Phase Transitions II Flashcards
What do n and K parameter of the JMAK equation depend on?
they depend on mode of growth (homogeneous or heterogeneous) and shape of created particles
What does JMAK model do with secondary crystallization?
- it assumes 100% conversion has been reached when in reality it has not and continues to increase.
How is crystallization measured?
- density change
- x-ray diffraction
- differential calorimetry (heat flow)
How is density changed used to measure crystallization?
measure the density at beginning and end of test in a manometer (thus getting the height changes) and can relate degree of crystallinity to the change in height
How is x-ray diffraction used to measure crystallization?
- tells us about polymorphism and lattice parameters
- for the diffraction angle can get species of chains, orientation of chains, and overall orientation
*however, takes a long time to get good results + very expensive (these are snapshot technologies not over the entire process)
How does differential calorimetry measure crystallization?
Packing into crystals is moving to low entropy states. The crystallization process shows an exothermic spike as the system is cooled
DSC measures heat flow to the sample relative to a blank and reports the heat difference (Q) required to change the temperature of the sample compared to the reference (or to hold the temperature)
For constant temperature, the heat to a certain point in time divided by the heat for the entire system gives the degree of crystallinity
Why is differential calorimetry good for measuring crystallization?
- real time measurement of multiple phase transitions + accurate and sensitive, excellent for measuring kinetics
What does non-isothermal differential calorimetry give us?
Multiple heating/cooling cycles allow discovery of the crystallization and melting temperatures as well as the enthalpy of the transition (integral of the peak)
