Biomedical Engineering Reference
In-Depth Information
is used as in conventional DSC. However, in MDSC a dif erential heat-
ing proi le is applied to the sample and reference. Specii cally a sinusoidal
modulation (oscillation) is overlaid on the conventional linear heating and
cooling ramp to yield a proi le in which the average sample temperature
continuously changes with time but not in a linear fashion.
h e net ef ect of imposing this more complex heating proi le on the
sample is the same as if two experiments are running simultaneously on
the material—one experiment at the traditional heating rate and another
at a sinusoidal heating rate. h e actual heating rate for these two simul-
taneous experiments is dependent on three operator-selectable variables,
which are underlying heating rates, period of modulation and temperature
amplitude of modulation. h e general equation which describes the resul-
tant heat l ow at any point in a DSC or MDSC experiments is
dQ/dt = Cpβ + f (T, t)
(13.11)
where dQ/dt is the heat l ow; β is the heating rate; Cp is the heat capacity,
and; f (T, t) is the heat l ow from kinetic (absolute temperature and time
dependent) processes.
It can be seen from the equation that the total heat l ow (dQ/dt), which
is the only heat l ow measured by conventional DSC, is composed of two
components. One component is a function of the sample's heat capacity
and rate of temperature change and the other is a function of absolute tem-
perature and/or time. Modulated DSC determines the total as well as these
two individual heat l ow components to provide a better understanding of
the complex transitions in materials. h e MDSC is able to do this based on
the two heating rates seen by the material:
i. the average heating rate which provides total heat l ow infor-
mation, and
ii. the sinusoidal heating rate which provides heat capacity
information from the heat l ow that respond to the rate of
temperature change.
13.6.2.2.1 Heat Capacity
h e heat capacity (Cp) of the sample is continuously determined by divid-
ing the modulated heat l ow amplitude by the modulated heating rate
amplitude. h is approach is based on the well-accepted procedure for
determining Cp in conventional DSC. In conventional DSC, Cp is gener-
ally calculated from the dif erence in heat l ow between a blank (empty
pan) run and a sample run under identical conditions
