Handbook of Solvents - George Wypych - ChemTech - Ventech!

12.1 Rheological properties, aggregation, permeability 701
Peroxide concentration, arbitrary units
0.5
0.4
0.3
0.2
0.1
0
0 0.5 1 1.5 2 2.5 3
Time, h
Figure 12.1.35. Peroxide formation from
tetrahydrofuran during irradiation at 254 nm. [Data
from J F Rabek, T A Skowronski, B Ranby, Polymer,
21, 226 (1980).]
Relative signal intensity, arbitrary units
by abstracting hydrogen from polyethylene chain in an α-position to the double bond (Figure
12.1.36).
3-hydroperoxyhexane cleaves with formation of carboxylic acid and hydrocarbon
radical (ethylene or propylene). 3 These known examples show that the presence of even
traces of solvents may change the chemistry and the rate of photo-oxidative processes because
of formation of radicals.
In another aspect of photo-oxidative processes, solvents influence the reactivity of
small molecules and chain segments by facilitating the mobility of molecules and changing
the absorption of light, the wavelength of emitted fluorescent radiation, and the lifetime of
radicals. Work on anthraquinone derivatives, which are common photosensitizers, has
shown that when photosensitizer is dissolved in isopropanol (hydrogen-donating solvent)
the half-life of radicals is increased by a factor of seven compared to acetonitrile (a non-hydrogen-donating
solvent). 4 This shows that the ability of a photosensitizer to act in this manner
depends on the presence of a hydrogen donor (frequently the solvent). In studies on
another group of photosensitizers − 1,2-diketones, the solvent cyclohexane increased the
absorption wavelength of the sensitizer in the UV range by more than 10 nm compared with
solutions in ethanol and chloroform. 5 Such a change in absorption may benefit some systems
because the energy of absorbed radiation will become lower than the energy required
to disrupt existing chemical bonds. But it may also increase the potential for degradation by
shifting radiation wavelength to the range absorbed by a particular material. Presence of
water may have a plasticizing action which increases the mobility of chains and their potential
for interactions and reactions. 6
Studies on photoresists, based on methacryloyethyl- phenylglyoxylate, show that, in
aprotic solvents, the main reaction mechanism is a Norrish type II photolysis leading to
chain scission. 7 In aprotic solvents, the polymer is photoreduced and crosslinks are formed.
5
4
3
2
1
0
10 -5
0.0001 0.001 0.01 0.1
Xylene concentration, mol l -1
Figure 12.1.36. Spectral intensity of n-octane radical
formed during irradiation of PE at -196 o C for 30 min
vs. concentration of p-xylene. [Adapted, by permission,
from H Kubota, M Kimura, Polym. Deg. Stab.,
38, 1 (1992).]