Chapter 31' Thermal Methods

1
Chapter 31. Thermal Methods

• What kinds of problems can be solved with thermal
  analysis?
• Polymers
• qualitative identification (what is it?),
  because polymers have characteristic thermal
  curves
• obtain glass transition temperature (Tg)
  temperature above which polymer chains are able
  to make segmental motions. Polymer goes from a
  glassy amorphous state to rubberlike flexible
  state.
• Other solids
• Melting, boiling, decomposition of organics,
  hydrates, and other solids
• Generating phase diagrams

2
Five types of thermal analysis

• TGA thermogravimetric analysis
• DTA differential thermal analysis
• DSC differential scanning calorimetry
• Pyrolysis GC/MS heating and combusting analyte
  to form fragments based upon combustion, then
  doing GC/MS on fragments
• Accelerating rate calorimetry good for studying
  behavior of explosives and other reactive
  compounds

3
TGA

• temperature of sample is increased (usually
  linearly with time)
• mass of sample in a controlled atmosphere is
  recorded continuously as a function of
  temperature or time
• Mass is plotted versus time - thermogram

4
DTA

• reference material is compared with sample
• Reference material is usually an inert substance
  like alumina, silicon carbide or glass beads
• temperature of both sample and reference material
  is changed (usually increasing linearly with
  time)
• Temperature difference b/w reference and sample
  is calculated and plotted versus sample
  temperature
• Considered a qualitative technique

5
DSC

• Sample and reference are placed in instrument
• Either temperature is ramped or held constant
• Heat flow between reference and sample is
  measured
• NRG flow is measured instead of temperature
  difference
• Considered a quantitative technique
• Technique of choice for Tg determination

6
General instrument diagram