1. Glass Transition & Melting3
2004 Training Seminars DSC
Interpreting DSC Data
Glass Transition & Melting

2. Glass Transitions
The glass transition is a step change in molecular mobility (in the amorphous phase of a sample) that results in a step change in heat capacity
The material is rigid below the glass transition temperature and rubbery above it. Amorphous materials flow, they do not melt (no DSC melt peak)

3. Glass Transitions
The change in heat capacity at the glass transition is a measure of the amount of amorphous phase in the sample
Enthalpic recovery at the glass transition is a measure of order in the amorphous phase. Annealing or storage at temperatures just below Tg permit development of order as the sample moves towards equilibrium

4. Heat Flow & Heat Capacity at the Glass Transition
Polystyrene

5. Measuring/Reporting Glass Transitions
The glass transition is always a temperature range
The molecular motion associated with the glass transition is time dependent. Therefore, Tg increases when heating rate increases or test frequency (MDSC®, DMA, DEA, etc.) increases.
When reporting Tg, it is necessary to state the test method (DSC, DMA, etc.), experimental conditions (heating rate, sample size, etc.) and how Tg was determined
Midpoint based on ½ Cp or inflection (peak in derivative)

6. Glass Transition Analysis
Polystyrene 9.67mg 10°C/min

7. Glass Transition Analysis
Polystyrene 9.67mg 10°C/min

8. Step Change in Cp at the Glass Transition
PET 9.43mg
% Amorphous = 0.145/0.353= 41%

9. Aged Epoxy Sample

10. Effect of Annealing Time on Shape of Tg

11. Importance of Enthalpic Relaxation
Is enthalpic recovery at the glass transition important? …Sometimes
Glass transition temperature, shape and size provide useful information about the structure of the amorphous component of the sample.
This structure, and how it changes with time, is often important to the processing, storage and end-use of a material.
Enthalpic recovery data can be used to measure and predict changes in structure and other physical properties with time.

12. Effect of Aging on Amorphous Materials
Decreases
Entropy
Enthalpy
Heat Capacity
Coefficient of
Expansion
Increases
Modulus
Specific Volume
Response on S
Physical Property
Storage Below Tg H
Max Tg
Storage V
time H
Equilibrium M
Liquid S
Equilibrium
Glass
Kauzmann
Temp; Lowest Tg
(Entropy of Crystal)
Temperature
Where H =
High relative cooling rate M =
Medium relative cooling rate S =
Slow relative cooling rate

13. Suggestions for Finding Weak Glass Transitions
Know your empty-pan baseline
Get as much material in the amorphous state
Cool slowly through the glass transition region
Heat rapidly through glass transition region
Use MDSC®
Or use Quasi-Isothermal MDSC

14. Glass Transition Summary
The glass transition is due to Amorphous material
The glass transition is the reversible change from a glassy to rubbery state & vice-versa
DSC detects glass transitions by a step change in Cp

15. Melting Definitions
Melting – the process of converting crystalline structure to a liquid amorphous structure
Thermodynamic Melting Temperature – the temperature where a crystal would melt if it had a perfect structure (large crystal with no defects)
Metastable Crystals – Crystals that melt at lower temperature due to small size (high surface area) and poor quality (large number of defects)

16. Definitions (cont.)
Crystal Perfection – the process of small, less perfect crystals (metastable) melting at a temperature below their thermodynamic melting point and then (re) crystallizing into larger, more perfect crystals that will melt again at a higher temperature
True Heat Capacity Baseline – often called the thermodynamic baseline, it is the measured baseline (usually in heat flow rate units of mW) with all crystallization and melting removed…. assumes no interference from other latent heat such as polymerization, cure, evaporation etc. over the crystallization/melting range

17. Extrapolated Onset Temperature
Melting of Indium
Extrapolated Onset Temperature
Heat of Fusion
For pure, low molecular weight materials (mw<500 g/mol) use Extrapolated Onset as Melting Temperature
Peak Temperature

18. Melting of PET For polymers, use Peak as Melting Temperature
Extrapolated Onset Temperature
Heat of Fusion
Peak Temperature

19. Comparison of Melting

20. Analyzing/Interpreting Results
It is often difficult to select limits for integrating melting peaks
Integration should occur between two points on the heat capacity baseline
Heat capacity baselines for difficult samples can usually be determined by MDSC® or by comparing experiments performed at different heating rates
Sharp melting peaks that have a large shift in the heat capacity baseline can be integrated with a sigmoidal baseline

21. Baseline Due to Cp

22. Baseline Type

23. DSC of Polymer Blend More on this sample in the MDSC® section
Where is the Cp baseline?

24. Is it a melt?
YES!
Onset shifts by only 0.3°C

25. Onset shifts by almost 30°C
Is it a Melt?
NO!
Onset shifts by almost 30°C

26. Effect of Heating Rate on Melting

27. Effect of Heating Rate on Polymorph
DSC at 1C/min
DSC at 1C/min
DSC at 10C/min
Comparison of all 3 heating rates
DSC at 50C/min

28. Effect of Impurities on Melting
Effect of p-Aminobenzoic Acid Impurity Concentration on the Melting Shape/Temperature of Phenacetin
99.3% Pure
100% Pure
Melting of Eutectic Mixture
96.0% Pure
95.0% Pure
As sample becomes less pure the melting peak broadens and shifts to a lower melting temperature. Higher levels of impurities forms an Eutectic – The lowest melting point of an alloy or soln that is obtainable by varying the proportions of the components
NBS Thermal Analysis Purity Set
Approx. 1mg Crimped Al Pans 2°C/min

29. Van't Hoff Purity Calculation

30. Crystallization, Heat Capacity,4
2004 Training Seminars DSC
Interpreting DSC Data
Crystallization, Heat Capacity,
and Crosslinking

31. Crystallinity Definitions
Crystallization – the process of converting either solid amorphous structure (cold crystallization on heating) or liquid amorphous structure (cooling) to a more organized solid crystalline structure
Crystal Perfection – the process of small, less perfect crystals (metastable) melting at a temperature below their thermodynamic melting point and then (re) crystallizing into larger, more perfect crystals that will melt again at a higher temperature

32. Change in Crystallinity While Heating
Quenched PET 9.56mg 10°C/min

33. Crystallization
Crystallization is a kinetic process which can be studied either while cooling or isothermally
Differences in crystallization temperature or time (at a specific temperature) between samples can affect end-use properties as well as processing conditions
Isothermal crystallization is the most sensitive way to identify differences in crystallization rates

34. Crystallization Crystallization is a two step process: Nucleation
Growth
The onset temperature is the nucleation (Tn)
The peak maximum is the crystallization temperature (Tc)

35. Effect of Nucleating Agents
POLYPROPYLENE
WITH NUCLEATING
AGENTS
WITHOUT NUCLEATING AGENTS
crystallization
melting
Effect of Nucleating Agents

36. What is Isothermal Crystallization?
A Time-To-Event Experiment
Annealing Temperature
Melt Temperature
Isothermal Crystallization Temperature
Temperature
Zero Time
Time

37. Isothermal Crystallization
Polypropylene

38. Specific Heat Capacity (Cp)
Heat capacity is the amount of heat required to raise the temperature of a material by 1°C from T1 to T2
True Heat Capacity (no transition) is completely reversible; the material releases the same amount of heat as temperature is lowered from T2 to T1
Specific Heat Capacity refers to a specific mass and temperature change for a material (J/g/°C)
See slide

39. Why is Heat Capacity Important?
Absolute thermodynamic property (vs. heat flow) used by engineers in the design of processing equipment
Measure of molecular mobility
Cp increases as molecular mobility increases.
Amorphous structure is more mobile than crystalline structure
Provides useful information about the physical properties of a material as a function of temperature
Heat Capacity is an absolute thermodynamic property to measure and an important physical property to measure as a function of temperature.

40. Does DSC Measure Heat Capacity?
DSC or MDSC® do not measure heat capacity directly. They measure heat flow rate which can be used to calculate heat capacity which is more appropriately called apparent heat capacity
DSC calculated Cp signals include all transitions because the heat flow signal is simply divided by heating rate (an experimental constant) to convert it to heat capacity units
A true value of Cp can only be obtained in temperature regions where there are no transitions

41. Calculating Heat Capacity (Cp)
Depending on the DSC that you have there are three different ways to calculate Cp
Three Run Method – ASTM E1269
Applicable to all DSC’s
Direct Cp – Single Run Method
Applicable to Q1000 only
MDSC® - Single Run Method
Any TA Instruments DSC w/ MDSC option
Most accurate determination

42. Cp by Standard DSC
Generally, three experiments are run in a DSC over a specific temperature range
Empty pan run
Sapphire run
Sample run
Traditionally, heat capacity is measured in a DSC by running three experiments. They are as shown in slide.

43. Calculating Cp by Standard DSC
Three experiments are run over a specific temperature range
Allow 5 minute isothermal at start
Use 20°C/min heating rate
Empty pan run
Match pan/lid weights to ± 0.05 mg
Used to establish a reference baseline

44. Calculating Cp by Standard DSC
Sapphire run
Used to determine calibration constant
Use same weight of pan/lid as for baseline ± 0.05 mg
Typical weight is 20 – 25 mg
Sample run
Typical weight is 10 – 15 mg
Use same weight of pan/lid as before ± 0.05 mg

45. Cp by Traditional DSC – 3 Runs
Heat Flow
Baseline Run
Sample Run
Calibration Run

46. Cp by Traditional DSC – 3 Runs

47. Specific Heat Capacity
MDSC® & Tzero™ DSC have the ability to calculate a heat capacity signal directly from a single run.
Benefits of using a heat capacity (instead of heat flow) signal include:
The ability to overlay signals from samples run at different heating rates
The ability to overlay signals from heating and cooling experiments
Advanced and patented techniques from TA Instruments offer the advantage of directly measuring heat capacity in a single run. The heat capacity signal offers additional benefits over using traditional heat flow signal.

48. Direct Cp from a Q1000 Absolute integral calculates total heat
Latent Heat of Melting is Not Heat Capacity
Absolute integral calculates total heat
Latent Heat of Crystallization is Not Heat Capacity

49. Heat Flow w/ Different Heating Rates
Heat Flow Signals Increase in Size with Increasing Heating Rate

50. Benefit of Plotting Heat Capacity
Heat Capacity Signals Are Normalized for Heating Rate and Permit Comparison of Experiments Done at Different Heating Rates
Remember, DSC and MDSC Cp signals are really Apparent Cp signals; crystallization and melting are latent heats, not Cp

51. Heat Flow & Cp Signals Polypropylene Size: 9.21 mg
DSC 10degC/min
Heat Flow on Cooling
Heat Flow on Heating
Heat Capacity on Cooling
Heat Capacity on Heating

52. Weak Tg Visible in Cp Signal
Heat Capacity on Cooling
Heat Capacity on Heating
Sample: Polypropylene
Size: 9.21 mg
DSC 10 C/min
Slide shows advantage of using heat capacity signal for picking out weak Tg in polypropylene after heating and cooling scans. Also you can compare Tg on heating and cooling because Cp overlays but you would not see same comparison in Heat flow.

53. Thermoset Curing & Residual Cure
A “thermoset” is a cross-linked polymer formed by an irreversible exothermic chemical reaction
A common example would be a 2 part epoxy adhesive
With a DSC we can look at the curing of these materials, and the Tg of full or partially cured samples

54. Curing of a Thermoset

55. Partially Cured System
2nd heat shows increased Tg, due to additional curing during 1st heat
Note: Small exotherm due to residual cure

56. Photopolymer Cure by PCA
The PCA is an accessory for the Q1000 / Q100 DSC modules that features a medium pressure mercury UV source and dual light guides. It is ideal for rapid initiation of unsaturated materials (e.g., acrylics)

57. Use PDSC to Study Phenolic Curing
With ambient pressure, curing is not visible due to volatization of water. Water comes from the condensation reaction during the curing of the phenolic
Decomposition
After an "equilibrate at 200 oC" method segment, the isothermal temperature is stable to within 0.04 oC during the first 5 minutes and is then stable to within 0.01 oC for the remaining 95 minutes.