Physical Processes - Isoconversional Kinetics of Thermally Stimulated Processes - page 74

Fig. 3.9 When glass is
reheated, its enthalpy crosses
(overshoots) the liquid line
and returns to it at higher
temperature

The enthalpy plot (Fig. 3.9 ) is helpful in understanding the DSC (heat flow)
traces measured on heating and cooling. According to Eq. 3.4, the heat flow related
to the glass transition originates from a change in the heat capacity between the
values for the liquid and glass phase. By virtue of Eq. 3.3, the heat capacity would
change as the first derivative of the plots presented in Fig. 3.9 . On cooling, DSC
shows some small stepwise change in the heat flow. On the other hand, on heating,
the DSC signal is complicated by a peculiar feature associated with enthalpy over-
shoot, which manifests itself as a relatively small endothermic peak at the end of the
glass transition step. Figure 3.10 displays an example of this feature observed in the
glass transition of polystyrene (PS) and PS-clay nanocomposite [ 30 ].
The nonequilibrium structure of the glass phase reveals itself in the relaxation
kinetics. When the glass freezes dynamically, it conserves a heterogeneous structure

Fig. 3.10 Heat capacity
change during the glass
transition in polystyrene
(PS100) and polystyrene-
clay nanocomposite (nPS90)
on heating at 10 ﾰC min −1 .
(Reproduced from Vyazovkin
and Dranca [ 30 ] with permis-
sion of ACS)

2.0

nPS90
PS100

1.9

1.8

1.7

∆ C p =0.34

1.6

∆ C p =0.24

1.5

1.4

1.3

1.2

40 60 80 100 120 140

T / o C