which direct microdomains via a sweeping propagation of an
ordering front.
in-plane
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