Differential ScanningCalorimetry

Stephen Collins

Technical Group Talk

Definitions

• A calorimeter measures the heat into or out of a sample.

• A differential calorimeter measures the heat of a sample relative to a reference.

• A differential scanning calorimeter does all of the above and heats the sample with a linear temperature ramp.

• Endothermicheat flows into the sample.• Exothermic heat flows out of the sample.

Technical Group Talk

• Differential Scanning Calorimetry (DSC) measures the temperatures and heat flows associated with transitions in materials as a function of time and temperature in a controlled atmosphere.

• These measurements provide quantitative and qualitativeinformation about physical and chemical changes that involve endothermic or exothermic processes, or changes in heat capacity.

DSC: The Technique

Technical Group Talk

Conventional DSC

Metal 1

Metal 2

Metal 1

Metal 2

Sample Empty

Sample Temperature

Reference Temperature

Temperature Difference = Heat Flow

•A “linear” heating profile even for isothermal methods

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What can DSC measure?

•Glass transitions

•Melting and boiling points

•Crystallisation time and temperature

•Percent crystallinity

•Heats of fusion and reactions

•Specific heat capacity

•Oxidative/thermal stability

•Rate and degree of cure

•Reaction kinetics

•Purity

Technical Group Talk

6

DSC Thermogram

Temperature

Hea

t Flo

w -

> e

xoth

erm

ic

GlassTransition

Crystallisation

Melting

Cross-Linking(Cure)

Oxidation

79.70°C(I)

75.41°C81.80°C

144.72°C

137.58°C20.30J/g

245.24°C

228.80°C22.48J/g

Cycle 1

-0.5

0.0

0.5

1.0

1.5

Hea

t Flo

w (W

/g)

0 50 100 150 200 250 300

Temperature (°C)

Sample: PET80PC20_MM1 1minSize: 23.4300 mgMethod: standard dsc heat-cool-heatComment: 5/4/06

DSCFile: C:...\DSC\Melt Mixed 1\PET80PC20_MM1.001Operator: SACRun Date: 05-Apr-2006 15:34Instrument: DSC Q1000 V9.4 Build 287

Exo Down Universal V4.2E TA Instruments

Technical Group TalkTechnical Group Talk

Example DSC - PET

Tg

Tc

Tm

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670

Influence of Sample Mass

Temperature (°C)

150 152 154 156

0

-2

-4

-6

DS

C H

eat Flow

(W/g)

10mg

4.0mg

15mg

1.7mg

1.0mg0.6mg

Indium at10°C/minute Normalized Data

158 160 162 164 166

Onset not influenced by mass

Technical Group Talk

6

Effect of Heating Rateon Indium Melting Temperature

154 156 158 160 162 164 166 168 170-5

-4

-3

-2

-1

0

1

Temperature ( °C)

Heat Flow (W/g)

heating rates = 2, 5, 10, 20°C/min

Technical Group Talk

DSC: Main Sources of Errors

•Calibration

•Contamination

•Sample preparation – how sample is loaded into a pan

•Residual solvents and moisture.

•Thermal lag

•Heating/Cooling rates

•Sample mass

•Processing errors

Technical Group Talk

99

Sample Preparation : Shape

• Keep sample as thin as possible (to minimise thermal gradients)

• Cover as much of the pan bottom as possible• Samples should be cut rather than crushed to obtain a

thin sample (better and more uniform thermal contact with pan)

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Other DSC Techniques

Hyper-DSCBased on principle that high heating rates give large broad transitions.

•Heating rates typically 400-500oC/min

•Need very small sample sizes (~nanograms)

Good for:

•A quick overview of new sample

•Picking out minute transition

Poor for:

•Accuracy: transitions can be shifted by as much as 40oC

•Repeatabiliy: Very sensitive to thermal lag.

Technical Group Talk

Other DSC Techniques

Modulated DSC

•Composite heating profile: •Determines heat capacity and separates heat flow into that due to reversible and non-reversible events.

Note that temperature is not decreasing during Modulation i.e. no cooling

Modulate +/- 0.42 °C every 40 secondsRamp 4.00 °C/min to 290.00 °C

52

54

56

58

60

62

Mod

ulat

ed T

empe

ratu

re (

°C)

52

54

56

58

60

62

Tem

pera

ture

(°C

)

13.0 13.5 14.0 14.5 15.0

Time (min)

Typicaly:

Heating rates: 0 - 50C

Modulation:

Period: 60 second

Amplitude: +/-10C

Benefits• Increased Sensitivity for Detecting Weak (Glass) Transitions

– Eliminates baseline curvature and drift• Increased Resolution Without Loss of Sensitivity

– Two heating rates (average and instantaneous)• Ability to Separate Complex Thermal Events and Transitions Into

Their Heat Capacity and Kinetic Components• Ability to Measure Heat Capacity (Structure) Changes During

Reactions and Under Isothermal Conditions

Downside• Slow data collection

Technical Group Talk

Modulated DSC

Technical Group Talk

Example MDSC

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

Non

rev

Hea

t F

low

(W

/g)

-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

Rev

Hea

t F

low

(W

/g)

-0.14

-0.12

-0.10

-0.08

-0.06

-0.04

-0.02

0.00

Hea

t F

low

(W

/g)

-50 0 50 100 150 200 250

Temperature (°C)Exo Up Universal V4.2E TA Instruments

Technical Group Talk

Modulated DSC

Reversible Transitions

•Glass Transition

•Melting

Non-reversible

•Crystallisation

•Curing

•Oxidation/degradation

•Evaporation