Biomedical Engineering Reference
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the substrate typically penetrate several domain spacings into
the film — perhaps only a small percentage of its thickness.
Even here, though, since a completely continuous internal pore
structure connected everywhere to the substrate is required for
electrochemical replication, these details of microphase morphology
and its orientation are extremely important, and an understanding
of behavior in thin films and at interfaces is crucial.
(a)
free surface
(b)
(n+1/2)
λ
λ
n
λ
substrate
ASYMMETRIC
SYMMETRIC
Quantization of film thickness in lamellae-forming copolymer
thin films. (a)
Figure 2.6
affinity of one block to the substrate and another
to the free-surface results in film thicknesses quantized in units of (
Asymmetric
n
+ 1/2)
λ
.
(b)
affinity of one block to both the substrate and free surface
results in quantization in units of
Symmetric
nλ
.
In the region of an external interface, differences in the surface-
free energy or affinity of each block toward the substrate (Δ
) have
a strong influence on copolymer morphology. When the asymmetry
is sufficiently large, the free energy of the system is reduced by
preferential segregation of one or other component to the interface.
Russell and coworkers first studied these effects experimentally
in
γ
lamellae-forming
poly(styrene)-
b
-(methylmethacrylate)
(PS-
b
-PMMA) copolymers using neutron reflectivity [9] and dynamic
secondary ion mass spectroscopy (SIMS) [10]. After phase separation,
the lamellae were found to orient parallel to both gold and silicon
substrates (a so-called
L
morphology). On silicon substrates, the
∥
total film thickness is (
n
+ 1/2)
λ
, where
λ
is the lamellar period and
n
. The
differences are explained by the symmetry or asymmetry of affinities
for each block at the free surface and substrate surface, illustrated in
Fig. 2.6. If the initial film is not commensurate with these quantized
thicknesses, then on thermal annealing, islands and holes will form
is the number of periods, while on gold, the thickness is
nλ
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