Page 130 - Photoreactive Organic Thin Films

4, PHOTOISOMERIZAT1ON AND PHOTO-ORIENTATION OF AZO DYE IN FILMS OF POLYMER

Several photoisomerization and photo-orientation studies have been
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reported in amorphous and liquid crystalline and hybrid azo-polymers, "
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Langmuir-Blodgett-Kuhn (LBK) multilayers, " alignment layers for liquid
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crystal molecules, " self-assembled monolayers {SAMs), " dendrimers,
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phospholipids, polypeptides, peptide oligomers, zeolites, and so on.
Photoinduced mass movement of azo-polymer chains has been reported and
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polarization-sensitive surface relief gratings have been fabricated. ' Recent
studies on optical ordering processes in amorphous polymers have addressed
the role of Tg and polymer structural effects, including the main chain
rigidity; the nature of the connection of the chromophore to a rigid,
semirigid, or flexible mainchain; and the free volume, the free-volume
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distribution, or both.
The polymer structure and Tg are not the only important parameters for
polymers; the molecular weight and its distribution are also important. In
fact, the glass relaxation is characterized by Tg, which is affected by several
factors including the molecular weight, swelling, cross-linking, and hydrostatic
pressure. Pressure effects on photoisomerization-induced molecular movement
processes in NLO azo-polymers substantially below Tg have also been
reported, and it has been shown that both photoisomerization and photo-
orientation in a poly{methy-methacrylate) polymer is strongly hindered under
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hydrostatic pressure. Photoisomerization of azobenzenes depends on the
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free volume. Even though azo dyes can sometimes trigger polymer segmental
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motion and swelling by photoisomerization, ' ' applied pressure can bury
the chromophores into the polymer by compression and free-volume reduction.
Many of the light-induced nonpolar orientation studies have been performed
in liquid crystalline polymers and poly(methyl-methacrylate) (PMMA) polymers
containing azo dye with Tgs around 130°C. Recently, photo-orientation in
higher-Tg polymers has been of interest, and the correlation of optical ordering
{nonpolar and polar) to the polymer structure in a series of very high Tg {up
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to 350°C) rigid or semirigid NLO polyimides has been reported. In particular,
it has been shown that sub-Tg molecular movement, which generally is believed
to be governed by the difference between Tg and the operating temperature
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T, strongly depends on the molecular structure of the unit building blocks
of the polymer, and that polymers with similar Tgs can exhibit significantly
different photoinduced properties. The long-term stability of induced molecular
order should, in principle, improve with the increased difference between the
use temperature and the glass transition temperature of the polymer.
In this chapter, we will show that the occurrence of near-pure photo-
orientation can also be strongly influenced by the polymer molecular structure
by using a series of polyurethane polymers containing azo dye, and we review
the work on high temperature azo-polyimides. We show in an NLO polyirnide
with a 350°C Tg containing no flexible connectors or tethers to an NLO azo
chromophore that is connected through the donor substituent as a part of the
polymer backbone, that photoisomerization is also capable of moving molecular
units at room temperature, whereas, in the absence of photoisomerization,
appreciable molecular movement is induced only by heating the polymer
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above its Tg. We will present evidence that the process of isomerization itself
depends on the polymer molecular structure of these high-Tg polyimides. The