PDF Thermal Transitions: Crystallization, Melting and the Glass ...
[Pages:28]Thermal Transitions: Crystallization, Melting and the Glass Transition
? This lecture: Crystallization and Melting
? Next Lecture: The Glass Transition Temperature
Today:
? Why do polymers crystallize in a chain
folded fashion? ? Why do polymers melt over a range of
temperatures? ? What are the factors that affect the Tm?
Chapter 8 in CD (Polymer Science and Engineering)
Thermal Transitions
Temperature Crystallization
Viscoelastic liquid
Melting
Glass Transition
Semi-crystalline Solid
Glassy Solid
Crystallization and the Glass Transition
Volume
Glassy Solid
Cool
Liquid or Melt
Crystalline Solid
Temperature Tg Tc
Kinetics, Crystallization and the Glass Transition
Volume Volume
Glassy Solid
Cool Quickly
Liquid or Melt
Does not Crystallize !
Cool Quickly Cool Slowly
Temperature
Tg
Crystallizable Polymer
Temperature
Tg Tg
Non-Crystallizable Polymer
Polymer Crystallization
WHAT DO WE KNOW FROM EXPERIMENTAL OBSERVATION ?
? Crystallization occurs relatively slowly
? At high undercoolings ? And results in the formation
of chain folded lamellae
Polymer Crystallization
Melt
SMALL MOLECULES
Specific
Cool
Volume
What is undercooling ?
Heat Tc Tm
Semi-crystalline Solid
Temp POLYMERS
Specific
Cool
Volume
Heat
Tc Tm Temp
Crystallization Kinetics General Features
? Induction period - formation of primary nuclei
? Primary crystallization - a period of fast spherulitic growth
? Secondary crystallization - a period of slower crystallization that occurs once the spherulites have impinged on one another
Degree of Crystallinity
Secondary Crystallization
Primary Crystallization Induction Period (Nucleation Step)
Time
Thermodynamic Considerations
The free energy of this primary nucleus is given by
Gcryst = (4 xl ) + (2 x 2 ) e - (x 2l)g
? The last term represents the free energy that we would obtain if all the segments were in the bulk.
? The first two terms are the excess free energy that must be "added in" to account for those segments at the surface.
l x
e
x
................
................
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