Development of novel plastic scintillators based on polyvinyltoluene for the hybrid j-pet/mr tomograph

Figure 8 Mayo's and Flory's mechanisms of free radical generation in styrene polymerization. The scheme is taken 
from the article [61].
 

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R
n
• + M → R
n+1
• 
where R
n
• denotes polymer chain with n mers, M – monomer, R
n+1
•– polymer chain 
extended by one mer. 
Termination can occur by combination: 
R
x
• + R
y
• → R
x+y
or disproportionation: 
R
x
• + R
y
• → R
x
+ R
y
[62]
.
Free radicals are highly reactive. A radical attacks a molecule of monomer, 
initiating a macroradical, to which other monomer units are added successively. Each time 
the active centre is transferred to the attached mer. This is the way in which propagation of 
polymer chain occurs. 
As the reaction proceeds, the amount of monomer decreases, interpolymeric 
reactions start to occur. Active parts of macroradicals meet each other and termination 
takes place. It leads to dead polymer and may occur via combination or disproportionation.
The main disadvantage of this kind of polymerization are problems with the 
dissipation of the exothermicity in the sample volume. In effect, local hot spots are
formed [63]. During free radical polymerization, an autoacceleration occurs, which is 
known as Trommsdorff or gel effect [64]. This is a phenomenon of spectacular increase of 
polymerization rate. When the reaction speeds up, viscosity grows and huge amounts of 
heat that are released accelerate this reaction further [65].
Trommsdorff effect occurs at high conversion, when termination reactions 
dominate and “dead” polymers, without active parts are produced. The vast majority of 
polymer chains are long and slightly mobile. They are so immobilized that their fastest 
termination is with short, mobile, unentangled chains. Gel effect leads to uncontrolled local 
rise of temperature what may result in the reactor explosion [66].
The other disadvantage of the bulk polymerization is that during the process the 
polymerization shrinkage occurs. The most of the shrinkage appears in the early stages of 
polymerization. It covers about 20 per cent of the initial volume. The shrinkage is a cause 
of cracks and many other flaws like empty void in a block of polymer [67]. This is very 

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33 
disfavourable in case of plastic scintillators because optical homogeneity of the material is 
essential to provide the proper performance of the detectors. 
However, conducting scintillators synthesis with the temperature schedule which is 
presented in Fig. 9 enables to obtain samples without defects. Firstly the reaction mixture 
is quickly heated to 100 °C and then to 140 °C within few hours. Next, the principal 
heating occurs and lasts about 70 hours. After this time, monomer is reacted into polymer 
and the sample is cooled slowly to 90 °C. The scintillator is stored in this temperature for 
three hours in order to anneal the sample. Then the scintillator is finally quenched.