9.3 Differential scanning calorimetry (DSC)

Differential scanning calorimetry (DSC) is a powerful thermal analysis technique that measures heat flow differences between a sample and reference. It's used to study material properties like melting points, crystallization, and glass transitions, providing insights into thermal behavior and composition.

DSC is crucial for various industries, from pharmaceuticals to polymers. By analyzing thermograms and calculating enthalpy changes, researchers can assess material purity, study polymorphism, and optimize manufacturing processes. It's a versatile tool for understanding and improving materials.

Principles and instrumentation of DSC

Basic principles and components

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• Differential scanning calorimetry (DSC) measures the difference in heat flow between a sample and a reference as a function of temperature or time
• Basic components of a DSC instrument:
  • Sample and reference pan
  • Heating block
  • Temperature sensors
  • Computer for data acquisition and analysis
• DSC operates under a controlled temperature program (heating, cooling, or isothermal stages) to study the thermal behavior of materials
• Principle of DSC based on measuring the heat flow required to maintain the sample and reference at the same temperature throughout the experiment

Thermal events and their effects on heat flow

• When the sample undergoes a thermal event (melting or crystallization), the heat flow to the sample changes relative to the reference
• Thermal events result in a characteristic peak or dip in the DSC thermogram
• Examples of thermal events:
  • Melting (endothermic, requires more heat flow to the sample)
  • Crystallization (exothermic, requires less heat flow to the sample)

Interpretation of DSC thermograms

Identification of thermal events

• DSC thermogram is a plot of heat flow versus temperature or time
• Provides information about the thermal events occurring in the sample
• Endothermic events (melting or glass transition) appear as downward peaks, indicating an increase in heat flow to the sample
• Exothermic events (crystallization or curing) appear as upward peaks, indicating a decrease in heat flow to the sample
• Examples of thermal events in a thermogram:
  • Melting point of a pure substance (sharp endothermic peak)
  • Crystallization of a material (sharp exothermic peak)
  • Glass transition (step change in the heat flow signal, indicating a change in the heat capacity of the material)

Analysis of peak characteristics

• Shape, position, and area of the peaks in a DSC thermogram provide information about the nature, kinetics, and enthalpies of the thermal events
• Peak shape:
  • Sharp, narrow peaks indicate a pure substance or a well-defined thermal event
  • Broad or multiple peaks suggest impurities, overlapping events, or complex thermal behavior
• Peak position:
  • The temperature at which a peak occurs provides information about the thermal event (melting point, crystallization temperature, glass transition temperature)
  • Shifts in peak position can indicate changes in sample composition, purity, or thermal history
• Peak area:
  • The area under a peak is proportional to the enthalpy change associated with the thermal event
  • Larger peak areas indicate greater enthalpy changes

Enthalpy changes from DSC data

Calculation of enthalpy changes

• Enthalpy change (ΔH) associated with a thermal event can be calculated from the area under the corresponding peak in the DSC thermogram
• Area under the peak is proportional to the enthalpy change
• Proportionality constant determined by calibrating the DSC instrument with a standard material of known enthalpy
• To calculate the enthalpy change:
  1. Establish the baseline of the thermogram
  2. Integrate the peak area using the software provided with the DSC instrument
• Specific enthalpy change (J/g) obtained by dividing the total enthalpy change by the mass of the sample

Applications of enthalpy data

• Enthalpy changes used to quantify:
  • Heat of fusion (melting)
  • Heat of crystallization
  • Heat capacity changes associated with thermal events
• Examples of applications:
  • Determining the purity of a substance by comparing the measured enthalpy of fusion with the theoretical value
  • Studying the kinetics of crystallization by analyzing the enthalpy change as a function of cooling rate
  • Investigating the effect of additives or processing conditions on the thermal behavior of materials

DSC applications for materials analysis

Thermal properties and purity assessment

• DSC widely used to characterize the thermal properties of materials:
  • Melting point
  • Crystallization temperature
  • Glass transition temperature
  • Heat capacity
• Purity of a substance assessed using DSC:
  • Pure substance exhibits a sharp, narrow melting peak
  • Impurities cause peak broadening and a decrease in melting temperature
• Examples of purity assessment:
  • Quality control of raw materials in the pharmaceutical industry
  • Identifying the presence of contaminants or degradation products in a sample

Polymorphism and stability studies

• Polymorphism is the ability of a material to exist in different crystalline forms
• DSC used to study polymorphism by identifying the presence of multiple melting or crystallization peaks
• Relative stability of polymorphs determined by comparing their melting points and enthalpies of fusion
• Examples of polymorphism studies:
  • Characterizing the polymorphic forms of a drug substance
  • Optimizing the crystallization conditions to obtain the desired polymorph
• DSC employed to assess the stability of materials:
  • Compatibility of drug-excipient mixtures in pharmaceutical formulations
  • Oxidative stability of polymers
  • Thermal degradation of materials

Specific applications in various fields

• Pharmaceutical industry:
  • Studying the compatibility of drug-excipient mixtures
  • Assessing the stability of drug formulations
  • Optimizing the manufacturing process
• Polymer science:
  • Investigating the curing kinetics of thermosets
  • Analyzing the thermal behavior of composites and blends
  • Determining the glass transition temperature and melting point of polymers
• Examples of other applications:
  • Characterizing the thermal properties of food ingredients and products
  • Studying the phase transitions in liquid crystals
  • Investigating the thermal stability of inorganic materials (ceramics, metals)