53
Basing on the Kuhn-Mark-Houwink-Sakurada equation:
(9)
molecular weight of the polymer (M) was determined.
Viscometry is an indirect method of
molecular weight determination and requires knowledge of constant values: α and K. For
polystyrene solution in toluene measured in 25 °C, these constants are equal to [87]:
K= 10.5×10
-3
[ml/g]
α = 0.73.
Three samples of polystyrene obtained in different syntheses but the same
conditions were tested using this method. Values of determined
molecular weight are
shown in Tab. 9.
Table 9 Molecular weight of polystyrene.
Sample number
Molecular weight
1
2.5×10
5
u
2
2.8×10
5
u
3
2.9×10
5
u
The exact values of polymers molecular weights are not very important in this
case. The most significant is that determined values of molecular
weights are higher
than
10
5
u.
Since plastic scintillators are prepared in the same conditions,
with identical
temperature schedule, it is assumed that molecular masses of J-PET scintillators are equal
to molecular weight of pure polystyrene. This means, that
the light output of J-PET
scintillators is maximized taking into account only polymer properties. Because
polystyrene molecular weight is more than 2 times larger than 10
5
u, light output of the
scintillators is not sensitive to small variations of this quantity.
J-PET scintillators are based mainly on polyvinyltoluene, not polystyrene. However
constant values: K and α are not determined
for polyvinyltoluene solutions, so we took
54
into consideration only results for polystyrene. We assume
that polyvinyltoluene as
polystyrene homolog, differing only by an additional methyl group in the mer, polymerizes
in the same way and its molecular weight is similar.
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