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

Figure 34 Thermogram of pure polyvinyltoluene. Red line denotes heating, blue one cooling of the sample.
 
In both cases around 350 K there are peaks corresponding to the glass transition 
temperature T
g
. In the scintillator, glass transition determined from the heating curve, 
occurs in temperature 354 K while in pure polyvinyltoluene in 364 K. T
g
values 
determined using DSC are more similar to each other than results obtained from PALS 
analysis. They are also comparable with literature data. Glass transition temperature of 
polyvinyltoluene, depending on its structure, lies in range 366 - 391 K [101]. The 
difference of 2 K between the literature and experimental data is negligible because results 
are dependent on many factors, including the rate of heating.
In general, T
g
obtained in DSC are significantly larger in comparison to results of 
PALS. It is due to the fact that T
g
is associated with the onset of a coordinated segmented 
motion about polymer axis. The pick-off process of o-Ps is much more sensitive to such 
changes and reorganization in the macromolecule than DSC, in which only macroscopic 
effects can be observed, occurring in the sample as a whole [102]. Therefore temperatures 
determined by PALS may be interpreted as the onset of changes in the structure of 
macromolecule.
The other issue that should be discussed is time scale of both experiments. In PALS 
the sample was stored in each temperature for 2 hours, while in DSC the rate was set to

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68 
10 K/min. Different rate of temperature changes plays an important role even using the 
same technique and it has a discernible impact on obtained results [102]. Obtaining such 
discrepancy in results when using two different methods with two radically different rates 
of heating was expected.
Generally it is assumed that the softening point of amorphous polymers is 20 K 
below the glass transition point [103]. Therefore, softening temperature of J-PET 
scinillator is at 334 K (61 ºC). The value of softening point of commercial scintillators 
produced by Saint Gobain, which are also based on polyvinyltoluene is 343 K (70 ºC) [12]. 
Discrepancy between both values is relatively large, but the method of determination of 
softening point used by Saint Gobain is not given, so they cannot be compared with the 
high precision. Softening point can be interpreted as the maximal temperature in which 
scintillator can be used. However, PALS analysis indicates that changes in the J-PET 
scintillator structure start to occur in much lower temperature region including also room 
temperature in which scintillators are being used.