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

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Endothermic Heat Flow

• Heat FlowEndothermic: heat flows into the sample as

a result of either heat capacity (heating) or some endothermic process (glass transition, melting, evaporation, etc.)

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Exothermic Heat Flow

• Heat FlowExothermic: 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 (transducer), thermopile (series of thermocouples) or PRT (Platinum Resistance Thermometers)

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 gasWhat 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

Rs Rr

qs qr

Cs Cr

Tr

T0

Ts

Heat FlowSensor Model

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

ddT

TRC

ssss

0

dTd

ddT

TTRC

srrr

0

Measuring the C’s & R’s

Repeat first experiment with sapphire disks on sample and reference (no pans)

Assume:d

dTcmq s

sapphss d

dTcmq r

sapphrr

Use time constants to calculate heat capacities

10

ss

sapphss

ddT

Tcm

C

10

rs

sapphrr

dTd

ddT

TTcm

C

Measuring the C’s & R’s

Use time constants and heat capacities to calculate thermal resistances

s

ss C

R

r

rr C

R

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 offPreferably enable the secondary purgeDo 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

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apac

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oule

/°C

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70

Sam

ple

Res

ista

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t)

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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.

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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 runsSamples spilling out of the panTransfer 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 dirtyClean 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 Bad SealedNot 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 measuringIf running an extremely reactive sample (like

an explosive) run very small samples (<1mg)Pure organic materials, pharmaceuticals

(1-5mg)Polymers - ~10mgComposites – 15-20mg

Effect of Sample Size on Indium Melt

Size: 0.4900 mg

Size: 1.2100 mg

Size: 5.7010 mg

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Temperature (°C)

Weight Onset Peak Width(mg) (°C) (°C) (°C)0.49 156.41 156.56 0.171.21 156.45 156.76 0.295.70 156.61 157.17 0.55

Purge Gas• Purge gas should always be used during DSC

experimentsProvides dry, inert atmosphereEnsures even heatingHelps sweep away any off gases that might

be released• Nitrogen

Most commonIncreases Sensitivity Typical flow rate of 50ml/min

Purge Gas• Helium

Must be used with LNCSHigh Thermo-conductivityIncreases ResolutionUpper temp limited to 350°CTypical flow rate of 25ml/min

• Air or OxygenUsed to view oxidative effectsTypical flow rate of 50ml/min

Sample Temperature Range

• Rule of ThumbHave 2-3 minutes of baseline before and

after transitions of interest - if possible DO NOT DECOMPOSE SAMPLES

IN DSC CELLTemperature range can affect choice of pansJust 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/min

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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 materialsEnthalpic recovery at the glass transition

• Run Heat-Cool Heat experiments to see effect of & eliminate thermal historyHeat at 10°C/minCool at 10°C/minHeat at 10°C/min

Heat-Cool-Heat of PET

Second HeatFirst Heat

Cool

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