Biomedical Engineering Reference
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and an
L
is favored. For intermediate fields,
E
<
E
<
E
, a mixed
⊥
1
2
orientation of microdomains is predicted with
L
near the interfaces
∥
and
in the center of the film.
The electric field strength, dielectric contrast, interfacial
energies, and film thickness, all play a role in achieving the desired
alignment. Complete vertical alignment of domains can be achieved
in two ways: (i) strengthening the electric field driving force by
increasing the field strength and the dielectric contrast, and/or
(ii) reducing the surface fields by making the substrate “neutral” to
both blocks. Xu
L
⊥
-PMMA
copolymer, complete alignment in a 40 V/μm field could only be
achieved once surface interactions were neutralized with a random
copolymer surface-grafted brush [38].
et al.
found that in a lamellae-forming PS-
b
(a)
(b)
100 nm
100 nm
(c)
(d)
100 nm
100 nm
Figure
-PLA
copolymer films. (a,c) Surface and cross-sectional SEM images of a cylinder-
forming PFS-
2.7
Electric field alignment of cylinder-forming PFS-
b
electric
field followed by selective removal of the minority PLA phase. (b,d)
Equivalent images of a film annealed with no electric field.
b
-PLA film thermally annealed under a 120 V μm
−1
The potential effectiveness of electric field BCP alignment of the
C phase is demonstrated in Fig. 2.7 comparing two cylinder-forming
poly(4-fluorostyrene-
b
-poly(lactide) (PFS-
b
-PLA) films thermally
annealed under a 120 V μm
−1
electric field. The film is heated above
the glass transition (
T
g
) temperatures of both blocks while the field
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