Biomedical Engineering Reference
In-Depth Information
with the SMA at the surface will determine the magni-
tude of the driving force leading to a SMA-dominated
surface. Second, the molecular mobility of the bulk
material and the SMA additive molecules within the
bulk will determine the rate at which the SMA reaches
the surface, or if it will get there at all. An additional
concern is the durability and stability of the SMA at the
surface.
A typical SMA designed to alter the surface properties
of a polymeric material will be a relatively low molecular
weight diblock or triblock copolymer (see Section 3.2.2).
The ''A'' block will be soluble in, or compatible with, the
bulk material into which the SMA is being added. The
''B'' block will be incompatible with the bulk material and
have lower surface energy. Thus, the A block will anchor
the B block into the material to be modified at the in-
terface. This is suggested schematically in Fig. 3.2.14-9 .
During initial fabrication, the SMA might be distributed
uniformly throughout the bulk. After a period for curing
or an annealing step, the SMA will migrate to the surface.
Low-molecular-weight end groups on polymer chains can
also provide the driving force to bring the end group to
the surface.
As an example, on SMA for a polyurethane might
have a low-molecular-weight polyurethane A block and
a PDMS B block. The PDMS component on the surface
may confer improved blood compatibility to the poly-
urethane. The A block will anchor the SMA in the
polyurethane bulk (the polyurethane A block should be
reasonably compatible with the bulk polyurethane),
while the low-surface-energy, highly flexible silicone B
block will be exposed at the air surface to lower the
interfacial energy (note that air is ''hydrophobic''). The
A block anchor should confer stability to this system.
However, consider that if the system is placed in an
aqueous
environment,
a
low-surface-energy
polymer
(the B block) is now in contact with water
a high in-
terfacial energy situation. If the system, after fabrica-
tion, still exhibits sufficient chain mobility, it might
phase-invert to bring the bulk polyurethane or the A
block to the surface. Unless the system is specifically
engineered to do such a surface phase reversal, this in-
version is undesirable. Proper choice of the bulk poly-
mer
d
and
the
A
block
can
impede
surface
phase
inversion.
An example of a polymer additive that was developed
by 3M specifically to take advantage of this surface
chemical inversion phenomenon is a stain inhibitor for
fabric. Though not a biomaterial, it illustrates design
principles for this type of system. The compound has
three ''arms.'' A fluoropolymer arm, the lowest energy
component, resides at the fabric surface in air. Fluo-
ropolymers and hydrocarbons (typical stains) do not mix,
so hydrocarbons are repelled. A second arm of hydro-
philic PEO will come to the surface in hot water and
assist with the washing out of any material on the surface.
Finally, a third arm of hydrocarbon anchors this additive
into the fabric.
Many SMAs for inorganic systems are known. For
example, very small quantities of nickel will completely
alter the structure of a silicon (111) surface (Wilson and
Chiang, 1987). Copper will accumulate at the surface of
gold alloys (Tanaka et al. , 1988). Also, in stainless steels,
chromium will concentrate (as the oxide) at the surface,
imparting corrosion resistance.
There are a number of additives that spontaneously
surface-concentrate, but are not necessarily designed as
SMAs. A few examples for polymers include PDMS,
some extrusion lubricants (Ratner, 1983), and some UV
stabilizers (Tyler et al. , 1992). The presence of such
additives at the surface of a polymer may be unplanned
and
Fig. 3.2.14-9 A block copolymer SMA comprising an A block and
a B block is blended into a support polymer (the bulk) with
a chemistry similar to the A block. During fabrication, the block
copolymer is randomly distributed throughout the support poly-
mer. After curing or annealing, the A block anchors the SMA
into the support, while the low-energy B block migrates to the
air-polymer interface.
they
will
not
necessarily
form
stable,
durable
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