Techniques for the characterization of physical gels - Physical Gels from Biological and Synthetic Polymers - page 29

Peak (endothermic)

Φ m

Baseline (interpolated)

Zero line

T i T onset T peak T o ffset T f

Temperature

Heat flux versus temperature in a DSC experiment with a sample undergoing a phase transition.

Figure 2.7

ow and so
to derive its heat capacity, the zero line has to be subtracted from the baseline. The zero
line is the curve measured with an empty sample pan and conditions identical to the
experiment itself. The second term in ( 2.19 ) is associated with the enthalpy change due to
a reaction or a phase transition. In this case, a peak appears during the temperature ramp,
as can be seen in Figure 2.7 , where heat

The first term in ( 2.19 ) is related to the baseline. To get the true sample heat

flux is plotted versus temperature.
By international convention, when heat is added to a system (as in melting transitions)
the endothermic peak is plotted upwards (or in a positive direction) and when heat is
released from a system, as in crystallization, the exothermic peak is plotted in the
opposite direction (this convention is not always applied in the literature).
This peak can be characterized by several points ( Figure 2.7 ): T i , the initial deviation from
the baseline; T peak , the maximum/minimum position; T onset , the extrapolated onset temper-
ature obtained from the intersection of the linear ascending peak slope with the baseline; and
T f , the temperature when the measured heat

flux rate again reaches the baseline.
In general, the melting of pure substances gives a very steep rise in the heat

ux, and
the melting temperature is determined at the onset temperature. The onset temperature,
peak position and peak shape depend on the rate of heating. In particular, some gels
contain very small crystalline domains and the peaks are rather wide. This requires that
any characteristic temperature chosen to describe the thermal evolution of system should
mention the rate of change of the temperature.
Another important parameter derived from DSC measurements is the area under the
peak, which provides the total enthalpy of the transition. The enthalpy change between
the limiting temperatures Ti i and T f can be calculated by integrating the heat flux, after the

baseline has been subtracted. When dealing with the melting of pure substances, the heat
capacities of the solid and the liquid states differ in absolute value and in their temper-
ature dependence, so the baseline is shifted after the transition. To derive the exact
melting enthalpy, the baseline correction must be performed very carefully. By contrast,
for gels the solvent phase is the major component and no changes are expected for the
baseline during gelation or melting.