Thermal Analysis Flashcards
What does thermal analysis measure?
Measures a substance physical or chemical properties under heating, cooling or when held at constant temperature
What can affect the reaction rate and change the shape or transition temperature of the TG curve?
Experimental parameters: pan type, sample mass, purge gas, ramp rate
What is a endothermic reaction?
When a sample absorbs heat and heat flow is positive
What instruments are used to measure thermal properties?
TGA and DSC
What does TGA measure?
mass loss
What does DSC measure?
Heat flow
What regions does Tg occur in?
amorphous regions ONLY
What regions does Tm occur in?
Crystalline regions ONLY
What kind of process is crystallization?
exothermic process– lowest energy state and entropically favorable
What process is melting?
endothermic – breaking of bonds
What is a first order transition and what does it reveal?
- Change in the latent heat
- A discontinuity in the free energy in first derivative
What is a second order transformation?
Continuous phase transformation
What kind of transition is a glass transition?
Second order
What is a single more important design parameter for polymers?
Glass transition region
What happens during the glass transition region?
- large drop in modulus
- onset of molecular motion without melting
What are two techniques to measure Tg?
-DMA
-DSC
What is a Tg?
- Second order phase transformation
- Large changes in Young’s modulus at temperatures around Tg
How can we influence Tg?
- Repeat unit structure
- Chain length
- Composition
What is Tg dependent on?
Pressure
What should Tg depend on?
- Main bonding
- Steric effects
- Isomerism
- Crosslinking
Which a larger number of chain ends what happens to the Tg?
Lower Tg
How many Tg values does a miscible polymer blend have?
A single one between the Tg values of the individual polymers
What does Tg do with increasing pressure?
increases
What is the effect of pressure in terms of volume and conformational freedom?
lower
What does the presence of crystallinity do to Tg?
- shift Tg to higher temps
- spreads the transition over a larger temperature range
