Instant Notes: Analytical Chemistry

The factors that influence the results of thermal methods (see SCRAM in

Topic G1) are considered below.

● The sample is generally about 10 mg of powder, fibers or reactants such as

monomers for plastic production. These are placed into thecrucible,which

should be unreactive and stable over the temperature range used. Platinum,

aluminum, silica or alumina crucibles are commonly used. The sample and

reference pans (either with alumina powder or sometimes an empty pan) are

placed in their holders within the furnace, generally a wire-wound electrical

heater controlled by the computer program.

● The rate of heatingis user-determined, often about 10 K min−^1 , but for the

best approach to equilibrium, low heating rates are needed, and isothermal

experiments may also be carried out. High heating rates save time, and can

simulate situations like burning, but they tend to raise the temperature of

recorded events.

● The atmospheresurrounding the samples can be controlled. A slow flow of

nitrogen gas will give an almost inert atmosphere and sweep away harmful

products. Oxygen may be used to study the oxidative stability of polymers.

Carbon dioxide will react with some oxides to form carbonates.

● The mass of the sample, together with its volume and packing is important

since these determine the heat transfer and the diffusion of gases across the

sample.

● The computer records the values of DT or DP and of the temperature Tand

time t.Computer software has been designed to correct the temperature by

calibration, to measure peak areas and onset points and to calculate reaction

parameters.

Typical physical changes in a sample (of a polymer) are shown in Figure 2. The

first part of the curve shows a small deflection due to the heat capacity of the

solid, glassy polymer. Around 80°C, the material changes to a rubbery nature,

and its heat capacity increases. This is theglass transition, Tg. At about 120°C,

the molecules in the polymer may move freely enough to form the crystalline

polymer and so anexothermic peakfor cold crystallization is observed. This

form is stable until it melts at about 250°C giving an endothermic peak.

Physical
properties and
changes

G2 – Differential thermal analysis and differential scanning calorimetry 313

0.4

0.3

0.2

0.1

0.0

–0.1

–0.2

–0.3

–0.4

50 100 150

Temperature (°C)

200 250 300

Heat flow (W/g)

Fig. 2. DSC of polyethylene terephthalate (PET).