Using DSC

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Using DSC
Krystyna R. Brzezinska Materials Research
Laboratory kbrzez_at_mrl.ucsb.edu
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Summary

• Introduction to DSC.
• Amorphous Structure (Tg).
• Crystalline Structure (Tm)

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DSC

• DSC measures the difference in heat absorbed or
  released by a sample, as compared to an inert
  reference (empty pan), as both are heated, cooled
  or held at constant temperature.

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DSC Cell
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DSC

• Measure Transitions
• Glass Transition Temperature (Tg)
• Melting Temperature (Tm)
• Crystallization Temperature (Tc)

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Think FirstHeat Later

• Does the sample contain volatile components?
• - 2 to 3 water/solvent can lower the glass
• transition temperature (Tg) by up to 100oC.
• - Evaporation creates endothermic peaks in
  standard (non-hermetic) DSC pans and can be
  suppressed with use of hermetic DSC pans.

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• 2. At what temperature does the sample
  decompose?
• Set the upper limit of the DSC experiment based
  on decomposition temperature (TGA). No meaningful
  DSC data can be obtained once decomposition
  results in a 5 weight loss.
• Decomposition affect the quality of the baseline
  due to both endothermic and exothermic heat flow,
  the quality of the baseline for future
  experiments and can affect the useful lifetime of
  the DSC cell due to corrosion.

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• 3. How does thermal history (temperature and
  time) affect DSC results on my sample?
• 4. Identical materials can look totally different
  based on
• Storage temperature and time.
• Cooling rate from a temperature above Tg or above
  the melting point.
• Heating rate.
• Different kinds of experiments may need to be
  performed in order to measure the current
  structure vs. comparing samples to see if the
  materials are the same.

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Amorphous Structure

• Glass Transition (Tg)
• Detectable by DSC due to a step increase in heat
  capacity as the sample is heated to a temperature
  above the glass transition temperature (Tg).
• Important transition because significant changes
  in physical properties, reactivity and storage
  stability occur at Tg.

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Glass Transition (Tg)

• Reporting Tg as a single temp., it is necessary
  to state
• What point in the step change (onset, midpoint or
  end) is being measured.
• The experimental conditions used to measure Tg
  heating rate, sample weight.

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Glass Transition (Tg)

• To increase sensitivity
• Use gt10mg samples.
• Quench cool sample from a temperature above the
  melt to maximize amorphous structure.

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Tg sensitivity
Use gt10oC/min heating rates.
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Glass Transition (Tg)

• As a little as 2-3 water can lower Tg by up to
  100oC.
• To measure an accurate Tg in a sample with a
  volatile component by running sample in a
  hermetic (sealed) pan.
• Use a dry-box or dry-bag to prepare samples in
  hermetic pans. This eliminates water absorption
  during preparation and loss water during the
  measurement.

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Crystalline Structure

• Crystalline structure in a sample is determined
  from the presence of an endothermic melting peak.
• Important complimentary techniques to DSC
  include
• Hot Stage Microscopy
• X-Ray Diffraction (XRD)
• Nuclear Magnetic Resonance (NMR)
• Infrared Spectroscopy

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Crystalline Structure

• Factors which complicate DSC analysis
• Endotermic peaks can be created by evaporation
  and decomposition as well as melting.
• TGA should be done on all new samples prior to
  DSC to determine volatile content and
  decomposition temperature.
• Dehydyration/Desolvation usually results in loss
  of crystalline structure.
• Melting is a thermodynamic transition and
  therefore, the onset of melting does not change
  significantly with heating rate.
• Decomposition is a kinetic (time-dependent)
  transition and therefore, the onset temperature
  of the peak shifts to a significantly higher
  temperature at higher heating rate.