Chemistry Reference
In-Depth Information
The measurement of fluorescence and phosphorescence spectra of photosensitizers is very important in
providing information about the energy of the excited states. It also allows identification of the phenomena.
The process of energy transfer requires that the excited donor diffuse to the proximity of an
acceptor within the time period of its excited lifetime. This is subject to the viscosity of the medium
and the efficiently of the collision process and the range
r
in which the collisions can occur. The
observed rate constant for energy transfer
k
ET
is governed by the molecular rate constant
k
diff
for
diffusion-controlled reaction. This is defined by the Debye equation:
k
diff
¼
RT=
;
8
3
000
k
ET
¼ ak
diff
a
R
T
where
is the probability of energy transfer.
is the universal gas constant,
is the temperature in
Kelvin,
is the viscosity of the medium in poise. The Schmoluchowski [
94
] equation defines the
diffusion constant in terms of the diffusion coefficient of the sensitizer and the acceptor:
0
:
5
k
diff
¼
4
p=
1
;
000
ðR
s
þ R
a
ÞðD
s
þ D
a
ÞN
a
=
2
Þf
1
þ½R
s
þ R
a
=ðt
0
ðD
s
þ D
a
Þ=
2
Þ
g
where
D
s
and
D
a
are the diffusion coefficients of the sensitizer and the acceptor
R
s
and
R
a
are the
molecular radii of the sensitizer and the acceptor,
N
a
is the Avogadro number, and
t
0
is the lifetime of
the excited state of the sensitizer.
10.5 Photocross-linkable Polymers
Some photocross-linking of polymers can be traced back to ancient days, when pitch was photocross-
linked for decorative purposes [
102
]. In modern times, wide varieties of photocross-linkable
polymers were developed. The early practice of photo imaging relied mainly upon the photo-
dimerization reactions. These reactions are common, photo-induced, reactions of organic chemistry,
namely intermolecular cyclization. This reaction of cyclization that takes place between two reactive
species, with one of them electronically excited, is actually predicted by the Woodward-Hoffmann
rule [
104
]. In contrast, the reactions of thermally excited ground states of molecules proceed by
different pathways. Many polymers were synthesized that possess pendant groups capable of photo-
cyclization intermolecularly to be used in the photo-imaging technology. Photocross-linking tech-
nology today, however, also uses coupling reactions of radicals, chain growth polymerizations that
result in photocross-linking, and some ionic reactions. The light-induced polymerizations of multi-
functional monomers that transform liquid resins into solid polymers almost instantly and selectively
in the illuminated areas are now versatile processes. They can, however, be achieved in a variety of
ways. Thus, for instance, Decker discussed work in the less explored areas of photo-curing, namely
laser-induced ultra-fast polymerization and UV curing of binary polymer systems [
105
]. By using
highly sensitive acrylate photoresists, relief images of micronic size can be obtained by fast scanning
with a focused laser beam. Also, polymer networks of different architectures can be obtained by UV
irradiation of various monomer blends, e.g., acrylate-epoxide, acrylate-vinyl ethers, acrylate-polyene,
vinyl ether-maleate, and thiol-polyene [
104
]. This does not mean, however, that photocross-linking of
polymers is now unimportant technologically or scientifically. The fact that considerable research
still continues in the field is a direct indication of that.
The light cross-linkable reaction, like all cross-linking reactions, results in
gelation
and the extent
of gelation is important in this process. This extent is tied to the quantity of the functional groups in
the reaction mixture. Carothers equations relate the critical extent of the gelation,
p
c
, at the gel point
to the functionality of the reactants:
