Biomedical Engineering Reference
In-Depth Information
Russell and coworkers reported an interesting method of
inducing standing alignment in strongly immiscible cylinder-forming
copolymer films by exposure to a controlled solvent atmosphere
(“solvent annealing”) followed by solvent extraction [25, 26]. It was
proposed that an ordering front propagates down through the layer
on removal of the solvent, as opposed to the horizontal sweeping
gradient present during zone-casting. After solvent annealing, the
vertical alignment is coupled to extremely high degrees of in-plane
hexagonal ordering.
Shear flow fields are a remarkably effective means of orienting
copolymer microdomains. Large amplitude oscillatory shear (LAOS)
and extrusion are common methods used to align bulk copolymer
melts and solutions [27]. Thomas and coworkers developed the
use of roll casting to expose relatively thick films to a shear flow
alignment field [28, 29]. Recently, a step-like deformation of precast
films confined under a rubber stamp was shown to align a single-
layer film of molten cylinders [30]. Shear flow fields are, however,
not well suited to producing film-spanning vertical alignment in
films many times thicker than the domain spacing.
Extremely precise local control of domain alignment can be
achieved in a wide range of sample geometries using electric fields.
While electric fields apply a weaker alignment force than shear flow,
they make up for this in the ability to apply spatially specific fields
either in-plane or out-of-plane by control of electrode geometry.
2.3.1.1 Electric ield alignment
Directing the orientation of an anisotropic dielectric object in an
electric field relies on the coupling of the electric field to induced
polarization charges, whose distribution depends on the orientation
of the object. When an external electric field is applied to a dielectric
material by means of a constant potential difference across external
electrodes, the electrostatic contribution to the free energy of the
system (
F
) is given by
1
2
F
=
F
− �
ε
(
r
) |
E
(
r
)|
2
d 
3
r
, (2.7)
0
 
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