Handbook of Solvents - George Wypych - ChemTech - Ventech!

702 George Wypych
log η 0
10 5
10 4
1000
100
10
1
10 4
10 5
log M w
Figure 12.1.37. Effect of viscosity on molecular weight
of polyethylene wax obtained by thermolysis in the
presence of phenylether at 370 o C. [Adapted, by permission,
from L Guy, B Fixari, Polymer, 40, No.10,
2845-57 (1999).]
10 6
Yield, %
Polarity of the solvent determines quantum yields in polyimides. 8 The most efficient
photocleavage was in medium-polar solvents. The selection of solvent may change the
chemical mechanism of degradation and the associated products of such reactions.
Singlet oxygen is known to affect material stability by its ability to react directly with
hydrocarbon chains to form peroxides. The photosensitizers discussed above are capable of
generating singlet oxygen. Solvents, in addition to their ability to promote photosensitizer
action, affect the lifetime of singlet oxygen (the time it has to react with molecules and form
peroxides). Singlet oxygen has very short lifetime in water (2 μs) but much longer times in
various solvents (e.g., 24μs in benzene, 200μs in carbon disulfide, and 700μs in carbon tetrachloride).
9 Solvents also increase the oxygen diffusion coefficient in a polymer. It was
calculated 10 that in solid polystyrene only less than 2% of singlet oxygen is quenched compared
with more than 50% in solution. Also, the quantum yield of singlet oxygen was only
0.56 in polystyrene and 0.83 in its benzene solution. 10
The photostabilizer must be durable. It was found that salicylic stabilizers are efficiently
degraded by singlet oxygen in polar alkaline media but in a less polar, non-alkaline
solvents these stabilizers are durable. 11 In hydrogen-bonding solvents, the absorption spectrum
of UV absorbers is changed. 12
Thermal decomposition yield and composition of polystyrene wastes is affected by
addition of solvents. 13 Solvents play two roles: First, they stabilize radicals and, second, by
lowering solution viscosity they contribute to homogeneity, increase the mobility of components
and increase reactivity. 85% benzyl and phenoxy radicals were stabilized by hydrogens
from tetralin. 13 In polyethylene thermolysis, the use of solvent to reduce viscosity
resulted in obtaining lower molecular weight, narrow molecular weight distribution and
high crystallinity in resultant polyethylene wax. 14 Figure 12.1.37 illustrates the relationship
80
70
60
50
40
30
20
10
0
methanol DMF DMSO
Solvent
Figure 12.1.38. Yield of condensation products depending
on solvent. [Adapted, by permission, from J
Jeczalik, J. Polym. Sci.: Polym. Chem. Ed., 34, No.6,
1083-5 (1996).]