Chemistry Reference
In-Depth Information
Fig. 1.9
POLPAC, model for a novel linear polarizer based
on polyacetylene.
construction principles. PVAPAC provides an all-polymer high-tech material alter-
native (Fig. 1.9).
The excellent catalytic control in the process of synthezising PVAPAC allows
the use of the molecular energy levels and symmetry of quasilinear highly conju-
gated
-systems in tailoring the macroscopic properties for LCD applications.
Catalysts that cause selfbranching are not useful for this purpose. An assembly of
linear polyenes can be synthesized in the acetylene polymerization, whose integral
spectral absorption curve extends over the whole visible light region (450-
680 nm), being optimized to fit the human eye sensitivity curve and the intensity
of the LCD light source. The polymer can be processed into thin films of precise
thickness. By stretch-orienting the film uniaxially, the chromophores have to be
arranged in a completely parallel fashion. When visible light passes through this
film, the electromagnetic radiation that vibrates within the direction of the poly-
acetylene chains is absorbed, while the perpendicular component of light passes
through unhindered, rendering the transmitted light linearily polarized.
When PVAPAC films, optimized with respect to conjugation length distribution
and PAC concentration, are stretched in a controlled manner they turn into
highly dichroic transparent neutral grey POLPAC
®
filters (Fig. 1.10). The absence
of coloration is indicative of an exceptionally highly ordered PAC state in these
novel all polymer broadband polarizers based on polyacetylene (POLPAC
®
).
1.7.4
MATPAC for NLO Devices
[24]
Another inherent property of polyacetylene chains, associated with the highly de-
localized
-system, is the polarizability in an electromagnetic field. The dipole mo-
ment induced by the oscillating electric field of electromagnetic radiation may be
expressed as the sum of the linear polarizability and nonlinear higher terms (hy-
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