Biomedical Engineering Reference
In-Depth Information
on the free surface to avoid the elastic penalty of stretching or
compressing chains required to alter the domain spacing.
Parallel domain alignment at an interface will propagate into the
bulk of a copolymer film to a degree dependent on the magnitude of
the substrate affinity asymmetry or “surface field” Δ
γ
. TEM studies
on a PS-
-PMMA lamellar system found a critical thickness (linearly
dependent on Δ
b
) below which surface fields are enough to cause
the entire film to adopt an
γ
alignment [11]. For example, the
critical thickness was ~10 lamellar periods when Δ
L
∥
,
corresponding to a film roughly 400 nm thick. In films thicker than
this, the coherence of the parallel alignment was lost some 3 lamellar
periods from the interface. Substrate interactions can be tuned in an
extremely well-controlled manner to favor either copolymer block,
by surface grafting random copolymer brushes with varying fractions
of each component [12]. A “neutral substrate,” with Δ
γ
~
50 nJ cm
−2
~ 0 has no
energetic preference for either copolymer block. Approximately
neutral surfaces can also be made using self-assembled monolayers
SAMs such as alkane thiol layers on gold [13]. On neutral surfaces,
the lamellar microdomains are found to spontaneously orient
at 90
γ
°
to the surface [12, 14], which removes commensurability
constraints between the film thickness and
λ
, and the surface of the
film remains smooth.
The behavior of cylinder-forming copolymers in the vicinity
of a planar interface is perhaps less intuitive than lamellae. In this
system, the bulk symmetry is broken regardless of orientation so
that the microdomains are forced to adjust. Here the balance of film
thickness and surface fields leads to various alignments of the phase,
or even a complete change from the bulk morphology known as
“surface reconstruction.” Knoll
et al.
studied this effect in great detail
using poly(styrene)-
-(styrene) triblock copolymer
thin films in conjunction with molecular dynamic simulations
[15]. In addition to parallel alignment of the cylinders (
b
-(butadiene)-
b
) similar
to the lamellar copolymer, they found wetting layers, perforated
lamellae, and lamellar reconstructions with increasing strength
of the surface field. The averaged mean curvature of the domain
interfaces decreases in order to adapt to the planar symmetry of
the surface. In light of the modulating confinement effects of film
thickness, cylinder-forming copolymers can be induced to form
C
∥
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