Influence of the Processes Parameters on the Properties of The ...

Chapter 3.
Analytical Methods and Designs <strong>of</strong> Experiments
Differential scanning calorimetry (DSC) is a <strong>the</strong>rmoanalytical technique in which <strong>the</strong> difference in
<strong>the</strong> amount <strong>of</strong> heat required to increase <strong>the</strong> temperature <strong>of</strong> a sample and reference is measured as a function
<strong>of</strong> temperature. Both <strong>the</strong> sample and reference are maintained at nearly <strong>the</strong> same temperature throughout <strong>the</strong>
experiment. Generally, <strong>the</strong> temperature program for a DSC analysis is designed such that <strong>the</strong> sample holder
temperature increases linearly as a function <strong>of</strong> time.
The technique was developed by Watson and O’neill [1962] and was introduced commercially at
<strong>the</strong> 1963, Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy [ISO - International
Organization for Standardization, 1963]. Heat-flux DSC and power-compensated DSC are <strong>the</strong> two types <strong>of</strong>
DSC that have been widely used.
One <strong>of</strong> <strong>the</strong> big advantages <strong>of</strong> DSC is that samples are very easily encapsulated, usually with little
or no preparation, ready to be placed in <strong>the</strong> DSC cell, so that measurements can be quickly and easily made
[Gabbott, 2008]. The specific heat <strong>of</strong> a material changes slowly with temperature in a particular physical
state, but alters discontinuously at a change <strong>of</strong> state. As well as increasing <strong>the</strong> sample temperature, <strong>the</strong>
supply <strong>of</strong> <strong>the</strong>rmal energy may induce physical or chemical processes in <strong>the</strong> sample, e.g. melting or
decomposition, accompanied by a change in enthalpy, <strong>the</strong> latent heat <strong>of</strong> fusion, heat <strong>of</strong> reaction etc.
In a heat flux DSC, <strong>the</strong> sample material, enclosed in a pan and an empty reference pan are placed
on a <strong>the</strong>rmoelectric disk surrounded by a furnace. The furnace is heated at a linear heating rate and <strong>the</strong> heat
is transferred to <strong>the</strong> sample and reference pan through <strong>the</strong>rmoelectric disk (cf. Figure 3.1). The temperatures
<strong>of</strong> <strong>the</strong> two <strong>the</strong>rmometers are compared, and <strong>the</strong> electrical power supplied to each heater adjusted, so that <strong>the</strong>
temperatures <strong>of</strong> both <strong>the</strong> sample and <strong>the</strong> reference remain equal to <strong>the</strong> programmed temperature, i.e. any
temperature difference which would result from a <strong>the</strong>rmal event in <strong>the</strong> sample is ‘zero’. The analogical
signal, <strong>the</strong> rate <strong>of</strong> energy absorption by <strong>the</strong> sample (e.g. W/s), is proportional to <strong>the</strong> specific heat <strong>of</strong> <strong>the</strong>
sample since <strong>the</strong> specific heat at any temperature determines <strong>the</strong> amount <strong>of</strong> <strong>the</strong>rmal energy necessary to
change <strong>the</strong> sample temperature by a given amount. In o<strong>the</strong>r words, <strong>the</strong> measuring principle is to compare <strong>the</strong>
rate <strong>of</strong> heat flow to <strong>the</strong> sample and to an inert material which are heated or cooled at <strong>the</strong> same rate.
1.2 First Order Transitions
(A)-Apparatus
(B)-Principle
Figure 3.1: Differential scanning calorimetry.
Changes in <strong>the</strong> sample, which are associated with absorption or evolution <strong>of</strong> heat, cause a change
in <strong>the</strong> differential heat flow which is <strong>the</strong>n recorded as a peak. The area under <strong>the</strong> peak is directly
proportional to <strong>the</strong> enthalpy change and its direction indicates whe<strong>the</strong>r <strong>the</strong> <strong>the</strong>rmal event is endo<strong>the</strong>rmic or
exo<strong>the</strong>rmic. Analysis <strong>of</strong> a DSC <strong>the</strong>rmogram enables <strong>the</strong> determination <strong>of</strong> two important parameters: <strong>the</strong>
transition temperature peak (taken at <strong>the</strong> maximum T m or at <strong>the</strong> onset T onset ), and <strong>the</strong> enthalpy <strong>of</strong>
melting/crystallization (H m /H c ). The extrapolated onset temperature (T onset ) corresponding to <strong>the</strong> transition
temperature at zero heating rate can be obtained by plotting peak temperatures as a function <strong>of</strong> heating rate
[Ruegg et al., 1977].
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