Civil Engineering Reference
In-Depth Information
concentrations of water in the range of 20-50 wt% were birefringent and showed opti-
cal rotation properties. h e authors attributed this behavior to the formation of a lyo-
tropic mesophase, where the water molecules occupy positions between the polymer
chains, increasing the mobility of the polymer backbone and allowing the polymeric
chains to align spontaneously parallel to each other.
Many other cellulose derivatives were studied and, among them, acetoxyproylcellu-
lose (APC) was found to develop a thermotropic cholesteric phase as well as a lyotropic
phase, in several organic solvents, at room temperature. Gray
et al.
[18] prepared this
cellulose derivative by the acetylation of hydroxypropylcellulose (a schematic of the
chemical reaction is shown in Figure 8.3).
APC solutions in acetone with concentrations higher than 50 wt% are birefringent.
h e colors rel ected by the solutions depend on the concentration; for 40 wt% the solu-
tions appear milky, between 65-80 wt% show faint colors and for concentrations higher
than 80 wt% samples show iridescent colors. h e fact that APC can form thermotropic
cholesteric phases, without the need of solvent, can be attributed to the mobility of
the cellulose main chain due to the chemical acetylation of HPC hydroxyl groups. h e
authors observed that a layer of APC heated in a hot stage placed under a microscope
show rel ection temperature depending on colors. h e increase of temperature was
found to be at the origin of the variation of the sample rel ection colors from blue to red
before the isotropization temperature was reached. Birefringence and optical activity of
the cholesteric mesophase are consequences of its structure. h e cholesteric mesophase
resembles a nematic mesophase with a natural twist around an axis normal to the direc-
tor. In each plane, the average orientation of the molecules is the same for all of them
and the director is rotated from one layer to the next, forming a structure whose helical
axis is perpendicular to the planes formed by the layers [19]. h is structure is showed
in Figure 8.4.
h e cholesteric phase is characterized by the distance in which the director rotates a
full turn of 360º (pitch), normally designated by P.
For values of the pitch in the range of 0.4-0.8 μm (corresponding to the visible light
range) the measuring of pitch can be performed by infrared or visible spectroscopy or
by optical rotatory dispersion.
A model to determine the birefringence of a cholesteric material was proposed by
de Vries [20] for a planar cholesteric organized in a stack of a large number of layers.
Each layer is considered to be rotated by a few degrees in relation to the ones immedi-
ately under and above in such a manner that a helical structure is formed. Illuminating
Figure 8.3
Schematic representation of the chemical acetylation reaction of hydroxypropylcellulose
(HPC) to obtain the cellulose derivative acetoxypropylcellulose (APC).
