Using DSC Krystyna R. Brzezinska Materials Research Laboratory firstname.lastname@example.org
Introduction to DSC. Amorphous Structure (Tg). Crystalline Structure (Tm) Summary
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 • Measure Transitions: • Glass Transition Temperature (Tg) • Melting Temperature (Tm) • Crystallization Temperature (Tc)
Think First………Heat 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.
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.