2004 Training Seminars
DSC
Interpreting DSC Data
Glass Transition & Melting
3
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)
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
Heat Flow & Heat Capacity at
the Glass Transition
Sample: Polystyrene
Size: 14.0200 mg
Method: Anneal80
Comment: MDSC.3/40@2; After Anneal @ 80øC various times
DSC
File: C:\TA\Data\Len\FictiveTg\PSanneal80.002
Polystyrene
-0.3
2.0
-0.4
1.5
Glass Transition is Detectable by DSC
Because of a Step-Change in Heat Capacity
Heat Capacity
Heat Flow
-0.7
Temperature Below Tg
- lower Cp
- lower Volume
- lower CTE
- higher stiffness
- higher viscosity
- more brittle
- lower enthalpy
1.0
-0.6
[ ––––– · ] Heat Flow (mW)
Heat Capacity (J/g/°C)
-0.5
-0.8
-0.9
0.5
-1.0
70
Exo Up
90
110
Temperature (°C)
Universal V3.8A TA Instruments
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)
Glass Transition Analysis
Polystyrene
9.67mg
10°C/min
Glass Transition Analysis
Polystyrene
9.67mg
10°C/min
Step Change in Cp at the Glass Transition
PET
9.43mg
% Amorphous = 0.145/0.353= 41%
Aged Epoxy Sample
Effect of Annealing Time on Shape of Tg
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.
Effect of Aging on Amorphous Materials
Physical Property Response on
S
Storage
Below Tg
Specific Volume Decreases
Modulus
Increases
Coefficient of
Expansion
Decreases
Heat Capacity
Enthalpy
Entropy
Decreases
Decreases
Decreases
Max Tg
Storage
V time
H
H
M
S
Equilibrium
Liquid
Equilibrium
Glass
KauzmannTemp; Lowest Tg
(Entropy of Crystal)
Temperature
Where H = High relative cooling rate
M = Medium relative cooling rate
S = Slow relative cooling rate
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
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
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)
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
Melting of Indium
0
Heat Flow (mW)
Extrapolated
-5 Onset
Temperature
-10
156.60°C
28.50J/g
Indium
5.7mg
10°C/min
Heat of
Fusion
For pure, low
molecular weight
materials (mw<500
g/mol) use
Extrapolated Onset as
Melting Temperature
-15
-20
Peak Temperature
-25
150
Exo Up
157.01°C
155
160
Temperature (°C)
165
Universal V4.0B TA Instruments
Melting of PET
-1
For polymers, use Peak as Melting Temperature
-2
Heat Flow (mW)
-3
Extrapolated
Onset
Temperature
236.15°C
52.19J/g
-4
Heat of
Fusion
-5
PET
6.79mg
10°C/min
-6
Peak Temperature
-7
200
Exo Up
210
220
230
240
Temperature (°C)
249.70°C
250
260
270
Universal V4.0B TA Instruments
Comparison of Melting
0
Heat Flow (mW)
-5
156.60°C
28.50J/g
PET
6.79mg
10°C/min
236.15°C
52.19J/g
249.70°C
-10
-15
Indium
5.7mg
10°C/min
-20
157.01°C
-25
140
Exo Up
160
180
200
220
Temperature (°C)
240
260
280
Universal V4.0B TA Instruments
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
Baseline Due to Cp
Baseline Type
DSC of Polymer Blend
More on this sample in
the MDSC® section
Where is the
Cp baseline?
Is it a melt?
YES!
Onset shifts by only 0.3°C
Is it a Melt?
NO!
Onset shifts by almost 30°C
Effect of Heating Rate on Melting
10
Melt
Heat Capacity (J/g/°C)
8
6
10°C/min
50°C/min
4
100°C/min
2
150°C/min
0
-40
0
40
80
120
160
Temperature (°C)
200
240
280
Effect of Heating Rate on Polymorph
DSC at 1C/min
DSC at 1C/min
DSC at 10C/min
DSC at 50C/min
Effect of Impurities on Melting
Effect of p-Aminobenzoic Acid Impurity Concentration
on the Melting Shape/Temperature of Phenacetin
99.3% Pure
Melting of
Eutectic Mixture
96.0% Pure
95.0% Pure
NBS 1514
Thermal Analysis
Purity Set
Approx. 1mg
Crimped Al Pans
2°C/min
100% Pure
Van't Hoff Purity Calculation
-0.8
135.0
125.20°C
137.75°C
-1.0
134.5
-1.4
134.0
-1.6
-1.8
-2.0
Purity: 99.53mol %
Melting Point: 134.92°C (determined)
Depression: 0.25°C
Delta H: 26.55kJ/mol (corrected)
Correction: 9.381%
Molecular Weight: 179.2g/mol
Cell Constant: 0.9770
Onset Slope: -10.14mW/°C
RMS Deviation: 0.01°C
133.5
Total Area / Partial Area
-2.2
122
Exo Up
-2
0
2
4
6
8
10
124
126
128
130
132
134
136
Temperature (°C)
133.0
138
Temperature (°C)
Heat Flow (W/g)
-1.2
2004 Training Seminars
DSC
Interpreting DSC Data
Crystallization, Heat Capacity,
and Crosslinking
4
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
•
Change in Crystallinity While Heating
Heat Flow (W/g)
0.5
60
0.0
40
-0.5
20
-1.0
Integral (J/g)
Quenched PET
9.56mg
10°C/min
1.0
134.63°C
230.06°C
71.96J/g
105.00°C
275.00°C
127.68°C
0.6877J/g
230.06°C
0
-1.5
-50
Exo Up
0
50
100
150
Temperature (°C)
200
250
300
350
Universal V4.0B TA Instruments
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
Crystallization
• Crystallization is a two step process:
 Nucleation
 Growth
• The onset temperature is the nucleation (Tn)
• The peak maximum is the crystallization
temperature (Tc)
Effect of Nucleating Agents
2.0
crystallization
POLYPROPYLENE
WITHOUT
NUCLEATING AGENTS
POLYPROPYLENE
WITH NUCLEATING
AGENTS
Heat Flow (W/g)
1.5
0.0
Heat Flow (W/g)
1.0
-0.5
-1.0
melting
0.5
-1.5
60
80
0.0
40
Exo Up
50
100
120
140
160
180
200
Temperature (°C)
Exo Up
60
70
80
90
100
110
Temperature (°C)
120
130
140
150
160
What is Isothermal Crystallization?
• A Time-To-Event Experiment
Annealing Temperature
Melt Temperature
Isothermal Crystallization
Temperature
Zero Time
Time
Isothermal Crystallization
5
117.4 oC
Polypropylene
Heat Flow (mW)
4
117.8 oC
3
118.3 oC
2
118.8 oC
119.3 oC
119.8 oC
1
120.3 oC
0
-1
1
3
5
Time (min)
7
9
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)
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
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
Calculating Heat Capacity (Cp)
• Depending on the DSC that you have there
are three different ways to calculate Cp
1) Three Run Method – ASTM E1269
 Applicable to all DSC’s
2) Direct Cp – Single Run Method
 Applicable to Q1000 only
3) MDSC® - Single Run Method
 Any TA Instruments DSC w/ MDSC option
 Most accurate determination
Cp by Standard DSC
• Generally, three experiments are run in a
DSC over a specific temperature range
– Empty pan run
– Sapphire run
– Sample run
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
1. Empty pan run
–
–
Match pan/lid weights to ± 0.05 mg
Used to establish a reference baseline
Calculating Cp by Standard DSC
2. 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
3. Sample run
–
–
Typical weight is 10 – 15 mg
Use same weight of pan/lid as before ± 0.05 mg
Cp by Traditional DSC – 3 Runs
Heat Flow
5
400
Baseline Run
0
300
Heat Flow (mW)
200
-10
Calibration Run
-15
100
-20
0
-25
-30
-100
0
10
20
Time (min)
30
40
Temperature (°C)
Sample Run
-5
Cp by Traditional DSC – 3 Runs
6
500
280.00 °C
454.6 J/g
Cp & Total Heat
for PET
4
300
150.00 °C
174.6 J/g
200
2
50.00 °C
1.161 J/g/°C
280.00 °C
1.924 J/g/°C
150.00 °C
1.609 J/g/°C
100
50.00 °C
34.94 J/g
0
0
0
50
100
150
Temperature (°C)
200
250
300
Total Heat (J/g)
Heat Capacity (J/g/°C)
400
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
Direct Cp from a Q1000
Sample: PET; Quenched
Size: 16.0000 mg
Method: Heat@20
Comment: Heat@20
DSC
File: C:...\Crystallinity\RIqPETcycle20.001
6
600
400
135.54°C
0.7311J/g
2
Latent Heat of
Crystallization is Not
Heat Capacity
Running Integral
0
-2
0
50
100
150
Temperature (°C)
200
250
200
[ ––––– · ] Integral (J/g)
4
Heat Capacity (J/g/°C)
275.00°C
530.8J/g
Latent Heat of
Absolute integral
Melting is Not Heat
Capacity
calculates total
heat(Single Run)
Heat Capacity
0
300
Universal V3.8A TA Instruments
Heat Flow w/ Different Heating Rates
Heat Flow Signals Increase in Size
with Increasing Heating Rate
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
Heat Flow & Cp Signals
Polypropylene
Size: 9.21 mg
DSC Cycle @ 10degC/min
Heat Flow on Cooling
Heat Flow on Heating
Heat Capacity on Cooling
Heat Capacity on Heating
Weak Tg Visible in Cp Signal
Heat Capacity on Cooling
Heat Capacity on Heating
Sample: Polypropylene
Size: 9.21 mg
DSC Cycle @ 10 C/min
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
Curing of a Thermoset
12
Method Log:
1: Ramp 10.00 °C/min to 190 °C
135.26°C
Heat Flow (mW)
10
8
6
98.35°C
258.3J/g
4
40
60
80
100
120
Temperature (°C)
140
160
180
200
Partially Cured System
2nd heat shows increased
Tg, due to additional
curing during 1st heat
Note: Small exotherm due to residual cure
Photopolymer Cure by PCA
250
1.08min
Cure of a Photopolymer by PCA
200
Heat Flow (mW)
150
Method Log:
1: Equilibrate at 35.00 °C
2: Isothermal for 1.00 min
3: Light: on @ 20mW/cm2
4: Isothermal for 5.00 min
5: Light: off
6: Isothermal for 2.00 min
7: End of method
1.01min
209.1J/g
100
50
0
0
2
4
Time (min)
6
8
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