Civil Engineering Reference
In-Depth Information
Figure 8.4
Schematic structure of a cholesteric phase. Each layer is rotated with respect to the adjacent
layers in such a way that the molecules form a helix;
n
represents the orientation of the director for
an arbitrary layer and
t
the direction perpendicular to it. h e schematic representation of the helical
structure corresponds to 1/2 of the pitch of the helix and does not rel ect any discontinuity in the
arrangement of the molecules.
the cholesteric with white light normally to the planes results in the rel ection of one
circularly polarized component of the light with a range of wavelengths that has the
maximum wavelength (λ
0
) given by the expression:
lm
=
P
8.1
0
represents the average refractive index of the cholesteric phase and P is the
value of the helical pitch. If the value of λ
0
corresponds to the visible range the cho-
lestric phases exhibit bright colors. However, if the light reaches the cholesteric with an
angle that is not normal to the cholesteric planes, the wavelength of the rel ected light
will be dependent on the incidence angle and the observed color of the cholesteric will
change with the angle of incidence [21]. h is phenomenon is termed iridescence and
the dependence of the value of the wavelength rel ected by the cholesteric structure
with the angle of incidence is given by the following expression [22]:
where
m
lm j
=
P
sin
8.2
0
where
j
represents the angle of incidence.
8.2.1
All-Cellulosic-Based Biomimetic Composite Systems
Composite materials are heterogeneous systems composed of at least by two dif er-
ent phases, commonly designated as matrix and reinforcement. h ese designations
are associated with the distinct properties (mechanical, thermal or electric) of the two
phases. h e reinforcement can be of micro- (microcomposites) or nanosize (nanocom-
posites). h e i eld of nanocomposites is a rapidly expanding area of research generat-
ing new materials with unique properties. Several new materials have been developed
within the last decade incorporating nanosized i ller materials in polymer matrices.
