Chemical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 168

of 2-hydroxyethyl methacrylate [ 12 ]. The glass transition temperature of poly(2-
hydroxyethyl methacrylate) is reported [ 13 , 14 ] to be around 90ﾰC. That is, all
polymerization runs in Fig. 4.3 are conducted below the glass transition temperature
of the polymer product. The lower the polymerization temperature falls below the
glass transition temperature of the polymer, the smaller the conversion at which the
reacting system vitrifies. If at 80 ﾰC the systems vitrifies at ʱ ~ 0.9, at 50 ﾰC the ʱ
value of vitrification is around 0.75. These results are obviously consistent with the
general trend shown in Fig. 4.2 .
It should be noted that vitrification of a reaction system introduces significant
complications into the polymerization kinetics. Examples of such complications
are discussed later. At this point, it should be mentioned that in most of polymeriz-
ing systems the isothermal kinetic measurements have to be performed at tempera-
tures below the glass transition temperature of the polymer product. This is because
above this temperature polymerization is too fast to follow with regular experimen-
tal techniques as differential scanning calorimetry (DSC). For example, it is seen
from Fig. 4.3 that at 80 ﾰC the process is completed in less than 10 min. Increasing
temperature above 90 ﾰC, which is approximately the glass transition temperature of
the polymer, would reduce the completion time to a few minutes. For a typical DSC
instrument, this would be insufficient time to equilibrate before taking the measure-
ments that would deem the latter impossible.
It is much simpler to avoid vitrification when performing kinetic measurements
under nonisothermal conditions. Simple heating at a constant heating rate may re-
sult in three different situations with respect to vitrification during polymerization.
They are illustrated in Fig. 4.4 (curves 1, 2, and 3). When heating occurs at a slow
heating rate, at some extent of conversion (point A on curve 1 in Fig. 4.4 ), the glass
transition temperature of the monomer-polymer system may rise above the sample
temperature. At this point, the system would vitrify causing polymerization to slow
down dramatically or practically stop. However, if temperature continues to rise, it
will ultimately reach the T g,p value. Then the glassy system would convert back to
the liquid state and polymerization would resume. A similar process may occur at
Fig. 4.3 Isothermal polym-
erization of 2-hydroxyethyl
methacrylate at different tem-
peratures. (Reproduced from
Achilias [ 12 ] with permission
of Springer)