Download Using DSC

Using DSC
Krystyna R. Brzezinska
Materials Research Laboratory
[email protected]
Summary
1. Introduction to DSC.
2. Amorphous Structure (Tg).
3. Crystalline Structure (Tm)
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.
DSC Cell
DSC
• Measure Transitions:
- Glass Transition Temperature (Tg)
- Melting Temperature (Tm)
- Crystallization Temperature (Tc)
Think First………Heat Later
1. 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.
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.
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.
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.
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.
Glass Transition (Tg)
• To increase sensitivity:
- Use >10mg samples.
- Quench cool sample from a
temperature above the melt to
maximize amorphous structure.
Tg sensitivity
Use >10oC/min heating rates.
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.
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
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.