Differential Scanning Calorimetry

1

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?

2

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

3

Timeor Tr

TemperatureTr

Ts

Tf

Time�T =Ts-Tr

0

-0.5

Tf

DSC raw signal

DSC working principle

4

DSC working principle

DSC raw signal,

Timeor Tr

�T =Ts-Tr

0

-0.5

Tf

Timeor Tr

Heat flow (mW)

0

-10DSC signal, Φ

Peak integral �� � �H

Φ=�T/Rth

Rth, thermal resistence of thesystem

�H

5

Baseline slope

Where,

m is the sample masscp is the specific heat capacity

of the sampleβ is the heating rate

Timeor Tr

Heat flow Φ(mW)

0

-10

Initial deflection

Φ

A normal DSC curve is not horizontal, its baseline shows a slope.

�⋅⋅=Φ pcm

6

ICTA and Anti-ICTA

ICTAC (International Confederation for Thermal Analysis and Calorimetry)

Direction of DSC signal

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mW

-20

-10

0

°C120 130 140 150 160 170

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ICTA (�T=Ts-Tr)endothermic downwards,

exothermic upwards.

Anti-ICTA (�T=Tr-Ts)endothermic upwards,

exothermic downwards.

7

Endothermic and exothermic effects

Endothermic:

When the sample absorbs energy, the enthalpy change is said to beendothermic. 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.

8

Exothermic effect

DSC raw signal

Timeor Tr

TemperatureTr

Ts

Time

0

�T =Ts-Tr

9

Schematic DSC curve of a polymer

1 23 4 5

6

Temperature

Hea

t flo

w

exo

endo

1. initial startup deflection; 2. glass transition; 3. crystallization; 4. melting; 5. vaporization; 6. decomposition.

10

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)

11

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

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Melting and Crystallization with DSC

12

How to evaluate melting peaks

• Pure materials:

- onset (independent of heating rate)

- ∆Hf baseline: line, integral tangential

• Impure materials:

- peak temperature (depends on β)

- ∆Hf baseline: line, tangential right - purity analysis for eutectic systems

(based on curve shape analysis)

• Polymers

- peak temperature (depends on β and m)

- ∆Hf baseline: line, spline, integral tangential

13

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

14

The glass transition with DSC

dTdH

mcp ⋅= 1

β⋅⋅=Φ pcm

Temperature

cp

Temperature

Φex

othe

rm

15

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, theycan be endothermic or exothermic.

16

Chemical reaction

Homogeneous decomposition of dibenzoyl peroxide; peak temperature and peak shape depend on heating rate; peak area is independent of heating rate.

17

MaterialsAdditivesPlasticizersImpuritiesFillers

ProcessingThermal treatment

Mechanical stressingShaping

Storage and use

MaterialProperties

Where to use DSC?

PolymersPharmaceuticalsChemicalsFoodCosmetics

18

DSC

� Multiple Thermocouples increase sensitivity and reduce noise

� For Best Resolution: Low signal time constant e.g1.7 s (20-�L Al crucible, N2 gas)

� High baseline stability using chemically resistant ceramic substrate and relatively inert silver furnace

Resolution and Sensitivity

19

Sensor technology

∆∆∆∆TSR

S R �Temperature gradients on the sensor lead to baseline deviation from zero

∆∆∆∆TR0 ∆∆∆∆TS0

S RT0

� A single sensor temperature (T0) is measured.

� Inhomogeneous temperature distribution of the sensor is not considered.

20

� 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.

TSS R

TS TR

TS0 TR0

Sensor technology

21

� 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

TSS R

TS TR

TS0 TR000 RSdt

dq Φ−Φ=

�=

−=Φ

N

i i

iSiSS R

TT

1

,0,0

�=

−=Φ

N

i i

iRiRR R

TT

1

,0,0

Sensor technology

22

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

23

Noise and sensitivity

24

Resolution

25

� How long does the system take to equilibrate?

� How fast does the signal come back to the baseline?

� Small τSignal �better resolution

ττττSignal = Rth⋅⋅⋅⋅Cs, Cs = Cpan+ Csample+ Csensor

FRS5 sensor (Rth ≈ 0.04 K/mW) & Al40 µl (50 mg)

Cpan ≈ 50 mJ/KCsample (10 mg, 1.5 J/gK) ≈ 15 mJ/K => τSignal ≈ 3 sCsensor ≈ 10 mJ/K

FRS5 sensor (Rth ≈ 0.04 K/mW) & Al20 µl (20 mg)

Cpan ≈ 20 mJ/K => τSignal ≈ 1.8 s

Signal time constant

26

Signal time constant

How to improve τSignal?

� 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

� τSignal can be further decreased by using He as purge gas.

27

Cooling behavior

0 5 10 15 20 25

-150

-100

-50

0

50

100

150

200

250

Air cooled Intracooler Liquid nitrogen

Tem

pera

ture

[°C

]

Time [min]

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