differential scanning calorimetry
TRANSCRIPT
1
DSC:
Differential Scanning Calorimetry
A bulk analytical technique
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
-0 .4
-0 .3
-0 .2
-0 .1
0 .0
0 .1
Heat Flow (W/g)
0 2 5 5 0 7 5 1 0 0 1 2 5 1 5 0
Tem pe ra tu re (°C )E xo U p
Endothermic Heat Flow
• 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 .1
0 .0
0 .1
Heat Flow (W/g)
0 20 40 60 80 100 120 140 160
Temperature (°C )Exo Up
Exothermic Heat Flow
• 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.)
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.
Constantan Body
Chromel Wire
Chromel Area Detector
Constantan Wire
Chromel Wire
Base Surface
Thin Wall Tube
Sample Platform
Reference Platform
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 many
DSCs. The absolute value of the signal is
not relevant, only absolute changes are
used.
• Absolute Heat Flow: used by TA’s Q1000.
Dividing the signal by the measured heating
rate converts the heat flow signal into a heat
capacity signal
DSC Heat Flow
t)(T,dt
dT Cp
dt
dHf+=
signal flowheat DSC dt
dH=
Weight SampleHeat x Specific Sample
CapacityHeat Sample Cp
=
=
Rate Heating dt
dT=
(kinetic) re temperatuabsolutean at
timeoffunction is that flowHeat t)(T, =f
Tzero Heat Flow Equation
R s R r
q s q r
C s C r
T r
T 0
T s
Heat Flow
Sensor Model
( )ττ d
TdC
d
dTCC
RRT
R
Tq r
ssr
rsr
∆−−+
−∆+
∆−=
110
How do we calculate these?
Besides the three
temperatures (Ts, Tr, T0);
what other values do we
need to calculate Heat
Flow?
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: 0≡= rs qq
Heat balance equations give sensor time constants
τ
τ
d
dT
TRC
ssss
0∆==
ττ
τ
d
Td
d
dT
TTRC
srrr ∆
−
∆−∆== 0
Measuring the C’s & R’s
Repeat first experiment with sapphire disks on sample
and reference (no pans)
Assume:τddT
cmq ssapphss = τd
dTcmq rsapphrr =
Use time constants to calculate heat capacities
10 −∆
=
ss
sapphs
s
d
dT
T
cmC
ττ
10 −
∆
−
∆+∆=
rs
sapphr
r
d
Td
d
dT
TT
cmC
τττ
Measuring the C’s & R’s
Use time constants and heat capacities to calculate
thermal resistances
s
s
sC
Rτ
=r
r
rC
Rτ
=
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
Can see that it is bad during
Tzero™ cal run
Bad Tzero™ Calibration 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
Example of Typical Results
40
50
60Reference Resistance (°C
/Watt)
0.02
0.03
0.04
Reference Capacitance (Joule/°C)
0.01
0.02
0.03
0.04
0.05
Sample Capacitance (Joule/°C)
30
40
50
60
70
Sample Resistance (°C
/Watt)
-200 -100 0 100 200 300
Temperature (°C)
Characteristics of the thermal resistances and heat capacities:
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.
This cell is very well balanced. It is acceptable and usual
to have larger differences between sample and reference.
-0.4
-0.2
0.0
0.2
0.4
0.6
Heat Flow (mW)
-100 0 100 200 300 400
Temperature (°C)
Conventional BaselineT zero Baseline
Tzero™ vs Conventional Baseline
Indium with Q Series Heat Flow Signals
Q1000
Q100
Q10
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
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
• When using Hermetic pans, a little more
pressure is needed
• Hermetic pans are sealed by forming a cold
wield on the Aluminum pans
Crimped Pans Hermetic Pans
Good BadSealed
Not
Sealed
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
Size: 0.4900 mg
Size: 1.2100 mg
Size: 5.7010 mg
-25
-20
-15
-10
-5
0
Heat Flow (mW)
150 152 154 156 158 160 162 164
Temperature (°C)
Weight Onset Peak Width
(mg) (°C) (°C) (°C)
0.49 156.41 156.56 0.17
1.21 156.45 156.76 0.29
5.70 156.61 157.17 0.55
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
Do not attempt to interpret transitionsbefore Heating rate has stabilized
9.56mg PET @ 10°C /m in
0
2
4
6
8
10
12
Deriv. Temperature (°C
/min)
-0 .25
-0 .15
-0 .05
Heat Flow (W/g)
-5 5 15 25 35
Temperature (°C )Exo Up
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
Second Heat
First Heat
Cool
-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
Heat Flow (W/g)
20 60 100 140 180 220 260
Temperature (°C)