1
2004 Training Seminars
DSC
Understanding DSC
Agenda
•
•
•
•
•
•
What does a DSC measure?
How does a DSC make that measurement?
How is a Tzero™ DSC different?
Tzero Calibration
Tzero Results
Advanced Tzero
What Does a DSC Measure?
A DSC measures the difference in heat flow rate
(mW = mJ/sec) between a sample and inert
reference as a function of time and
temperature
Endothermic Heat Flow
0.1
• Heat Flow
 Endothermic: heat flows into the sample as
a result of either heat capacity (heating) or
some endothermic process (glass
transition, melting, evaporation, etc.)
0.0
Heat Flow (W/g)
-0.1
-0.2
-0.3
-0.4
0
Exo Up
25
50
75
Temperature (°C)
100
125
150
Exothermic Heat Flow
Heat Flow (W/g)
0.1
• Heat Flow
 Exothermic: heat flows out of the sample
as a result of either heat capacity
(cooling) or some exothermic process
(crystallization, cure, oxidation, etc.)
0.0
-0.1
0
Exo Up
20
40
60
80
Temperature (°C)
100
120
140
160
Temperature
• What temperature is being measured and
displayed by the DSC?
Sensor Temp: used by most DSCs. It is
measured at the sample platform with a
thermocouple, thermopile or PRT.
Sample Platform
Chromel Area Detector
Reference Platform
Constantan Body
Thin Wall Tube
Base Surface
Constantan Wire
Chromel Wire
Chromel Wire
Temperature
• What temperature is being measured and
displayed by the DSC?
Pan Temp: calculated by TA Q1000 based
on pan material and shape
 Uses weight of pan, resistance of pan, &
thermoconductivity of purge gas
What about sample temperature?
 The actual temperature of the sample is
never measured by DSC
Temperature
• What other temperatures are not typically
being displayed.
Program Temp: the set-point temperature
is usually not recorded. It is used to control
furnace temperature
Furnace Temp: usually not recorded. It
creates the temperature environment of
the sample and reference
Understanding DSC Signals
Heat Flow
• Relative Heat Flow: measured by all DSCs
except TA Q1000. The absolute value of the
signal is not relevant, only absolute changes
are used.
• Absolute Heat Flow: used by Q1000.
Dividing the signal by the measured heating
rate converts the heat flow signal into a heat
capacity signal
DSC Heat Flow
dH
 DSC heat flow signal
dt
Cp  Sample Heat Capacity
 SampleSpecific Heat xSample Weight
dH
dT
 Cp
 f (T, t)
dt
dt
dT
 Heating Rate
dt
f (T ,t)  Heat flow thatis functionof time
at an absolute temperatu
re (kinetic)
Tzero Heat Flow Equation
Heat Flow
Sensor Model
qr
qs
Cs
Cr
Tr
Ts
Besides the three
temperatures (Ts, Tr, T0);
what other values do we
need to calculate Heat
Flow?
Rr
Rs
How do we calculate these?
T0
 1
T
1
dTs
dT
q
 T0     Cr  Cs 
 Cr
Rr
d
d
 Rs Rr 
Measuring the C’s & R’s
• Tzero™ Calibration calculates the C’s & R’s
• Calibration is a misnomer, THIS IS NOT A
•
CALIBRATION, but rather a measurement of
the Capacitance (C) and Resistance (R) of
each DSC cell
After determination of these values, they can be
used in the Four Term Heat Flow Equation
showed previously
Measuring the C’s & R’s
• Preformed using Tzero™ Calibration Wizard
1. Run Empty Cell
2. Run Sapphire on both Sample & Reference
side
Measuring the C’s & R’s
Empty DSC constant heating rate
Assume:
qs  qr  0
Heat balance equations give sensor time constants
T0
 s  Cs Rs 
dTs
d
T0  T
 r  Cr Rr 
dTs dT

d
d
Measuring the C’s & R’s
Repeat first experiment with sapphire disks on sample
and reference (no pans)
dTs
dTr
Assume:
qs  ms csapph
qr  mr csapph
d
d
Use time constants to calculate heat capacities
ms csapph
Cs 
T0
1
dTs
s
d
mr csapph
Cr 
T0  T
1
 dTs dT 


 r
d 
 d
Measuring the C’s & R’s
Use time constants and heat capacities to calculate
thermal resistances
Rs 
s
Cs
Rr 
r
Cr
A few words about the Cs and Rs
• The curves should be smooth and continuous,
•
•
•
without evidence of noise or artifacts
Capacitance values should increase with
temperature (with a decreasing slope)
Resistance values should decrease with
temperature (also with a decreasing slope)
It is not unusual for there to be a difference
between the two sides, although often they are
very close to identical
Good Tzero™ Calibration Run
Bad Tzero™ Calibration Run
Can see that it is bad during
Tzero™ cal run
Before Running Tzero™ Calibration
• System should be dry
• Dry the cell and the cooler heat exchanger
using the cell/cooler conditioning template and
the default conditions (2 hrs at 75°C) with the
cooler off
Preferably enable the secondary purge
Do not exceed 75°C cell temperature with
the cooler off, although the time can be
extended indefinitely
Stabilization before Calibration
• System must be stable before Tzero™
•
•
Calibration
Stabilization is achieved by cycling the baseline
over the same temperature range and using the
same heating rate as will be used for the
subsequent calibration
Typical systems will stabilize after 3-4 cycles, 8
cycles recommended to ensure that the system
has stabilized
Tzero™ Calibration Conditions
• Normally, Heat Only calibration is all that is
•
•
necessary
Heating Rate should be 20°C/min
Temperature Limits based on cooler type
 RCS; –90 to 400°C
 LNCS; –180 to 300°C
• Use Diagnostic signals to improve
troubleshooting capability
Enable and Select Diagnostic Signals
Check this box!
Enable and Select Diagnostic Signals
Select 1-8 for an RCS
or all of them for an LNCS
More on Diagnostic Signals later
Example of Typical Results
50
50
40
40
This cell is very well balanced. It is acceptable and usual
to have larger differences between sample and reference.
30
-200
-100
0
100
Temperature (°C)
200
0.05
0.04
0.04
0.03
0.03
0.02
0.02
0.01
300
Sample Capacitance (Joule/°C)
60
Reference Capacitance (Joule/°C)
60
Reference Resistance (°C/Watt)
Sample Resistance (°C/Watt)
Characteristics
of the thermal resistances and heat capacities:
70
Both curves should be smooth, with no steps, spikes or inflection points.
Thermal resistances should always have negative slope that gradually decreases.
Heat capacities should always have positive slope that gradually decreases.
Tzero™ vs Conventional Baseline
0.6
Conventional Baseline
T zero Baseline
Heat Flow (mW)
0.4
0.2
0.0
-0.2
-0.4
-100
0
100
200
Temperature (°C)
300
400
Indium with Q Series Heat Flow Signals
Q1000
Q100
Q10
2
2004 Training Seminars
DSC
How to Get Better
DSC Results
Agenda
•
•
•
•
Keeping your DSC cell clean
Calibration
Sample Preparation
Thermal Method
Agenda
•
•
•
•
Keeping your DSC cell clean
Calibration
Sample Preparation
Thermal Method
Keeping the DSC Cell Clean
• One of the first steps to ensuring good data is
•
to keep the DSC cell clean
How do DSC cells get dirty?
Decomposing samples during DSC runs
Samples spilling out of the pan
Transfer from bottom of pan to sensor
How do we keep DSC cells clean?
• DO NOT DECOMPOSE SAMPLES IN
THE DSC CELL!!!
• Run TGA to determine the decomposition
•
•
•
temperature
Stay below that temperature!
Make sure bottom of pans stay clean
Use lids
Use hermetic pans if necessary
TGA Gives Decomposition Temperature
Cleaning Cell
• If the cell gets dirty
Clean w/ brush
 Brush gently both sensors and cell if
necessary
 Be careful with the Tzero™ thermocouple

Blow out any remaining particles
Brushing the Sample Sensor
Calibration
• Heat Flow (Cell Constant) (All DSC’s)
• Temperature Calibration (All DSC’s)
• Direct Cp (Q1000)
Heat Flow Calibration (Cell Constant)
• Heat Flow Calibration of Differential Scanning
•
•
•
Calorimeters – ASTM E-968
Enthalpy Calibration
Performed using Calibration Wizard
One Run
Indium metal




Sample Weight 1-5mg
Pre-melt sample the first time you run it
Heating rate of 10°C/min
Dependent upon purge gas
Cell Constant
• The cell constant is calculated as the ratio of
the theoretical heat of fusion of a standard
material, to the measured heat of fusion
Cell Constant  Hf (Theoretical) / Hf (Measured)
• Cell Constant should be 0.95-1.20 in N2
Calibration
• Heat Flow (Cell Constant) (All DSC’s)
• Temperature Calibration (All DSC’s)
• Direct Cp (Q1000)
Temperature Calibration
• Temperature Calibration of Differential
•
•
•
Scanning Calorimeters – ASTM E-967
Performed using Calibration Wizard
Indium Cell constant run also performs
temperature calibration
Can do up to 5 standards
Pure metals typically used - In, Sn, Zn, Pb
We’ve found that on the Q series DSC’s one
temperature calibration point is all that is
usually needed
Calibration
• Heat Flow (Cell Constant) (All DSC’s)
• Temperature Calibration (All DSC’s)
• Direct Cp (Q1000)
Direct Cp Calibration (Q1000 Only)
• Required to measure the absolute value of
•
•
Heat Capacity (Cp) with a single run
Reset previous calibration value to 1.0
Run standard material (sapphire) in standard
mode, @ 10-20°C/min
Set to 1.0
Direct Cp Calibration
• The heat capacity calibration constant, K, is
calculated as the ratio of the theoretical heat
capacity of a standard material, to the
measured heat capacity of the material
Theo.
Cp
K  Meas.
Cp
Use sapphire
encapsulated in pan
Setup to do Cp Constant
(Direct)
Direct Cp Calibration
1.2
Theoretical Values
Heat Capacity (J/g/°C)
1.0
56.85°C
0.8380J/g/°C
106.85°C
0.9168J/g/°C
0.8
56.85°C
0.7932J/g/°C
0.6
106.85°C
0.8567J/g/°C
156.85°C
0.9775J/g/°C
156.85°C
0.9054J/g/°C
206.85°C
1.025J/g/°C
206.85°C
0.9467J/g/°C
Measured Values
0.4
0.2
0.0
0
50
100
150
Temperature (°C)
200
250
Cp Constant (Direct)
• Get data table from UA
• Calculate Cp constant @ a single point or
average values (see below)
Temperature Heat Capacity
°C
J/g/°C
56.85
0.7932
106.85
0.8567
156.85
0.9054
206.85
0.9467
Theoretical
kCp
0.8380
0.9168
0.9775
1.0250
1.0565
1.0702
1.0796
1.0827
Avg
1.0722
Cp Constant (Direct)
• Type new value into calibration table
Agenda
•
•
•
•
Keeping your DSC cell clean
Calibration
Sample Preparation
Thermal Method
Sample Pans
• Type of pan depends onSample form
Volatilization
Temperature range
• Use lightest, flattest pan possible
• Always use reference pan of the same type as
sample pan
Standard DSC Pans (Crimped)
• Pan & lid weighs ~23mg, bottom of pan is flat
• Used for solid non-volatile samples
• Always use lid (see exceptions)
•
Lid improves thermal contact
Keeps sample from moving
Exceptions to using a lid
Running oxidative experiment
Running PCA experiment
Standard DSC Pans (Crimped)
• Crimped pans are available in:
•
Aluminum - up to 600°C
Copper
- up to 725°C (in N2)
Gold
- up to 725°C
Standard Pans without lids
Graphite
- up to 725°C (in N2)
Platinum
- up to 725°C
Hermetic Pans (Sealed)
• Pan & Lid weigh ~55mg, bottom of pan is not
•
•
•
as flat as std pans
Used for liquid samples and samples with
volatiles
Always use lid (same exceptions as before)
After sealing pans, should form dome
Hermetic Pans (Sealed)
• Hermetic Pans are available in:
Aluminum – <600°C; <3 atm (300 kPa gage)
Alodined Aluminum - <600°C; <3 atm (300 kPa gage)

•
(For aqueous samples)
Gold – <725°C, <6 atm (600 kPa gage)
Specialized Sealed Pans
High Volume - 100µL; <250°C; 600 psig(4.1 MPa)
High Pressure - 35µL; <300°C; 1450 psig(10 MPa)
• Note: 3 atm is approximately 44 psig
It Does Matter What Pan you use
Monohydrate
Pharmaceutical
sample
Sample Shape
• Keep sample thin
• Cover as much as the bottom of pan as
possible
Sample Shape
• Cut sample to make thin, don’t crush
• If pellet, cut cross section
Sample Shape
• Cut sample to make thin, don’t crush
• If pellet, cut cross section
• If powder, spread evenly over the bottom of the
pan
Using Sample Press
• When using crimped pans, don’t over crimp
• Bottom of pan should remain flat after crimping
Crimped Pans
Hermetic Pans
Good
Not
Sealed Sealed
Bad
• When using Hermetic pans, a little more
•
pressure is needed
Hermetic pans are sealed by forming a cold
wield on the Aluminum pans
Sample Size
• Larger samples will increase sensitivity
•
but…………….
Larger samples will decrease resolution
• Goal is to have heat flow of 0.1-10mW going
through a transition
Sample Size
• Sample size depends on what you are
measuring
If running an extremely reactive sample (like
an explosive) run very small samples (<1mg)
Pure organic materials, pharmaceuticals
(1-5mg)
Polymers - ~10mg
Composites – 15-20mg
Effect of Sample Size on Indium Melt
0
Size: 1.2100 mg
Heat Flow (mW)
-5
Size: 0.4900 mg
Size: 5.7010 mg
-10
-15
-20
-25
150
Weight
(mg)
0.49
1.21
5.70
152
154
156
158
Temperature (°C)
Onset
(°C)
156.41
156.45
156.61
160
Peak
(°C)
156.56
156.76
157.17
162
Width
(°C)
0.17
0.29
0.55
164
Agenda
•
•
•
•
Keeping your DSC cell clean
Calibration
Sample Preparation
Thermal Method
Purge Gas
• Purge gas should always be used during DSC
•
experiments
Provides dry,inert atmosphere
Ensures even heating
Helps sweep away any off gases that might
be released
Nitrogen
Most common
Increases Sensitivity
Typical flow rate of 50ml/min
Purge Gas
• Helium
•
Must be used with LNCS
High Thermo-conductivity
Increases Resolution
Upper temp limited to 350°C
Typical flow rate of 25ml/min
Air or Oxygen
Used to view oxidative effects
Typical flow rate of 50ml/min
Sample Temperature Range
• Rule of Thumb
Have 2-3 minutes of baseline before and
after transitions of interest - if possible
 DO
NOT DECOMPOSE SAMPLES
IN DSC CELL
Temperature range can affect choice of pans
Just because the instrument has a
temperature range of –90°C to 550°C (with
RCS) doesn’t mean you need to heat every
sample to 550°!
Start-up Hook
12
9.56mg PET @ 10°C/min
10
Do not attempt to interpret transitions
before Heating rate has stabilized
Heat Flow (W/g)
8
6
-0.15
4
2
-0.25
0
-5
Exo Up
5
15
Temperature (°C)
25
35
Deriv. Temperature (°C/min)
-0.05
Heating Rate
• Faster heating rates increase sensitivity
•
but…………….
Faster heating rates decrease resolution
• Good starting point is 10°C/min
Effect of Heating Rate
PMMA
10.04mg
Thermal History
• The thermal history of a sample can and will
•
affect the results
The cooling rate that the sample undergoes can
affect :
 Crystallinity of semi-crystalline materials
 Enthalpic recovery at the glass transition
• Run Heat-Cool Heat experiments to see effect
of & eliminate thermal history
 Heat at 10°C/min
 Cool at 10°C/min
 Heat at 10°C/min
Heat-Cool-Heat of PET
1.5
1.0
Cool
Heat Flow (W/g)
0.5
Second Heat
0.0
First Heat
-0.5
-1.0
-1.5
20
60
100
140
180
Temperature (°C)
220
260
6
2004 Training Seminars
DSC
Calculating %
Crystallinity from the
Latent Heat of Fusion
Calculating % Crystallinity
Background
• DSC or MDSC® measure the energy required (latent
heat of fusion) to convert crystalline structure to
amorphous structure at that temperature.
• The heat of fusion is temperature dependent; i.e., it
takes more heat to melt crystalline structure at higher
temperatures.
• The increase in the heat of fusion with temperature is
due to the difference in the heat capacity curves for
amorphous and crystalline material.
• In order to correctly calculate % crystallinity from the
heat of fusion requires a knowledge of how the heat of
fusion changes with temperature.
Calculating % Crystallinity (cont.)
1- If 100% crystalline and amorphous material is
available, create Cp and enthalpy plots from
measured heat flow
 Enthalpy plots are obtained by measuring the absolute
integral of a Cp plot over some temperature range
 All enthalpy plots are normalized to the same value (J/g)
at a convenient temperature above the melting point
where all samples should theoretically be the same
because all have the same liquid structure (mobile
amorphous)
 See Figures 1-3
Enthalpy Plots Are Integrals of Heat
Capacity Plots
Integrals of 100% Crystalline and 100% Amorphous Heat Capacity
Curves Can Be Used to Create an Enthalpy Plot
Figure 1
Drug 3.75mg
MDSC® .159/60/1
Figure 2
Effect of the Temperature-Dependence of the Heat of Fusion on
Crystallization and Melting Peak Areas for a Drug
The Enthalpy Plot Can Be Used to
Calculate % Crystallinity
Illustrating the Temperature Dependence of the Heat of
Fusion on the Monohydrate Form of the Drug
Figure 3
Calculating % Crystallinity (cont.)
2. For polymers, enthalpy values for 100% crystalline and
3.
100% amorphous structure are available from ATHAS
Databank as a function of temperature
 See ATHAS Figures
Calculate % Crystallinity by dividing measured heat of
fusion by the difference (*) in enthalpy for 100% crystalline
and 100% amorphous structure at that temperature
(Fig. 4)
% Crystallinity = 37/113*  33b % @ 160ºC
*113 J/g = Heat of Fusion = (88323a – 66589a) J/mole 192a g/mole
a = values obtained from ATHAS Databank
note b; use of 140J/g for H = 26% Crystallinity
ATHAS – Page 1
ATHAS - Page 2
These are the Major Polymer
Categories for ATHAS
ATHAS – Page 3
Each Polymer Category has a Sub-listing
ATHAS – Page 4
ATHAS – Page 5
----------------------
Calculate g/mole from molecular structure
which equals 192 g/mole for PET
Convert ºK to ºC
Specific Heat Capacity
Absolute Enthalpy
ATHAS – Page 6
----------------------
Figure 4; % Crystallinity of PET @160 °C
Use of ATHAS Databank to Calculate % Crystallinity on 12.64mg
Sample of Quench Cooled PET after Cold Crystallization
20°C/min