Biomedical Engineering Reference
In-Depth Information
3.2
Particle size distribution of a typical poly(ethyleneoxide)-PbS
nanocomposite.
17
attractive for optical studies and applications. The refractive index of some
nanocomposite has been found to depend linearly on the volume fraction of
the particles. When the particles become very small, the refractive index
changes.
17
In optical applications, the refractive index is a key feature.
Typical refractive indices for organic polymers are between 1.3 and 1.7.
8,15
Isotropic refractive indices around 2.1 have also been found (e.g. for poly
(thiophene) or aromatic poly(imide)s
19-21
) and these values are extraordi-
narily high for polymers. The theoretical lower limit of the refractive index
of organic polymers has been estimated to be close to 1.29.
22
So far, an
isotropic refractive index above 2.5 ('ultrahigh' refractive index
23
) or below
1.25 ('ultralow' refractive index
24
) has never been reported for a pure
polymer. Inorganic materials cover a more extended range of refractive
indices (Table 3.1). In technical applications, the refractive index of
inorganic fillers is commonly between 1.4 and 1.7,
25,26
and is occasionally
up to 2.7 (TiO
2
)
25,27
). By incorporating of inorganic colloids with extreme
refractive indices in organic polymers, an attempt has been made to obtain
composite materials with refractive indices outside the typical range of
polymers and hitherto unknown for polymer composites. High refractive
index polymers have many applications, ranging from anti-reflection
coatings for solar cells to high refractive index lenses.
After transmission of visible light through composites comprising a
transparent polymer matrix with embedded particles, the intensity loss by
scattering is substantially reduced for small particles, i.e. particles with
diameters below 50-100 nm (nanoparticles) (Fig. 3.2).
17
Similarly, infrared
(IR) windows are typically made from single-crystal ceramics as single-
crystal windows have excellent optical properties, but they cannot withstand
the mechanical demands of some applications, leading to catastrophic
failures. Ceramic nanocomposites can be used in making IR transparent
