Differential Scanning Calorimetry Queens University Belfast 16/02/12

What is a DSC? What is 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

DSC working principle Ice Air 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?

DSC working principle Tr Ts Tf Tf Temperature Time ∆T =Ts-Tr DSC raw signal -0.5 Time or Tr Tf

DSC working principle ∆T =Ts-Tr DSC raw signal, -0.5 Time or Tr -0.5 Tf Heat flow (mW) -10 DSC raw signal, =∆T/Rth Rth, thermal resistence of the system ∆H DSC signal,  Peak integral -> ∆H

Baseline slope Time or Tr Heat flow F (mW) -10 Initial deflection Where, m is the sample mass cp is the specific heat capacity of the sample  is the heating rate A normal DSC curve is not horizontal, its baseline shows a slope.

Direction of DSC signal ICTA and Anti-ICTA ICTAC (International Confederation for Thermal Analysis and Calorimetry) Direction of DSC signal ICTA (∆T=Ts-Tr) endothermic downwards, exothermic upwards. Anti-ICTA (∆T=Tr-Ts) endothermic upwards, exothermic downwards.

Endothermic and exothermic effects 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.

Exothermic effect Tr Ts Temperature ∆T =Ts-Tr DSC raw signal Time or Tr Temperature Tr Ts ∆T =Ts-Tr DSC raw signal

Schematic DSC curve of a polymer 1. initial startup deflection; 2. glass transition; 3. crystallization; 4. melting; 5. vaporization; 6. decomposition.

What is melting and crystallization? Melting of Indium: 156.6 °C, -28.6 J/g (endothermic) crystalline amorphous Crystallization of Indium: 153.5 °C, +28.6 J/g (exothermic)

Melting and Crystallization with DSC Warum kalibrieren nur beim Heizen?

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

What is glass transition? Glassy state Glass transition Rubbery state amorphous solid, rigid, brittle liquid (non polymers) rubber like (polymers) Glass transition is cooperative molecular movement.

The glass transition with DSC Temperature cp F exotherm

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. Examples: A freshly-cut apple turns brown; A bicycle fender becomes rusty; A copper penny turns green.

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

Where to use DSC? Material Properties Processing Polymers Pharmaceuticals Chemicals Food Cosmetics Materials Additives Plasticizers Impurities Fillers Processing Thermal treatment Mechanical stressing Shaping Storage and use Material Properties

DSC Resolution and Sensitivity 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

Sensor technology Temperature gradients on the sensor lead to baseline deviation from zero A single sensor temperature (T0) is measured. Inhomogeneous temperature distribution of the sensor is not considered.

Sensor technology R S TS TR TS0 TR0 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.

Sensor technology R S TS TR TS0 TR0 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

Noise and sensitivity

Noise and sensitivity

Resolution

Signal time constant Signal = RthCs, Cs = Cpan+ Csample+ Csensor How long does the system take to equilibrate? How fast does the signal come back to the baseline? Small Signal better resolution Signal = RthCs, 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 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.

Cooling behavior

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