Biomedical Engineering Reference
In-Depth Information
9.3.1
Fabrication of HAp-Commodity Polymer Nanocomposite
Microspheres
As was described above, a polymer having ester groups functions as an
adsorption promoter (wettability modifi er) of the HAp nanoparticles (or
nanocrystals). A similar function can be expected even for polymers having
no ester groups by introducing other functional groups that interact with
HAp. An example of such functional groups is the carboxyl group, which
is already known to ionically interact with the HAp surface [62]. We have
found that even a polymer having no ester groups in main/side chains acts
as an adsorption promoter (wettability modifi er) for HAp by introducing
a carboxyl group only at the polymer chain end. We investigated the effect
on the emulsion stability of the polymer chain end groups of polystyrene
(PS), which is dissolved in the oil phase [51]. PS having a carboxyl group
as a polymer chain end group (PS-COOH) and PS having a methyl group
as a chain end group (PS-CH 3 ) were used as samples. They were synthe-
sized by solution polymerization using 4,4'-azobis(cyanovaleric acid) or
2,2'-azobisisobutyronitrile as a free-radical polymerization initiator. When
a CH 2 Cl 2 solution of PS-COOH was used, a stable O/W emulsion was
formed. This result suggests that adsorption of the HAp nanoparticles
onto the oil/water interface was achieved by the interaction of the car-
boxyl group at the PS chain end and the HAp nanoparticles at oil-water
interface. It has been confi rmed that the PS-COOH with molecular weight
of ranging from 6,700 to 64,500 can lead to formation of spherical emul-
sion droplets. On the other hand, when a CH 2 Cl 2 solution of PS-CH 3 was
used as the oil phase and homogenized with an aqueous dispersion of
HAp nanoparticles, macro-phase separation of the water phase and oil
phase took place or an unstable emulsion was formed. These results indi-
cate that the stability of the emulsion depends on the difference in the
polymer chain end group, which is often overlooked compared with the
polymer main chain or side chain. These results are crucial for the design
of polymers, which can be applied to the Pickering emulsion method [51].
Subsequently, the fabrication of nanocomposite microspheres with PS
core/HAp shell morphology was achieved by evaporation of CH 2 Cl 2 from
the formed Pickering-type emulsion. The solubility of CH 2 Cl 2 in water
is 1.3 g/100 mL (20
C); therefore, CH 2 Cl 2 in oil droplets can gradually
evaporate into the gas phase through the continuous water phase. When
polymer solutions of the same concentration were used in PS-COOH sys-
tems with molecular weights of 26,100 and 64,500, the volume of the oil
droplets decreased, accompanied by the evaporation of CH 2 Cl 2 , while
maintaining their spherical shape. In these systems, spherical micro-
spheres were formed. On the other hand, in the PS-COOH system with
the molecular weight of 6,700, the spherical oil droplets were defl ated
after evaporation of CH 2 Cl 2 , and microspheres with concavities were
°
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