1. Polymer Thermal Transitions: Crystallization, Tm and Tg
高分子專題演講
黃耀正
張豐志教授實驗室
交通大學應用化學所
2009/10/19
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

2. Glass Transition Temperature (Tg) & Crystalline Melting Point (Tm)
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

3. Thermal transitions of polymers
Completely amorphous polymer:
glass transition temperature (Tg) only
Completely crystalline:
crystalline melting point (Tm) only
Semi-crystalline: Both Tg & Tm
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

4. Semicrystalline polymers
Two major types of thermal transitions:
Crystalline melting point (Tm)
Above Tm, polymer is liquid
Below Tm, polymer forms flexible crystalline solid
Glass transition temperature (Tg)
Above Tg, polymers are rubbery
Below Tg, polymers are glassy
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

5. Glass transition temperature (Tg) From glassy state to rubbery state
The temperature where polymer loses its glasslike properties and assumes those more commonly identified with a rubber.
Glassy
glassy transition
rubbery plateau
Rubbery flow
Viscous flow
From glassy state to rubbery state
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

6. Factors affecting Tg Chain length Chain flexibility Side groups
Branching
Cross-linking ‧
etc.
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

7. Chain length
Each chain end has some free volume associated with it. A polymer with shorter chains will have more chain ends per unit volume, so there will be more free volume. Hence Tg' for shorter chains will be lower than Tg for long chains. Note that the shorter-chained polymer also has more free volume frozen in below Tg than the long-chained polymer.
Free volume
A small amount of unfilled volume is associated with the end of a polymer chain. This volume is called the free volume.
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

8. . Chain Flexibility Side Groups Branching Cross-linking
A polymer with a backbone that exhibits higher flexibility will have a lower Tg. This is because the Ea for conformational changes is lower. Therefore, conformational changes can take place at lower temperatures.
Side Groups
Larger side groups can hinder bond rotation more than smaller ones, and therefore cause an increase in Tg. Polar groups such as Cl, CN or OH have the strongest effect.
Branching
Polymers with more branching have more chain ends, so have more free volume, which reduces Tg, but the branches also hinder rotation, like large side groups, which increases Tg. Which of these effects is greater depends on the polymer in question, but Tg may rise or fall.
Cross-linking
Cross-linking reduces chain mobility, so Tg will be increased. It also affects the macroscopic viscosity of the polymer, since if there are cross-links between the chains, then they are fixed relative to each other, so will not be able to slide past each other. .
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

9. Crystalline Melting Point (Tm)
Further increase in temperature, provides more energy to the molecules and the rubber will now melt (For simplicity we are assuming that the hose is made out of thermoplastic, non cross linked material). This is the melt phase. The temperature at which the molecules transition from a rubbery state to a melt state is called the melt temperature or the Tm. Tm Tm
Tmo
Tmo Tc
1/lc
Lc∞
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

10. Factors affecting Tm
Same factors affect both Tg & Tm. Usually high Tg & high Tm are found together and vice versa.
High polarity and hydrogen bonding increases Tm.
Molecular symmetry leads to high Tm. Eg: Polyethylene has high Tm due to tightly packed crystals although Tg is low due to flexibility of the chain.
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

11. Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

12. Crystallization Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

13. Crystalline and amorphous polymers
Amorphous polymers are generally found in a random coil
conformation and have a disordered chain structure.
Crystalline polymers are predominantly in the all-trans conformation, and the chains are arranged in lamellae, as below:
Folded chains into a two-dimensional lamella
One-dimensional chain-folded sequences
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

14. Structure of Crystalline Polymers
The Fringed Micelle Model
According to the fringed micelle model, the crystallites are about 10 nm long.
The chains are long enough to pass several crystallites, binding them together.
amorphous
crystalline
Interconnected by amorphous regions
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

15. Polymer Single Crystals
1957 Keller prepared single crystals of PE.
The Folded-Chain Model --- Composed of polymer chains folded back on themselves
Adjacent reentry
10~20 nm thick
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

16. The Unit Cell of Polyethylene
Orthorombic
TEM image
ED pattern
Top view
Side view
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

17. Single crystal of Nylon 6
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

18. PEO-b-PS : the crystals reject the amorphous portion (PS), which appears on the surfaces of the crystals.
Optical micrograph
Electron micrograph
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

19. Switchboard Model
In the switchboard model the chains do not have a reentry into the lamellae by regular folding; they rather reenter more or less randomly.
tie molecule lc
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

20. Spherulitic Morphology
Polarizing Optical Microscope
 Maltese cross
Formation
1° nucleus
2° column
3° additional
4° fill
When the spherulites are nucleated simultaneously, the boundaries between them are straight. However, when the spherulites have been nucleated at different times, their boundaries form hyperbolas.
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

21. SEM Image Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

22. Mechanism of Spherulite Formation
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

23. Kinetics of Crystallization
Experimental Observations of Crystallization Kinetics
Slope = grown rate(μm/min)
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

24. Thermodynamic control
Poor chain mobility
High supercooling
degree (ΔT)
Good chain mobility
Low supercooling
degree (ΔT)
ΔT = Tf - Tc
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

25. Example Calculation of Percent Crystallinity
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

26. PEO-b-PS Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

27. Ac: Intergrated area of crystalline region
Aa: Intergrated area of amorphous region
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

28. Methods of Determining Crystal Structure
(1) X-Ray Methods (2) Electron Diffraction of Single Crystals (3) Infrared Absorption (4) Raman Spectra
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU

29. Thanks for your attention
Chang’s group, Polymer Research Center
Institute of Applied Chemistry, NCTU