4: Differential Scanning Calorimetry (DSC)
- Page ID
- 527188
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\(\newcommand{\avec}{\mathbf a}\) \(\newcommand{\bvec}{\mathbf b}\) \(\newcommand{\cvec}{\mathbf c}\) \(\newcommand{\dvec}{\mathbf d}\) \(\newcommand{\dtil}{\widetilde{\mathbf d}}\) \(\newcommand{\evec}{\mathbf e}\) \(\newcommand{\fvec}{\mathbf f}\) \(\newcommand{\nvec}{\mathbf n}\) \(\newcommand{\pvec}{\mathbf p}\) \(\newcommand{\qvec}{\mathbf q}\) \(\newcommand{\svec}{\mathbf s}\) \(\newcommand{\tvec}{\mathbf t}\) \(\newcommand{\uvec}{\mathbf u}\) \(\newcommand{\vvec}{\mathbf v}\) \(\newcommand{\wvec}{\mathbf w}\) \(\newcommand{\xvec}{\mathbf x}\) \(\newcommand{\yvec}{\mathbf y}\) \(\newcommand{\zvec}{\mathbf z}\) \(\newcommand{\rvec}{\mathbf r}\) \(\newcommand{\mvec}{\mathbf m}\) \(\newcommand{\zerovec}{\mathbf 0}\) \(\newcommand{\onevec}{\mathbf 1}\) \(\newcommand{\real}{\mathbb R}\) \(\newcommand{\twovec}[2]{\left[\begin{array}{r}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\ctwovec}[2]{\left[\begin{array}{c}#1 \\ #2 \end{array}\right]}\) \(\newcommand{\threevec}[3]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\cthreevec}[3]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \end{array}\right]}\) \(\newcommand{\fourvec}[4]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\cfourvec}[4]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \end{array}\right]}\) \(\newcommand{\fivevec}[5]{\left[\begin{array}{r}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\cfivevec}[5]{\left[\begin{array}{c}#1 \\ #2 \\ #3 \\ #4 \\ #5 \\ \end{array}\right]}\) \(\newcommand{\mattwo}[4]{\left[\begin{array}{rr}#1 \amp #2 \\ #3 \amp #4 \\ \end{array}\right]}\) \(\newcommand{\laspan}[1]{\text{Span}\{#1\}}\) \(\newcommand{\bcal}{\cal B}\) \(\newcommand{\ccal}{\cal C}\) \(\newcommand{\scal}{\cal S}\) \(\newcommand{\wcal}{\cal W}\) \(\newcommand{\ecal}{\cal E}\) \(\newcommand{\coords}[2]{\left\{#1\right\}_{#2}}\) \(\newcommand{\gray}[1]{\color{gray}{#1}}\) \(\newcommand{\lgray}[1]{\color{lightgray}{#1}}\) \(\newcommand{\rank}{\operatorname{rank}}\) \(\newcommand{\row}{\text{Row}}\) \(\newcommand{\col}{\text{Col}}\) \(\renewcommand{\row}{\text{Row}}\) \(\newcommand{\nul}{\text{Nul}}\) \(\newcommand{\var}{\text{Var}}\) \(\newcommand{\corr}{\text{corr}}\) \(\newcommand{\len}[1]{\left|#1\right|}\) \(\newcommand{\bbar}{\overline{\bvec}}\) \(\newcommand{\bhat}{\widehat{\bvec}}\) \(\newcommand{\bperp}{\bvec^\perp}\) \(\newcommand{\xhat}{\widehat{\xvec}}\) \(\newcommand{\vhat}{\widehat{\vvec}}\) \(\newcommand{\uhat}{\widehat{\uvec}}\) \(\newcommand{\what}{\widehat{\wvec}}\) \(\newcommand{\Sighat}{\widehat{\Sigma}}\) \(\newcommand{\lt}{<}\) \(\newcommand{\gt}{>}\) \(\newcommand{\amp}{&}\) \(\definecolor{fillinmathshade}{gray}{0.9}\)After completing this chapter, you should be able to:
- Explain the principle and working of DSC.
- Draw and interpret DSC curves.
- Compare and contrast DSC with TG and DTA.
- Calculate heat of any reaction from the given data.
- Explain the applications of DSC to polymeric materials and pharmaceuticals.
- Explain the applications of simultaneous TG-DTA-DSC analysis.
Differential scanning calorimetry (DSC) has become the most widely used thermal analysis technique. In this technique, the sample and the reference materials are subjected to a precisely programmed temperature change. DSC is very similar to DTA and gives much the same sort of information but DSC is more often used for quantitative measurement of energy changes.
Principle
In DSC, the difference in temperature (∆T) between the sample and an inert reference is maintained at zero as they are subjected to controlled heating or cooling. The instrument is provided with a separate heater for the sample and the reference. When a thermal transition occurs in the sample, thermal energy is added to either the sample or the reference container in order to maintain both the sample and the reference at the same temperature. Because the energy transferred is exactly equivalent in magnitude to the energy absorbed or evolved in the transition, the balancing energy yields a direct calorimetric measurement of the transition energy. Since DSC can measure directly both temperature and the enthalpy of a transition or the heat of a reaction, it is often substituted for differential thermal analysis as a means of determining these quantities except in certain high temperature applications.
4.1 Instrumentation and working
A typical DSC cell uses a constantan (Cu-Ni) disk as the primary means of transferring heat to the sample and the reference positions and also as one element of the temperature-sensing thermoelectric junction. The sample and a reference are placed in separate pans that sit on raised platforms on the disk. Heat is transferred to the sample and reference through the disk. The differential heat flow to the sample and reference is monitored by the chromel/constantan thermocouples formed by the junction of the constantan disk and the chromel wafer covering the underside of each platform. Chromel and alumel wires connected to the underside of the wafers form a chromel/alumel thermocouple, which is used to directly monitor the sample temperature.
Figure 4.1b. DSC cell cross section
The diagram shown above is of a Differential Scanning Calorimetry (DSC) heat flux apparatus. The setup features a dynamic sample chamber enclosed in a block with a gas purge inlet and lid at the top.
Inside the chamber:
· A reference pan (gray) and a sample pan (orange) are placed side-by-side.
· Both pans sit on chromel discs, which are positioned on a thermoelectric disc (constantan).
· These are connected to a heating block that provides uniform heating.
Below the pans:
· Chromel and alumel wires lead to a thermocouple junction, which detects temperature differences.
The setup allows simultaneous heating of both pans and comparison of their thermal responses using the heat flux method.
In a DTA set up there is a single heater and in DSC there are separate heaters for sample and the reference. Why?
Activity 4.1: Read each of the statements and mark them as True or False
(i) In DSC the sample and reference positions are provided with their own separate
heating sources, so that the assembly may be operated on a ‘null balance’ basis.
(ii) In DTA the equipment is so designed that the temperature of the sample is equal
to that of the reference material at every point in the heating programme.
(iii) Chemical decompositions which give rise to weight changes may be detected by
DTA and DSC.
(iv) The main components of a conventional differential thermal analyser consist of following
- The sample/reference holder
- The thermocouple
- The furnace
- The amplifier
- The recorder
4.2 DSC curves and its interpretation
The enthalpy of a sample refers to its heat content. Exothermic/Endothermic changes in a sample give rise to enthalpy changes. Enthalpy changes may be taken to correspond to a heat of reaction are usually written as ∆H.
∆H= Hp – HR
Hp = Enthalpy of products
HR = Enthalpy of Reactants
A typical DSC curve of a polymer (PET) is shown in Figure 4.2 given below
Figure 4.2. DSC curve of PET
Description of the diagram:
1. Glass Transition at 79.8°C – marked by a step-like change in baseline.
2. Crystallization Peak at 148.0°C – represented by a sharp exothermic peak (upward).
3. Melting Point at 230.6°C – represented by a sharp endothermic peak (downward).
Each event is labelled in red: “Glass Transition,” “Crystallization,” and “Melting.” The heat flow becomes more negative with melting, indicating energy absorption, and more positive during crystallization, indicating energy release.
Activity 4.2: Choose the correct option (James and Tonge, 2008)
- If ∆H < 0, the system has undergone an endothermic/ exothermic change which means
Ts __ TR [Choose the correct option: =, <, >]
2. Conversely ∆H > 0, means _______change and Ts ___ TR [Choose the correct option: =, <, >]
3. In order to keep ∆T = 0 [∆T = Ts – TR],
- In case of an endothermic reaction we must provide heat to sample/reference.
- In case of an exothermic reaction we must provide heat to sample/reference
Activity 4.3: Based on the DSC curve given in figure 4.2 answer the following questions.
- How many transitions are recorded?
- Are there any endotherm/s or exotherm/s? If yes, how many?
- What does an endo or exothermic nature tell you about the transition or ∆H value?
- Do you think DSC curve of PET is useful in predicting its stability? Justify your answer.
Let us learn more about heat capacity, glass transition temperature and the role of DSC in characterization of polymeric materials.
What does change in Heat capacity (Cp) means?
Heat Capacity (specific heat) is denoted as CP. It is the energy required to raise the temperature of one mole of material through one degree kelvin.
4.3 Applications of DSC
I. Characterisation of polymeric materials: The ‘glass transition temperature’ (Tg) is an important parameter for many polymeric, ceramics and glasses. On cooling the material from the liquid state, there is (at the Tg) a change from the liquid state to amorphous state or glassy state. At this point there is discontinuity in the rate of change of the volume. There is also a change in the specific heat which allows study by DSC. Both these effects are illustrated in the figure 4.4a and 4.4b respectively.
Figure 4.3a. Figure 4.3b. [Graphs depicting specific volume and specific heat changes at the glass transition temperature (Tg) of a material.]
Two diagrams illustrating thermal behaviour at the glass transition temperature (Tg):
1. Left Graph – Specific Volume vs. Temperature:
a. Y-axis: Specific volume
b. X-axis: Temperature
c. A kink in the curve marks Tg, separating two linear regions:
i. Below Tg: Glassy state
ii. Above Tg: Liquid state
d. An arrow at Tg indicates the transition point from glass to liquid.
2. Right Graph – Specific Heat vs. Temperature:
a. Y-axis: Specific heat
b. X-axis: Temperature
c. At Tg, there is a step increase in specific heat (Cp), indicated by a vertical arrow labelled ΔCp.
d. The graph shows a gradual increase before Tg, then a sudden rise, followed by a slower increase above Tg.
II. Drug analysis for purity assessment: DSC analysis can be used to assess purity of drug. Fig 4.3c shows comparative melting points of 98%, 99% and 100 mole % phenacetin. Since melting is an endothermic process but does not involve change in weight, it cannot be detected by TGA. DTA or DSC is the most suitable technique in such cases.
Pure compounds give sharp endothermic peak in DSC, which is evident from the peaks observed for 98% and 100% mole phenacetin. Impure compounds will melt at a temperature lower than the corresponding pure compounds. Hence, melting point and nature of peak can be used to comment on the purity of any drug.
Key points:
· The x-axis represents temperature in degrees Celsius (°C), ranging roughly from 125°C to 140°C.
· The y-axis represents heat flow in mcal/sec, with a scale marker showing 1 mcal/sec.
· The traces show endothermic peaks indicating melting points or phase transitions.
· As purity increases from 96% to 100%, the melting point peak becomes sharper and shifts slightly.
· The heating rate is 1° per minute, and the sample size is 5.5 mg.This DSC analysis helps characterize the purity and thermal properties of Phenacetin.
4.4 Comparison of DSC with DTA
| DSC | DTA | |
| 1 | It involves measurement of energy changes whilst the sample is subjected to controlled heating. | It is a technique in which the difference in temperature between the sample and inert reference material, is measured as a function of temperature. |
| 2 | It can detect all chemical and physical transitions including change in heat capacity. | It can detect all physical and chemical transitions. |
| 3 | It is a quantitative method. | It is a semi-quantitative method. |
| 4 | This technique is used to study purity of compounds, heat of reaction and characterization of polymers. | This technique is used to study phase transitions. |