Thermal Analysis: Differential Scanning Calorimetry (DSC)

Professor Maria L. Auad E-mail: auad@auburn.edu

Phone: 334 844-5459 Office: 103 Textile Building

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Introduction Any measurement of a change in properties as the temperature changes qualifies as a thermal analysis technique.

When performing thermal analysis procedures, usually a controlled temperature program heats or cools a sample at a certain rate, and physical or chemical property changes are monitored as a function of temperature.

Some of the measured properties include mass, temperature, heat flow, size, malleability, sound transmission, magnetic characteristics, optical characteristics, electrical conductivity, and tensile strength.

Thermal analysis techniques give useful information about polymers, inorganic compounds, alloys, drugs, and other organic materials.

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Techniques in this category include TG or TGA: thermal gravimetric analysis or thermogravimetry. Measures the mass change of the sample with a thermobalance. A variation on this is DTG, or derivative thermogravimetry, which measures the slope or derivative of the mass change with temperature, dm/dt. EGD: evolved gas detection. Measures and usually further analyzes evolved gas as the sample decomposes. DRS: dynamic reflectance spectroscopy. Measures the amount of light reflected from the sample as the temperature changes. TMA: thermo-mechanical analysis measures the expansion or contraction of a sample as the temperature changes. DSC and DTA: differential scanning calorimetry, and a related technique, differential thermal analysis. DSC measures the amount of heat flowing into the sample (endothermic, +dH/dt) or out of the sample (exothermic, -dH/dt), and DTA (the older technique) measures small differences in temperature between a sample and reference as the same amount of heat energy is applied to both.

Differential Scanning Calorimetry - DSC Differential Scanning Calorimetry (DSC) is concerned with the measurement of energy changes in materials. It is thus the most generally applicable of all thermal analysis methods, since every physical or chemical change involves a change in energy.

Small, flat samples are contained in shallow pans, with the aim of making a good thermal contact between sample, pan and heat flux plate. Symmetrical heating of the cell, and therefore S and R, is achieved by constructing the furnace from a metal of high thermal conductivity. Note the provision for establishing a gas flow through the cell, to sweep away volatiles, provide the required atmosphere, and to assist in heat transfer.

Traditional Heat Flux DSC: Cell Schematic Diagram Dynamic Sample Chamber

Reference Pan Sample Pan

Lid

Gas Purge Inlet

Chromel Disc

Heating Block

Chromel Disc

Alumel Wire

Chromel Wire

Thermocouple Junction

Thermoelectric Disc (Constantan)

DSC working range: typically up to 700°C, and down to -140°C with a liquid nitrogen cooling system.

-Temperature calibration is carried out by running standard materials, usually very pure metals with accurately known melting points. -Energy calibration may be carried out by using either known heats of fusion for metals, commonly indium, or known heat capacities. -

DSC: operational details

A variety of sample pans can be used for different purposes. The best quantitative results for polymers are obtained from thin samples crimped flat between the pan and a lid. Hermetically-sealed pans capable of holding a few atmospheres pressure are used for liquids, or when it is necessary to retain volatiles. Very high-pressure seals can be achieved using O-ring or screw-threaded seals. For materials that react with aluminium, or for higher temperatures, pans may be made from stainless steel, inconel, gold, alumina, graphite, silica or platinum.

Typical purge gases are air and nitrogen, though helium is useful for efficient heat transfer and removal of volatiles. Argon is preferred as an inert purge when examining samples that can react with nitrogen. The experiment can also be carried out under vacuum or under high pressure using instruments of the appropriate design.

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 qualitative information about physical and chemical changes that involve endothermic or exothermic processes, or changes in heat capacity.

DSC: The Technique

DSC: What DSC Can Tell You? • Glass Transitions • Melting and Boiling Points • Crystallization time and

temperature • Percent Crystallinity • Heats of Fusion and Reactions • Specific Heat / Heat Capacity • Oxidative/Thermal Stability • Rate and Degree of Cure • Reaction Kinetics • Purity

Typical DSC Transitions

Temperature

Hea

t Flo

w

Glass Transition Crystallization

Melting

Cross-Linking (Cure)

Oxidation or Decomposition

Exothermic:

Understanding DSC Signals Heat Flow Where: = measured heat flow rate Cp = sample heat capacity = specific heat (J/g°C) x mass (g) = measured heating rate f (T,t) = heat flow due to kinetic processes

(evaporation, crystallization, etc.)

dtdH

dtdT

Calibration of Differential Scanning Calorimeters Differential Scanning Calorimeters are almost universally calibrated for temperature and enthalpy using the melting temperatures of highly pure metals. Recommended values for the melting temperature (Tm) and heat of fusion (Hf) are given below. Many of these substances will react with standard aluminum crucibles. This may be overcomes by annealing the empty crucible (and lid) air above 400°C in order to build up a protective layer of aluminum oxide.

Calibration of Differential Scanning Calorimeters

Material Tm (°C)

Hf (J/g)

Mercury -38.8344 11.469 Gallium 29.7646 79.88 Indium 156.5985 28.62 Tin 231.298 7.170 Bismuth 271.40 53.83 Lead 327.462 23.00 Zinc 419.527 108.6 Aluminum 660.323 398.1

Melting of Indium

157.01°C

156.60°C28.50J/g

Indium5.7mg10°C/min

-25

-20

-15

-10

-5

0

Heat

Flow

(mW

)

150 155 160 165Temperature (°C)

Exo Up Universal V4.0B TA Instruments

Peak Temperature

Extrapolated Onset

Temperature

Heat of Fusion

For pure, low molecular weight

materials (mw<500 g/mol) use

Extrapolated Onset as Melting Temperature

Indium Melt, 5.7mg at 10°C/min

Second Order Transitions: Tg

The DSC curve may show a step change, as at S in the curve, reflecting a change in heat capacity not accompanied by a discrete enthalpy change. The most common example, and a major application area of DSC, is the glass transition (Tg) seen in amorphous polymers. This important region, in which the material changes from a rigid glassy state to a rubber, or very viscous liquid state, may be analyzed to give a wealth of information about the material.

Temperature

Hea

t Flo

w

Glass Transition Crystallization

Melting Cross-Linking (Cure)

Oxidation or Decomposition

Exo up

15

Examples: •The amount or effectiveness of a plasticizer may be judged by how much it reduces Tg or affects the shape of the transition.

•Examination of the transitions in polymer blends gives information as to their compatibility.

•Curing reactions result in an increase in Tg and measurements can be used to monitor the extent of cure.

•Tg also varies with chain length for a related group of polymers.

•Additional features occurring in the glass transition region, often a superimposed endothermic peak, are related to the aging undergone by the material in the glassy state.

The temperature Tg may be used to identify polymers, as it varies over a wide range for commonly used materials.

Peaks may be characterized by: Position (i.e., start, end, extrapolated onset and peak temperatures) Size (related to the amount of material and energy of the reaction) Shape (which can be related to the kinetics of the process)

Enthalpy Changes (first order transition)

The DSC/DTA curve may show an exothermic or endothermic peak,. The enthalpy changes associated with the events occurring are given by the area under the peaks. In general, the heat capacity will also change over the region, and problems may arise in the correct assignment of the baseline. In many cases the change is small, and techniques have been developed for reproducible measurements in specific systems.

Process Exothermic Endothermic

Solid-solid transition * * Crystallization *

Melting * Vaporization * Sublimation * Adsorption * Desorption *

Desolvation (drying) * Decomposition * *

Solid-solid reaction * * Solid-liquid reaction * * Solid-gas reaction * *

Curing * Polymerization *

Catalytic reactions *

Applications

• Thermoplastics • Thermosets • Heat Capacity • Glass Transition • Melting and Crystallization • Additional Applications

Thermoplastic Polymers

Semi-Crystalline or Amorphous

Crystalline Phase melting temperature Tm (endothermic peak)

Amorphous Phase glass transition temperature (Tg) (causing ∆Cp)

Tg < Tm

Crystallizable polymer can crystallize on cooling from the melt at Tc (Tg < Tc < Tm)

DSC of Thermoplastic Polymers • Tg • Melting • Crystallization • Oxidative Induction Time • General Recommendations

– 10-15mg in crimped pan – @ 10°C/min

Thermoplastic: Heat/Cool/Heat

0 20 40 60 80 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0

50

100

150

200

250

300

Time (min)

Hea

t Flo

w (W

/g)

[

] Te

mpe

ratu

re (°

C)

First Heat Cooling Second Heat

Thermoplastic: Heat Flow vs. Temperature for Heat/Cool/Heat

Second HeatFirst Heat

Cool

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5H

eat F

low

(W/g

)

20 60 100 140 180 220 260Temperature (°C)

The Glass Transition (Tg) The glass transition is a step change in molecular mobility (in the

amorphous phase of a sample) that results in the change of many physical properties

The material is rigid below the glass transition temperature and

rubbery above it. Amorphous materials flow, they do not melt The change in heat capacity at Tg is a measure of the amount of

amorphous phase in the sample. Enthalpic recovery at Tg is a measure of order in the amorphous

phase. Annealing or storage at temperatures just below Tg permit

development of order as the sample moves towards equilibrium

Advantages of DSC Techniques • Differential Scanning Calorimetry

– Most common technique for Tg – Small sample size – Faster analysis (fast heating, automation)

• Modulated - DSC (MDSC®)

– Can separate kinetic and heat capacity events

Typical Polymer Tg by DSC

92.87°C(H)0.2638mW

-2.0

-1.5

-1.0

-0.5

0.0

Heat

Flo

w (m

W)

20 40 60 80 100 120 140 160Temperature (°C)

Sample: PMMA Size: 10.47mg Heating Rate: 10°C/min

Effect of Plasticizer on Tg

• Plasticizers are generally low molecular weight organic additives which are used to soften rigid polymers

• Plasticizers are typically added to a polymer for two reasons: – 1. To lower the glass transition to make a rigid

polymer become soft and rubbery. – 2. To make the polymer easier to process.

• Plasticizers make it easier for a polymer to change molecular conformation.

• Therefore plasticizers will have the effect lowering and broadening the glass transition.

Crystallization • Crystallization is a kinetic process which can be

studied either while cooling, heating or isothermally

• Differences in crystallization temperature or time (at a specific temperature) between samples can affect end-use properties as well as processing conditions

• Isothermal crystallization is the most sensitive way to identify differences in crystallization rates

Crystallization Crystallization is a two step process: Nucleation Growth

The onset temperature is the nucleation (Tn)

The peak maximum is the crystallization temperature (Tc)

Effect of Cooling Rate on Crystallization

A temperature shift is seen in the cooling data on the next slide. In this example, the samples were cooled from 285°C to room temperature at 2 to 16°C/min. The higher rates of temperature change broaden the crystallization process and shift it further in temperature from the starting point.

0.0

0.5

1.0

1.5

2.0

Heat

Flow

(W/g)

40 50 60 70 80 90 100 110 120 130 140 150 160Temperature (°C)Exo Up

POLYPROPYLENE WITH NUCLEATING AGENTS

POLYPROPYLENE WITHOUT NUCLEATING AGENTS

-1.5

-1.0

-0.5

0.0

Heat

Flow

(W/g)

60 80 100 120 140 160 180 200Temperature (°C)Exo Up

crystallization

melting

Effect of Nucleating Agents

• Sample must be pure material, not copolymer or filled

• Must know enthalpy of melting for 100% crystalline material (∆Hliterature)

• You can use a standard ∆Hliterature for relative crystallinity

Calculation of % Crystallinity

For standard samples:

% crystallinity = 100* ∆Hm / ∆Hliterature

For samples with cold crystallization:

% crystallinity = 100* (∆Hm - ∆Hc)/ ∆Hlit

Crystallinity Calculation

78.99°C(I)

75.43°C

80.62°C

134.62°C

127.72°C53.39J/g

256.24°C

242.91°C74.71J/g

-1.5

-1.0

-0.5

0.0

0.5

1.0

Hea

t Flo

w (W

/g)

50 100 150 200 250 300Temperature (°C)

( ) %15140

39.5371.74100 =−

×

% crystallinity = 100* (∆Hm - ∆Hc)/ ∆Hlit

Melting Definitions • Melting – the process of converting crystalline

structure to a liquid amorphous structure • Thermodynamic Melting Temperature – the

temperature where a crystal would melt if it had a perfect structure (large crystal with no defects)

• Metastable Crystals – Crystals that melt at lower temperature due to small size (high surface area) and poor quality (large number of defects)

Melting of Indium

157.01°C

156.60°C28.50J/g

Indium5.7mg10°C/min

-25

-20

-15

-10

-5

0

Heat

Flow

(mW

)

150 155 160 165Temperature (°C)

Exo Up Universal V4.0B TA Instruments

Peak Temperature

Extrapolated Onset

Temperature Heat of Fusion

For pure, low molecular weight

materials (mw<500 g/mol) use

Extrapolated Onset as Melting Temperature

Melting of PET

249.70°C

236.15°C52.19J/g

PET6.79mg10°C/min

-7

-6

-5

-4

-3

-2

-1

Heat

Flow

(mW

)

200 210 220 230 240 250 260 270Temperature (°C)

Exo Up Universal V4.0B TA Instruments

Extrapolated Onset

Temperature

Peak Temperature

Heat of Fusion

For polymers, use Peak as Melting Temperature

Comparison of Melting

249.70°C

236.15°C52.19J/g

157.01°C

156.60°C28.50J/g

Indium5.7mg10°C/min

PET6.79mg10°C/min

-25

-20

-15

-10

-5

0

Heat

Flow

(mW

)

140 160 180 200 220 240 260 280Temperature (°C)Exo Up Universal V4.0B TA Instruments

Thermosetting Polymers: polymerization Rxn

Thermosetting polymers react (cross-link) irreversibly. A+B will give out heat (exothermic) when they cross-link (cure). After cooling and reheating C will have only a glass transition Tg.

A + B C

GLUE

DSC Thermoset Cure: First and Second Heat

0 50 100 150 200 250 300-0.24

-0.20

-0.16

-0.12

-0.08

-0.04

Temperature (°C)

Hea

t Flo

w (W

/g)

Tg

Tg

155.93°C

102.64°C20.38J/g

Residual Cure

First

Second

General Recommendations

10-15 mg in crimped pan if solid; hermetic pan if liquid @ 10°C/min

Determination of % Cure

79.33J/g75.21 % cured

NOTE: Curves rescaled and shifted for readability

145.4J/g54.55 % cured

Under-cured Sample

Optimally-cured Sample-5.27°C(H)

DSC Conditions:Heating Rate = 10°C/min.Temperature Range = -50°C to 250°CN2 Purge = 50mL/min.

-12.61°C(H)

-0.5

0.0

0.5

1.0

1.5

2.0

Heat

Flow

(W/g)

-50 0 50 100 150 200 250Temperature (°C)Exo Up Universal V2.4F TA

245.22°C

125.54°C

-20

-10

0

10

20He

at F

low

(mW

)

50 100 150 200 250 300Temperature (°C)

Polyethylene Oxidation Onset Temperature

Oxidation

Another type of thermal stability is that of stability to combustion. By heating a hydrocarbon sample in air or oxygen, the temperature at which the sample begins to burn is an indicator of the thermal stability of the material.

OXIDATION INDUCTION TIME – EFFECT OF PRESSURE

Increased pressure of O2 decreases oxidation time

Modulated Differential Scanning Calorimetry

(MDSC®)

What it’s all about & how to get better results

What Does MDSC® Measure? • MDSC separates the Total heat flow of

DSC into two parts based on the heat flow that does and does not respond to a changing heating rate

• MDSC applies a changing heating rate on top of a linear heating rate in order measure the heat flow that responds to the changing heating rate

• In general, only heat capacity and melting respond to the changing heating rate.

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 15.0Time (min)

Average & Modulated Temperature

Modulated Temperature

Average Temperature

(Heat-Iso)

Modulated DSC® Theory

• MDSC® uses two simultaneous heating rates – Average Heating Rate

• This gives Total Heat Flow data which is equivalent to standard DSC @ same average heating rate

– Modulated Heating Rate • Purpose is to obtain heat capacity information

at the same time as heat flow

MDSC® of Quench-Cooled PET

Nonreversing

Reversing

Total

-0.4

-0.2

0.0

Nonr

ev H

eat F

low (W

/g)

-0.4

-0.2

0.0

0.2

0.4

Rev H

eat F

low (W

/g)

-0.4

-0.2

0.0

0.2

Heat

Flow

(W/g)

0 50 100 150 200 250 300Temperature (°C)Exo Up

ΔH total = ΔH rev. + ΔH nonrev.

When & Why to Run MDSC®

• Always run a standard DSC @ 10°C/min first

• If you’re looking for a glass transition -- – If the glass transition is detectable and can be

routinely analyzed, then you don’t need to use MDSC

– However, if the Tg is hard to detect, or has an enthalpic recovery, then run MDSC

When & Why to Run MDSC®

• If looking at melting and crystallization – – If the melting process looks normal (single

endothermic peak) and there is no apparent crystallization of the sample as it is heated, then there is no need to use MDSC

– However, if melt is not straightforward, or it is difficult to determine if crystallization is occurring as the sample is heated, use MDSC

Where’s the Tg?

Tablet Binder, 44%RH 3.08mg MDSC® 1/60/5 Vented pan

Here’s the Tg!

MDSC® Aids Interpretation

Xenoy 13.44 mg MDSC .318602

DSC of Amorphous PET/PC Mixture…Where is the PC Tg ?

120.00°C 170.00°C

30.74J/g

215.00°C270.00°C

42.95J/g

120.00°C 270.00°C13.31J/g

Standard DSC @ 10°C/min57% PET; 43% PC

DSC Heat Flow AnalyzedTwo Different Ways

-16

-12

-8

-4

0

4

[ –––

–– · ]

Hea

t Flow

(mW

)

-22

-18

-14

-10

-6

-2

Heat

Flow

(mW

)

50 100 150 200 250Temperature (°C)

Exo Up Universal V3.8A TA Instruments

It is often difficult to accurately measure the crystallinity of polymers by DSC because the crystallinity increases as the sample is being heated in the DSC cell.

To measure the correct crystallinity requires the ability to: determine the true heat capacity (no transitions) baseline quantitatively measure how much crystallinity developed during the heating process

MDSC® Shows Two Tgs in Polymer Mixture

Decrease in Heat CapacityDue to Cold Crystallization

Glass Transitionof Polycarbonate

True Onset of Melting

Cold Crystallization PeakSeen Only in Total Signal

Total Heat Flow

Reversing Heat Flow

-3.2

-3.0

-2.8

-2.6

-2.4

-2.2

-2.0

[ –––

–– · ]

Rev

Hea

t Flow

(mW

)

-3.2

-3.0

-2.8

-2.6

-2.4

-2.2

-2.0

Heat

Flow

(mW

)

50 100 150 200 250Temperature (°C)

Exo Up Universal V3.8A TA Instruments

MDSC® .318/40/3

MDSC® Gives Correct Crystallinity of Zero