Chemistry Reference
In-Depth Information
High viscosity is the main drawback for cholesteric liquid crystalline
polymers in applications. However, cholesteric polymers have their advan-
tages. They may exhibit the memory effect. These polymers make process-
ing and handling easily. Therefore, cholesteric liquid crystalline polymers
have become the new materials for optical filters, temperature indica-
tors,
etc
. The Merck product with the trade name Transmax is actually
associated with the cholesteric liquid crystalline polymers.
Ueno
et al
. (1986) synthesized polysiloxane cholesteric copolymers. The
polysiloxane in a solvent was coated on the biphthalate polyethylene (PET),
a black substrate, making the helical axis perpendicular to the substrate.
The cholesteric liquid crystal was in the Granjean texture. The displayed
information was written by an Argon ion laser and the cholesteric polymers
were thus heated up into an isotropic state. When cooled down, the helical
axes were randomly distributed and thus the sample was opaque. The rest
of such polymers remained in the Granjean texture and displayed a bright
color because of the selective reflection. If one observed through a circu-
lar polarizer of the same sense as the cholesteric polymers, the scattering
light was blocked. Consequently, a black image was displayed on a color
background. If observed through a circular polariser of the opposite sense,
a white image was displayed on a black background.
The polysiloxane cholesteric polymers doped by diphenyl ketone were
used as an optical storage material (Pinsl
et al.
, 1987). The display was
based on the convertible photo-reaction. The information was written by
a semiconductor laser of wavelength
λ
= 800 nm. The storage density of
1
.
33
×
10
9
bytes/cm
2
was achieved. The image can be kept up to two or
three months.
Table 6.10. Liquid crystalline polymers C6M and chiral molecules CB15
(28 wt%): pitch and reflective peak wavelength as functions of polymer-
ization temperature before and after the removal of CB15. (From Hikmet
et al.
, 1993.)
T/
◦
C
λ
max
/nm
with CB15
P
/nm
with CB15
λ
max
/nm after
removal of CB15
P
/nm after
removal of CB15
30
543
337
380
236
40
551
343
422
263
50
583
362
433
269
60
598
372
449
281
70
606
378
462
288
