D ifferential S canning C alorimetry
description
Transcript of D ifferential S canning C alorimetry
Differential Scanning Calorimetry
Queens University Belfast 16/02/12
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What is a DSC?
Differential: measurement of the difference in heat flow from sample and reference side
Scanning: the common operation mode is to run temperature or time scans
Calorimeter: instrument to measure heat or heat flow.
Heat flow: a transmitted power measured in mW
What is DSC?
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Ice
Ts Tr
Hot Plate
Heat the hot plate from -20 °C to 30 °C,
What will happen to the ice?
How do Ts and Tr react?
How do the Ts and Tr relate to each other?
Air
DSC working principle
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Timeor Tr
TemperatureTr
Ts
Tf
Time∆T =Ts-Tr
0
-0.5
Tf
DSC raw signal
DSC working principle
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DSC working principle
DSC raw signal,
Timeor Tr
∆T =Ts-Tr
0
-0.5
Tf
Timeor Tr
Heat flow (mW)
0
-10DSC signal,
Peak integral -> ∆H
=∆T/Rth
Rth, thermal resistence of the system
∆H
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Baseline slope
Where,
m is the sample masscp is the specific heat capacity of the sample is the heating rate
Timeor Tr
Heat flow (mW)
0
-10
Initial deflection
A normal DSC curve is not horizontal, its baseline shows a slope.
β pcm
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ICTA and Anti-ICTA
ICTAC (International Confederation for Thermal Analysis and Calorimetry)
Direction of DSC signal
melting
In, 6.0000 mg
mW
-20
-10
0
°C120 130 140 150 160 170
exo
STARe SW 9.10MSG Lab: NJ
melting
In, 6.0000 mg
mW
0
5
10
15
20
°C120 130 140 150 160 170
endo
STARe SW 9.10MSG Lab: NJ
ICTA (∆T=Ts-Tr) endothermic downwards,
exothermic upwards.
Anti-ICTA (∆T=Tr-Ts) endothermic upwards, exothermic downwards.
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Endothermic and exothermic effects
Endothermic:
When the sample absorbs energy, the enthalpy change is said to be endothermic. Processes such as melting and vaporization are endothermic.
Exothermic:
When the sample releases energy, the process is said to be exothermic. Processes such as crystallization and oxidation are exothermic.
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Exothermic effect
DSC raw signal
Timeor Tr
TemperatureTr
Ts
Time
0
∆T =Ts-Tr
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Schematic DSC curve of a polymer
1 23 4 5
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Tem perature
He
at f
low
e xo
endo
1. initial startup deflection; 2. glass transition; 3. crystallization; 4. melting; 5. vaporization; 6. decomposition.
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What is melting and crystallization?
amorphouscrystalline
Melting of Indium: 156.6 °C, -28.6 J/g (endothermic)
Crystallization of Indium: 153.5 °C, +28.6 J/g (exothermic)
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In: 6.18 mg10 K/min
Integral -193.95 mJ normalized -28.52 Jg -1Onset 156.58 °C
Integral 194.27 mJ normalized 28.57 Jg -1Onset 153.43 °C
mW
-10
0
10
20
°C140 145 150 155 160 165
exo prod 1 13.03.2006 13:58:29
STARe SW 9.01MSG2006: Marco
Melting and Crystallization with DSC
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How to evaluate melting peaks
• Pure materials:
- onset (independent of heating rate)
- Hf baseline: line, integral tangential
• Impure materials:
- peak temperature (depends on )
- Hf baseline: line, tangential right - purity analysis for eutectic systems (based on curve shape analysis)
• Polymers
- peak temperature (depends on and m)
- Hf baseline: line, spline, integral tangential
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amorphous solid,rigid, brittle
liquid (non polymers)rubber like (polymers)
What is glass transition?
Glass transition is cooperative molecular movement.
Glassy state Rubbery stateGlass transition
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The glass transition with DSC
dT
dH
mcp
1
pcm
Temperature
cp
Temperature
ex
othe
rm
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Chemical reaction
A chemical reaction is a process that one or more substances (reactants) are converted to one or more new chemical substances (products) with different properties. e.g. oxidation, decomposition, polymerization etc.
Chemical reactions always involve a change in energy. Depending on whether the energy is absorbed or released during the process, they can be endothermic or exothermic.
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Chemical reaction
Homogeneous decomposition of dibenzoyl peroxide; peak temperature and peak shape depend on heating rate; peak area is independent of heating rate.
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Materials
Additives
Plasticizers
Impurities
Fillers
Processing
Thermal treatment Mechanical stressing
Shaping Storage and use
MaterialProperties
Where to use DSC?
Polymers
Pharmaceuticals
Chemicals
Food
Cosmetics
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DSC
Multiple Thermocouples increase sensitivity and reduce noise
For Best Resolution: Low signal time constant e.g 1.7 s (20-μL Al crucible, N2 gas)
High baseline stability using chemically resistant ceramic substrate and relatively inert silver furnace
Resolution and Sensitivity
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Sensor technology
TSR
S R Temperature gradients on the sensor lead to baseline deviation from zero
TR0 TS0
S R T0
A single sensor temperature (T0) is measured.
Inhomogeneous temperature distribution of the sensor is not considered.
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Inner ring of thermocouples measure TR and TS
Outer ring measures sensor temperatures at reference and samples sides, TS0 and TR0
Thermocouples act as thermal resistence, R.
TS
S R
TSTR
TS0 TR0
Sensor technology
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Heat flow on the sample and reference sides are separately measured
N is the number of thermocouples (TC) per ring.FRS5 sensor (56 TC): N = 14
HSS7 sensor (120 TC): N = 30
TS
S R
TSTR
TS0 TR0
00 RSdt
dq
N
i i
iSiSS R
TT
1
,0,0
N
i i
iRiRR R
TT
1
,0,0
Sensor technology
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Noise and sensitivity
Peak Height 137 uW
Lysozyme dissolved in 0.1 M HCl/glycinebuffer at pH = 3Heating rate 3 K/minSample Mass around 105 mg
1.7 % Lysozyme
uW100
°C50 60 70 80 90
Peak Height 6.5 uW
Step 0.69 uW
0.1 % Lysozyme
uW5
min
°C60 65 70 75 80
18 20 22 24
exo Lysozyme 1.7% and 0.1% 29.01.2008 16:41:22
STARe SW 9.10MSG Lab: NJ
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Noise and sensitivity
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Resolution
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How long does the system take to equilibrate?
How fast does the signal come back to the baseline?
Small Signal better resolution
Signal = RthCs, Cs = Cpan+ Csample+ Csensor
FRS5 sensor (Rth 0.04 K/mW) & Al40 l (50 mg)
Cpan 50 mJ/K
Csample (10 mg, 1.5 J/gK) 15 mJ/K => Signal 3 s
Csensor 10 mJ/K
FRS5 sensor (Rth 0.04 K/mW) & Al20 l (20 mg)
Cpan 20 mJ/K => Signal 1.8 s
Signal time constant
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Signal time constant
How to improve Signal?
Cs can be strongly influenced by the choice of the pan.
Recommendations for better resolution:
-> Al20 ul pan instead of Al40 ul pan
-> Al pans instead of alumina pans
Cs can be reduced by using smaller sample size
Signal can be further decreased by using He as purge gas.
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Cooling behavior
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