Differential Scanning Calorimetry (DSC) Flashcards
What are the basic principles of DSC?
- Measures the difference in the heat flow (and electrical power)
- between the sample and inert reference
- as a function of time or temperature.
What are the parameters measured in DSC?
- Heat capacity Cp
- Temperature of transitions
- Enthalpy of transitions
What are the units used in DSC?
mW = mJ/sec
Of what material is the sample pan made of, and at what temperature?
- Aluminum
- Gold
- Glass
- Carbon
Aluminum pan used up to 600 C (~ 650 C melts).
What are important properties of the experiment of DSC,mention calibration?
- Sample is weighed and sealed with a cover.
- Purge gases - nitrogen or helium.
- Calibration is done by:
- baseline calibration
- heat flow calibration: Indium
- heat capacity calibration: Sapphire
What is an Endothermic Heat Flow, give examples?
When heat flows into a sample as a result of an endothermic process, for example:
1. glass transition
2. melting
3. evaporation
Graph: downwards
What is an Exothermic Heat Flow, give examples?
When heat flows out of the sample as a result of some exothermic process, for example:
1. crystallization
2. cure
3. oxidation
Graph: upwards
What are the factors affecting DSC measurement?
- Sample shape (thin for optimisation)
- Type of pan
- Heating rate
- Purge gas
What properties make the ideal purge gas?
- Dry, inert atmosphere
- Ensured even heating
- Helps sweep away gases that might be released
What purge gas is used and why?
Nitrogen
1. Most common
2. Increases sensitivity
3. Typical flow rate of 50 ml/min
What is calibration of DSC based on?
1. Transitions:
- melting (pure metals, In, Sn, Pb)
- polymorphic transitions (KNO3)
2. Enthalpies of high purity metals (In)
What transitions does DSC detect, and explain each.
- First order transitions (melting and polymorphic changes)
- Second order transitions (change in heat capacity)
(no enthalpy change)
How do you calculate the degree of crystallinity of a polymer, and what is the ΔH for a 100% crystalline material?
Calculated from enthalpy of melting, with ΔH = 290 J/g.
What is the effect of crystallinity on Tg?
- Low % amorphous: negligible heat capacity change
- 100% amorphous: large heat capacity change
What does the bulk state involve?
Both amorphous and crystalline polymers
What is the glass transition temperature (Tg)?
- The temperature at which the polymer softens, changing from a solid ‘glassy’ state to a soft ‘rubbery’ state.
- Measures changes caused by mass loss.
What occurs at the Tg?
Sufficient thermal energy is present allowing segments of the main molecular chains to rotate, causing an increase in the free volume of the polymer.
What are the factors affecting the Tg?
- chemical composition and structure
- secondary bonding
- crosslinking
- presence of plasticizers
What happens during plasticization of the polymer?
- Internal stresses are generated
- due to the accumulation of water molecules
- in the polymeric network.
What is the reduction of Tg caused by?
The breaking of existing hydrogen bonds by water which facilitates chain rotation.
What information do we get from thermal analysis?
Changes in material properties as function of temperature.
What is the effect of the sample purity on the curves?
- Peak height is reduced.
- Melting occurs over a broader temperature range.
- Melting point is lowered.
What are necessary properties for a successful DSC experiment?
- sample weight 1 - 5 mg
- Atmosphere N2 (flow rate - 50cm/3min)
- Heating rate - 1C/min (slow)
What is the procedure of DSC?
- Weigh sample and seal.
- Set temperature program.
- After DSC scan measure ΔH.
- Select temperature values, calculate partial areas and fraction melted.
(F=Apartial/Atotal).
What is the initial and final temperature of DSC?
Initial: 10C below melting T
Final: 20C above melting T
